Corrosion Behavior of Titanium Based Ceramic Coatings Deposited on Steels (Korrosionsverhalten Einer Auf Stahl Abgeschiedenen Keramikbeschichtung Mit Titanbasis)

Corrosion Behavior of Titanium Based Ceramic Coatings Deposited on Steels (Korrosionsverhalten einer auf Stahl abgeschiedenen Keramikbeschichtung mit Titanbasis)

Der Technischen Fakultät der Universität Erlangen-Nürnberg zur Erlangung des Grades

DOKTOR-INGENIEUR

vorgelegt von Frau. Dipl.-Ing. Rania Ali

Erlangen- 2016

Als Dissertation genehmigt von der Technischen Fakultät der Universität Erlangen-Nürnberg

Tag der Einreichung: 11.12.2012 Tag der Promotion: 13.11.2015 Dekan: Prof. Dr. Peter Greil Berichterstatter: Prof. Dr. Sannakaisa Virtanen Prof. Dr. Andreas Roosen

Acknowledgements:

One of the joys of completion is to look over the journey past and remember all the friends and family who have helped and supported me along this long but fulfilling road.

First of all I would like to express my heartfelt gratitude to my supervisor, Prof. Dr. Sannakaisa Virtanen. This thesis would not have been possible without her help, support and patience. She unconditionally and readily shared her knowledge and offered support, providing me with valuable insight and many ideas for the research.

I would also like to thank my examining committee, Prof. Dr. Andreas Roosen, and Prof. Dr. Nadja Popovska-Leipertz, Prof. Dr. de Ligny, who provided encouraging and constructive feedback.

I am very thankful to my friends and colleagues, Dr. Manuela Killian, Dr. Emad Alkhateeb, Dr. Florian Kellner, Dr. Florian Seuss, Dr. Leonhard Klein, Dr. Hanadi Ghanem, Dr. Giorgia Obigodi-Ndjeng, for always being there and ready to share their experience with me over my PhD period.

I also thank Prof. Dr. Patrik Schmuki for giving me the possibility to carry out my thesis at LKO.

I appreciate all my colleagues and friends who have helped me with surface analysis and technical stuff. Also many thanks to the LKO staff, particularly, the corrosion group, who enabled me to enjoy every day during this research process.

Special thanks go to Mr. Hans Rollig and his family. You have been like surrogate family sheltered me over the years.

I would like to thank my parents, my sisters and brothers. Without their love and support I would have not completed this road.

Finally, I would like to thank my husband, Ghadeer Diab. He was always there cheering me up and stood by me through the good times and the bad times.

Dedicated to:

My small family,

my beloved husband,

and my little angel, my daughter Leah

You are the sunshine of my life…

Abstract

Titanium based ceramic films are increasingly used as coating materials because of their high hardness, excellent wear resistance and superior corrosion resistance. Using electrochemical and spectroscopic techniques, the electrochemical properties of different coatings deposited on different steels under different conditions were examined in this study.

Thin films of titanium nitride (TiN), titanium diboride (TiB2), and titanium boronitride with different boron concentrations (TiBN-1&2) were deposited on stainless steel and low carbon steel by chemical vapor deposition using the hydrogen reduction of TiCl4, BCl3 and N2 at a reduced pressure of 600 mbar and a temperature of 900°C. The factors evaluated were the substrate material, the coating composition, the boron content, the thickness and the boron content. Different alternating current and direct current electrochemical methods (corrosion potential screening, potentiodynamic techniques at low scan rate and electrochemical impedance spectroscopy) were used to study the electrochemical behavior of the different coated steels in different electrolytes at ambient temperature. The porosity of the deposited coatings which is essential for the estimation of the corrosion resistance of coated components was also measured. Results showed that different coatings deposited on different steels have different morphologies and crystal structures and consequently different corrosion resistance. The resistance to corrosive attack of the coatings deposited on stainless steel was relatively poor for TiN, better for TiBN-1&2 and best for TiB2.

On coated low carbon steel, TiB2 showed the worst corrosion resistance, followed with TiN and TiBN-1 with relatively better resistance; TiBN-2 was the best. Thicker TiBN-3 with higher boron content deposited on low carbon steel was tested in simulated soil solution, simulated seawater and 1 M HCl. The corrosion resistance was also evaluated with immersion tests. The effect of different temperatures (15, 35, and 45°C) was evaluated.

Finally, applied cathodic protection and interrupted cathodic potential measurements were also carried out. To elucidate the corrosion protection mechanisms of the coatings, coating morphology, chemical composition and crystal structure was studied by different characterization techniques before and after corrosion testing. Results showed that coating with good corrosion resistance has to meet the following requirements: fine and dense structure with low porosity, good adhesion to the material substrate. Surface analyses indicate that the coatings do not only offer a physical barrier which protects the substrate material from aggressive species, but also oxidize to form an oxide passive layer on the coating surface, which consists mainly of titanium oxide and titanium oxynitride. This layer enhances the corrosion protection due to its chemical inertness; it also fills the cracks existing on the surface and decreases the number of pathways which allow the electrolyte to penetrate into the underlying substrate. It is also shown that coatings with nano-crystal structure, and intermixed phases with different crystal orientation, as TiBN-3 on low carbon steel, can provide a superior corrosion protection in neutral test solution up to 90 days immersion days. In acid medium, the coating is less protective due to the dissolution of the oxide layer. Applying cathodic protection was found to decrease the protection effect of the coating due to the reduction of the oxide film. Interrupting the applied cathodic potential leads to coating damage and peeling off due to hydrogen embrittlement and the reduction and reformation of the oxides filling the cracks which leads to chipping off of the deposited coating.

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Zusammenfassung

Titan basierte keramische Beschichtungen werden mit zunehmender Häufigkeit wegen ihrer hohen Härte, guten Abriebresistenz und überragender Korrosionsbeständigkeit verwendet. Mittels elektrochemischer und spektroskopischer Untersuchungen wurden in dieser Arbeit die elektrochemischen Eigenschaften unterschiedlicher Beschichtungen auf unterschiedlichen Stählen und mit unterschiedlichen Beschichtungsbedingungen evaluiert.

Dünne Filme aus Titannitrid (TiN), Titanborid (TiB2) und Titanboronitrid mit variierender Borkonzentration (TiBN-1&2) wurden sowohl auf Edelstahl als auch auf kohlenstoffarmem Stahl mittels chemischer Gasphasenabscheidung aufgebracht. Dabei wurden TiCl4, BCl3 und N2 unter reduziertem Druck (600 mbar) bei einer Temperatur von 900°C reduziert. Das Substratmaterial, die Beschichtungszusammensetzung, -dicke und der Borgehalt wurden ausgewertet. Um das elektrochemische Verhalten der unterschiedlichen, beschichteten Stahlproben in verschiedenen Elektrolyten bei Raumtemperatur zu prüfen, wurden unterschiedliche elektrochemische Methoden mit Wechsel- oder Gleichstrom angewandt (Screening des Korrosionspotentials, potentiodynamische Techniken mit geringer Rasterfrequenz, elektrochemische Impedanzspektroskopie). Die Porosität der abgeschiedenen Beschichtungen, welche im Hinblick auf die Bestimmung der Korrosionsbeständigkeit der beschichteten Komponenten essenziell ist, wurde ebenfalls bestimmt. Die Messungen zeigten, dass unterschiedliche Beschichtungen auf unterschiedlichen Stählen unterschiedliche Oberflächenstrukturen und Kristallstrukturen aufweisen, was wiederum in einem unterschiedlichen Korrosionsverhalten resultiert. Die Beschichtung von Edelstahl mit TiN wies relativ geringe Resistenz gegenüber korrosiven Angriffen auf, TiBN 1&2 wiesen bessere Beständigkeit auf, am besten schnitt TiB2 ab. Auf kohlenstoffarmem

Stahl wiederum zeigte TiB2 die schlechteste Korrosionsbeständigkeit, gefolgt von

TiN und TiBN-1, welche im Vergleich stabiler waren; TiBN-2 zeigte die besten Resultate.Dickere Beschichtungen mit erhöhtem Borgehalt (TiBN-3) auf kohlenstoffarmem Stahl wurden in künstlicher Bodenlösung (simulated soil solution), künstlichem Meerwasser und 1M HCl getestet. Die Korrosionsbeständigkeit wurde auch mittels Tauchtests evaluiert. Der Einfluss von Temperatur (15°C, 35°C, 45°C) auf die Korrosion wurde ebenfalls untersucht. Schlussendlich wurden kathodischer Schutz und kathodische Potentialmessungen durchgeführt.

Um den Mechanismus der Korrosionsprotektion aufzuklären, wurden die Oberflächenbeschaffenheit, chemische Zusammensetzung und Kristallstruktur der Beschichtungen mit verschiedenen Untersuchungsmethoden vor und nach den Korrosionsbeständigkeitstests bestimmt. Die Ergebnisse lassen den Rückschluss zu, dass für gute Korrosionsbeständigkeit folgende Voraussetzungen notwendig sind: feine und dichte Struktur mit geringer Porosität und gute Substratanhaftung. Oberflächenanalytische Untersuchungen legen nahe, dass die Beschichtungen nicht nur eine physische Barriere gegenüber aggressiven Medien darstellen, welche das Substrat schützt, sondern auch durch Oxidation stabilen Passivfilme auf ihrer Oberfläche ausbilden, welche hauptsächlich aus Titandioxid und Titanoxynitrid bestehen. Diese Schicht erhöht den Korrosionsschutz wegen ihrer chemischen Inertanz, füllt gleichzeitig auf der Oberfläche vorhandene Risse auf und verringert die Anzahl der möglichen Wege, auf denen der Elektrolyt das unterliegende Substrat erreichen kann. Es wird auch gezeigt, dass Beschichtungen mit nanokristalliner Struktur und vermischten Phasen unterschiedlicher kristalliner Orientierung (z.B. TIBN-3 auf kohlenstoffarmem Stahl) für bis zu 90 Tage überragenden Korrosionsschutz in neutraler Testlösung bieten können. In saurem Medium ist die Beschichtung weniger effizient, da der passive Oxidfilm angegriffen werden kann. Das Anlegen eines kathodischen Schutzpotentials zeigte eine Verringerung des Schutzeffekts der Beschichtungen, da diese während des Prozesses reduziert wurden. Unterbrechung des angelegten kathodischen Potentials führte zu

Beschädigungen an den Beschichtungen und deren Ablösung durch Wasserstoffversprödung und Reduktion und Umgestaltung der Oxidschicht, was zu Volumenvergrößerungen des Oxids in den Rissen der Beschichtung und daraus resultierendem Absplittern der Beschichtung führte

Table of contents

Chapter 1: INTRODUCTION …………………………………………….1 Chapter 2: LITERATURE REVIEW……………………………………..5 2.1: Steel…………………………………………………………………………………5 2.2: Corrosion and corrosion protection…………………………………………..6 2.2.1: Corrosion forms…………………………………………………………..6 2.2.2: Corrosion protection……………………………………………………..7 2.2.3: Passive Procedures………………………………………………………8 2.2.3.1: Organic coatings (polymer coatings or paints)…………………9 2.2.3.2: Anodic protection: Passivation…………………………………11 2.2.3.3: Cathodic protection………………………………………………12 2.2.3.4: Barrier protection………………………………………………...12 2.3: Titanium based ceramic coatings……………………………………………13 2.3.1: Titanium Nitride coatings (TiN)……………………………………….13

2.3.2: Titanium diboride coatings (TiB2)……………………………………15 2.3.3: Boron nitride (BN)………………………………………………………16 2.3.4: Titanium boronitride coatings (TiBN)…………………………...... 18 2.4: Corrosion protection of titanium based ceramic coatings……………...20 2.5: Deposition process: Chemical vapor deposition (CVD)…………………24 2.5.1: Process principle and deposition mechanism……………………..24 2.5.2: Chemical precursors and reaction chemistry……………………...26 2.5.3: Advantages and limitations……………………………………………27 2.5.4: Applications………………………………………………………………28 2.5.5: CVD process parameters………………………………………………29 2.5.5.1: Temperature and pressure……………………………………..29 2.5.5.2: Coating-substrate adhesion…………………………………….30 2.6: Research Objectives: Protective coatings…………………………………32

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Table of contents

2.7: The electrochemical testing methods………………………………………34 2.7.1: Potentiodynamic polarization methods (potential-current diagrams)………………………………………………………………...35 2.7.2: Electrochemical Impedance Spectroscopy (EIS)………………….38

Chapter 3: EXPERIMENTAL WORK AND METHODS…………….40 3.1: Materials and electrolytes…………………………………………………….40 3.2: Samples preparation…………………………………………………………..42 3.3: Coating of steel…………………………………………………………………42 3.3.1: Coating of stainless steel……………………………………………..42 3.3.2: Coating of low carbon steel…………………………………………..44 3.4: Coating characterization………………………………………………………46 3.4.1: X-ray Photoelectron Spectroscopy (XPS)………………………….46 3.4.2: Scanning Electron Microscopy and Energy Dispersive X-ray (SEM & EDX)……………………………………………………………..46 3.4.3: X-Ray Diffraction (XRD)………………………………………………..47 3.4.4: Glow Discharge Optical Emission Spectroscopy (GD-OES)……48 3.4.5: Focused Ion Beam Microscopy (FIB)……………………………….48 3.4.6: Metallographic microstructural study………………………………49 3.5: The electrochemical measurements………………………………………...50 Chapter 4: RESULTS…………………………………………………...55 4.1: Characterization of different coatings on stainless steel……………….55 4.1.1: Surface morphology and optical metallographic cross-sections of different coatings……………………………………………………55 4.1.2: Chemical composition of the deposited layer……………………..59 4.1.3: Crystal structure of the deposited coatings (XRD)……………….60

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Table of contents

4.2: Characterization of different coatings on low carbon steel……………..62 4.2.1: Surface morphology and optical metallographic cross-sections of different coatings……………………………………………………62 4.2.2: Crystal structure of the coatings…………………………………….67 4.2.3: Chemical composition of the coatings……………………………..68 4.3: Characterization of thick TiBN-3 coating deposited on low carbon steel……………………………………………………………………………….78 4.4: Electrochemical investigations on coated metals……………………….91 4.4.1: Electrochemical characterization of coated stainless steel……91 4.4.1.1: Open-circuit potentials and potentiodynamic polarization measurements…………………………………...91 4.4.1.2: Electrochemical impedance spectroscopy measurements...... 94 4.4.2: Electrochemical characterization of coated low carbon steel….96 4.4.2.1: Open-circuit potentials and potentiodynamic polarization measurements…………………………………...96 4.4.2.2: Electrochemical impedance spectroscopy measurements...... 99 4.4.2.3: Surface characterization after electrochemical measurements……………………………………………….101 4.4.3: Summary: The electrochemical and corrosion behavior of different coatings on different steel substrates…………………103 4.4.4: Electrochemical characterization of TiBN-3 coated low carbon steel……………………………………………………………………..104 4.4.4.1: The results of 48 hours measurements……………………104 4.4.4.1.1: Open circuit potential measurements in simulated soil solution and simulated seawater…………104

Table of contents

4.4.4.1.2: Potentiodynamic polarization measurements in simulated soil solution and simulated seawater …………………………………………………...106 4.4.4.1.3: Electrochemical measurements of TiBN-3 coated low carbon steel in 1M HCl……………………108 4.4.4.1.4: Electrochemical impedance spectroscopy measurements of TiBN-3 coated low carbon steel in different test solutions: 48 hours results …………………………….……………………..110 4.4.4.2: The results of long time immersion (90 days) measurements……………………………………………………...113 4.4.4.3: Potentiodynamic cyclic voltammograms of TiBN-3 coated low carbon steel at different immersion times in simulated soil solution and simulated seawater…………………….……………………..116 4.4.4.4: The electrochemical behavior at different temperature…………121 4.4.4.4: The electrochemical behavior under cathodic potential………..121 4.4.4.5: Pitting corrosion……………………………………………………..122 4.4.4.6: The electrochemical behavior TiBN-3 coated low carbon steel under applied cathodic potential...... 125 4.4.4.7: Interrupted cathodic polarization measurements………………..127 4.4.4.8: Surface analysis of TiBN-3 coating after different corrosion tests………………………………………………………………….131 4.4.4.8.1: SEM and FIB-cut analysis……………………………..131 4.4.4.8.2: X-ray diffraction analysis………………………………132 4.4.4.8.3: XPS surface analysis…………………………………..133 4.4.4.9: XPS analysis after measurements at different temperatures….136 4.4.4.10: XPS analysis after measurements at 48h in 1M HCl………….138 4.4.4.11: XPS analysis after interrupted cathodic protection measurements……………………………………………………...139

Table of contents

4.4.5: Summary: The electrochemical and corrosion behavior of TiBN-3 deposited on low carbon steel ………………………………………………..…141

4.5: Equivalent circuit for CVD coated steels………………………………….142

Chapter 5: DISCUSSION……………………………………………. 146 5.1: CVD process parameters…………………………………………………….146 5.1.1: The effect of substrate microstructure and chemical composition on different deposited coatings……………………………………146 5.1.2: The influence of boron flow rate on the morphology of TiN…..148 5.2: Corrosion and electrochemical behavior of different coatings on different steels………………………………………………………………...150 5.2.1: The electrochemical and corrosion behavior of different coatings on stainless steel and low carbon steel…………………………..151 5.2.2: The electrochemical behavior of TiBN-3 coating on low carbon Steel……………………………………………………………………..154 5.2.2.1: The influence of test solution………………………………..157 5.2.2.2: The effect of test temperature……………………………….159 5.2.2.3: Passivity and localized corrosion……………………………159 5.2.2.4: The effect of interrupted cathodic polarization…………….161

6: CONCLUSIONS……………………………………………………..163 7: OUTLOOKS (FUTURE WORK)…………………………………...165 8: BIBLIOGRAPHY…………………………………………………….166

Chapter 1: Introduction

1 Introduction

Metallic Materials have always been an important part of human culture and civilization; different types of materials are strongly needed in daily life applications. Figure 1.1 shows the world consumption of diverse materials in the mid-eighties. Likewise, today’s advanced technologies involve sophisticated materials, since all of them utilize devices, products and systems that must consist of various advanced materials. The current technical development is strongly dependent on new materials with particular mechanical, chemical, electrical, magnetic or optical properties.

Figure 1.1: World consumption of various materials in the middle of 1980’s [1]

Steels, in particular, are unquestionably the dominating industrial construction materials. The combination of low cost, good mechanical properties and manufacturing characteristics make their unique universal usefulness, although steels, are from a corrosion viewpoint relatively poor materials since they rust in air, corrode in aggressive environments.

Chapter 1: Introduction

Figure 1.2: Steel production worldwide from 1950 to 2007

Figure 1.2, introduced by the World Steel Association (WSA), shows the permanent increase in steel production worldwide. Corrosion and/or degradation of steel structures can cause dangerous and expensive damage to everything from automobiles, home applications, and drinking water systems to pipelines, bridges, and public buildings, like other natural hazards such as earth quakes or severe weather disturbances. The first significant report about the cost of corrosion was introduced by Uhlig in 1949 [2], where the annual cost of corrosion to the United States was estimated in the report to be $5.5 billion or 2.1 percent of the 1949 GNP. According to another corrosion study in the U.S. in 2002, made by NACE International Association, the direct cost of metallic corrosion was $276 billion on an annual basis representing 3.1% of the U.S. Gross Domestic Product (GDP). Unlike weather-related disasters, corrosion can be controlled.

Chapter1: Introduction

However, preventing and controlling corrosion depend on the specific material to be protected, environmental concerns such as a soil resistivity, humidity, and exposure to saltwater or industrial environments, and many other factors [3].

The most commonly used methods in controlling corrosion include organic and metallic protective coatings, plastics, and polymers, corrosion-resistant alloys; corrosion inhibitors and cathodic protection- a technique mainly used on pipelines, underground storage tanks, and offshore structures [4]. A combination of one or more protection techniques (e.g., protective coating and cathodic protection in pipelines) is in many applications very necessary to achieve effective protection [5].

Conventional coatings (organic, and inorganic), considered as the most widely applied protection method, lose their effectiveness with time and need maintenance and/ or must be replaced, which is again very costly [6]. This made the demand of developing new coatings with excellent corrosion resistance, especially for structure exposed to erosive-corrosive environments it is an issue of great importance [7].

Ceramic coatings, known as hard coatings, are promising candidates for this aim

[8]. Titanium based ceramic coatings (TiN, TiB2, and TiBN) characterized by high hardness, wear resistance, and corrosion resistance, are obtaining widespread use for strengthening and protection of constructional steels subject to wear and corrosion [9, 10]. The properties of the coatings i.e., morphology, porosity and other defects, strongly vary in function depend on deposition parameters, and consequently, influence their corrosion behavior [11-13].

Chapter 1: Introduction

Since many potential applications require a suitable corrosion resistance, the understanding of surface degradation processes and the mechanisms influencing corrosion is one of the key factors for the optimization of the materials. Electrochemical techniques are powerful tools, which can be used very effectively for probing corrosion processes, and life prediction, where relatively simple techniques can yield powerful information.

The present study had multiple objectives. The first goal was to study the corrosion behavior of four different types of titanium based ceramic films (TiN,

TiB2, TiBN with different boron contents) deposited on two different types of steels in different test solutions. Furthermore, the influence of changing deposition parameters on the morphology and the corrosion resistivity of TiBN coating deposited on low carbon steel was studied. Thicker TiBN was deposited on low carbon steel in order to produce coating free of defects. This coating was tested in solutions with varying chloride contents, at several temperatures, and under cathodic polarization. It was of our interest to evaluate the corrosion protection of the coatings by electrochemical methods and to develop a propitiate model from electrochemical impedance spectroscopy measurements (EIS), which can be used to fit the experimental data and extract the parameters which can characterize the corrosion process.

Chapter 2: Literature Review

2 Literature Review

2.1 Steels The term steel usually refers to an iron-based alloy containing carbon in amounts less than about 2%. Carbon steels can be defined as steels that contain only residual amounts of elements other than carbon, for deoxidation and better corrosion resistance (such as manganese, silicon and aluminum), to improve the mechanical properties (e.g., copper, nickel to improve strength; molybdenum to help resisting embrittlement).

Carbon steel, although susceptible to corrosion, is one of the most widely used materials in the industry and in daily life applications. This material is used in water- and steam- pressure systems of power plants, in oil structures (pipes, tanks and transporting ships), and as support for many other structures throughout the world due to their fairly low cost (compared to other different materials e.g. Ti, Cr, Al,…), good properties, high strength and ease of fabrication, availability, weldability.

As the description implies, the primary alloying element of these iron-based materials is carbon. Because carbon is such a powerful alloying element in steel, there are significant differences in the strength, hardness, and ductility achievable with relatively small variations in the levels of carbon in the composition. However, other important factors- such as heat treatment and fabrication processes can result in significant changes to the properties of the carbon steel components. Steels are from a corrosion viewpoint poor materials since they rust in air, corrode in acids and scale in furnace atmosphere, thus, providing steel structure with additional corrosion protection is of high importance to prolong the service life of steel structures.

Chapter 2: Literature Review

2.2 Corrosion and corrosion protection Corrosion is defined as the deterioration of a material, usually a metal, because of a reaction with its environment which leads to a measurable alteration of the material (properties, behavior), and may cause functional impairment of a component or the whole system. Thermodynamics and kinetic basic principles of the corrosion reaction allow the prediction of whether a corrosion reaction is possible or not (thermodynamics) and how fast it proceeds (kinetics).

2.2.1 Corrosion forms

Almost all corrosion problems and failures encountered in service can be associated with one or more of the seven basic forms of corrosion [14]: 1. Uniform surface corrosion General corrosion occurs on the entire surface at nearly the same rate. 2. Pitting corrosion Corrosion with locally different abrasion rates; caused by the existence of corrosion elements, known as shallow pit corrosion. Local corrosion resulting in holes, that is, in cavities expanding from the surface to the inside of the metal. 3. Crevice corrosion Local corrosion occurs in any confined spaces caused by component design or joints (metal or nonmetal). 4. Galvanic corrosion Dissimilar metal corrosion occurs at contact surfaces of different metals. 5. Intergranular corrosion Corrosion takes place in or adjacent to the grain boundaries of a metal.

Chapter 2: Literature Review

6. Stress corrosion cracking (SCC) SCC is the result of straining a metal (residual or applied stresses) in a corrosive environment. 7. Erosion corrosion Corrosion of a metal which is caused or accelerated by the relative motion of the environment and the metal surface Mechanisms and characteristics of different corrosion forms were thoroughly discussed by many authors [15-24].

2.2.2 Corrosion protection All methods, measures, and procedures aimed at the avoidance of corrosion damages are called corrosion protection [3, 15, 25-30]. Modifications of a corroding system in so far as corrosion damages are minimized. Figure 2.1 gives an overview about how corrosion can be mitigated.

Corrosion Protection

Passive Corrosion Protection Active Corrosion Protection Keeping corrosion substances Avoidance of corrosion away from steel substrate

Intervention in the Artificial cover and Metallic coatings and corrosion process protection layers organic layers

Corrosion Protection Planning Removal of aggressive Influencing aggressive substances substances Practical design Suitable for the material Construction selection Intervention in the electrochemical process Figure 2.1: Methods, measures, and procedures of corrosion protection (van Oeteren, Korrosionsschutz – Fibel [31]).

Chapter 2: Literature Review

2.2.3 Passive Procedures In passive corrosion protection, corrosion is prevented or at least decelerated through the isolation of the metal from the corrosive environment by the applied protective layers. These protective layers must fulfill the following technical preconditions:

 It has to be pore-free; impermeable to ionic moieties [32] and if possible, to oxygen.  Maintain adhesion (to the metal) under wet service condition.  Corrosion resistant.  It must be resistant to external mechanical stress; and possess certain ductility.

The main types of protective coatings are classified as follows, Figure 2.2.

Protective coatings

Metallic coatings Non-metallic coatings Organic coatings

1. Hot dipping 1. Surface or chemical 1. Paints (a) Galvanizing conversion coatings 2. Varnishes (b) Tinning (a) Chromate coating 3. Lacquers (b) Phosphate coating 4. Enamels 2. Metal spraying (c) Oxide coating

3. Cladding 2. Anodizing

4. Cementation (a) Sherardizing (b) Chromizing (c) Calorizing

5. Electroplating or electrodeposition Figure 2.2: Main types of protective coatings [33].

Chapter 2: Literature Review

2.2.3.1 Organic coatings (polymer coatings or paints)

Organic coatings are inert organic barriers applied to metals substrates. They can be effective barriers to protect steels when it is anticipated that the coating can be applied to cover essentially all of the substrate surface and when the layer remains intact in service. The key to maintaining corrosion protection by an intact coating is sufficient adhesion to resist displacement forces, since; at a point of weak adhesion between the surface and the substrate, the stress can lead to disbandment. If the coating covers the entire surface of the steel on a microscopic as well as a macroscopic scale, and if perfect wet adhesion could be achieved at all areas of the interface, the coating would protect steel against corrosion indefinitely. Practically, it is very difficult to achieve both of these requirements in applying coatings and to assure full coverage of the entire metal surface as required for barrier coating. Furthermore, coatings that were intact initially may be damaged during their service lives, even those designed to minimize the probability of mechanical failure. In such cases it is generally desirable to design coatings to suppress electrochemical reactions rather than primarily for their barrier properties. This can be achieved by the use of passivating pigments, which promote the formation of a barrier layer over anodic areas, passivating the surface. For the pigment to be effective, the binder must permit diffusion of water to dissolve the pigment, which in turn must have minimum solubility. However, if the solubility of the pigment is too high, the pigment would leach out of the coating film too fast, limiting the time that it is available to inhibit corrosion. The use of passivating pigments may lead to blistering after exposure to humid environments if water permittivity was too high.

Chapter 2: Literature Review

Anticorrosive pigments be divided into three types [34]:

1. Pigments with a physical protective action are chemically inactive or passive. These lamellar pigments are packed in layers; they lengthen the pathways of ions and inhibit their penetration.

In addition, they improve adhesion between substrate and coating and protect the underlying binder. An example is micaceous iron ore.

2. Pigments with chemical protective action contain soluble components and can maintain a constant pH value in the coating. Their action depends on reactions in the interfacial areas between the pigment and substrate, pigment and binder, or between pigment and ions that penetrate into the coating. An example is red lead. Redox reactions can occur to form protective compounds (oxides or oxide hydrates that may contain pigment cations). The somewhat soluble PbO raises the pH and neutralizes any fatty acids formed over time. The toxic hazards of red lead have resulted in prohibition of its use.

3. Pigments with an electrochemical protective action passivate the metallic surface. Those that prevent corrosion of the iron by forming a protective coating (e.g., phosphate pigments) are regarded as being active in the anodic region of the metal surface (anodic protection). Pigments that prevent rust formation due to their high oxidation potential (e.g., chromate) are said to be active in the cathodic region (cathodic protection). Soluble chromate has also been established as carcinogenic to humans. They must be handled with appropriate caution.

Electrochemical corrosion can also be mitigated without the use of organic coatings; this can be achieved by suppressing the anodic reaction; suppressing cathodic reaction; or by preventing water, oxygen, and corrosion stimulants from contacting the surface.

Chapter 2: Literature Review

2.2.3.2 Anodic protection: Passivation

The mechanism of anodic protection is based on the theory, that if the oxygen concentration near the anode is high enough; ferrous ions are oxidized to ferric ions soon after they are formed at the anodic surface. Ferric hydroxide forms a barrier over the anodic areas, since; it is less soluble in water than ferrous hydroxide. Suppression of corrosion by retarding the anodic reaction is called passivation. However, a variety of oxidizing agents are used as passivators. Chromate, nitrite, molybdate, and tungstate salts are examples. As with oxygen, a critical concentration of these oxidizing agents is needed to achieve passivation, since lower concentrations may promote corrosion by cathodic depolarization. The most extensively studied reaction is the reaction with chromate salts. Partially hydrated mixed ferric and chromic oxides are deposited on the surface, where they presumably act as a barrier to suppress the anodic reaction. Other certain nonoxidizing salts, such as alkali metal salts of boric, phosphoric, and carbonic acids, also act as passivating agents. Their basicity may result in passivating action. By increasing pH, they may reduce the critical oxygen concentration for passivation below the level reached in equilibrium with air.

Alternatively, it has been suggested that the anions of these salts may combine with ferrous or ferric ions to complex salts of low solubility to form a barrier at the anode. Possibly, the corrosion protection effect could be a mixture of both mechanisms to some extent.

Recently, a new approach to passivation is developed, the use of a film of electrically conductive polymer to a steel surface to protect it from corrosion. An example is polyaniline; it is deduced to be effective by leading to the formation of an adhesive, very thin, metal oxide passivating layer on the surface of the metal.

Chapter 2: Literature Review

2.2.3.3 Cathodic protection

This type of corrosion protection is achieved by coating steel with zinc to make galvanized steel. The steel is protected in two ways: Zinc has a more negative electrode potential than does steel in most environments, so zinc is again the anode when it is coupled to steel. When defects such as a pinhole or a crack develop in the outmost zinc coating, the underlying steel is protected by the sacrificial corrosion of zinc. The surface of zinc becomes coated with a mixture of zinc hydroxide and zinc carbonate, after exposure to the atmosphere. Both are somewhat soluble in water and strongly basic.

2.2.3.4 Barrier protection

Barriers are films that can prevent corrosion by hindering oxygen and water from reaching the surface. The zinc layer on galvanized steel acts as a barrier. Tin coating on steel in tin cans acts as a barrier and it is effective as long as the can is closed. After a can has been opened, the cut bare edges expose both steel and tin to water and oxygen, and the steel corrodes relatively fast, because tin is nobler than iron in the electromotive series. Nevertheless, those coatings degrade by time elapsing [35-37], particularly, when environments become severe i.e., in oil production and refinery and chemical process industries, where conditions are aggressive and mechanical loads are high, those coatings do not provide sufficient protection and become economically not feasible, therefore the use of more corrosion resistive coatings becomes very important. In the last few decades a new group of coatings was developed and found their way into steel industrial applications, these coatings exhibit a high wear and corrosion resistance, they are titanium based ceramic films (known as hard coatings). 12

Chapter 2: Literature Review

2.3 Titanium based ceramic coatings

In modern production technology, the surface treatment of tools and other parts are important to improve the properties of work pieces, such as the corrosion resistance, wear resistance, etc... Titanium based ceramic coatings are widely used in practice, in particular titanium based ceramic films.

The high interest in Ti-based ceramic coatings, TiN, TiC, TiB2 and TiBN, as coatings arises from the unusual combination of properties that characterize these compounds and fulfill the requirements for good coatings. These compounds exhibit, on the one hand, ultra hardness and high melting points, typical characteristic of covalently bonded compounds. On the other hand, they also, because of their metallic-like bonding, display metallic properties, such as high thermal conductivity. The chemical inertness of those compounds favors their application as corrosion protective coatings in wide variety of aggressive environments [38]. In a relatively short time, these coatings have become major industrial materials with numerous applications such as cutting and grinding tools, drilling, bearing, textile machinery and many others. These coatings can be deposited by a wide variety of methods such as chemical and physical vapor deposition (CVD, PVD), ion plating and sputtering.

2.3.1 Titanium Nitride coatings (TiN)

Titanium nitride (TiN) belongs to the family of refractory transition metal nitrides, it has a cubic structure identical to TiC with exceptional combination of chemical, physical, mechanical and electrical properties, and a decorative golden appearance [39, 40]. It is not as hard as TiB2 and TiC but is more chemically resistant and has a lower coefficient of friction.

Chapter 2: Literature Review

Its basic application is in improving the wear-resistance of cutting and cold forming instruments [41, 42], it is most commonly used as a coating for drill bits, saw blades, and other grinding and shaping tool. Drill bits coated with TiN last up to three times longer than those without it. TiN coatings are deposited on stainless steels as well for medical applications (artificial hips, knees or teeth).

The protective properties of TiN coatings on various constructional materials including corrosion-resistant steel, are discussed in [43-46]. The conclusion was that the properties and the protection efficiency of TiN coating depend on the method of deposition, the coating thickness and the substrate type.

Synthesis of TiN can be performed by different deposition techniques, i.e. physical vapor deposition (PVD) [47, 48] and chemical vapor deposition (CVD) [49, 50]. The first commercial TiN coating was deposited on tools by CVD.

Several different CVD reactions have been used to deposit TiN, the most ¯ commonly used one is the reaction of titanium tetrachloride TiCl4 with molecular nitrogen. The range of temperature for this reaction is 900-1200 °C [51]. This deposition temperature can be relatively high for some materials; therefore, the use of ammonia, NH3, is sometimes preferred as a source of nitrogen, since is facilitates the reduction of the deposition temperature to about 500 to 700 °C [40, 52]. The corrosion behavior of deposited TiN was thoroughly studied in various media [48, 53-56], the evaluation of the electrochemical oxidation behavior of TiN showed that it has a high stability against oxidation in wide-pH range. The electrochemical oxidation resistive properties were attributed to the presence of the nitrogen-enriched surface layer of titanium oxynitride with a large electron density that screens the underlying titanium ions and inhibits the oxidation reaction [54, 57].

Chapter 2: Literature Review

Although TiN is already used in many industrial applications, its use as protective corrosion coating (especially on steels) is still limited due to the presence of pores, microcracks and other defects [48, 58, 59].

The porosity decreases when increasing the thickness but at the same time strains increase, causing spalling in the deposited film [60]. Since, the effectiveness of the corrosion protectiveness by the TiN coatings is determined by its continuity and its good adhesion to the substrate, more work and efforts are demanded to develop coatings with better protection quality.

2.3.2 Titanium diboride coatings (TiB2)

Titanium diboride (TiB2) is a ceramic material with a hexagonal structure in which boron atoms form a covalently bonded network within metallic Ti matrix. TiB2 is well known for its outstanding chemical and mechanical characteristics, such as high hardness, high stability at high temperature, high resistivity to corrosion, oxidation and chemical attacks [61]. In addition, because of its metallic-like bonding, TiB2 also exhibits very good thermal and electrical properties [62]. This combination of properties makes TiB2 very interesting as a coating material for various applications, especially for cutting tools.

Similar to TiN, the deposition of TiB2 can be carried out by different techniques. Chemical vapor deposition (CVD), in particular, was reported to have several advantages over the other conventional methods, e.g., good reproducibility and ease of controlling the growth rate, which are of great importance for good coatings [63].

The most extensively studied reaction for CVD deposition of TiB2 is between

TiCl4 and BCl3 by Peshev and Niemyski, 1965 [64].

Chapter 2: Literature Review

It was shown that the different morphological forms of TiB2 strongly depend on the deposition temperature.

TiB2 films are characterized by a strong [001] texture of the columnar grains and the grain boundaries perpendicular to the surface represents short cracks path that can also impair the toughness [65].

The literature on the corrosion behavior of titanium diboride is rare and mainly concerns acid environments [66, 67].

Under these conditions, the corrosion products of titanium diboride are found to be the titanyl ion, TiO2+ and the boric acid [68]. In addition to these compounds, also Ti3+ forms under acidic deaerated conditions. In simulated ocean water, the complex TiO2.H2O is formed which reduces the material dissolution rate [67]. In NaCl solution at room temperature, titanium diboride behaves like a passive metal due to the formation of a surface oxide film, whose protectiveness decreases with the temperature and disappears at 65° C [69].

The use of TiB2 films as hard protective coatings has been extensively studied due to their mechanical and tribological properties [70, 71]. Results indicated that

TiB2 coatings have better wear resistance but a lower adhesion level than TiN which limits the real applications of TiB2 coatings [72, 73].

2.3.3 Boron nitride (BN)

Boron nitride (BN) has been utilized as a significant coating material for cutting tool applications in recent years due to its superior mechanical and chemical properties. BN coatings possess good thermal conductivity, high electrical resistivity, high wear resistance and chemical inertness at high temperature.

Chapter 2: Literature Review

It is also superior to diamond due to its chemical stability against oxygen and ferrous materials at high temperature [74, 75]. BN exists in two main crystalline polymorphs: the cubic BN (c-BN) and the hexagonal BN (h-BN) phases. Hexagonal boron nitride has a layered structure similar to graphite but is a transparent material, refractory and corrosion-resistant material. It is soft, lubricating at low and high temperature, has a low friction coefficient, and an electrically insulating and thermally conductive material. It has wide applications as a solid lubricant in metal forming dies and metal forming processes at high temperature in any environment [76]. In contrast, c-BN has an extremely high hardness next only to diamond. The combination of outstanding thermal, electrical, optical, and mechanical properties of c-BN puts it forward as a suitable coating material for fabricating cutting tools.

Recently, sintered cubic boron nitride cutting tools have been used extensively in the market. The problem associated with making the use of a sintered c-BN cutting tool possible includes its high cost, poor ductility and difficulty of forming them into various cutting tool shapes [77]. Currently, different deposition processes have been explored to synthesize BN films. Among them PVD [78-80] and CVD [81-83] processes.

Of all the techniques employed so far, chemical vapor deposition is the most common and involves formation from reactive compounds by thermal means, i.e. thermal decomposition of BCl3 using NH3 [84, 85]. These techniques generally require a high substrate temperature. The handling of toxic and hazardous precursors such as B2H6, BCl3 and BBr3 for the deposition of BN by thermal CVD technique is also required. Phani et al. [86] proposed the use of aminodiboride (ADB) as single-source precursor to solve this problem.

Chapter 2: Literature Review

Boron nitride films were successfully deposited on different metal substrates. i.e. Nickle and low carbon steel [87]. It was also used as interfacial coating for SiC fibers [88]. As for the majority of hard coatings, the conducted investigations were mainly focused on the influence of deposition parameters on the growth of BN films, the microstructure, and the mechanical properties. Recently, attention was drawn to their corrosion resistance properties. According to Moreno et al. [89], it was deduced that applying a multilayer system of TiN[BCN/BN] n/c-BN on stainless steel substrate would improve its corrosion resistance by 15 times higher that the uncoated steel.

2.3.4 Titanium boronitride coatings (TiBN)

The development of TiBN coatings rose from the necessity of developing new, higher performance coating systems more closely matched to particular applications, with better mechanical, physical and chemical properties than those of single phase coatings i.e., TiN, TiB2 and BN.

Adding borides to titanium nitrides coatings was reported to increase the hardness, improve the corrosion resistance, and yet maintain good toughness [90]. In a defined composition range, TiBN coatings exhibit wear resistance at least 3 times better than for TiN or CrN [91] and hardness over 50 GPa [92, 93].

This high level of hardness was explained by the nanocomposite structure of the film [94].

Ti-B-N coatings can be synthesized by almost all deposition techniques, having been initially prepared by chemical vapor deposition (CVD) with deposition temperatures above 1050 °C [95].

Chapter 2: Literature Review

Subsequently, various film types within the system Ti-B-N-C based on TiB2 were successfully synthesized by adding nitrogen to TiB2 films grown by non-reactive and reactive DC magnetron sputtering [96, 97], it was reported that the addition of nitrogen caused further improvement of technically relevant properties such as morphology, hardness and oxidation resistance of Ti-B films. This was attributed to the formation of mixed-phase structure consisting of compounds based on TiB2, TiN, and BN.

The tribological [98] and corrosion [99] properties of Ti-B-N coatings prepared by a similar sputter technique [100] were found to be promising. On the other hand, a systematic investigation of the electrical properties of reactively sputtered films in the Ti-B-N system in connection with the nanocomposite structure of the films was reported in [101].

In order to synthesize coatings of different Ti-B-N compositions several other methods have been employed such as a plasma-assisted CVD (PACVD) [102, 103].

C. Pfohl et al. reported that optimizing the PACVD process parameters permits the deposition of dense titanium based coatings, hard and corrosion-resistant at the same time. However, substrate corrosion and a consequent delimitation were observed, which was related to the presence of pores in the deposited coating. Other different deposition methods were arc physical vapor deposition [104], Ti- implantation into (hexagonal) BN [105], interdiffusion of Ti/BN multilayer films [106] and co-sputtering from Ti and BN targets [107].

Chapter 2: Literature Review

CVD of TiBN coatings was reported only by few authors. After Peytavy et al. [95] Holzschuh [108], deposited very good adherent TiBN coatings onto cemented carbides using CVD technique at moderate temperatures (700-900 °C). The deposition was performed by the addition of boron to the TiN coating. CVD-TiBN was used as interlayer for the deposition of adherent diamonds films of high quality onto steel substrates [109]. Despite the great importance and the huge relevance between the corrosion resistance of this coating and its possible application, the major focus of the conducted studies about TiBN coatings was about their mechanical and tribological properties, whereas little information on the electrochemical behavior and corrosion resistance has been published for this material [99, 104, 110].

2.4 Corrosion protection of titanium based ceramic coatings

The corrosion resistance of titanium based ceramic coatings is not related to their barrier effect only but also connected to the passivation of the coatings. TiN and

TiB2 are not stable in aqueous solutions, but react spontaneously to titanium dioxide forming a thin (few nanometer) passive layer, which prevents further diffusion and dissolution of oxygen in the lattice [39, 111] and hence, the further oxidation of the coating. The electrochemical oxidation of TiN was evaluated by many research studies [54, 112], it was reported that the electrochemical oxidation of TiN in a potential window of (0.5 to 0.8-0.9V) leads to the formation and the growth of oxide/oxynitride layer.

+ ¯ TiN + 2H2O → TiO2+1/2 N2 + 4H + 4e (2-1) 3+ TiN→ Ti + 1/2N2 + 3e¯ (2-2)

Chapter 2: Literature Review

Whereas at potential between 1.0-1.5 V: the oxidation results in formation of hydroxide [112]

+ ¯ TiN+ 3H2O→ Ti(OH)3 +N2 + 3H + 3e (2-3) + ¯ TiN + 3H2O→ TiO2.H2O + 1/2N2 + 4H + 4e (2-4)

Milośev et al. [54] attributed the increase in current density in this potential region to the formation of TiO2 on the surface of TiN rather than hydroxides (reaction (2-

3)), but accompanied by a similar liberation of N2.

The explanation was that the hydroxides are more soluble and less protective than the oxides, and can cause an increase in current in this region. At potentials above 2.0 V, oxygen evolution takes place with a simultaneous oxidation of titanium nitride to TiO2.

The electrochemical oxidation of TiB2 was also studied [111]. The authors studied the kinetics, formation mechanism, and the composition of oxide films resulting from the oxidation of SiC-TiB2-B4C ceramics containing 10 wt% and 40 wt% TiB2 in 3 % NaCl solution. In this composite only TiB2 is the corrosive component, SiC and B4C are fully inert to corrosive environments. It was reported that the oxide film formed in both cases fundamentally changes its composition in transfer from one type to another: at 10 wt% TiB2, an internal oxide layer about

100 nm thick contains of trivalent titanium oxide Ti2O3 forms in the lower layer of the oxidized sample over the potential range -0.18 to 1.20 V.

3- + ¯ 2TiB2 + 15H2O → Ti2O3 + 4BO3 + 30H + 18e (2-5)

Chapter 2: Literature Review

At anodic potentials between 1.20 V and 1.90 V, an external oxide layer containing higher titanium oxide TiO2 forms. This layer is about 50 nm thick.

3- + ¯ TiB2 + 8H2O → TiO2 + 2BO3 + 16H + 10e (2-6)

At 40 wt% TiB2, unlike the composite containing 10 wt% TiB2, TiB2 dissolves insignificantly at -0.40 to -0.20 V to first TiO2+ ions and then successively Ti3+ ions pass into solution as follows:

2+ 3- + ¯ TiB2 + 7H2O → TiO + 2BO3 + 14H + 10e (2-7) 3+ 3- + ¯ TiB2 + 6H2O → Ti + 2BO3 + 12H + 9e (2-8)

However, those reactions are too slow and the sample surface is immediately passivated to form stable TiO at -0.05 to +0.40 V:

3- + ¯ TiB2 + 7H2O → TiO + 2BO3 + 14H + 8e (2-9)

This film is stable only up to 1.00 V. With further increase of the potential the film becomes unstable and destroys at potentials more positive than 1.00V as Ti3+ ions pass into solution. The measured thickness of the film is 250 nm with the following chemical composition: the uppermost layer (out of four) contains physically absorbed molecular oxygen and (deep inside the sample): nonstoichiometric β-Ti1–xO3, stoichiometric β-Ti2O3, and β-TiO1–x as the lowest layer.

The efficiency of the corrosion protection of titanium based ceramic films is strongly related to the quality and uniformity of the passive film, which in turn very strongly depends on the microstructure of the coatings (presence of pores and defects in the deposited coatings), and their adhesion to the metal substrate.

Chapter 2: Literature Review

Small pores in the coating may accelerate the corrosion by many mechanisms, e.g., galvanic corrosion, crevice or pitting corrosion mechanisms. As the corrosion reactions are initiated at the coating-substrate interface, measurements of the porosity are essential in order to estimate the corrosion resistance of the whole coated component. The determination of porosity is possible by means of optical methods but difficult because of the small defect sizes.

By using electrochemical measurements, oxidation and reduction rates on the sample surface can be measured and porosity can be estimated from these values. If the coating is of a good quality, no significant changes occur at the sample surface before measuring the anodic polarization curves. Very porous films (of porosity greater than 1%) will, however, already have failed after analysis of the corrosion potential. On the assumption that the coating is electrochemically inert at low anodic overpotentials, the porosity of the coating was calculated using the following equation [113]:

−|푬풄풐풓풓| 푷 = (푹풑,풔⁄푹풑)ퟏퟎ /풃풂 (ퟐ − ퟏퟎ)

2 Where, P is the total coating porosity of the coating, Rp,s Ω.cm the polarization 2 resistance of the substrate, Rp Ω.cm is the measured polarization resistance of the coated steel system, and is calculated according to:

ퟏ 풃풂풃풄 푹풑 = (ퟐ − ퟏퟏ) ퟐ. ퟑퟎퟑ풊풄풐풓풓 풃풂 + 풃풄

ΔEcorr is the difference of the corrosion potential between the coating and the substrate, ba and bc the anodic and cathodic Tafel slopes, icorr the corrosion current density in A.cm-2.

Chapter 2: Literature Review

2.5 Deposition process: Chemical vapor deposition (CVD) In the last years chemical vapor deposition (CVD) has been gaining in popularity, becoming a widely used materials-processing technique. CVD is a distinctly different coating process than the physical vapor deposition (PVD) process or vacuum evaporation, ion plating, or sputtering. A heat-activated process, CVD can be defined ‘’according to Broadly’’ with the formation of solid products on a heated substrate via chemical reactions of gaseous precursors introduced into a reactor [114].

2.5.1 Process principle and deposition mechanism

In general, the CVD process involves the following keys steps [115, 116] 1. Generation of active gaseous reactant species. 2. Transport of the gaseous species into the reaction chamber. 3. Gaseous reactants undergo gas phase reactions forming intermediate species. Depending on the process conditions, homogeneous reactions may lead to the creation of gaseous intermediates.

4. The precursors and reactive intermediates diffuse to and adsorbs on the surface, where the heterogeneous reaction occurs at the gas-solid interface (i.e., heated substrate) which produces the deposit and the by- product species. The deposits will diffuse along the heated substrate surface forming the crystallization center and growth of the film [117, 118]. 5. This is accompanied by the production of chemical by-products that are exhausted out of the chamber along with unreacted precursor gases.

Chapter 2: Literature Review

The different CVD process steps are illustrated in Figure 2.3.

main gas flow

gas phase reaction

diffusion and deposition and adsorption of reactive diffusion of volatile species reaction products

film growth surface diffusion and reactions

Figure 2.3: Schematic representation of the basic process steps during CVD [119].

For the deposition of dense films and coatings, the heterogeneous reaction is favored. In contrary, a combination of heterogeneous and homogeneous gas phase reaction is preferred for the deposition of porous coatings.

Chapter 2: Literature Review

2.5.2 Chemical precursors and reaction chemistry

The common precursors used in CVD process are metals and metal hydrides, halides, and halo-hydrides, and metalorganic compounds. Generally, metal halides and halo-hydrides are more stable than the corresponding hydrides. The selection criteria of a suitable chemical precursor for coating applications are that the precursor should:  be stable at room temperature.  have low vaporization temperature and high saturation of vapor pressure.  have suitable deposition rate, i.e., high deposition rates for thick coatings applications.  generate vapor that is stable at low temperature.  undergo decomposition/chemical reaction at a temperature below the melting temperature and phase transformation of the substrate. For instance, the deposition of hard coatings (e.g., carbides, nitrides, and borides) can use halides which tend to react at high temperatures and offer high deposition rates.  have low toxicity, explosivity and inflammability for safety of handling chemicals and deposition of the unreacted precursors.  be cost-effective for coating deposition.

Chapter 2: Literature Review

2.5.3 Advantages and limitations

CVD has several features which make it the preferred process in many cases [120, 121]:

 It is not restricted to a line-of-sight deposition which is a general characteristic of sputtering, evaporation and other PVD processes. Deep recesses, holes, and other difficult three-dimensional configurations can usually be coated with relative ease.  CVD films are quite conformal, i.e., the film thickness on the sidewalls of features is comparable to the thickness on the top.  In addition to the wide variety of materials that can be deposited, they have high purity. This results from the relative ease with which impurities are removed from gaseous precursors using distillation techniques. The deposition rate is high and thick coatings can be obtained.  CVD does not normally require ultrahigh vacuum as PVD processes. Its flexibility such that it allows many changes in composition during deposition; co-deposition of elements or compounds is readily achieved.

However, CVD does not just present advantages:

 One of the major disadvantages lies in the properties of the precursors. Ideally, the precursors need to be volatile at near-room temperature. This is non-trivial for a number of elements, although the use of metal-organic precursors has eased this situation.

Chapter 2: Literature Review

 Another primary disadvantage is that the films are usually deposited at elevated temperatures of 600°C and above; many substrates are not thermally stable at these temperatures. Moreover, it leads to stresses in films deposited on materials with different thermal expansion coefficients, which can cause mechanical instabilities in the deposited film.  CVD precursors can be highly toxic, explosive, or corrosive; the by- products of reactions can also be hazardous and must be neutralized, which may be a costly operation.

2.5.4 Applications

The major applications of CVD take advantage of the unique characteristics of the process, such as good throwing power, the ability to deposit refractory materials at temperatures far below the normal ceramic processing temperatures, and the capability of producing materials of exceptionally high purity. Typical cases for the CVD process include the fabrication or coating of tubing, tungsten boride crucibles and dinnerware. Its applications in solid-state microelectronics are of prime importance. Thin CVD films of insulators, dielectrics (oxides, silicates, and nitrides), element and compound semiconductors and conductors are extensively utilized in the fabrication of solid-state devices [122].

A substantial field of CVD exists for the hard and wear-resistant coatings such as nitrides, borides, carbides, oxides, oxy-nitrides and carbo-nitrides of almost all the transition metals, these coatings have found important applications in tool technology [123], corrosion resistant coatings of cutting tools and surfaces needing erosion and/or corrosion protection [124-127], decorative coatings, anti- reflection and spectrally selective coatings on optical components.

Chapter 2: Literature Review

CVD processes are also used in the manufacturing of objects with complex shapes (e.g., refractory crucibles) out of materials such as tungsten, molybdenum, and rhenium which resist conventional machining and fabrication [128].

2.5.5 CVD process parameters

The deposition process and processing parameters such as temperature, pressure, reactant gas concentration and total gas flow, and substrate cleanliness influence the deposition rate, film growth, and the properties of the deposited coatings. Therefore the thermodynamics and kinetics need to be defined.

2.5.5.1 Temperature and pressure

The temperature at which the coating is deposited is critical as it controls both the thermodynamics and kinetics of the coating process. The deposition temperature must be achieved and maintained in order for the reaction to occur on the substrate and not in the gas phase, and to form coatings with appropriate microstructure (e.g., grain size and shape) [12, 129-131]. Small changes in the temperature may change the reaction, and/or its kinetics, resulting in an inferior coating. Increasing the substrate temperature results in an increase in the deposition rate, mainly due to an increase in the chemical reaction rate [132]. Substrate temperature affects the growth and crystallographic structure of the deposited films, the grain size and the preferred crystal orientation.

The total pressure of the reactor and the reactant gas partial pressure control the transportation of the reactant gases to the substrate surface. CVD processes are performed from atmospheric pressure to high vacuum.

Chapter 2: Literature Review

At atmospheric pressure, the growth processes are often considered to be ‘’transport controlled’’. Parameters such as the substrates temperature, gas flow rates, reactor geometry and gas viscosity all affect the transport phenomena in the boundary layer. This influences the structure and composition of the deposited films. In order to reduce the dependence of growth rate and film composition on the hydrodynamics in the CVD reactor, many CVD processes are carried out at total gas pressure below 1 atm where chemical reactions become rate controlling, rather than the mass-transfer processes in determining the characteristics of the deposited films [132]. Deposition rate varies with the deposition temperature, pressure and gas flow, consequently, the properties and characteristics of the deposited films will vary. The relative changes between the partial pressure and the total pressure strongly influence the deposition rate [133].

2.5.5.2 Coating-substrate adhesion

The adhesion of the coating to the substrate can be enhanced by avoiding [116]:  Substrate contamination (e.g., an inherent oxide layer due to oxidation), therefore substrates must be cleaned prior coating deposition. This can be achieved by mechanical grinding and polishing up to (desired or requested) roughness. Additional sputter etching of the substrates can also be performed directly before the deposition [96, 104].  The attack of corrosive unreacted precursors and/or by-products on the substrate, hence, this will lead to the formation of weakly bonded compounds at the interface of the coating-substrate.  Depletion of a gaseous precursor which can cause differences in gas composition and coating thicknesses with different stress concentration.

Chapter 2: Literature Review

 Stress due to the deposition conditions or resulting from a mismatch in thermal expansion coefficients between the substrate and the coating when cooling down after deposition. Stress can be reduced to a certain extent by depositing a ductile buffer layer prior to the final CVD process. Total stress can also be reduced by decreasing the thickness of the coating as well as by changing the grain size and morphology of the coating.

Chapter 2: Literature Review

2.6 Research Objectives: Protective coatings

Titanium based ceramic films were reported to be promising coatings that are economically feasible and shows good corrosion protection for parts subject to high stresses and corrosion environment [100]. However, a main part of the studies carried out on this type of coatings was concerning the mechanical properties (e.g., hardness, toughness, and adhesion) where there is still a substantial lack of literature on the corrosion behavior of the coatings, in particular, the deposited titanium based ceramics on steel substrates. In this study, the aim was to evaluate the electrochemical and corrosion behavior of three types of titanium based ceramic coatings (TiN, TiB2, and TiBN) deposited on stainless steel and low carbon steel in different natural solutions which mainly simulate the soil and seawater environments, since most of coated steel structures, made for outdoor applications, are in contact with one/or both of these environments. In many industrial applications, the mechanical loads occurring are superimposed by corrosive attack. Requirements for coatings therefore are chemical inertness, a smooth surface, a dense morphology without micro pores and diffusion pathways, a good adhesion between the coating and the substrate material.

As reported in the literature, there are many factors influence the corrosion resistivity of these coatings, begins with the material of interest, to the coating process and deposition conditions, the properties of the deposited coatings, applications and the environment. For better understanding of the corrosion and protection mechanism of the coated systems, each of the previously mentioned factors needs to be closely investigated. Parameters, such as the morphology of the deposited coatings and its relation to the deposition conditions (e.g., flow rates of gaseous precursors, deposition time and coating thickness, and substrate preparation), will be investigated in this study.

Chapter 2: Literature Review

There are, as known in wet corrosion studies, many factors which precisely affect the corrosion process and its velocity (e.g., pH value, chloride concentration, chemical composition of solution, salt concentrations, and temperature). Therefore, will also be explicit investigated in the study. There are different corrosion tests to determine the corrosion resistant of materials (e.g., salt spray test, electrochemical method). In this work, electrochemical methods were chosen since they permit the separate determination of coating and substrate corrosion. Therefore, a short introduction to the electrochemistry and corrosion protection is provided in the next section.

Chapter 2: Literature Review

2.7 The electrochemical testing methods

Electrochemical techniques are powerful tools for the study of corrosion [134]. These techniques provide the technologist with the ability to monitor corrosion rates is service, giving early warning of conditions that could adversely affect performance and integrity. They also provide the experimentalist with the ability to determine the corrosion rate with high sensitivity, assess rate controlling mechanisms, and in some cases make life prediction. The applicability of various test methods depends on the exposure conditions e.g., immersion in solution and atmospheric exposure. In this study the immersion is used for predicting the corrosion performance of the coating systems. The used immersion test solutions were mainly neutral solutions simulating natural environments, i.e., seawater, and soil. In this study, the primary used electrochemical methods are: potentiodynamic Polarization methods (PDP), Tafel Extrapolation Method, Electrochemical Impedance Spectroscopy (EIS).

Chapter 2: Literature Review

2.7.1 Potentiodynamic polarization methods (potential-current diagrams)

It is very useful to know if a metal/ composite is immune to corrosion in given circumstances using the potential-pH diagram (Pourbaix diagram), but practical situations are most often completely different than the standard diagram and must be independently evaluated. For achieving this, polarization methods are very useful. They involve changing the potential of the working electrode and monitoring the current which is produced as a function of time or potential. Several methods may be used in polarization of specimens for corrosion testing [135]. Potentiodynamic polarization, PDP, is a technique where the potential of the electrode is varied at a selected rate by application of a current through the electrolyte (Figure 2.4). It is used for determining the corrosion current and to identify specific corrosion reactions, such as pitting and crevice corrosion.

Figure 2.4: Schematic polarization curves for Fe in an aqueous solution in the presence of hydrogen ions.

Chapter 2: Literature Review

A variant of potentiodynamic polarization is the cyclic potentiodynamic polarization, CPP, test, described in [136], which provides a reasonable, rapid method for qualitatively predicting the propensity of an alloy to suffer from localized corrosion in the form of pitting and crevice corrosion. This technique was developed for stainless steels and nickel-base alloys but has been increasingly used for other alloys. This ASTM (American Society for Testing and Materials) standard provides details on conducting CPP tests but is very limited with respect to interpretation. Beavers et al. [137] has published a paper concluding that from the analysis of CPP curves for the stainless steel-aqueous chloride system and other alloy-environment systems, both a forward and reverse scan should be performed in order to maximize the information on localized corrosion obtainable from the test technique.

For passivating systems (e.g., containing titanium, zirconium, chromium, etc), the cyclic potentiodynamic polarization technique is probably the most useful tool in assessing localized corrosion [138]. In this technique, the voltage applied to an electrode under study is ramped at a continuous rate in the anodic direction (forward scan) up to a chosen current or voltage. At that point, the voltage scan direction is reversed toward the cathodic or active direction (backward or reverse scan) to a chosen voltage (usually either the corrosion potential or some active potential) where the scan is terminated. The corrosion behavior of the system is predicted from the structure of the polarization potential. If the reverse scan traces, nearly, the same path of the forward scan in the region beyond the critical current density, the material has little tendency to pit. If the current density was different between the forward and reverse portions of the scan, a hysteresis loop will be created [139, 140]. This difference is a result of the disruption of the passivation chemistry of the surface as the potential increases; it reflects the ability of the system to restore that passivation as the potential decreases back toward the corrosion potential [141].

Chapter 2: Literature Review

The risk of localized corrosion is greater when the current density of the return portion is greater relative to the forward portion; this case is referred as negative loop. Pitting and repassivation potentials are two characteristic potentials in terms of localized corrosion can as well be defined from the hysteresis loop during the cyclic polarization [142, 143]. The tested specimen would be expected to resist localized corrosion if the corrosion potential if the corrosion potential lay cathodic with respect to the repassivation potential [144]. The repassivation potential can be chosen by several ways: as the potential at which the anodic forward and the reverse scan cross each other, or alternatively, as that potential at which the current density reaches its lowest readable value on the reverse portion of the polarization scan. Choosing the latter case is done when the forward and backward portions of the polarization scan do not cross each other [138]. Figure 2.5 represents a schematic representation of a cyclic potentiodynamic polarization scan; it shows the case of pitting/repassivation. The first potential at which the current density increases significantly with the applied potential is the break down potential Ebd, the second feature is the potential at which the hysteresis loop is completed during reverse scan after localized corrosion. This potential is the repassivation potential or sometimes called protection potential

Epro.

Figure 2.5: Schematic representation of a cyclic potentiodynamic polarization curve [139].

Chapter 2: Literature Review

2.7.2 Electrochemical Impedance Spectroscopy (EIS)

The EIS technique involves the application of a time varying voltage and measuring the current response. The ratio of two gives the frequency-dependent impedance [145]. The use of EIS in the evaluation of protective coatings on metals and their interaction with corrosive environments provides new information which cannot be obtained with traditional dc techniques, such as, open circuit potential measurements, OCP, and/or polarization resistance, and polarization curves [139, 146, 147]. An important advantage of EIS over other techniques is the possibility of using tiny a.c. voltage amplitudes (10 to 50 mV) exerting a very small perturbation on the system, and hence, EIS is considered as a non-destructive technique relative to some dc techniques e.g., PDP.

Impedance is usually measured over a domain of discrete frequencies which is determined according to the system under investigation [148]. For corrosion studies, the high frequency end of the measurement domain is determined by the frequency required to short the interfacial capacitance. Under these conditions, only the cell solution resistance will be contained in the complex impedance. The frequencies required to short the interfacial capacitance are related to the measured system, i.e., uncoated/coated metal, coating types. For instance, for bare metals, conversion coated metals; the interfacial capacitance is shorted at frequencies ranging from 5 to 20 kHz. As the frequency is lowered, interfacial resistances and reactances will contribute to the complex impedance.

Electrochemical and diffusional processes associated with corrosion are detected at frequencies between about 10 and 10-6 Hz. As mentioned, the low frequency limit of the impedance magnitude can be related to Rp which can be obtained from Bode plot.

Chapter 2: Literature Review

Thus, with Tafel slopes, the corrosion rate can be calculated using the Stern- Geary equation. Figure 2.6 shows a schematic representation of a Bode plot.

Figure 2.6: Schematic diagram of a Bode [149]

Electrochemical reactions consist of electron transfer at the electrode surface; mainly involve electrolyte resistance, adsorption of electroactive species, charge transfer at the electrode surface, and mass transfer from the bulk solution to the electrode surface. Each process can be considered as an electric component or a simple electric circuit. The whole reaction process can be represented by an electric circuit composed of resistance, capacitors, or constant phase elements combined in parallel or in series. All this information can be extracted from EIS measurements [150]. This is usually done by fitting the impedance data to an equivalent electrical circuit which is representative of a physical process taking place in the system under investigation. Through appropriate modeling, EIS can provide some critical information on barrier and/or passive coatings, coating degradation, development of coating defects, coating delamination, and under film corrosion mechanisms [151, 152].

Chapter 3: Experimental Work and Methods

3 Experimental Work and Methods

3.1 Materials and electrolytes

The chemical composition of the materials covered in this study is given in Tables 3.1 & 3.2. The first coated metal was stainless steel type X46Cr13. It has a martensitic microstructure with very good mechanical properties and a moderate corrosion resistance.

C Si Mn P S Cr 0.43-0.50 Max.1.0 Max.1.0 Max.0.04 Max.0.015 12.5-14.5

Table 3.1 The nominal composition in weight percent of the stainless steel

The corrosion resistance is limited due to the relatively low content of chromium (ca. 13%). At this level of chromium, a thin protective passive film forms spontaneously on steel, this acts as a barrier to protect the steel from corrosion, but for a limited range, especially in Cl‾ containing environments [153].

While the second was a commercial low carbon steel from production pipelines, it has API (American Petroleum Institute) grade X-52 with ferrite-pearlite structure.

C Mn Si P Al Cr Cu V Ni S 0.1544 1.262 0.313 0.0155 0.31 0.027 0.0464 0.03 0.0279 0.01

Table 3.2 The nominal composition in weight percent of the low carbon steel

Chapter 3: Experimental Work and Methods

As previously discussed, the alloying elements that are used in low carbon steel are limited primarily to carbon, manganese, and silicon with very small amounts of other elements, such as chromium, aluminum and copper. Such steels are hot rolled at elevated temperatures when they have an austenite () crystal structure followed by relatively rapid cooling [154]. During the rapid cooling, the austenite partially transforms to proeutectoid ferrite, (α), and the remaining () transforms to pearlite, which leads to their microstructure a plus pearlite. The pearlite consists of pearlitic ferrite and cementite [155, 156].

In the last part of this study and additional to low carbon steel, tantalum specimens were coated with the same coating. Its immunity could help for a better understanding of the protection mechanism of the coating.

The major part of the electrochemical and corrosion study was performed in artificial types of water: a. Simulated soil solution (SSS) b. Simulated seawater (SSW) The chemical composition of both solutions is presented in Table 3.3.

Solution Compound Concentration g/L Simulated soil NaCl 0.272 solution Na2SO4 0.71 NaHCO3 0.21 NaCl 24.53 MgCl2 5.20 Simulated CaCl2 1.16 seawater KCl 0.70 Na2SO4 4.09 NaHCO3 0.20 KBr 0.10

Table 3.3: The chemical composition of test solutions (SSS) and (SSW)

Chapter 3: Experimental Work and Methods

The used aqueous media have different chemical compositions, different salt concentrations (in particular the chloride content), and different conductivities. This gives them unique properties which are expected to differently affect the tested specimens. Conventional electrolytes, as well, e.g., 0.5 M NaCl and 1 M HCl were additionally used for the simplicity.

3.2 Samples preparation

The as received stainless steel has cylindrical shape with 24 mm diameter; it was cut into smaller pieces of 6 mm height, whereas low carbon steel samples were prepared into square shape with dimensions of 20mm*20mm*4 mm. The samples were ground with silicon carbide paper up to 2400 grit, rinsed thoroughly with ethanol in ultrasonic bath for 15 min. After that, the samples were dried in a stream of air. In addition to the ground low carbon steel samples, prepared for the deposition of thicker TiBN-3 coating, part of samples were sand blasted under highly pressurized air (4 bars) with 30 µm SiC powder for cleaning and providing a rough surface. This was done for the purpose of studying the effect of surface preparation on the corrosion behavior of the coating system.

3.3 Coating of steel

3.3.1 Coating of stainless steel The deposition of the different hard coatings on different steel types was carried out in CVD equipment from Surmetal Company (Switzerland) with a hot wall reactor at a reduced pressure of 600 mbar and a temperature of 900°C. Figure 3.1 shows a schema of the CVD equipment used.

Chapter 3: Experimental Work and Methods

Exhaust Gases

Reactor

Neutralizer

Evaporator BCl3 N2 H2

Vacuum Pump

Figure 3.1: Schema of the chemical vapor deposition (CVD) equipment.

The deposition parameters of the different coatings were as following:

a. Titanium nitride (TiN): TiN was deposited from a system TiCl4/ N2/H2. The

partial pressure of TiCl4 was 11.3 mbar and the N2/H2 ratio was 1:1. Deposition of TiN occurs according to the following reaction.

2TiCl4 (g) + N2 (g) + 4H2 (g) → 2TiN (s) + 8HCl (g) (3-1)

b. Titanium diboride (TiB2): Precursor mixture of TiCl4, BCl3 and H2 were

used for deposition of TiB2. The partial pressure of TiCl4 was 11.3 mbar

and the H2/BCl3 ratio was 35:1. The deposition of TiB2 occurs according to the following reaction.

TiCl4 (g) + 2BCl3 (g) + 5H2 (g) → TiB2 (s) + 10HCl (g) (3-2)

Chapter 3: Experimental Work and Methods

c. Titanium boronitride (TiBN): The deposition of TiBN was carried at the

same condition as the TiN but with addition of different flow rates of BCl3.

The TiBN-1 was obtained with BCl3 flow rate of 0.16 Nl/min, and TiBN-2

with BCl3 flow rate of 0.32 Nl/min.

Based on the mass balance, all coatings had a thickness of approximately 2 µm.

3.3.2 Coating of low carbon steel

Based on the mass balance, the deposition time was adjusted according to the growth rate to reach a coating thickness of approximately 3 µm. For deposition of

TiN precursor system of TiCl4 /H2 / N2 was used. The flow rate of H2 was 12

Nl/min and the ratio of H2 to N2 was 1:1. The TiCl4 was introduced to the reactor as vapor by bubbling hydrogen through the evaporator; the flow rate of TiCl4 was adjusted to 0.65 Nl/min. The deposition of TiB2 takes place from a system TiCl4/

BCl3/ H2. The flow rate of the precursor TiCl4 was adjusted to 0.65 Nl/min and the flow are of the H2 gas was 12 Nl/min, the H2 to BCl3 ratio was 35:1. TiBN deposition was performed from TiCl4/ BCl3/ H2/ N2 at the same process parameters as TiN but with variable BCl3 flow rate. The TiBN-1 was deposited at a BCl3 flow rate of 0.16 Nl/min and TiBN-2 at BCl3 flow rate of 0.32 Nl/min.

Thicker TiBN-3 coating of 6- 8 µm was achieved by increasing the deposition time to have a free pin holes coating. Process gases of H2 / N2 / BCl3 and TiCl4 vapor were used for coating of TiBN. The flow rate of H2, BCl3 and N2 was 12

Nl/min, 0.32 Nl/min and 12 Nl/min, respectively. The TiCl4 was introduced to the reactor as vapor by bubbling hydrogen through the evaporator; the flow rate of

TiCl4 was adjusted to 0.65 Nl/min. The deposition temperature 900°C was chosen, since, the simultaneous deposition of TiB2 and TiN takes place only between 750°C and 950°C [104].

Chapter 3: Experimental Work and Methods

A full screening study on the dependence of TiBN deposition on the process temperature and the concentration ratio of BCl3 and TiCl4 was carried out by the authors is shown in Figure 3.2 [104]. Additional to low carbon steel samples, tantalum samples were pretreated in the same way and coated in the same patch, to be dealt as references for better comparison. Since Ta is an inert metal and thus the contribution of substrate in the electrochemical tests can be minimized or even excluded.

Figure 3.2: Dependence of TiBN deposition domain TiBN on the temperature and the BCl3 /TiCl4 ratio.

Chapter 3: Experimental Work and Methods

3.4 Coating characterization

Different analytical techniques were used to characterize the deposited coatings before and after the electrochemical measurements and corrosion tests.

3.4.1 X-ray Photoelectron Spectroscopy (XPS) The elemental composition and chemical state of the deposited layers were analyzed by using X-ray Photoelectron Spectroscopy (PHI 5600 XPS Spectrometer) with Al Kα radiation at an incident angle of 45°. In the measurements, the XPS spectra of the coatings were referenced to the C 1s peak at 284.6 eV. Meanwhile sputtering by argon of (TiBN-3 with 6-8 µm thickness) was carried out, the chemical state of the composition was analyzed up to 500 nm. XPS is a surface analytical technique; it provides detailed chemical information from the top 1-10 nm of a sample surface. In this technique, electrons are emitted from the sample upon its irradiation with X-ray beam to a detector that finds out their energies [157]. The kinetic energy of the emitted electrons Ekin is:

Ekin= hν- EB (3-3)

Where EB is the binding energy of the ejected electrons, hν is the energy of incident X-ray beam.

3.4.2 Scanning Electron Microscopy and Energy Dispersive X-ray (SEM & EDX) The coatings surface morphology was characterized by HITACHI FE-SEM S4800 Scanning Electron Microscopy (SEM). The SEM is a technique uses a highly focused electron beam that scans the surface of the solid specimen [158].

Chapter 3: Experimental Work and Methods

The produced signals by electron-sample interactions include secondary electrons (that produce SEM images), backscattered electrons and characteristic X-rays. The desired signal is captured and converted to electron pulse that are then used to reconstruct the image [159]. Additional compositional chemical analysis was conducted by Energy Dispersive X-Ray analyzer (EDX) equipped with the Hitachi FE-SEM S4800 to determine the atomic ratio of the different deposited coatings. EDX technique is based on the interaction of the electron beam with atoms within the sample, exciting the emission of X-ray with energies characteristic of the atomic number of the atoms involved [160]. These X-rays are collected and analyzed according to energy, and counted using the technique of Energy Dispersive Analysis of X-rays (EDX). Depending on the electron energy and their absorption by the solid, the spatial resolution is around 1-3 µm.

3.4.3 X-ray Diffraction (XRD)

The crystallographic structure was investigated by X-Ray Diffractometer (XRD) from Philips using X’Pert PRO diffractometer with monochromatic Cu Kα radiation. XRD is a non-destructive analytical technique which provides detailed information about the internal lattice of crystalline substance, including unit cell dimensions, bond-length, bond-angles, and details of site-ordering [161]. The reacted sample is simply placed in the specimen holder of a diffractometer, so that the X-ray beam falls on the flat scale surface, and the intensity of diffracted beams measured. The resulting diffraction pattern is then matched with tabulated standards to get phase identifications.

Additional techniques were used to characterize TiBN-3 since it showed a highly promising corrosion resistance in test solutions.

Chapter 3: Experimental Work and Methods

3.4.4 Glow Discharge Optical Emission Spectroscopy (GD-OES) A quantitative depth profiling of the chemical composition was investigated by Glow Discharge Optical Emission Spectroscopy (GD-OES) using an ARL spectrometer. The diameter of the glow discharge lamp was 8 mm and argon plasma was used for surface sputtering. GD-OES is an analytical technique used to measure the elemental concentrations of solid materials; it does not require complex sample preparation and can be used for bulk, surface and depth profile analysis of metals, oxides and semiconductive materials. Its ability to perform elemental depth profiling with a fine spatial resolution is particularly useful for the characterization of surfaces [162]. In this technique, a stream of argon ions mills materials from the sample surface. The sputtered material is then excited in a low pressure plasma discharge and the resulting light emission is used to characterize and quantify the sample’s composition.

3.4.5 Focused ion Beam Microscopy (FIB) Focused ion beam microscopy (FIB) using dual-beam SEM/FIB 1540 EsB (Zeiss- Company- Germany) was used to prepare and image precise cross-sections of TiBN-3 before and after electrochemical investigations. These measurements were conducted at the institute of Metals Science and Technology (WTM) of the Friedrich-Alexander University of Erlangen-Nuremberg. The focused ion beam milling is a technique uses an energetic beam of gallium (Ga+) ions to selectively sputter regions of a material, whilst also functioning as a scanning ion microscope. The milling accuracy is of order of the beam size allowing very precise sectioning to be carried out [163, 164]. Cross-sections with dimensions of 13-15*25*15 µm (respectively width, length and depth) were milled with beam current of 10 nA for initial cuts, 2 nA for the final cleaning.

Chapter 3: Experimental Work and Methods

The aim was to investigate the possible presence of interfacial corrosion or coating detachment that might probably resulted from long exposure times in test solutions. Direct SEM images from the cross-sections were captured. Information on the distribution of the chemical composition across the sections was obtained from line-scan analysis. Line-scan analysis gives the possibility to analyze the sample composition along a line by scanning a fine beam of the exciting particles across the sample [165].

3.4.6 Metallographic microstructural study In order to observe the microstructure and the thickness of the different deposited films on different steels, cross-sections were prepared. For this, coated samples were firstly cut with Accutom 5R (Struers- Germany), Figure 3.3 shows the cutting device.

Figure 3.3: Cutting- off machine

Cross-sections, of the samples used to examine the microstructure, were cold embedded, ground with SiC-paper up to 1200 grit and then were polished for 5 min with diamante paste of 3&1µm particle size. After each step, samples were cleaned in ethanol in an ultrasonic bath.

Chapter 3: Experimental Work and Methods

This procedure was followed with chemical etching of the prepared cross- sections; low carbon steel in (2% HNO3) electrolyte and stainless steel in

(glycerin, HCl and HNO3, with the ratio 3:2:1, respectively). Etched samples were rinsed in ethanol and then were used for optical microscoping. The TiBN-3 coated LCS sample used for SEM was embedded in Epo-Black at temperature of 180°C and pressure of 70 bar, ground up to 2400 grit and then polished for 5 min with diamond paste of 3µm particle size followed with OPU-10 N for another 5 min with ultrasonication in ethanol between different steps. The polished cross-section was first examined by optical microscopy, using different magnifications to estimate the thickness of the coating. Higher magnification images were obtained using scanning electron microscopy (SEM).

3.5 The electrochemical measurements

In this study, conventional electrochemical techniques were employed to evaluate the corrosion behavior of the coated stainless steel and low carbon steel samples in different test solutions. As the corrosion reactions are initiated at the coating-substrate interface, measurements of the porosity are essential in order to estimate the corrosion resistance of the whole coated system. The determination of porosity is possible by means of optical methods but difficult because of the small defect size. By using electrochemical measurements, oxidation and reduction reaction rates on the sample surface can be measured and porosity can be estimated from these values.

Chapter 3: Experimental Work and Methods

All electrochemical measurements were carried out under atmospheric conditions (i. e., the electrochemical cell was not deaerated). The experiments were conducted in a three-electrode compartment cell (Figure 3.4) using IM6 electrochemical system (Zahner Company, Germany) for data acquisition.

Figure 3.4: Electrochemical cell [166].

A saturated Ag/AgCl connected to a Luggin capillary with 3 M KCl and a platinum foil served as reference electrode (RE) and counter electrode (CE). The working electrodes were the different coated steel samples with 1 cm2 test surface area. The corrosion behavior of coated stainless steel was studied in 0.5 M NaCl by measuring the electrochemical impedance spectroscopy (EIS) and the polarization curves. For EIS measurements, an alternating current signal with the frequency range from 100 kHz to 10 mHz and amplitude of ± 10 mV was applied to the working electrodes at the corrosion potential. EIS spectra were collected at 0, 3 and 6h of immersion in test solution.

Chapter 3: Experimental Work and Methods

At the end, polarization measurements were conducted in the potential range from 100 below the corrosion potential to 2000 mV with a potential scan rate of 1 mV/s. The electrochemical and corrosion behavior of the coated low carbon steel were extensively studied in a variety of test solutions.

The first stage of the study was the evaluation of the different coatings (TiN, TiB2, TiBN-1 and TiBN-2) in simulated soil solution (SSS) to simulate the real case of a coated steel structure in contact with soil environment; the chemical composition of SSS is presented in Table 3.3.

The succession of each experiment started by measuring open circuit potential OCP for 15 min followed with EIS (EIS at 0h), then the OCP was further recorded for 3 h in the test solution and again EIS was measured (EIS at 3h); this consequence was repeated and (EIS at 6 h) was obtained. Electrochemical impedance spectroscopy test was conducted, in the frequency range from 5 mHz to 100 kHz applied to the electrode at its corrosion potential. Finally, polarization tests were carried out at the end of each measurement series with a scan rate of

1mV/s in the potential rage from -1 to 1.5 Vvs Ag/AgCl. Relaying on the fact that TiBN-2 has showed the best corrosion behavior among the different tested coatings and after increasing the coating thickness to get TiBN-3, an extensive electrochemical study in different test solution was performed. In addition to the previously mentioned simulated soil solution, a simulated seawater electrolyte (SSW), resembling one of the most corrosive environments for steel structures, was used (see Table 3.3). 1M HCl was used as a corrosive acidic media. The regular electrochemical test of TiBN-3 was measuring EIS at the corrosion potential in test solutions for 48 h at constant time intervals of 3 h.

Chapter 3: Experimental Work and Methods

The perturbation signal of the applied potential and frequency range were the same as previously mentioned for coated low carbon steel (LCS) samples.

At the end of each experiment potentiodynamic scanning or cyclic potentiodynamic polarization measurements were conducted in the same test -1 solution in the potential range -1 to 4 Vvs Ag/AgCl at a scanning rate of 1 mV.s . For the comparison, bare LCS samples were tested in the same way and taken as reference samples. The same regular test of the TiBN-3 coated LCS was carried out at different temperature i.e. 15, 25, 35 and 45 °C in both SSS and SSW.

Long-time experiments of the TiBN-3 coated LCS were performed in SSS and SSW. Coated samples were soaked in test solution for 90 days where OCP was recorded and EIS was measured at corrosion potential in the frequency range 1 mHz to 50 kHz at constant time intervals of 3 to 5 days. After 90 days cyclic potentiodynamic polarization curves were performed in the potential range -1 to 4

Vvs Ag/AgCl.

Porosity and corrosion parameters (corrosion potential Ecorr, corrosion current density Icorr, passivity current density Ipass) were obtained from the polarization curves by Tafel’s method. EIS results were used to determine the total impedance (|Z|) and phase angle and to model the corrosion process, using a simple equivalent circuit.

Chapter 3: Experimental Work and Methods

Different type of electrochemical tests was carried out at cathodic polarization potential of -1 Vvs Ag/AgCl. Two different measurements were set up to study the effect of cathodic polarization on the TiBN-3 coated LCS, the first one was by measuring EIS for 48h under applied cathodic potential (CP) at frequency ranged from 100 kHz to 5 mHz. The second test was performed by interrupting the applied CP, where EIS was measured at applied cathodic potential for 24h with time intervals of 3 h, after that the applied CP was cut and the EIS was measured at OCP in the same solution for another 24 h with same time intervals. These measurements were alternately repeated for 10 days, i.e., 4 days on/ 4 days off. At the end, potentiodynamic polarization curves were recorded and the samples were further analyzed with the same previously mentioned techniques.

Chapter 4: Results

4 Results

4.1 Characterization of different coatings on stainless steel 4.1.1 Surface Morphology and optical metallographic cross- sections of different coatings

To obtain information of the different coatings deposited on stainless steel, different characterization techniques were applied. Scanning Electron Microscopy (SEM) was used to examine the morphologies of the different deposited coatings; results are shown in Figure 4.1.

While the TiN coating shows a faceted microstructure (4.1.a), which has usually defects in the structure providing good paths for the electrolyte onto the substrate surface, the TiB2 has uniformly distributed fine grain morphology (4.1.b). Adding a small amount of boron about 11% to the TiN layer leads to crystal refinement (4.1.c). By increasing the boron content to 20%, the TiN showed a dome structure with needles like shape (4.1.d). The competitive reaction between TiB2 and TiN on the steel substrate results in structure refinement, the TiB2 formed dome like structure in the presence of TiN, and enhances the crystal refinement of the faceted TiN. The fine-domed surface and the roughness of the surface can be adjusted by the growth rate [39].

Chapter 4: Results

a b

5µm 5µm

c d

5µm 5µm

Figure 4.1: SEM micrographs of coated stainless steel, a) TiN, b) TiB2 c) TiBN-1 d) TiBN-2

Details of the microstructure of the coated stainless steel can be observed on microscopic micrographs in Figure 4.2. It can be seen that boron containing coatings deposited on stainless steel have a compact and smooth morphology with different thicknesses. Cross-sections of coated stainless steel samples showed a relative good adhesion at the interface between stainless steel and boron containing coatings.

Especially good was TiB2, it has as well the highest thickness. Titanium nitride layer can hardly be seen after cross-section preparation; it is completely detached from the surface and is very thin. It is also interesting to note that the coating-substrate interfaces show no readily apparent intermediate phase.

Chapter 4: Results

The microstructure of the substrate shows a two-phase structure resulting from furnace cooling to room temperature. This structure is known as spheroidite, it is a dispersion of cementite particles in alpha ferrite, partly lamellar and partly spheroidal cementite in a ferrite matrix.

Figure 4.2: Optical micrographs of the cross- sections of TiN and TiB2 coatings deposited on stainless steel

Chapter 4: Results

Figure 4.2: Optical micrographs of the cross- sections of TiBN-1 and TiBN-2 coatings deposited on stainless steel

Chapter 4: Results

4.1.2 Chemical composition of the deposited layer

Results of survey scans of the coatings using the X-ray Photoelectron Spectroscopy (XPS) are listed in Table 4.1. The carbon signal mostly stems from surface contamination, due to the high surface sensitivity of XPS, and therefore will not be further discussed. Presence of oxygen indicates that oxidation of the surface of the coatings has taken place. This effect seems to be the strongest for the TiN coating. Apart from surface contamination and oxidation, the composition of the titanium nitride layer corresponds to Ti-N with at-% ratio of 1:1. The boron to nitrogen ratio in TiBN-1 was 1.1 and in TiBN-2 the ratio was 2.4, about two times higher. The oxidation of the surface may suggest that not all titanium has reacted to nitrides and carbides, and some metallic Ti was present on the surface, which upon exposure to air oxidized. As the amount of oxygen detected on the surface decreases with the boron content, the amount of not reacted titanium seems to be minimized by boron.

Element Ti N O C B Si Fe Layer TiN 16.1 16.7 23.2 42.4 ---- 1.4 0.2 TiBN-1 1.7 10.5 15.5 59.3 11.7 1.1 0.2 TiBN-2 1.1 8.2 12.6 57.2 20.1 0.7 0.1 TiB 19.7 ---- 5.4 38.6 33.2 2.1 1.0 2 Table 4.1: XPS elemental composition in at% of the deposited coatings on stainless steel

Chapter 4: Results

4.1.3 Crystal structure of the deposited coatings (XRD)

The X-ray diffraction pattern of the coated stainless steel is presented in Figure 4.3.

TiB2 {101} TiN

TiBN-1

TiBN-2

TiB2

Position (2Ө), [°]

Figure 4.3: XRD patterns of (TiN, TiB2, TiBN-1 and TiBN-2) coatings deposited on stainless steel

The XRD pattern of the TiBN coating indicates the presence of two main phases, titanium nitride and titanium diboride (TiB2), in addition to a phase of titanium boride (TiB). The intensity of TiB2 to TiN increases as the flow rate of BCl3 increases. The ratio of TiB2 to TiN was 1:1 in TiBN-1 and it was increased to 4:3 in TiBN-2. The peak of the boron also results in broadening of the TiN {200}.

Chapter 4: Results

The preferred orientation of the TiN coatings was evaluated by the texture coefficient (TC) according to the following equation : 풏 푰 (풉풌풍) ퟏ 푰 (풉풌풍) 푻푪(풉풌풍) = 풎 / ∑ 풎 푰풓(풉풌풍) 풏 푰풓(풉풌풍) 풊=ퟏ

Where: Im(hkl) is the measured X-ray relative intensity of the (hkl) plane, and

Ir(hkl) is the relative intensity in the powder pattern.

From the texture coefficient results (Table 4.2) can be seen that the TiN single layer had a preferred orientation of {200}. Also, in TiBN layers the preferred orientation of the TiN was {200} as found for the single TiN coatings. The TiB2 coating layer had a crystal-preferred orientation of the {100}.

T hkl {111} {200} {220} {311} {222} Layer TiN 1.08 1.17 1.08 0.86 0.81 TiBN-1 0.95 1.31 1.14 0.49 1.05 TiBN-2 0.49 1.78 1.16 0.44 1.13

Table 4.2: Texture coefficient of TiN in the presence of boron

Chapter 4: Results

4.2 Characterization of different coatings on low carbon steel

4.2.1 Surface Morphology and optical metallographic cross- sections of different coatings

The morphologies of the same coatings on the low carbon steel are shown in Figure 4.4. The TiN coating has a star shaped crystal intermixed with lenticular crystals (4.4.a); this type of microstructure is usually associated with high stress in the coatings. On the other hand, TiB2 coating shows a needle like morphology (4.4.b) with paths to the substrate, in addition, this coating was not uniform and continuous on the whole surface showing many defect spots where the coating seemed to be flaked off from the surface (4.4.e), and this was confirmed by EDX measurements where a large difference in atomic ratio of Ti/ Fe was detected between the different surface sites. Adding boron (about 34.5 at%) to the TiN layer leads to crystal refinement (4.4.c) and disappearance of the star-shaped crystals, but the microstructure still has many grain boundaries. Further increase of the boron content to 40 at% leads to finer and denser crystal size (4.4.d).

Chapter 4: Results

Figure 4.4: SEM images of coated low carbon steel coated with, a) TiN, b) TiB2, c) TiBN-1, d)TiBN-2, e) TiB2 with defects

Chapter 4: Results

The etched cross-sections of low carbon steel coated with (TiN, TiB2, TiBN-1, and TiBN-2) were observed under the light metallographic microscope, a cross- section of received low carbon steel sample is shown for comparison. The micrographs are shown in Figure 4.5.

As in SEM micrographs, TiB2 cross-section shows a rough non-uniform coating with many defects. TiN coating has almost a uniform thickness but few cracks and defects were detected. The structure of TiBN-1&2 is almost free of defects and the coating became thicker after increasing the flow rate of boron, since the deposition rate of titanium boron nitride is higher here than the deposition rate of titanium nitride or titanium boride by themselves. An interlayer is found between the coating and the steel in every case. This layer seems to be formed before the coating develops, in terms of diffusion interaction between the substrate and the coating material. At the steel interface between the steel and the coating a reaction zone with a ferritic microstructure is built which resulted from the slow cooling rate. The steel substrate shows quite clearly that carbon is lost from the steel in the vicinity of the deposited films i.e. decarburization. The loss of carbon is accompanied with a decrease in pearlite population of the substrate and a grain growth.

Chapter 4: Results

Figure 4.5: Optical micrographs of the cross- sections of the as received LCS,

TiN, and TiB2 coatings on low carbon steel

Chapter 4: Results

Figure 4.5: Optical micrographs of the cross- sections of TiBN-1, TiBN-2 coatings on low carbon steel

Chapter 4: Results

4.2.2 Crystal structure of the coatings

The crystallographic structures of the deposited coatings on LCS were investigated by XRD and are shown in Figure 4.6. The deposited titanium nitride revealed that it is crystalline and exhibited a single phase of fcc NaCl structure with a preferred orientation {111}. On the other hand, the TiB2 has a hcp structure with a strong {101} preferred crystal orientation. The TiBN-1 and TiBN-2 show overlapping of the peaks suggesting incorporation of different phases, especially the peak of TiB2 {200} overlaps with the TiN {311} and the peak of TiB2 {101} overlaps with the TiN {200}. The peak intensities of {111} and {220} of TiN decrease by increasing the boron content until the boron content reached 40 at%. In TiBN-2 the peak {111} of TiN disappeared. On the other hand, the peak intensity of TiB2 {100} increases with increasing boron content. The TiN {200} overlaps with the TiB2 {101}, and the peak intensities decrease with increase in the boron content.

TiBN-1 and TiBN-2 have the same preferred TiB2 crystal orientation of {100}. The TiBN-2 coated steel shows high intensity peaks with a good coincidence with the line positions of TiB2 {100}, indicating TiB2 to be the predominant phase in this layer. A new peak of hexagonal boron nitride (h-BN) appears at 2 of 30o in TiBN-1, and the peak intensity increases in TiBN-2 as boron content increases. XRD measurements did not detect any oxides in the coatings. Hence, as already indicated by the comparison of EDX and XPS data with different information depths, oxygen is mainly present on the surface of the coatings and therefore the thin surface oxide layers cannot be detected by XRD.

Chapter 4: Results

TiB2

TiBN-2

TiBN-1

TiN

Position (2Ө), [°]

Figure 4.6: XRD patterns of (TiN, TiB2, TiBN-1 and TiBN-2) coatings deposited on low carbon steel

4.2.3 Chemical composition of the coatings The elemental composition of the four different coatings determined by XPS is given in Table 4.3. Oxygen and carbon contaminations on the top surface of the coatings were expected since the coatings were not cleaned by sputtering before analysis. The carbon originates mainly from surface contamination and/or diffusion from the low carbon steel substrate. Presence of oxygen on the surface is usually associated with spontaneous oxidation of the coatings [167, 168].

Chapter 4: Results

The TiB2 coating has higher oxygen content than all other TiN based coatings, hence the oxidation rate of TiB2 seems to be higher than TiN. The oxygen content in TiBN-2 coating was lower than in the TiBN-1, as the ratio of nitrogen to boron increases.

Element Ti O C N B Layer TiN 24.0 27.0 33.6 15.4 --- TiBN-1 6.3 11.2 20.4 27.5 34.5 TiBN-2 4.2 5.6 14.6 34.7 40.5 TiB 17.7 45.9 22.6 2.8 11.0 2 Table 4.3: XPS elemental composition of the deposited coatings on low carbon steel

High resolution XPS spectra of the different coatings are shown in Figures 4.7.1 to 4.7.4. Figure 4.7.1 shows the high-resolution XPS spectra of Ti 2p, N 1s and C 1s peaks of TiN coated steel. The Ti 2p spectra can be deconvoluted into four overlapping peaks. The Ti 2p3/2 peak at 455.4 eV and Ti 2p1/2 spin-orbit at 461.4 eV are associated with titanium nitride (TiN) . The peak at 458.2 eV and its spin orbit at 464.1 eV refers to the oxide component (TiO2) [169]. The C 1s peak is deconvoluted into three peaks, the dominant peak is located at binding energy of 284.6 eV corresponding to C-C / graphite, the other peak located at binding energy of 286.3 eV is associated with C-O and the peak at 288.1 eV is associated with C-N [170]. The N 1s peak is deconvoluted into three peaks, the peak at lower binding energy at 395.8 eV is related to the presence of C-N, 397.3 eV to TiN [171] and 399.1 eV corresponds to N-O in TiN coatings [170] . The O 1s deconvoluted into three peaks at 530 eV, 531.7 eV corresponding to N-

O and TiO2 respectively and at 533.5 eV to C-O or H2O.

Chapter 4: Results

The most intense peak at 530 eV is related to N-O which dominates the other peaks especially TiO2, which constitute half of the former peak. This indicates that the nitrogen is found free on the top coating and the reaction between titanium and nitrogen at this temperature is slow.

Figure 4.7.1: XPS high-resolution spectra of the TiN coated low carbon steel

Chapter 4: Results

The XPS high-resolution spectra of the TiB2 coated steel are shown in Figure 4.7.2. The B 1s is deconvoluted into three peaks; the peaks at 187.6 eV, 190.7 eV and 192.4 eV correspond to TiB2, BN and B2O3, respectively [172, 173] with small amount of BN. The Ti 2p spectrum can be fitted into four peaks at binding energies of 454.7 eV (Ti 2p3/2) and 460.4 eV (Ti 2p1/2) due to the presence of

TiB2, while the peak obtained at 459.1 eV (Ti 2p3/2) and at 465.0 eV (Ti 2p1/2) is assigned to the TiO2 phase [174, 175]. The peak intensities show that the most intense peak is characterized by TiO2. The N 1s is deconvoluted into two peaks at 395.8 eV and 397.9 eV corresponding to C-N and B-N respectively [170, 174].

The TiO2 peak intensity dominate the B2O3 peak intensity, which indicates that oxidation of boron is slower than titanium.

Chapter 4: Results

Figure 4.7.2: XPS high-resolution spectra of the TiB2 coated low carbon steel

Chapter 4: Results

Figure 4.7.3 shows the high-resolution spectra of TiBN-1 coating. The B 1s spectra deconvoluted into two peaks at 187.4 eV and 190.7 eV correspond to

TiB2 and BN respectively. The TiB2 peak intensity was 1/3 the peak intensity of BN. The corresponding N 1s shows a main peak at 398.3 eV corresponding to BN and small peak at 397.4 eV corresponding to TiN. The O 1s deconvoluted into three peaks at 528.9 eV, 531.0 eV and 533.0 eV corresponding to C-O, TiO2 and H2O respectively. The Ti 2p spectrum was difficult to analyze due to the strong overlapping of the peaks, however the main constituents were identified as TiB2, TiN and TiO2. Oxidation of boron in TiBN-1 coating was not detected. It seems that the presence of nitrogen prevents the oxidation of boron. By increasing the BCl3 flow rate to 0.32 Nl/min in TiBN-2, a preferable formation of BN phase is observed and an absence of the TiN phase. However, the peaks intensities ratio of TiB2/BN was close to the TiBN-1. The B 1s, N 1s and O 1s spectra deconvolution of the TiBN-2 coating are shown in Figure 4.7.4. The binding energy and chemical state of the coatings are listed in Table 4.4.

Chapter 4: Results

Figure 4.7.3: XPS high-resolution spectra of the TiBN-1 coated low carbon steel

Chapter 4: Results

Figure 4.7.4: XPS high-resolution spectra of the TiBN-2 coated low carbon steel

Chapter 4: Results

Coating Binding energy (eV) Chemical state TiN Ti 2p 455.4 TiN 458.2 TiO2 N 1s 395.8 C-N 397.3 TiN 399.1 N-O O 1s 530.0 N-O 531.7 TiO2 533.5 C-O TiB2 Ti 2p 454.7 TiB2 459.1 TiO2 B 1s 187.6 TiB2 190.7 BN 192.4 B2O3 N 1s 395.8 C-N 397.9 B-N TiBN-1 B 1s 187.4 TiB2 190.7 B-N N 1s 398.3 B-N 397.4 TiN O 1s 528.9 C-O 531.0 TiO2 533.0 H2O TiBN-2 B 1s 187.4 TiB2 190.7 B-N N 1s 398.3 B-N O 1s 528.9 C-O 531.0 TiO2 533.0 H2O Table 4.4: Binding energy and chemical state of the different coatings obtained from fitting the main XPS peaks.

Chapter 4: Results

The chemical analysis performed by EDX of the deposited coatings is presented in Table 4.5. Oxygen was only found in TiN and TiB2 coatings and could not be detected in TiBN-1 and TiBN- 2 coatings; hence oxidation of the TiN and TiB2 coatings was stronger than in TiBN-1 and TiBN-2. A comparison of the EDX and XPS data indicates that most of the oxygen signal stems from the surface (significantly higher at-% for oxygen determined by surface-sensitive XPS). No carbon signal was measured by EDX indicating that carbon is present only as contamination of the top surface. Furthermore, the composition of TiBN-1 and TiBN-2 coatings show that the coatings are not uniform in the depth [176].

Element Ti N O B Fe Layer TiN 47.61 47.69 3.68 ---- 0.75 TiBN- 1 17.68 19.92 ---- 62.10 0.12 TiBN- 2 16.78 15.22 ---- 67.87 0.09 TiB 14.63 ---- 2.79 82.02 0.56 2 Table 4.5: Atomic percent compositions (by EDX) of different deposited coatings on low carbon steel- as received

Chapter 4: Results

4.3 Characterization of thick TiBN-3 coating deposited on low carbon steel

Previous results showed that increasing the boron content leads to denser TiBN with smaller grain size, making it an adequate protective agent against corrosion. Nevertheless considerable amount of defects were still present in the coating, this might affect the performance of the coating over a long time leading to unsatisfactory results on corrosion resistance. This problem can be resolved by increasing the coating thickness and modifying the film microstructure. This is supposed to reduce the pinhole and improve the corrosion resistance of the coating. Thicker TiBN coating of 6-8 µm (TiBN-3) with 75% boron content was deposited on low carbon steel. Coating morphology was examined by SEM; it has a very dense and fine structure consisting of leaf shaped crystals as shown in Figure 4.8. When the coating thickness was 3 µm and boron content was 40 at%, the microstructure had fine crystals, by increasing the coating thickness to about 6-8 µm and the boron content to 75 at%, the shape changed to leaf shaped assessing the role of thickness and boron content on the microstructure. The microscopic cross sectional micrograph of the coated sample, Figure 4.9, confirms the presence of a good adhesive defect-free coating layer.

Chapter 4: Results

Figure 4.8: SEM micrographs of as-deposited TiBN-3 on low carbon steel with increased thickness and boron content

TiBN-3

Figure 4.9: Optical micrograph of the cross section of TiBN-3 coated low carbon steel

Chapter 4: Results

The XRD patterns of TiBN coated low carbon steel with different thicknesses (3µm, 6µm) are shown in Figure 4.10. The patterns suggested the formation of different phases of TiB, TiB2 and TiN. The predominant phase of the TiBN coating was TiB2 with a preferred orientation of (100). Other diffraction peaks corresponding to TiN and TiB were also observed. An overall increase of peak intensities of different detected phases was observed for thicker TiBN coating in comparison with the thinner one. 1000 TiB 2 {100}

800

TiB 2 TiB {200} TiB TiN TiB 2 600 {200} 2 {201} {200} {101} TiB 2 {102}

Y Axis Title Axis Y 400 TiB 2 TiN (b) {110} {220}

200

(a)

0 20 30 40 50 60 70 80 o Position (2), [ ] Figure 4.10: XRD patterns of TiBN coated low carbon steel, a) 2-3µm thickness- b) 6-8 µm thickness

The SEM cross sectional view of TiBN-3 coated LCS exhibited a well adhered and continuous coating with a compact structure, as shown in Figure 4.11. No observation of detachments or cracks was seen in the layers. The good adhesion of the TiBN layer and the cracks free structure gives a good indication of the possibility to use the coating for efficient corrosion protection.

Chapter 4: Results

The concentration depth profiles of the elements Ti, B, N, C and Fe were measured by Glow Discharge Optical Emission Spectroscopy (GDOES) as shown in Figure 4.11. Deposition of TiBN results in formation of three different sublayers: the first sublayer has a thickness of about 3 µm is formed from diffusion of titanium in the iron substrate. It prevents iron boriding which is a beneficial as formation of the FeB compound causes embitterment of the steel sample [177]. On the other hand as seen in the depth-profile, outward diffusion of carbon from the substrate takes place and increasing the carbon amount in the second sublayer. The second sublayer consists of iron, carbon and boron with small amounts of titanium and nitrogen, resulting in formation of complex compounds of iron boride, iron nitride and iron carbides, with small amount of titanium nitride and titanium carbide; this sublayer is about 1 µm in thickness. The third sublayer consists of B, N and Ti which consists of phases of TiB, TiN and TiB2 as measured by XRD. It has a thickness of 5 µm.

Chapter 4: Results

Each of these compounds is considered to be stable against corrosion. In this sublayer, the iron concentration decreases rapidly at the TiBN interface, supporting the hypothesis that TiBN works as a good barrier for diffusion of iron.

Figure 4.11: SEM micrographs of the prepared cross section of the TiBN coated LCS and the GDOES elemental composition depth profile in at% of carbon, iron, titanium, nitrogen and boron of TiBN-3 coated low carbon steel.

Chapter 4: Results

Cross-sections of the bulk composite coating of original samples using the FIB is shown in Figures 4.12 (a&b). The coating thickness was measured from SEM imaging, it is about 8-10 µm, (since the specimen is tilted, the viewing angle, Ө, is considered when the thickness of the coating is measured). The first notable observation from the cross-section was the fine grained porous-free TiBN coating, and the good coating/substrate adhesion with almost no voids. The several FIB sections revealed the presence of an interfacial non-uniform diffusion layer. The layer thickness varied in different samples, from few nanometers (Figure 4.12.b) to 2-3 µm (Figure 4.12.a). EDX line scan measurements showed that the layer is rich with Ti (Figure 4.13.c). The results are in a good consistence with those obtained from GD-OES analysis.

Figure 4.12.a: SEM after FIB cut of original coated low carbon steel TiBN-3

Chapter 4: Results

Figure 4.12.b: SEM cross-sectional view of TiBN-3 coated low carbon steel after FIB cut

5 µm

Figure 4.12.c: Line-scan of the interfacial layer in TiBN-3 coated low carbon steel- FIB cut

Chapter 4: Results

Cross-section milling was performed for coated Ta samples; SEM image is presented in Figure 4.13.a. The section shows two discrete phases, the darker phase being the TiBN coating, and the lighter phase the Ta substrate. The measured coating thickness was similar to the thickness of the film deposited on LCS. No indication of the presence of interfacial diffusion layer was detected as in the case of coated LCS. Figure 4.13.b is a line-scan of the cross-section, a steep drop in Ti concentration at the coating/metal substrate can be observed, with a rapid increase in Ta concentration. Extensive examination of the cross- section with EDX mapping was carried out and can be observed Figure 4.13.c. It proved that there was no inward diffusion of Ti into Ta, in addition, few voids were observed in Ta substrate underneath the coating.

Chapter 4: Results

Figure 4.13.a: SEM cross-sectional view of TiBN-3 coated Ta after FIB cut

Figure 4.13.b: Line-scan of the interfacial layer in TiBN-3 coated Ta- FIB cut

Chapter 4: Results

Figure 4.13.c: EDX-mapping images of the cross-section in the TiBN-3 coated Ta

The chemical composition of the coating was investigated by X-ray photoelectron spectroscopy (XPS) and energy dispersive X-ray (EDX). XPS depth profile of the top 500 nm of the TiBN film has different composition from the bulk of the coating as measured by GDOES, Figure 4.14. The XPS results show carbon signal which originates from environment contamination and/or further outward diffusion of carbon from the TiBN layer which results in graphite formation. This value decreased when reaching the TiBN film.

Chapter 4: Results

In comparison to XRD and GDOES results, presence of oxygen indicates that oxidation has taken place only at the surface of the coatings due to exposure of the coating to the ambient atmosphere. The nitrogen concentration decreases by depth, while the titanium and boron show a slight increase of concentration by depth.

100 B1s C1s 80 N1s O1s Ti2p 60

at (%) At At % 40

0 0 100 200 300 400 500 600 Sputter depth (nm) Sputter depth (nm)

Figure 4.14: XPS depth profile of TiBN coated LCS

High-resolution elemental spectra were recorded during sputter etching of TiBN film to determine the chemical state and the phase composition of the coating [174].

Figure 4.15 shows the change in the chemical state of B 1s, N 1s and O 1s with sputtering depth. The B 1s spectra deconvoluted into two peaks at 187.4 eV and

190.7 eV corresponds to TiB2 and BN, respectively. The peak intensity ratio of

TiB2 /BN was 0.7 at the top of the layer (sputter depth of 0 nm) and increased to 1.2 at sputter depth of 500 nm.

Chapter 4: Results

The N 1s peak deconvoluted into two main peaks, the main peak at binding energy of 397.4 eV corresponds to TiN and a small peak at 398.3 eV corresponds to BN. The nitrogen amount decreases in depth, but the ratio of TiN to BN almost keeps constant at 0.1. BN phase was not detected by the XRD which emphasizes that BN only formed on the top surface as this phase decreases and diminish in the bulk coating. The O 1s spectra deconvoluted into three peaks, the peaks at 529.1 eV and

531.2 eV corresponding to CO and TiO2 respectively, while the peak at 533.2 eV is assigned to oxygen or B2O3 phase. However, no evidence was found in the B

1s spectra for the formation of the B2O3, indicating that only titanium oxidation takes place.

Figure 4.15: High resolution XPS spectra of B 1s, N 1s and O 1s of TiBN-3 coated LCS showing the change in the chemical composition as function of depth. 89

Chapter 4: Results

The binding energy and chemical state of the TiBN are listed in Table 4.6.The elemental concentration was determined from XPS peak areas using the Ti 2p, N 1s and B 1s peaks and appropriate atomic sensitivity factors.

Element B 1s N 1s O 1s Binding energy (eV) 187.4 190.7 397.4 398.3 529.1 531.2 533.2

Chemical state TiB2 BN TiN BN CO TiO2 Oxygen

Table 4.6: Binding energy and chemical state of TiBN coated LCS obtained from fitting the main XPS peaks.

The EDX spectra of the coating showed the presence of B, Ti and N elements from the phases of TiB2, TiN and TiB as the results obtained by XRD investigations besides a small amount of carbon and iron. These results are presented in Figure 4.16.

Element Wt% At%

B 47.36 75.54

C 1.14 1.64

N 4.97 6.12

Ti 45.63 16.43

Fe 0.91 0.28

Figure 4.16: The EDX elemental composition of TiBN-3 coated LCS.

Chapter 4: Results

4.4 Electrochemical investigations on coated metals

4.4.1 Electrochemical characterization of coated stainless steel

4.4.1.1 Open-circuit potentials and potentiodynamic polarization measurements The open-circuit potential values (OCP) of bare stainless steel (SS) and the different coatings are shown in Table 4.7. The open circuit potential values of coated SS are all higher than that on the bare SS which started relatively low and decreased further with immersion time, suggesting that the SS is not passive in chloride containing environment (0,5M NaCl). Initially, the OCP of the TiN was more positive than TiB2, TiBN-1, or TiBN-2, but after 6 hours of immersion in the sodium chloride solution, the corrosion potential of the TiN coating almost reached those of the steel substrate. Adding boron to TiN nitride coating seemed to enhance the corrosion resistance of the coatings since the OCP values were relatively nobler than the OCP of bare SS and did not undergo a big change with elapsing time, in contrary, the OCP of TiBN-2 was even better after 6h immersion time. The best OCP value was shown by TiB2 coated SS.

Type OCP1 at 0h (mV) OCP2 at 6h (mV) X46Cr13 -545 -602 TiN -436 -524 TiBN-1 -460 -490 TiBN-2 -474 -444 TiB -457 -409 2 Table 4.7: The open circuit potential values of the blank and coated stainless steel in 0.5 M NaCl after 6 hours immersion

Chapter 4: Results

The completion of the potentiodynamic measurements were performed after 6h immersion time in test solution, results are presented in Figure 4.17. Clearly, all coated steel samples show a significantly better corrosion behavior than the bare steel, in terms of corrosion current density (near the corrosion potential) and passivity. The uncoated metal undergoes a continuous active dissolution, as can be expected for this steel in chloride containing medium. All coated samples show a very similar, improved corrosion behavior in the vicinity of the corrosion potential. Upon anodic polarization, significant differences in the electrochemical behavior of the different coatings can be observed. The TiN coating shows almost no region of passivity, but instead a steady increase of the current, reaching the values of the bare metal sample at higher anodic potentials. Boron containing coatings, TiB2, TiBN-1 and TiNB-2 exhibit a passive behavior over a wide potential range, with low current densities in comparison to

TiN coated steel with a superior behavior for TiB2 which is in a good agreement with OCP measurements. Table 4.8 presents the results of the electrochemical measurements of different coatings and their porosity. The calculated porosities,

(Eq. 2-10), of these samples differ, and also do the corrosion behavior. TiB2 and TiBN-2 have quite similar polarization curves and the lowest porosity and therefore the best general corrosion resistance. TiN film has the highest porosity and current density and thus the worst corrosion resistance.

Chapter 4: Results

Figure 4.17: Polarization curves of the coated steel in 0.5 M NaCl.

Coating Ecorr Rp Calculated icorr mV Ω.cm2 porosity mA.cm-2 % TiN -611 26*103 0.7 1.04*10-3

TiBN-1 -523 17.2*103 0.08 1.26*10-3

TiBN-2 -396 30*103 0.008 0.9*10-3

3 -3 TiB2 -423 35*10 0.007 0.7*10

Table 4.8: Results of electrochemical experiments for different coatings on stainless steel

Chapter 4: Results

4.4.1.2 Electrochemical impedance spectroscopy measurements

Further information on the electrochemical behavior at the corrosion potential was obtained by electrochemical impedance spectroscopy. Figure 4.18 shows the EIS spectra as a function of immersion time in 0.5 M sodium chloride for uncoated/coated stainless steel. The EIS of the coated samples (Figures b, c, d and e) show different behavior than the uncoated SS (Figure a).

Initially, the coated SS samples show high impedance values in the low frequency range, indicating a good corrosion resistance. The uncoated steel and TiN coated steel show low impedance values of about 10 KΩcm2, while, boron containing coatings show higher impedance values ranging from 40 to 500 KΩcm2. After 3 hours of immersion, the impedance values at low frequencies decrease to lower values, which can be attributed to the penetration of the solution through the coatings onto the steel substrate. Just after immersion in the NaCl solution, the impedance values decrease rapidly, the impedance value of TiN decreased 2 2 2 to 4 KΩcm , TiBN-1 decreased to 10 KΩcm , TiBN-2 to 20 KΩcm and TiB2 to 10 KΩcm2, while, the uncoated steel drops to 1 KΩcm2. By increasing the immersion time to 6 hours, the coatings TiBN-1 and TiN as well as the uncoated steel result in a small increase of the impedance values at the low frequency range.

TiB2 shows the highest initial value of impedance among the coatings, in the low frequency range the impedance is ca. 500 KΩcm2, while TiBN-2 shows a value of 100 KΩcm2. However, the decrease in the low-frequency impedance values is faster for the TiB2 coating than for the TiBN-2 coating. At the end of 6 h immersion, all boron-containing coatings still show somewhat higher values of impedance (10-20 KΩcm2) than the uncoated and TiN-coated steel (1-4 KΩcm2), indicating a decrease of the average corrosion rate by a factor of ca. 10.

Chapter 4: Results

) 6 ) 6 2 90 10 2 10 90 5 (Stailess steel) 0 h 80 5 TiN 0 h 80 10 3 h 10 .cm 70 .cm 3 h 70

4 ) 6 h )

 4 o

60  10 o 6 h 60

( 10

( (

( 3 50 50 10 103 40 40 2 10 30 102 30

20 phase phase

1 phase 1 20 - 10 - 10 10 10 0 10 0 100 0 -2 -1 0 1 2 3 4 5 -2 -1 0 1 2 3 4 5

Impedance Impedance 10 10 10 10 10 10 10 10 Impedance Impedance 10 10 10 10 10 10 10 10 Frequency (Hz) Frequency (Hz)

6 )

) 6 90 90 2 10 2 10 TiBN-2 0 h 5 80 5 TiBN-1 80 10 10 70 3 h 70 .cm

.cm )

) 4 o

4 6 h  10 60 (

(  60 10 ( ( 50 50 3 103 10 40 40 2 2 10 30

10 30 0 h phase phase

phase phase 1 20 - 20 - 3 h 101 10 10 6 h 10 0 0 10 0 10 0 -2 -1 0 1 2 3 4 5 -2 -1 0 1 2 3 4 5

Impedance Impedance 10 10 10 10 10 10 10 10

Impedance Impedance 10 10 10 10 10 10 10 10 Frequency (Hz) Frequency (Hz)

) 6

2 10 90

5 TiB 80 10 2

.cm 70

4 )

 10 60 o

(

( 50 103 40 102 30 0 h

1 20 phase 10 3 h - 10 6 h 100 0 -2 -1 0 1 2 3 4 5

Impedance Impedance 10 10 10 10 10 10 10 10 Frequency (Hz)

Figure 4.18: Bode plot of EIS data obtained of the uncoated/ coated stainless steel as function of immersion times exposed to 0.5 M NaCl measured at the corrosion potential.

Chapter 4: Results

4.4.2 Electrochemical characterization of coated low carbon steel

4.4.2.1 Open-circuit potential and potentiodynamic polarization measurements The values of measured open circuit potentials (OCP) of bare and coated steel (LCS) are shown in Figure 4.19. While OCP of bare metal in test solution was around -700 mV, which is expected for steel in soil, all other samples showed higher OCP values. However, a slight decrease in OCP of TiB2 coating was observed, due to the defects in the coating, where the solution could penetrate fast onto the substrate and accelerate the corrosion process. The TiN-coated steel initially shows relatively high OCP value. However, after ca. 3 hours a drastic decrease of the OCP is observed, indicating onset of substrate corrosion. TiBN-1 and TiBN-2 coated samples were much nobler than TiN coated sample; the TiBN-1 coatings show a high OCP value during the 6 h experiment, however, the slightly decreasing trend of the OCP together with the cathodic potential transients indicates that the system is not completely stable. Only the TiBN-2 coated sample shows a stable, high OCP value, which even slightly increases with time.

Chapter 4: Results

0,0

-0,1

)

-0,2

vs Ag/AgCl vs -0,3 LCS TiB -0,4 2 TiN -0,5 TiBN-1 -0,6 TiBN-2

Potential (V (V Potential -0,7

-0,8 0 5000 10000 15000 20000 25000 time (sec)

Figure 4.19: Open circuit potential at 6 h in simulated soil solution.

Potentiodynamic polarization curves were used to estimate the electrochemical activity of the coating and for comparison of the different types of coatings. Figure 4.20 presents the polarization curves of bare and coated low carbon steel. Specimens coated with TiB2 have high current density polarization curves and their calculated porosity is also high. Compared to the other coatings studied its corrosion performance was very poor. TiN and TiBN coated samples show nobler corrosion potential and lower anodic current densities, as shown in Table 4.9. The calculated porosity of TiBN coatings was found to decrease with increasing boron content and consequently the corrosion behavior was much better, as TiBN-2 shows the best corrosion behavior amongst all the investigated coatings. The calculated porosity values correlate with the corrosion current densities of the polarization curves at low overpotentials. The higher the calculated porosity, the higher is the current density. These results are in good agreement with the OCP measurements.

Chapter 4: Results

10-1

)

-2 10-2 10-3 10-4 10-5 10-6

-7 LCS 10 TiB 2 10-8 TiN -9 TiBN-1 10 |Current Density| (A.cm Density| |Current TiBN-2 10-10 -1,0 -0,5 0,0 0,5 1,0 1,5 Potential (V ) vs Ag/AgCl

Figure 4.20: Polarization curves of the coated low carbon steel in simulated soil solution.

Coating Ecorr Rp Calculated icorr mV Ω.cm2 porosity mA.cm-2 % TiN -541 28*103 0.19 1.83*10-3

TiBN-1 -583 84*103 0.13 0.5*10-3

TiBN-2 -445 480*103 0.0017 0.04*10-3

3 -3 TiB2 -598 9.6*10 1.68 3.5*10

Table 4.9: Results of electrochemical experiments for different coatings on low carbon steel

Chapter 4: Results

4.4.2.2 Electrochemical impedance spectroscopy measurements

The EIS measurements of uncoated/coated LCS as a function of time are displayed in the form of Bode plot, Figure 4.21. In contrast to bare metal, which exhibits very low polarization resistances after 3 h (1.76 kΩ·cm²) and 6 h (1.55 kΩ·cm²), all coated samples have much better corrosion resistance in the SSS. TiN coating showed relative high impedance values after 3 h (38.74 kΩ·cm²) and

6 h (17.76 kΩ·cm²), whereas TiB2 coating revealed only a marginal protection, in which the lowest values for RP for all coatings were measured. It must be mentioned here that a higher resistance (5.21 kΩ·cm²) was measured after longer immersion time than for shorter one (4.57 kΩ·cm²); this could be due to the accumulation of corrosion products on the delaminated spots of the coating which hindered the dissolution process. The reported Rp values refer to impedance values at low frequency.

Contrary to TiN and TiB2, TiBN layers seems to provide a very effective protection to the underneath substrate i.e. TiBN-1 layer reveals high RP values of approximately 120 kΩ·cm², with almost no change with time. Moreover, the boron content strongly influences the deposited layer and its corrosion resistance. Increasing the boron content from 34.5 at% to 40.5 at%, leads to impedance increment to several MΩ·cm² (2.19 MΩ·cm²) after 3 h and (4.47 MΩ·cm² after 6 h). These values demonstrate – in agreement with the polarization curves - the very efficient barrier properties of the TiBN-2 coatings.

Chapter 4: Results

)

2 107 90 2 107 90 LCS 3hr 80 TiB 0hr 80 6 6 2

.cm 10 6hr 70 .cm 10 3hr 70 )

) 

 o

o 6hr (

( 60 60 (

5 ( 5 10 50 10 50 40 40 104 104 30 30 3 20 3 20 10 10 -phase

-phase -phase 10 10 102 0 102 0

-2 -1 0 1 2 3 4 Impedance -2 -1 0 1 2 3 4 Impedance Impedance 10 10 10 10 10 10 10 10 10 10 10 10 10 10 Frequency (Hz) Frequency (Hz)

7 ) 7

) 10 90 2 90 2 10 TiN 0 h 80 TiBN-1 0 h 80 106 6 3 h 70 .cm 10 3 h 70

.cm

) )

o 6 h 60 

6 h 60 o



(

5 ( 5 ( ( 10 50 10 50 40 4 40 104 10 30 30 20 3 103 20 10 -phase 10 10 -phase 0 2 102 10 0 -2 -1 0 1 2 3 4 -2 -1 0 1 2 3 4

10 10 10 10 10 10 10 Impedance 10 10 10 10 10 10 10 Impedance Impedance Frequency (Hz)

Frequency (Hz) ) 2 107 90 TiBN-2 80 6

.cm 10 70



) (

60 o 5 10 50 ( 40 104 30 0hr 103 20

3hr -phase 10 6hr 102 0 -2 -1 0 1 2 3 4 Impedance Impedance 10 10 10 10 10 10 10 Frequency (Hz)

Figure 4.21: Bode plot of EIS data obtained of the uncoated/ coated low carbon steel as function of immersion times exposed to simulated soil solution measured at the corrosion potential at different immersion times.

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4.4.2.3 Surface characterization after electrochemical measurements

The morphology of the coatings after the electrochemical measurements is shown in Figure 4.22. The observations are in agreement with OCP results.

Clearly the surface of TiN and TiB2 coatings have been attacked by the corrosive medium and became more porous. Localized corrosion was observed on some sites of the surface, they were mainly on delaminated spots of TiB2.

On the contrary, TiBN-1 and TiBN-2 did not show visible changes on their surface morphology, although the OCP of TiBN-1 started to decrease after 4 h immersion time. Different localized corrosion morphologies of TiN and TiB2 were observed as shown in Figure 4.22. The pits were initiated under the coating in defective sites. The pit propagation occurs underneath the coating, as the coating itself is stable in the solution. After substantial localized dissolution of the substrate, breaking of the coatings occurs as was observed in the optical micrograph of the cross-section done after the electrochemical tests, Figure 4.23.

On the TiB2 coated surface the pits look different; they were mostly formed on sites of local low coating thickness.

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Figure 4.22: SEM micrograph of coated steel after the electrochemical measurements in simulated soil solution.

Figure 4.23: Optical micrograph showing the pitting corrosion on TiN coated low carbon steel after electrochemical test in simulated soil solution.

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4.4.3 Summary: The electrochemical and corrosion behavior of different coatings on different steel substrates

The deposited ceramic thin films on both stainless steel and low carbon steel have significantly improved the corrosion resistance of the substrate materials in the different electrolytes used during this study. The corrosion performance was found to be strongly influenced by the porosity related to the coating defect density and by the uniformity of the deposited film layers and their adhesion to the substrate. According to this, the coatings with the best corrosion resistance were TiB2 on stainless steel and TiBN-2 on low carbon steel. Galvanic corrosion couple was created between the active base metal and the coating in the presence of defects and microcracks in the deposited films and led to sever local attacks in forms of crevice and pitting corrosion. Adding boron to TiN was found to decrease the grain size and form coating with mixed phases and mixed orientation with less defects which enhanced further the corrosion resistance of the coating. These coatings, TiBN, with different boron content seem to be a promising candidate for corrosion protection of steels in different environments. Therefore, the second part of the study was focused on improving further the coating properties to achieve a better corrosion resistance.

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4.4.4 Electrochemical characterization of TiBN-3 coated low carbon steel

4.4.4.1 The results of 48 hours measurements

Thicker TiBN coating (6-8µm) with higher boron content (75%) was deposited on low carbon steel samples (LCS) and on tantalum. Coated samples were electrochemically tested in different electrolytes (simulated soil solution, SSS, simulated seawater, SSW, and 1 M HCl) for different times (0, 2 and 90 days).

4.4.4.1.1 Open circuit potential measurements in simulated soil solution and simulated seawater

Figures 4.24 (a, b) show the time variations of free corrosion potentials for uncoated/ TiBN-3 coated LCS and Ta in SSS and SSW, respectively, in an open circuit conditions, pure titanium was measured for comparison. The data illustrate that all tested specimens in both solutions possess nobler potentials than the uncoated low carbon steel with an overall slight increase with time elapsing. The law value of the potentials at the beginning of immersion can be explained by the time necessary for the formation of hydroxide/ oxide passive layer on the surface. The corrosion potential of the uncoated LCS was less noble and decreased steeply upon immersion due to the less protective behavior of the formed oxide layer and due to the presence of the chloride in both solutions which increase the probability of pitting corrosion to take place. Moreover, the open circuit potential values of TiBN-3 coated LCS are almost identical in both test solutions and show a small deviation from the coated Ta in SSW, this indicates that the measured OCP was of the coating and that the interference of the substrate could be excluded from the measured reaction. In SSS, OCP values of uncoated/coated Ta were higher; this might be attributed to the nature of test solutions and their salt content.

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Figure 4.24: Open circuit potential measurements of different specimens in SSS and SSW

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4.4.4.1.2 Potentiodynamic polarization measurements in simulated soil solution and simulated seawater

The potentiodynamic polarization measurements on coated Ta and LCS samples in SSS and SSW after 48 h immersion in test solutions at ambient temperature are presented in Figure 4.25, the polarization curves of uncoated metals (LCS, Ti, and Ta) are shown for comparison. As can be seen the coated LCS specimens exhibited active-passive behavior, with much lower anodic current densities about three orders of magnitude and more noble corrosion potential than the uncoated LCS. Moreover, no pitting corrosion could be observed with increasing the polarization potentials. The behavior of TiBN-3 coating on Ta and LCS substrates is slightly different from the behavior of uncoated Ti and Ta in both test solutions, with identical anodic and cathodic current densities. However, the active-passive peak observed on coated LCS and Ta specimens at potential of 0.6 V did not exist in the case of bare Ti and Ta specimens, which strongly suggests that it is related to the coating itself. All coated samples present, as a common feature, a marked anodic current peak during the anodic polarization with a maximum about 2.0 V. this anodic peak is not found on the untreated Ti and could be referred to the oxidation of TiN to TiO2 [47, 178], following the oxidation reaction:

+ ¯ TiN + 2H2O → TiO2+1/2 N2 + 4H + 4e (4-1) Anodic polarization of the coating in different test solutions is accompanied by the formation of areas of passivation of same length, characterized by very low corrosion current densities indicating the low rate of dissolution of these compounds in the passive state. An exception is represented by low carbon steel in both solutions where it underwent continuous dissolution, represented by high anodic current densities which were higher in SSW.

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100

) -1 -2 10 a 10-2 10-3 10-4 10-5 10-6 10-7 uncoated LCS TiBN-3 coated LCS -8 10 uncoated Ta -9 TiBN-3 coated Ta 10 uncoated Ti

|Current Density| (A.cm Density| |Current 10-10 -1,0 -0,5 0,0 0,5 1,0 1,5 2,0 2,5 3,0 Potential (V ) vs Ag/AgCl

100

) -2 10-1 b 10-2 10-3 10-4 10-5 10-6 10-7 uncoated LCS TiBN-3 coated LCS 10-8 uncoated Ta -9 TiBN-3 coated Ta

|Current Density| (A.cm Density| |Current 10 uncoated Ti 10-10 -1,0 -0,5 0,0 0,5 1,0 1,5 2,0 2,5 3,0 Potential (V ) vs Ag/AgCl

Figure 4.25: Potentiodynamic curves of uncoated / TiBN-3 coated LCS and Ta and of bare Ti after 48h in: (a) SSS, (b) SSW

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4.4.4.1.3 Electrochemical measurements of TiBN-3 coated low carbon steel in 1M HCl

The open circuit potential of TiBN-3 coating on low carbon steel in 1M HCl corrosion is shown in Figure 4.26. The coating shows OCP values close to potentials measured in neutral test solutions with markedly continuous increase over the 48 h immersion time. The observed fluctuations may be due to a probable break down in the titanium oxide passive layer followed by a spontaneous repassivation. Figure 4.27 represents the polarization curve of TiBN-3 measured in 1M HCl; the measured I/E curves in SSS and SSW were added for comparison. The different coated samples are characterized by different cathodic and anodic polarization behaviors with similar Ecorr. The cathodic current density is higher and related to hydrogen reduction reaction [125]; the high cathodic current density indicates the high corrosion rate of the coating in acidic solution. At more anodic potentials the passive oxide layer TiO2 forms again but with higher corrosion current density icorr which is about two times of magnitude higher than of icorr in SSS and SSW. A major difference in the anodic polarization part between the samples in acidic HCl and neutral SSS and SSW solutions is the spontaneous formation of the passive oxide layer in HCl. This can be explained by the reduction of the oxides during the cathodic polarization and the slow dissolution of the coating in the acidic medium which results in quicker oxidation of the freshly exposed coating. Additional observation is the shifting of the anodic current peak related to TiN oxidation to higher potential than 2.0 V, as also observed by other authors [48, 175, 179] .

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-150 -160

) -170 -180

vs Ag/AgCl vs -190 -200 -210 -220 OCP of coated LCS in 1M HCl

Potential (mV Potential -230 -240 0 10 20 30 40 50 time (days)

Figure 4.26: Open circuit potential of TiBN-coated LCS in 1M HCl

Figure 4.27: Potentiodynamic curves off TiBN-3 coated low carbon steel after 48 h exposure in: SSS, SSW and 1M HCl

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4.4.4.1.4 Electrochemical impedance spectroscopy measurements (EIS) of TiBN-3 coated low carbon steel in different test solutions: 48h results

The EIS spectra of uncoated LCS, Ti and Ta, measured in SSS, SSW over 48 hours are shown in Figure 4.27. The spectra of the EIS were taken at an interval of three hours between two consequent spectra, but for data simplicity only a 12- hour interval was represented by a Bode plot. The uncoated LCS showed the lowest impedance values with phase angle around 30o. Compared to LCS, both Ti and Ta exhibited a significant higher impedance values in both solutions, about 2-3 times of magnitude higher than the impedance of LCS with phase angle bigger than 80°. This can be attributed to the passive nature of both metals (Ti, Ta) in SSW and SSS. Nevertheless, this behavior was more pronounced in SSW. Nevertheless, coated Ta in SSS showed two time constants. This was attributed, according to optical observation, to the breakdown in the coating and the penetration of the solution into Ta substrate. Figure 4.28 represents the EIS spectra on TiBN-3 coated LCS and Ta in SSS and SSW. Both coated metals exhibited impedance dispersion steady in time with very high impedances in low frequency region. No macro-defects were present and there was no visible sign of local corrosion attack after the exposure. The same observation was made for coated samples; the impedance magnitude over wide frequency range was higher in SSW. On the contrary, the changes in impedance spectra of TiBN-3 coated LCS in 1M HCl were very significant and occurred after certain exposure time interval (6 hours) indicating higher corrosion rate in HCl, Figure 4.28. The changes included decrease of impedance and phase shift in the middle at low frequency range, which indicates a decrease in the capacitive nature of the film as well as a decrease in the polarization resistance of the whole coating system.

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107 90 ) 2 uncoated LCS in SSS 80 6

10 70

.cm

) 

60 o ( 105 ( 50 40 104

30 -phase -phase 103 20

10 Impedance Impedance 102 0 10-2 10-1 100 101 102 103 104 Frequency (Hz)

7 107 90 10 90

) ) 80 80

2 6 2 106 10 70 70

)

.cm 5

o 10 60 o 10 60

(



( ( 50 50 4 104 10 40 40

3 103 30 10 30

-phase -phase

-phase -phase 20 20 102 102 bare Ti in SSS 10 bare Ta in SSS 10

Impedance Impedance

Impedance Impedance 101 0 101 0 10-2 10-1 100 101 102 103 104 10-2 10-1 100 101 102 103 104 Frequency (Hz) Frequency (Hz)

7 10 90 7 10 90

) 80 ) 2 6 80

10 2 6 70 10 70

)

.cm 5

o ) 10 60 .cm 5

( 60 o

 10

7 ( 

(

50 ( 10 490 50 10 4 40 10

) 40 3

2 30 3 10 30 80 -phase 10 6 20 -phase 2 20 10 10 2 10 10 bare Ta in SSS 10

Impedance Impedance bare Ta in SSW 170 0 Impedance 1 10 0 .cm -2 -1 0 1 2 3 4 10 10 10 10 10 10 10 10 -2 -1 0 1 2 3 4 10 10 10 10 10 10 10 5 )

Frequency (Hz) Frequency (Hz) 

10 60 o

( ( 50 at 0h, at 12h, at 24h 104 at 36h, at 48h 40 Figure 4.27: The impedance spectra of uncoated LCS-Ti and Ta in different test 3

10 solutions30- exposure 0- 48 hours. -phase -phase 20 2 10 111

coated LCS in SSW 10 Impedance Impedance 101 0 10-2 10-1 100 101 102 103 104 105 Frequency (Hz) Chapter 4: Results

107 90 10 90 )

) 2

2 80 80 6 106 10

70 70 .cm

.cm

5 )

5 )  10 60 o

 ( 10 60 (

(

( 50 at 0h, at 12h, at 24h 50 4 104 10 at 36h, at 48h 40 40 3

103 30 10 30 -phase -phase

-phase -phase 20 20 102 102 10

10 Impedance Impedance Impedance coated LCS in SSS coated LCS in SSW 1 0 101 0 10 -2 -1 0 1 2 3 4 5 10-2 10-1 100 101 102 103 104 10 10 10 10 10 10 10 10 Frequency (Hz) Frequency (Hz)

107 90 10 90 )

) 2 2 80 80 6 106 10

7 70 70 .cm

.cm

) 5 5 )

90 o  10  10 60 60

( 10 (

( ( 50 50 ) 4 10 104 2 80 40 40 3 6 10 30 103 30

-phase -phase -phase -phase 10 20 2 20 10 102 10

70 Impedance coated Ta in SSS coated Ta in SSW 10 Impedance Impedance 1 1 .cm 10 0 -2 -1 0 1 2 3 4 10 0 10 10 10 10 10 10 10 -2 -1 0 1 2 3 4 5 5 ) 10 10 10 10 10 10 10 10 Frequency (Hz)

 Frequency (Hz)

10 60 o

( ( 50 at 0h, at 12h, at 24h 4 at 36h, at 48h 10 40107 90

107 90

) )

2 80 3 6 2 6 80 10 10 10 30 70 70 0h 0h

.cm 6h

5 )

) .cm -phase -phase 6h

5 60 o 

10 o 12h (

( 60 10 (  12h 18h

( 4 50 20 10 50 18h 24h 2 4 40 24h 30h 10 36h 10 3 10 40 30 30h 42h coated LCS in SSW 10 -phase 36h 48h 3 2 20 Impedance Impedance 10 30 42h 10 -phase 10

Impedance Impedance coated LCS in 1M HCl 48h 1 1 20 2 10 0 0 10 -2 -1 0 1 2 3 4 10 10 10 10 10 10 10 10 -2 -1 0 1 2 3 4 5 coated LCS in 1M HCl 10 Impedance Impedance Frequency (Hz) 1 10 10 10 10 10 10 10 10 10 0 Figure10 4-2.28:10 The-1 impedance100 101 spectra102 10of3 TiBN10-43 coated LCS and Ta in different Frequency (Hz) Frequency (Hz) test solutions- exposure 0- 48 hours.

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4.4.4.2 The results of long time (90 days) measurements

Long time immersion tests were carried out to examine the corrosion resistance of the deposited TiBN-3 coating on LCS. This type of experiments enables the detection of initiation and growth of the corrosion pits if they exist in the coating. Moreover it helps to clarify the nature of the occurrence of the corrosion. The corrosion potential of coated samples for long immersion time in SSS and SSW are presented in Figure 4.29; the starting potential for both samples in both test solutions was around -320 mV. In SSS the potential was increased constantly up to 25 days immersion time where it reached a value of about -175 mV. Then it decreased to -200 mV in the 40th day, followed by increase and so on. Whereas in SSW, a steep increase was observed up to 10 days immersion where the potential value was about +100 mV followed with a decrease in the next 10 days measuring about +20 mV. After that the potential underwent constant changes up to the 70th day where the changes became less and more stable potentials values were measured. This stable behavior was earlier established in SSS where both samples reached almost the same OCP value after 90 days immersion.

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150 100 ) 50 0 -50

vs Ag/AgCl vs -100 -150 -200 -250 -300 in simulated soil solution

Potential (mV Potential -350 in simulated seawater -400 0 10 20 30 40 50 60 70 80 90 time (days) Figure 4.29: The corrosion potential as function of time of TiBN coated LCS measured in SSS and SSW over 90 days.

Impedance spectra for TiBN-3 coating in SSS and SSW are given in the Bode plot representation in Figures 4.29. The time evolution of the spectra gives a clear picture of the changes in corrosion behavior of the CVD/low carbon steel system during 90 days exposure. In both figures, log |Z| is linear with log f and phase angle ө has values close to 90°, with high impedance magnitudes at very low frequencies indicating that the TiBN-3/LCS coating system was in passive state in the simulated environments. Slight changes in impedance values were recorded with time elapsing. Comparing EIS spectra and Ecorr values measured at different time indicate that EIS impedance increased/decreased slightly when

Ecorr changed [180]. Both solutions possess different resistances, which were also slightly changed with immersion time. This can be due to the change in salt concentrations after adding fresh solution to the running experiment.

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108 90

) 2 a 80 8 107 90

10 70 )

.cm 0days

2 6 10 days

 80 ) 7 10 60

( o 25 days

10 ( 7050 51days 5 64days

.cm 10 0days

6 40 79days 10 days  10 60 ) 90 days

4 (

10 30o 25 days ( 50 phase 51days 5 20 3 64days 10 10 4010 79days Impedance Impedance 90 days 4 102 0 10 -3 -2 -1 0 1 2 3 304

10 10 10 10 10 10 10 10 phase Frequency (Hz) 20 103 8 8 1090 Impedance Impedance 10 10 90

)

2 2 ) 7 0 80 2 10 10 7 -3 -2 -1 0 1 2 3 b 480 1010 10 10 10 10 10 10 10 70 6 0 days

.cm 10 ) 0 days Frequency (Hz) 70 o 9 days  60

6 ( .cm

(

) 913 days days 10 5 10 o 25 days

 6050 13 days (

( 33 days 5 25 days 10 104 40 38 days 50 3355 days

phase phase 38 days 3 30 67 days 104 10 40 5578 days 20 90 days 2 67 days 3 10 3010 phase 78 days 10 Impedance 90 days 101 200 2 10-3 10-2 10-1 100 101 102 103 104 10 10

Impedance Impedance Frequency (Hz)

1 10 Figure 4.30: Electrochemical impedance spectra of TiBN0 -3 coated LCS -3 -2 -1 0 1 2 3 4 10presented10 as 10Bode plot10 as function10 of immersion10 10 time for10 a period of 90 days in: (a) SSS, (b) SSWFrequency. (Hz)

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4.4.4.2 Potentiodynamic cyclic voltammograms of TiBN-3 coated low carbon steel at different immersion times in simulated soil solution and simulated seawater

In order to evidence differences in the passivation capability of coated samples at different exposure times (0, 2 and 90 days), potentiodynamic cyclic voltammetry tests were performed Figures 4.31 (a, b). The corresponding corrosion parameters Ecorr and icorr values were determined by Tafel extrapolations for the measured curves, results are summarized in Table 4.9. The anodic current densities at different immersion times were almost identical, the characteristic peaks at 0.6 V was vanished in SSW after 2 days while it can still be seen on samples in SSS, moreover, the second peak at 2.0 V was present in both solutions after different exposure. After the first potential sweep in the noble direction, the anodic current densities in the reverse scan were less indicative the formation of a robust protective passive film which was not damaged although exposed for long time to the test mediums. The cathodic current densities remain almost unchanged after different immersion times. Collectively the results indicate the high corrosion resistivity of the TiBN-3 coating in both solutions.

At the end of the test, the surface of the coated samples appeared slightly pale gold but no sign of corrosion attack were visible; the surface of the untreated Ti resulted as well with the same color. This change in color is an indication to the formation of titanium oxide layer on the surface of the coated samples, with thickness similar to the oxide layer formed on bare Ti upon anodization in test electrolytes. It is well known that the thickness of titanium oxide is directly proportional to the potential applied [181]; with growing potential, the oxide layer show different colors, i.e. yellow- purple-blue- light blue- silver- yellow… etc.

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The voltammogram of untreated Ti in SSS and SSW carried out after 48h immersion are shown in Figure 4.32. From the figures it can be seen that Ti has the same active-passive behavior as in the coating system in same solutions but with lower anodic current densities and absence of characteristic peaks observed for the coated samples. The reverse scan curves proceeded towards the low current density region. This type of the cyclic polarization curve is known to resist localized corrosion.

10-3

)

-2 -4 a 10

10-5

10-6

10-7

-8 at 0 days 10 at 2 days at 90 days 10-9

|Current Density| (A.cm Density| |Current 10-10 -1 0 1 2 3 4 Potential (V ) vs Ag/AgCl Figure 4.31: Cyclic voltammograms of TiBN-3 coated LCS at different immersion times in a) SSS

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10-3

)

-2 10-4 b

10-5

10-6

10-7 at 0 days 10-8 at 2 days at 90 days 10-9

|Current Density| (A.cm Density| |Current 10-10 -1 0 1 2 3 4 Potential (V ) vs Ag/AgCl Figure 4.31: Cyclic voltammograms of TiBN-3 coated low carbon steel at different immersion times in b) SSW

10-3

)

-2 10-4

10-5

10-6

10-7

10-8 pure untreated Ti in 10-9 SSS

|Current Density| (A.cm Density| |Current SSW 10-10 -1 0 1 2 3 4 Potential (V ) vs Ag/AgCl Figure 4.32: Cyclic voltammograms of untreated bare Ti at 48h in SSS and SSW

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Corrosion potential and corrosion current density measurements gave information on the reactions around the corrosion potential. The corrosion current densities were determined by measuring Tafel slops of potentiodynamic curves of different studied TiBN-3 coated systems in different solutions at different immersion times. Those values were used to estimate the porosity of the different systems after electrochemical tests; results are listed in Table 4.9.

TiBN-3 coated LCS samples showed low current densities and porosity in SSS and SSW although after 90 days immersion. The increase in porosity between 2 and 90 days was very minimum in SSS while in SSW it increased by a factor of 10, this can be due to the high Cl‾ concentration in SSW. Compared to SSS and SSW, the corrosion performance of TiBN-3 coated LCS in 1M HCl was poor, this indicates the high corrosion current density and the high calculated porosity after only 2 days immersion. It can be concluded that the current density higher the higher is the calculated porosity,

Porosity determination of TiBN-3 on Ta from the polarization data is difficult because the response of the substrate is very small [125]. Thus, the correlation between the calculated porosity and current density could not be applied on TiBN-3 coated Ta, as almost no change in the current densities was observed after electrochemical measurements.

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Calculated Sample name time (day) E (mV) i (µA.cm-2) corr Porosity corr %

Uncoated LCS 2 -789 - 2.05 in SSS

Uncoated LCS 2 -858 - 6 in SSW TiBN-3 coated 2 -390 3.8*10-3 6.38 in 1M HCl 0 -331 4.4*10-6 50.5*10-3 TiBN-3 coated LCS 2 -405 2.1*10-5 51.1*10-3 in SSS 90 -400 4.7*10-5 52.3*10-3 0 -329 1.3*10-7 22.6*10-3 TiBN-3 coated LCS 2 -368 1.5*10-6 22.1*10-3 in SSW 90 -460 1.18*10-5 49.1*10-3

Table 4.9: Results of electrochemical experiments for uncoated/ TiBN-3 coated LCS in SSS, SSW and 1M HCl, and for uncoated/ TiBN-3 coated Ta in SSS and SSW

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Chapter 4: Results

4.4.4.4 The electrochemical behavior at different temperature

Figures 4.32 represents the polarization curves of TiBN-3 coated low carbon steel after 48 hours immersion in SSS and SSW at different temperatures 15, 35 and 45°C. It is seen that the corrosion potentials of the coated samples were not appreciably affected with the increase in temperature. Moreover, while the cathodic polarization current density increased slightly, the increase in the anodic polarization current density was more significant with the increasing temperature; this increase can be seen in the active-passive transition part around the corrosion potential. This trend, coupled to the marked stimulation of the anodic process at the end of the exposure time, induces a correspondent shift of the two characteristic peaks at (0.6 and 2.0 V), respectively, towards lower potentials, accompanied with higher anodic current density for the peak at 0.6V. This trend was clearer for the coated samples in SSS.

10-3

)

-2 (2) a (3) 10-4 (1)

10-5

10-6

10-7

10-8

10-9

|Current Density| (A.cm Density| |Current 10-10 -1 0 1 2 3 Potential (V ) vs Ag/AgCl Figure 4.32: Potentiodynamic curves of coated low carbon steel at different temperature after 48h in: (a) SSS, (b) SSW

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10-3

)

-2 (1) -4 b

10-3 10 )

-2 (1) (3) 10-5 10-4 -6 (2) (1)-at 15°C (3) 10 10-5 (2) at 35°C -7 (3) at 45°C (2)10 10-6 (1)-at 15°C 10-8 (2) at 35°C -7 (3) at 45°C 10 10-9 10-8 (A.cm Density| |Current 10-10 -9 -1 0 1 2 3 10 Potential (V ) |Current Density| (A.cm Density| |Current vs Ag/AgCl -10 10 Figure 4.32: Potentiodynamic curves of coated LCS at different temperature after -1 0 1 2 3 48h in:Po (a) t SSS, en ti (b) al SSW ( V ) vs Ag/AgCl

4.4.4.5 Pitting corrosion

Potentiodynamic cyclic voltammogram measurements were used to evaluate the pitting corrosion resistance of TiBN-3/LCS system. Figures 4.37(a-c) represent the curves of bare/TiBN-3 coated LCS, bare/ TiBN-3 coated Ta, and of TiBN-3 coated LCS and Ta, respectively; measurements were conducted in SSW. When a defect exists in the coating, the coated samples underwent a transition course: from passivation to pitting.

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The initiation of pitting corrosion is confirmed by the steep increase in the anodic current density at high anodic potential and the corresponding reverse curve which was about the same as that of the low carbon steel substrate Figure 4.37(a).

) 10 -2 10-1 10-2 10-3 10-4 10-5 10-6 a 10-7 10-8 bare LCS -9 TiBN-3 LCS

|Current Density| (A.cm Density| |Current 10 10-10 -1 0 1 2 3 4 Potential (V ) vs Ag/AgCl Figure 4.37.a: Potentiodynamic cyclic voltammetry curves off TiBN-3 coated LCS showing pitting corrosion behavior in SSW.

Coated LCS and Ta show identical anodic behavior on the positive going scans

Figure 4.37(b); pitting initiation starts at anodic potentials higher than 3 VvsAg/AgCl. On the reverse potential scan, the coated LCS remains in active dissolution state with high anodic current density, whereas the coated Ta show a repassivation behavior, which indicated by the gradual decrease of current reaching values cathodic with respect to the forward positive scan. Moreover, the characteristic trend of the reverse scan is rather identical with those presented in Figure 4.31 of pitting-free coated samples, and not with that of the bare Ta Figure 437(c).

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Chapter 4: Results

-1

) 10

-2 10-2 10-3 10-4 10-5 -6 10 b 10-7 -8 10 TiBN-3 coated Ta -9 TiBN-3 coated LCS

|Current Density| (A.cm Density| |Current 10 10-10 -1 0 1 2 3 4 Potential (V ) vs Ag/AgCl

-1

) 10

-2 10-2 10-3 10-4 10-5 10-6 c 10-7 -8 10 bare Ta -9 TiBN-3 coated Ta

|Current Density| (A.cm Density| |Current 10 10-10 -1 0 1 2 3 4 Potential (V ) vs Ag/AgCl Figure 4.37(b, c): Potentiodynamic cyclic voltammetry curves off TiBN-3 coated LCS& Ta showing pitting corrosion in SSW

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4.4.4.6 The electrochemical behavior of TiBN-3 coated low carbon steel under applied cathodic potential

Cathodic protection is a very important electrical mean of mitigating corrosion; it is applied mainly to buried and submerged metallic structure (primarily steel). Coatings of more noble metals than the substrate metal like titanium based ceramic coatings on steels are only protective when there are no pores. In other cases local corrosion occurs due to cell formation (bimetallic corrosion). Cathodic protection is theoretically possible, but this protection combination is not very efficient since the coating usually consumes more protection current than the uncoated steel. Nevertheless, in some special cases a high protection is of a great importance, where good coatings have to be accompanied with cathodic protection despite its high costs (e.g. pipelines in industrial installations, cables and storage tanks in power stations, refineries and tank farms). Therefore, in this study we tried to simulate most real field practical conditions, including cathodic protection test and their influence on this type of coatings.

TiBN-3 coated LCS samples were cathodically polarized at potential of -1 V for 48 h in aerated SSS and SSW. Figure 4.33 (a & b) shows the Bode diagrams measured on TiBN-3 coated low carbon steel under -1 V, EIS of coated LCS at OCP are shown for comparison. The effect of cathodic potential on the AC impedance behavior of the coated specimens can be obtained by comparing the figures. It can be seen that the total impedance of the specimens under cathodic potential of -1 V is about two times of magnitude lower than those EIS measured at OCP Figure 4.33 (c & d). Nevertheless, the impedance increased after 8 hour immersion and reached a stable value over the time period of cathodic polarization.

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Chapter 4: Results

6 10 90 106 90

)

80 2 80 5 a 5 b 10 70 10 70

.cm

)



o 60  60

(

( ( 4 104 10 50 50

40 3 40 103 10

30 phase 30 phase

2 20 102 20 10

Impedance 10

Impedance Impedance 10 1 1 10 0 10 0 -2 -1 0 1 2 3 4 5 -2 -1 0 1 2 3 4 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 Frequency (Hz) Frequency (Hz)

107 90 10 90 )

) 2

2 80 80 6 106 10

c 70 d 70 .cm

.cm

5 )

5 )  10 60 o

7  # ( 10 # 60 (

(

( 10 90 50 at 0h, at 12h, at 24h 50 4 104 10 at 36h, at 48h ) 40 40

2 3

80103 30 10 30 -phase -phase 6 -phase 20 20 10 102 102 10

10 Impedance Impedance Impedance 70 coated LCS in SSS coated LCS in SSW 1 0 101 0 .cm 10 -2 -1 0 1 2 3 4 5 10-2 10-1 100 101 102 103 104 10 10 10 10 10 10 10 10 5 )

Frequency (Hz) Frequency (Hz) 

10 60 o

( ( 50 at 0h, at 12h, at 24h 104 at 36h, at 48h 40 Figure 4.33: Bode plot of cathodically polarized bare/TiBN-3 coated low carbon 103 steel30:

(a&b) bare low -phase carbon steel samples in SSS and SSW respectively 20 102 (c&d) TiBN-3 coated low carbon steel in SSS and SSW respectively

coated LCS in SSW 10 Impedance Impedance

101 0 10-2 10-1 100 101 102 103 104 105 Frequency (Hz)

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4.4.4.7 Interrupted cathodic polarization measurements

Cathodic protection system might fail while operating due to different reasons, e.g. cut-off of the power supply which can result from human mistakes and/or nature external influences. If these failures happen consequently protected structures will gradually corrode, and when they suddenly fail, it can lead to consequences ranging from annoying to potentially catastrophic. In these terms, titanium-based ceramic coatings are still not well studied and a comprehensive work must be done in this field. In this experiment the aim was to study the effect of interrupting applied cathodic current on the coating and to screen the changes in the corrosion resistance.

These measurements are divided into two parts:

a) Current-on potential measurements, in which a cathodic potential of -1 V was applied on the coated LCS for 24h. b) Current-off potential measurements, in which the applied potential was cut-off for 24 h. During this period OCP and EIS was measured.

Figure 4.34 shows the open circuit potential measurements during cut-off periods. OCP values in SSS (left diagram) remained almost unchanged during the 4 cut-off days and reached the value of ~ -50 mV which was even nobler than the measured value under normal condition ~ -300 mV, (Figure 4.23). In SSW, the measured OCP values were also nobler than those shown in (Figure 4.23), but this ennoblement was seen in the 3rd and 4th cut-off days. This increase in OCP after removing the applied potential could be attributed to the repassivation of the coating surface and the reformation of titanium oxides which were reduced during cathodic polarization period.

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A possible local increase in pH value during cathodic polarization might also enhance OCP after interrupting the applied cathodic potential. This was not

extensively tested.

50 50 )

) 0 0 -50 -50

-100 Ag/AgCl vs -100

vs Ag/AgCl vs -150 -150 OCP- off1 -200 OCP- off2 -200 -250 OCP- off3 -250 OCP- off1 SSS OCP- off4 -300 -300 OCP- off2

Potential (mV Potential OCP- off3 Potential (mV Potential -350 -350 SSW OCP- off4 -400 -400 0 4 8 12 16 20 24 0 4 8 12 16 20 24 time (hours) time (hours)

Figure 4.34: Open circuit potential values measured during current-off potential periods.

Experimental impedance spectra collected during the current-off potential periods are presented in the Bode plots in Figures 4.35 and 36 for SSS and SSW respectively. Generally, two time constants are observable; the one appearing at high ω represents the dielectric characteristic of the CVD coating, while the one at low ω corresponds to the low carbon steel in pores.

Two maxima could be distinguished in the phase angle curves. The high frequency maximum decreased with time, while the low frequency maximum increased with time. This revealed that some change related to the coating and to the steel substrate in the pores occurred in the corrosion process. A better polarization resistance can still be seen for the specimens in SSW.

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106 90 106 90

) a ) b

2 a 80 2 b 80 5 5 10 70 10 70

.cm .cm

) )



o  60 60 o

(

( ( 104 4 ( 50 10 50

3 40 40 10 103 30 30

-phase -phase -phase -phase 2 20 2 10 10 20 10

Impedance Impedance 10 Impedance Impedance 101 0 1 -2 -1 0 1 2 3 4 10 0 10 10 10 10 10 10 10 10-2 10-1 100 101 102 103 104 Frequency (Hz) Frequency (Hz)

6 10 90 106 90

) c )

2 c 80 2 dd 5 80 5 10 70 10 70

.cm .cm

) )



60 o

 o

(

4 ( 60 ( 10 4 ( 50 10 50 3 40 40 10 103 30 30

-phase -phase -phase -phase 2 20 2 10 10 20 10

Impedance Impedance 10 Impedance Impedance 1 10 0 101 0 7 10-2 10-1 100 101 102 103 104 10-2 10-1 100 101 102 103 104 10 90Frequency (Hz) Frequency (Hz)

) d 2 6 80 10 at 0h, at 8h at 16h, at 24h 70 5

.cm 10 Figure 4.35: Bode plot spectra of coated samples during current-off potential )

 60

o (

period in SSS: a) day1( -off, b) day2- off, c) day3-off, d) day4-off 4 10 50

103 40

2 30

10 -phase 20 1

10 10 Impedance Impedance 100 0 10-2 10-1 100 101 102 103 104 105 Frequency (Hz) 129

Chapter 4: Results

7 107 90 10 90 ) b

) a 2 6 b 2 6 a 80 80 10 10 5 70 5 70 .cm

.cm

10 )

) 10  60 o

 (

o 60 (

(

( 4 4 10 50 10 50 3 3 40 10 40 10 2 30 2 30 10 -phase 10 -phase 20 20 1 101 10 10 10 Impedance Impedance Impedance 0 0 10 0 10 0 -2 -1 0 1 2 3 4 5 10-2 10-1 100 101 102 103 104 105 10 10 10 10 10 10 10 10 Frequency (Hz) Frequency (Hz)

7 107 90 10 90

) d

c 2 6 ) 6 c 80 d 80 day1 off, day2 off 2 10 10 day3 off, day4 off 70 5 70 5 .cm

10 )

)

.cm 10  60 o

o (

60 (

(  4 4 ( 10 50 10 50 3 3 40 10 40 10 2 30 2 30 10 -phase 10 -phase 20 20 1 1 10 10 10 10 Impedance 0 Impedance Impedance 0 10 0 10 0 -2 -1 0 1 2 3 4 5 7 10-2 10-1 100 101 102 103 104 105 10 10 10 10 10 10 10 10 10 90 Frequency (Hz) Frequency (Hz) ) d 2 6 80 10 at 0h, at 8h 70 at 16h, at 24h 5 .cm 10

Figure 4.36: Bode plot) spectra of coated samples during current-off potential

 60

o ( 4 period in SSW: a) day1( -off, b) day2- off, c) day3-off, d) day4-off 10 50

103 40

2 30

10 -phase 20 1

10 10 Impedance Impedance 100 0 10-2 10-1 100 101 102 103 104 105 Frequency (Hz) 130

Chapter 4: Results

4.4.4.8 Surface analysis of TiBN-3 coating after different corrosion tests

4.4.4.8.1 SEM and FIB-cut analysis

The coating surface was examined by SEM after different immersion times. In comparison to the original coating, the tested coated samples did not indicate the presence of visible corrosion attacks such as localized attacks, i.e. pitting corrosion. Figure 4.38 shows the SEM micrographs of the coated specimens after 90 days immersion in SSS and SSW.

FIB cross-sectional analysis was done for the coating after 90 days immersion to look at the bulk and coating after long-time exposure; images are presented in Figure 4.39. It confirms the excellent adherence of the coating to the substrate, and it also shows that there was no sign of detachment or separation of the coating from the steel substrate. Moreover, no microcracks were formed within the coating bulk during the immersion time.

Figure 4.38: SEM images of TiBN-3 coated low carbon steel after 90 days immersion in: (a) SSS, (b) SSW

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Figure 4.39: FIB images of the cross-sections of TiBN-3 coated LCS after long immersion test: (a) SSS, (b) SSW

4.4.4.8.2 X-ray diffraction analysis

The X-ray diffraction patterns recorded after 48h immersion on the samples exposed to SSS and SSW test solutions did not show a significant difference from the one recorded from the original sample. It seems, therefore, that the possible transformations of the coatings induced by the corrosive conditions at this immersion time did not affect the bulk of the materials and occurred only at the surface. On the other hand, this was not the case for the samples tested in same solutions after 90 days; hence, XRD spectra showed increase/decrease in some peak intensities as presented in Figure 4.40. A drastic change in TiB2 {100} and TiB2 {201} peak intensities was observed, the formers was decreased while the later was increased. Moreover a new peak at 2θ= 30° was observed, this peak is related to c-BN. The peak at 43.69° and 44.43°, related to TiN and TiB2 respectively was almost vanished after the immersion test.

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TiB 1500 2 1400 1300 c-BN 1200 TiB TiN TiB 2 2 1100 TiB TiN TiB TiN 1000 900 TiB 800 (a) 2 700

Y Axis Title 600 500 (b) 400 300 (c) 200 100 0 20 25 30 35 40 45 50 55 60 65 70 75 80 ( ), [o] Position 2 Figure 4.40: XRD patterns of TiBN-3 coated low carbon steel after long term immersion test a) as- received, b) in simulated soil solution, c) in simulated seawater

4.4.4.8.3 XPS surface analysis

For more precise information, XPS was used to follow all transformations of the coated samples after the long-immersion experiment. Figure 4.41 represents a comparison of XPS spectra between the original TiBN-3/LCS and the samples immersed in SSS and SSW for 90 days. Both samples show remarkable changes in peak sizes in both test solutions with comparison to original specimen. For both surfaces titanium displays two main peaks with highest intensities at 458.6

[182] and 464.5 eV, respectively, those peaks are attributed to TiO2.

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On the sample in SSS Ti 2p peaks shows as well a single peak at 453.8 eV related to elemental Ti [174, 183], and a small peak at 454.6 eV corresponds to

TiB2. Those peaks did not exist on the sample in SSW where the main components of Ti 2p were TiO2. The increase in TiO2 intensity was accompanied with an increase of O 1s peak intensity; and it was more significant in SSW, indicating that the oxidation rate of the coating is higher in SSW than SSS.

The O 1s spectra in SSW is formed by a contribution of N-O at 530.1 eV, TiO2 at

531.2 eV and H2O at 532.2 eV [54]. In simulated soil solution the main contribution in O1s was from TiO2 and H2O at the same binding energies, no signal from N-O was observed; instead a small peak at 529.1 eV related to CO, this can be due to the low concentration of the oxynitride.

In the N 1s spectra a decrease and broadening on the tested specimens was observed, the peak corresponding to TiN disappeared completely, and the mean components of N1s were the peak centered at 397.9 eV in accordance with BN bonding [184] and N-O at 400 eV [170].

The electron binding profile of B 1s in SSS shows a peak at 187.1 eV which corresponds to TiB2; this peak was completely disappeared for the sample soaked in SSW. The second peak of B 1s at binding energy 190.7 eV corresponds to BN still existed on two tested specimens in both solutions but with lower intensities. The conclusions derived from the XPS database show good agreement with the XRD results and allow the identification of the relevant changes in the deposited coating after the immersion test. It is important to mention that no iron was detected on both samples after the long immersion time. These results indicate that the coating is very corrosion resistive and that no microcracks were formed during immersion time where substrate corrosion might have taken place.

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18000 11000 16000 10000 O1s Ti2p 9000 14000

8000 12000 7000 10000 6000

5000 8000

Y AxisTitle Y

4000 Title Axis Y 6000

3000 4000 2000 2000 1000

0 0 468 464 460 456 452538 536 534 532 530 528 526 Binding Energy (eV)12000 Binding Energy (eV) 3500 B1s N1s 3000 10000

2500 8000

2000 6000

1500

Y Axis Title Axis Y Y Axis Title Axis Y 4000 1000

2000 500

0 0 196 194 192 190 188 186 184 404 402 400 398 396 394 Binding Energy (eV) Binding Energy (eV)

11000 10000 Ti2p Original TiBN-3 9000 after 90 days in SSS after 90 days in SSW 8000

7000 Figure 4.41: XPS spectra of Ti2p, B1s, N1s and O1s of TiBN-3 coated LCS 6000 before and after long-immersion test in SSS and SSW

5000 Y Axis Title Axis Y 4000

3000

2000 135

1000

0 468 464 460 456 452 Binding Energy (eV) Chapter 4: Results

4.4.4.9 XPS analysis after measurements at different temperatures

XPS analysis was performed on TiBN-3 samples tested at different temperatures in simulated soil solution. For chemical state determination high resolution spectra of Ti 2p, O 1s, B 1s, and N 1s were recorded and presented in Figure 4.42. The main observation is the increase in O 1s peak intensity accompanied with a decrease in N 1s and B 1s peak intensities with increasing temperature.

The N 1s distribution reveals three contributions; the major peak is centered at 398.2 eV with two shoulders at 396.1 and 399.9 eV. The peak at 398.2 eV is attributed to BN; the other two peaks are due to N-C and N-O bonds in oxynitride compounds respectively [185]. The peaks observed on B 1s spectrum peaks at

187.1 eV, 190.3 and around 191.7 eV binding energies correspond to TiB2, BN, and the last one may be related to boron oxide [62]. The O 1s distribution contains three contributions situated at 530.2, 532 and 533.1 eV, and are assigned with N-O, TiO2 and O2 or a combination of O2 and B2O3 [174].

The Ti 2p spectrum is a contribution of four peaks; two peaks with fixed positions at binding energies of 454.2 and 464.6 eV, these peaks are attributed to TiB2 and

TiO2, respectively, with increasing temperature the first peak decreases and the second one slightly increases. The other two peaks, a contribution at 455.1 eV is assigned to TiN [54], it disappeared when increasing the temperature to 45°C and a new peak at 458.8 eV appeared, it is related to TiO2 indicating the full oxidation of TiN to TiO2 with increasing test temperature. The peak at 459.5 eV is a contribution of TiO2, its intensity increased with increasing temperature.

The results indicate that elevating temperature enhances the oxidation of the coating components, particularly; TiN oxidizes and induces TiO2 on the top of the coating, which is partially covered the surface.

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Chapter 4: Results

The other coating components, BN, TiB2, seem to be more resistive to elevated temperature. Both seem to have a slower oxidation rate at these conditions than TiN.

19000 at 15°C Ti2p 10000 at 35°C 18000 O1s

at 45°C 17000 9000 16000

8000 15000 14000

7000 13000

Y AxisTitle Y Y AxisTitle Y 12000 6000 11000

10000 5000 9000

4000 8000 470 468 466 464 462 460 458 456 454 452540450538 536 534 532 530 528 526 Binding Energy (eV) Binding Energy (eV)

22000 at 15°C 7000 20000 N1s B1s at 35°C 18000 at 45°C 6000 16000

5000 14000

12000 Y Axis TitleAxis Y Y Axis Title Y 4000 10000

8000 3000 6000

2000 4000

194 192 190 188 186 184 404 402 400 398 396 394 392 Binding Energy (eV) Binding Energy (eV)

Figure 4.42: XPS spectra of Ti2p, B1s, N1s and O1s of TiBN-3 coated LCS after electrochemical measurements at different temperature in SSS.

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Chapter 4: Results

4.4.4.10 XPS analysis after measurements at 48h in 1 M HCl

High resolution spectra of Ti2p, B1s, N1s and O1s peaks of the XPS measurements performed on TiBN-3/LCS samples tested in 1M HCl are shown in Figure 4.43. Generally, Ti 2p, B 1s and N 1s peak intensities decreased drastically after 48h in 1M HCl. The main component of Ti 2p was titanium oxide at binding energy of 458.8 eV [171]. N 1s was deconvoluted into two components located at binding energies of 398.3 eV and 400.3 eV. The peak at 398.3 eV is characteristic for BN [186] and at 400.3 for N-O or N-C [53]. Deconvolution of B 1s spectrum reveals two components at 190.3 eV and 192.1 eV. The first one is assigned to BN [186] while the second one is probably associated to B2O3 [187]. The XPS spectrum of O 1s is fitted to two components, the first one at 529.4 eV corresponds to titanium oxide [170] and at 532.7 eV corresponds to oxygen in adsorbed H2O [188]. It can be concluded that the coating in acidic medium is less resistive due to the dissolution of formed oxide layer; the main remaining phase of the coating surface was BN.

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Chapter 4: Results

7000 Ti2p 6000 4000 Original TiBN-3 O1s 5000 At 48h in 1M HCl

4000

3000

Y AxisTitle Y 2000 Y Axis Title Axis Y

2000

1000

0 0 466 464 462 460 458 456 454 452 450538 536 534 532 530 528 Binding Energy (eV) Binding Energy (eV)

5000 12000 B1s N1s 4500 10000

4000 8000 3500

6000 3000

Y Axis Title Y Y Axis Title Y

2500 4000

2000 2000

1500 0 196 194 192 190 188 186 184 402 400 398 396 394 Binding Energy (eV) Binding Energy (eV)

Figure 4.43: XPS spectra of Ti2p, B1s, N1s and O1s of TiBN-3 coated LCS after electrochemical measurements in 1M HCl.

4.4.4.11 Analysis after interrupted cathodic protection measurements Similar characterization of cathodically polarized TiBN-3 coated LCS was carried out. SEM micrographs in Figure 4.44, show peel-off of theTiBN-3 layer at different spots of the cathodically polarized coated LCS specimens.

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Chapter 4: Results

Figure 4.44: SEM micrographs of TiBN-3 coated LCS tested under interrupted cathodic polarization.

XPS analysis confirmed the presence of iron in form of iron oxides on the surface of the specimen tested under interrupted cathodic polarization, Figure 4.45.

Figure 4.45: XPS high resolution peak of Fe before and after interrupted cathodic polarization measurements in SSW

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Chapter 4: Results

4.4.5 Summary: The electrochemical and corrosion behavior of TiBN-3 deposited on low carbon steel

The electrochemical data obtained indicate that the ternary TiBN coatings on low carbon steel exhibit corrosion properties better than those exhibited by binary TiN and TiB2 coatings.

Electrochemical results give the experimental evidence that he TiBN-3 is very dense and present less defects compared to the other coatings (TiN, TiB2, TiBN- 1, and TiBN-2), thus, the electrolyte did not penetrate onto the substrate though long immersion times in the aqueous solution. Localized corrosion is the corrosion mechanism expected for the failure of the coated samples.

Over the frequency range applied, the equivalent circuit employed for the description of the EIS spectra for the coated samples provides the best fit of the experimental data. The electrochemical behavior of the materials can be depicted as a metal covered with a porous film. Charge transfer values obtained from this provide a quantitative basis for the monitoring of corrosion of substrate covered by a ceramic film. A standard procedure is to consider that a protective surface 6 2 is exhibiting Rct values well above 1*10 Ω cm [189]. As the Rct values of (TiN, 6 2 TiB2, TiBN-1, and TiBN-2) coatings were found to be smaller than 1*10 Ω cm after 6 h exposure to SSS, the progress of corrosion at pinholes or pores must be considered. In contrary, the Rct of TiBN-3 was found after 90 days immersion in SSS and SSW to be still higher than 1*106 Ω cm2 indicating the unique corrosion resistance of the deposited coating. The corrosion protection of the coating in acidic medium was found to be not sufficient; this was attributed to the dissolution of titanium oxides upon exposing to 1M HCl.

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Chapter 4: Results

4.5 Equivalent circuit for CVD coated steels

For the interpretation of the electrochemical behavior of the system from impedance data interpretation, an equivalent circuit depicted in Figure 4.43 was used. The proposed model suggests that the physical behavior is equal to metal coated with porous films. This widely accepted scheme has been deduced to represent the electrochemical behavior of a metal covered with unsealed porous layers [145, 190-192]. The equivalent circuit consists of the following elements: a solution resistance Rs of the test electrolyte, a charge transfer resistance Rct and a capacitance Cdl for defects in the coatings, and a capacitance Cc and a polarisation resistance Rp for the reminder of the coating layer regarded as intact (non-defective). During the fitting process, the capacitances were represented by a constant phase elements to account the deviations from ideal dielectric behavior related to surface inhomogeneities [193].

Electrolyte

Cc oating Cdl Rct C

LCS

Figure 4.43: The equivalent circuit used in modelling the electrochemical impedance spectroscopy results for the different coatings on low carbon steel.

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Chapter 4: Results

The same circuit was applied to model the EIS data for different coating systems. Tables 4.10 and 4.11 include the EIS parameters obtained from fitting the experimental EIS data with the equivalent circuit for coatings with 3 μm in SSS, and for the thicker TiBN-3, respectively. The data in Table 4.12 summarizes the parameters at certain times of immersion in SSS.

In Table 4.10, TiBN-1 and TiBN-2 coatings show higher Rp than TiN and TiB2 which indicates that the coatings are denser and compacter, with less pores and defects. The simulation is in correlation with data measured after 6 h immersion.

Table 4.10: Impedance parameters of different coatings deposited on coated steel in SSS after 6h immersion in SSS

The simulated data of TiBN-3/ LCS confirm the EIS measurements, showing again the behavior of a typical porous coating on a metallic substrate. Nevertheless, the very high polarization resistance values and the pure capacitive of the coated samples indicate that the coatings porosity (i.e., number and size of defects) is very low. Furthermore, the time-dependence of the data showed an increase in the polarization resistance Rp as well as the capacitive Cc followed by a decrease, reflecting that progressively the pores in the coating are filled with precipitates of corrosion products followed by dissolution of these corrosion products in the SSS.

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Chapter 4: Results

However, the impedance resistance and the capacitive values are still high after 90 days of immersion; similar to the values in the beginning of immersion. From the results, one can conclude that these small micro-cracks did not reach the LCS substrate; otherwise, the impedance and phase angle would be expected to drop to the uncoated LCS values. The high values of α obtained for the TiBN-3/LCS system should also be noticed, which indicate a smooth surface of the coating film.

time R C R C R ct dl α1 p c α2 s Error (%) days MΩ cm2 µF cm-2 MΩ cm2 µF cm-2 Ω cm2

0 7.9 1.3 0.81 8.9 3.0 0.95 347 2.3

10 19.4 1.2 0.85 15.4 3.1 0.95 298 3.3

25 18.4 1.3 0.83 13.8 3.1 0.94 174 2.6

51 29.8 1.2 0.88 20.2 3.9 0.95 279 3.0

64 28.7 1.2 0.88 23.0 3.8 0.95 222 4.4

79 25.4 1.2 0.87 18.8 4.2 0.95 348 3.5

90 26.8 1.0 0.87 11.2 4.2 0.95 251 2.2

Table 4.11: Electrochemical parameters obtained with equivalent circuit simulation for TiBN-3 coated low carbon steel in SSS at different immersion times

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Chapter 4: Results

The experimental and simulated spectra of TiBN-3 coated LCS in SSS for two exposure times are presented in Figure 4.44. Both figures show that the equivalent circuit satisfies well the electrochemical behavior of the coated sample measured by EIS.

Figure 4.44: The measured and the simulated EIS data of TiBN coated sample in SSS at: a) 79 days and b) 90 days.

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Chapter 5: Discussion

5 Discussion

The corrosion protection of steels by hard coatings is one of the most important and versatile means of improving component performance. It is well known that the constitution of material systems and the fabrication parameters determine the coating properties and the microstructure, consequently, their protection behavior. Therefore, for better understanding of the corrosion behavior of the various coating systems, the influence of the deposition parameters must be considered.

5.1 CVD process parameters Surface morphology and microstructure of CVD deposited coatings are controlled by many factors that are often interrelated, such as substrate, temperature, deposition rate, impurities, temperature gradients, and gas flow.

In this study the variable parameters were the input gas composition, gas flow rate, deposition time, and the used substrates i.e., stainless steel and low carbon steel, while maintaining the total pressure and the temperature constant during the deposition.

5.1.1 The effect of substrate microstructure and chemical composition on different deposited coatings The different coatings deposited at same deposition conditions show different morphologies on different substrates. TiN coating morphologies were a mixture of lenticular plate shape on stainless steel and icosahedral on low carbon steel, with five-fold symmetry crystallites. TiB2 on the other hand, were relatively smooth and coherent on stainless steel, whereas, on low carbon steel it showed larger and more clearly defined crystal facets with spallation.

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Chapter 5: Discussion

Since the initial surface preparation was identical the differences in the morphologies of different deposited coatings were thought to be due to the different chemical composition of the used substrate materials. The different microstructures of different steels, i.e., martensite and ferrite-pearlite, in addition to the presence of chromium and chromium oxide film on stainless steel, which had probably re-formed on the previously polished surface, had affected the initial growth pattern, consequently, the texture of the deposited films, hence, the growth on the deposited first layer of each deposited film proceeds easily [87, 131]. Among other things, the efficiency of a CVD hard coating depends on the adhesion to the substrate and indirectly on the properties of the substrate-coating interfacial region [41]. Generally, all studied coatings on two different steels showed good adherence onto metal substrates except TiB2 on low carbon steel, it was failed by spalling at the interface. The spallation of deposit TiB2 from low carbon steel substrate is probably due to the different thermal stresses: thermal expansion mismatch, thermal stresses and blistering. These stresses induce microcracks in the coating with the subsequent weakening of the final product.

TiB2 has a thermal expansion coefficient of (5.5-5.6) which is less than that of the low carbon steel (10.0) [194]. Thus, the combination low carbon steel-TiB2 faces high stresses due to the effect of thermal expansion differences, therefore, tensile stresses in the steel and corresponding compressive stresses in TiB2 layer reach high values and lead to cracking and/or spalling. On the other hand, chipping was not observed on TiB2 coated stainless steel, although it has a thermal expansion coefficient two or three times higher than that of TiB2, hence, this could not be the only reason for chipping. Blistering could also appear when coating and substrate interact to form intermediate layer. Bad adhesion of TiB2 on low carbon steel was observed by Takahashi et.al and Pierson et al. [87, 194] and this behavior was attributed to the formation of iron- boride interlayer on iron substrate prevented the good adhesion of the deposited coating on low carbon steel substrate.

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Chapter 5: Discussion

The better adhesion between TiB2 and stainless steel can be due to the better structural match between the deposited coating and the metal substrate [195].

5.1.2 The influence of boron flow rate on the and morphology of TiN

In TiNB coatings, even small amounts of BCl3 added to the reactant gas phase of

TiN resulted in a pronounced grain refinement. By increasing the BCl3 flow rate from 0.16 to 0.32 0 Nl/min grains decreased in size even more. Compared to the nitrides, the surface of borides and boronitrides of Ti showed more metallic luster. The atomic concentrations as function of the boron flow rate and deposition time are listed in Table 5.1. Results showed that the increase of the boron flow rate in the deposition atmosphere results in an increase of the boron content in the deposited films (TiBN-1&2), furthermore, increasing the deposition time while maintaining the boron flow rate constant leads to thicker coating with higher boron content (TiBN-3). The increment of boron content is accompanied with the reduction of titanium and nitrogen content, and of the oxygen contamination upon boron incorporation.

Table 5.1: Atomic percent chemical compositions (by EDX) of different deposited TiBN films as function of the boron flow rate and deposition time during deposition

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Chapter 5: Discussion

Optically, TiN which initially presents a golden color, becomes silver gray upon boron addition, this is in a good correlation with XRD results where TiB2 is the predominant phase in the deposited coating.

Contrary to TiN and TiB2 coating layers which generally had a preferred orientation, TiBN coatings was revealed to have multi-oriented structure by XRD analysis. BCl3 incorporation into TiN process caused the coating layer to be a composite of TiN, TiB, TiB2, and BN crystallite with more pronunciation of TiB2. Boron has a higher reactivity in comparison to nitrogen and is able to react faster with titanium forming the titanium-boride composites whereas nitrogen evaporates [196].

It is also evident from the x-ray diffraction (XRD) broad diffraction peaks that the average grain sizes of TiBN films are in the nanometer range, which could also be seen from SEM micrographs of the different deposited coatings, where increasing the boron content lead to finer crystal size, denser and compacter structures. Additionally, FIB cross-sections of the bulk coating prepared did not show the presence of more than one phase within the Ti-B-N matrix. The individual grains in the polished cross-section of the coating could not be discerned, this is a typical feature for nanocrystalline coatings [197, 198].

The change in microstructure was associated with an increase in deposition rate by adding more BCl3. Increasing the flow rate, increases the super saturation and nucleation rate, while decreasing the crystal size [49]. Moreover, boron interrupts the columnar coating growth of TiN, which yields a fine-grained structure and smooth surface at the highest content [199].

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Chapter 5: Discussion

Adhesion is mainly influenced by the resulting substrate-coating interface which is developed at the early stages of the deposition [200]. The adhesion of different deposited coating layers onto different steel substrates was not mechanically actuated. Optical micrographs of cross-sections and conducted FIB cuts of TiBN-3/LCS indicated a good adhesion which is supposed to be resulting from the interdiffusion of carbon and titanium between the substrate and the coating during CVD coating [201]. Furthermore, the presence of mixed-phase layers in TiBN prevent the adhesion problems that may result in carbon diffusion from the steel, since the freed carbon can be absorbed by the cubic TiN lattice without causing adhesion problems [202].

5.2 Corrosion and electrochemical behavior of different coatings on different steels

Under most conditions, corrosion of chemical vapor deposited hard titanium based ceramic layers on steel usually takes a localized form, due to the establishment of an elecropotential difference between the coating material and the less noble steel substrate. At the defects, localized galvanic corrosion can occur, leading to accelerated attack at the coating/substrate interface [125, 203- 206].

Electrochemical measurements were used to evaluate the corrosion behavior of each different coating system. As known, open-circuit potential measurement is good to assess any existing through-porosity (or open porosity) in a coating structure.

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Chapter 5: Discussion

Because Ti based ceramic coatings are applied on stainless steel and low carbon steel substrates, any through-porosity (allowing the test solution to penetrate the coating into the interface of coating and substrate) makes the open-circuit potential of the coating approach that of the substrate. On the other hand, if the coating is dense (no existing cracks or through-porosity), the measured open- circuit potential is supposed to introduce the behavior of the coatings material in the used solution and the surrounding environment.

Additional polarization measurements give information on the reactions around the corrosion potential. Moreover, the corrosion phenomena (pitting, crevice corrosion) taking place during the corrosion attack can be evidenced.

In acid solution the cathodic reaction is hydrogen evolution, and in neutral solution is oxygen reduction. The electrons consumed there must be supplied by an anodic reaction, i.e. the dissolution of the steel substrate. Because anodic reaction on the coatings themselves is slow, measured corrosion currents indicate porous coatings [207]. The corrosion current densities and polarization resistance are inversely proportional. A high polarization resistance therefore indicates a low corrosion current density, consequently, low coating porosity.

5.2.1 The electrochemical and corrosion behavior of different coatings on stainless steel and low carbon steel

On low carbon steel the corrosion resistance of the TiN and TiB2 coatings was insufficient, probably due to the insufficient film thickness for the coatings and the unsatisfactory structure and high porosity. Open circuit potential of TiN coated specimen descended to a potential of the substrate, that is, the specimen became an activated state at certain time.

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Chapter 5: Discussion

It is known that an activation time, which is a period until OCP reaches that potential, can be used for evaluation of the corrosion resistance of ceramic films coated on the steels [13, 208].

On low carbon steel, TiB2 coatings have high current densities after polarization which is attributed to the bad adhesion of the coating on low carbon steel substrate and the chipping of the coating at some spots. In contrast, the best corrosion behavior of coated stainless steel was introduced by TiB2 with the lowest open circuit potential values and anodic current density, whereas TiN was found to improve the corrosion resistance very slightly.

The behavior of TiBN films deposited onto both different steel substrates was found to improve the corrosion resistance in neutral test solutions. TiBN-2 showed the best performance for coated LCS and was equal to TiB2 for coated stainless steel.

The nature of the chemical bonding character may affect the corrosion behavior of the single layer films. According to Holleck [209], all titanium-based hard layers tested here are metallic hard materials since metallic bonding is predominant.

However, besides primary metallic bonding localized metal-non-metal bonds can also be exist. The level of metallic bonding increases when going from films based on the group IV to group VI of the transition metals. In the same way it increases when going from nitrides to carbides and borides. For the coating tested, this means that TiB2 has the lowest and TiN has the highest amount of direct metal-non-metal bonds. According to this stability the free enthalpy ΔG is lowest for TiB2 and highest for TiN, whereas TiBN fall somewhere in between. This correlation could not be proven in this study.

152

Chapter 5: Discussion

The corrosion resistance of the investigated titanium-based coating systems was found to be mainly influenced by the structure, thickness, and the porosity of the deposited layers [58, 210, 211], enhanced by the formation of a passive oxide layer on the deposited films as shown in the electrochemical results and surface analysis carried out after measurements.

The effect of structure and porosity was confirmed from the calculated porosity and the polarization resistance (Rp) for each coating system. It was found that Rp values were inversely proportional to the calculated porosity, and those coatings with the highest Rp values and lowest porosity showed the best corrosion resistance. According to many studies [39, 68, 212], TiN and TiB2 themselves are not very stable in water solutions but it can be oxidized to a more stable compound Ti(OH)3 and further to TiO2.H2O at pH higher than 2 by the hydrogen evolution reaction.

The formed TiO2 oxide layer has a passive nature and very resistive to localized attack. This phenomenon was studied for coating deposited on low carbon steel, where XPS analysis of the original deposited coatings before the electrochemical measurements confirmed the presence of TiO2 on the surface of all deposited coatings.

TiN: consists of TiN, TiO2, NO and small amount of CN.

TiB2: partially oxidized to TiO2 and B2O3, contains also a small amount of BN and CN.

TiBN-1: consists of TiB2 and BN with small amount of TiN; oxide phases are detected in a very small extend. TiBN-2: similar to TiBN-1; much lower TiN content; much lower amount of oxide phases (TiO2 and B2O3). The BN phase seems to enhance the stability to oxidation.

153

Chapter 5: Discussion

These results indicate that titanium based coatings oxidize once they are objected to the surrounding atmosphere and build a thin oxide layer, the composition of this layer varies according to the existing phases in the coating film. The role of these oxides was excessively investigated for TiBN-3/LCS system.

5.2.2 The electrochemical behavior of TiBN-3 coating on low carbon steel

TiBN-3, the coating with its fine size crystallites, mixed orientations, and dense structure showed the best corrosion performance between all studied coatings on low carbon steel. The development of the open circuit potential of TiBN-3 coating in both solutions SSS and SSW over 90 days immersion indicate the occurrence of insoluble corrosion products on the film surface; a passive thin film was formed.

This film is mainly composed of TiO2 and NO as shown in XPS analysis after immersion tests. The passivation is correlated with the relatively high positive values of the potential. The fluctuation in the measured potential may be due to the slow rate of the accumulated corrosion products of titanium and/or substrate in the pinholes or the microcracks followed by dissolution in test solution. One reason for the local dissolution could be due to the boron resulting from TiB2 oxidation/dissolution will form boric acid, which locally prevents the full passivation of the coating surface [68].

However, no breakdown events are seen during the 90 days of immersion, as those would be expected to lead to a sudden significant decrease of the potential.

154

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This behavior was further confirmed by potentiodynamic polarization curves (Figures 4.25). Coated samples exposed to the atmosphere or the test solutions after coating process are covered spontaneously by an oxide film. As soon as potentiodynamic polarization was performed in test solutions between -1.0 and

+3.0 V, the process of dissolution of natural oxide film TiO2 begins first, with 3+ 2+ transfer of Ti , TiO ions or/and H3BO3 into the solution. Simultaneously, self- passivated film formation of TiNxOy or TiO2 also begins. The latter oxide inhibits and slows down the dissolution of the composites [46]. An anodic current peak can be observed at ≈2.0 V in each case, independent from the chemical composition of the neutral solutions or from the substrate material. A slight decrease of anodic current as the potential becomes more positive is noticed, most probably due to a decrease of the real surface area as the film thickness increases. Furthermore, cyclic potentiodynamic polarization curves carried out after different immersion time (Figure 4.31) showed that whether the measurements were performed directly or after long immersion time, the results were very identical. The region of constant current with increasing potential suggested that TiBN surface was passive.

Moreover, the negative hysteresis observed on the reverse scan as the current decreased suggested that the passive layer did not break down under the conditions used in the present study. The corrosion in the first stage could be assumed as follow, when test solution reached LCS substrate through coating defects, e.g., micro-cracks and pinholes.

The impedance spectra in SSS and SSW do not change during the immersion time which indicates that the corrosion rate is very low.

155

Chapter 5: Discussion

The positive corrosion potential, the high polarization resistance and the low fraction of anodic current flowing through the pores indicate that the substrate is passive and the measured response describes the coating.

Thus electrochemical and corrosion stability of the coating can be attributed to:  The increase in coating thickness which reduces the possibility of through-coating defects (e.g. pores).  The presence of different phases with different crystal orientations which decreases the opportunity of pore formation due to the discontinuous in crystallite boundaries in the structure.  The different compositions of the different phases in the coating with redirected current flow between coating and substrate due to the different electrical behavior.  The different preferred crystal orientations in the coating texture which commonly affects its properties (microhardness, adhesion) [213, 214], consequently its corrosion resistance. As a result, it could be concluded and according to surface analysis performed after electrochemical

experiments, that TiB2 with {201} and {200}, TiN with {311} orientation in addition to c-BN has better corrosion resistance when exposed to chloride containing neutral solutions.

Moreover the electrochemical potentials between the phases are different, thus the corrosion penetration towards the substrate can be reduced owing to the current flowing within the coating body.

 The presence of the oxide and oxynitride components on the surface which form a barrier protects the surface from corrosion.

156

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 The shielding effect of nitrogen anions (N3-) layer at the surface, inhibiting the oxidation of the underlying titanium ions [215].  It is possible that the iron initially absorbed by the coating may reduce its corrosion resistance; especially at low layer thicknesses.  Titanium oxides have a strong basic character and dissolve easily in acidic solution [216, 217] without acting as a barrier. Therefore the protection was insufficient in acidic HCl.

5.2.2.1 The influence of test solution

The OCP shift in the noble direction for the TiBN-3 coated samples suggests the formation of a passive film that acts as a barrier for metal dissolution and reduces the corrosion rate. The potential increase shows that the coating becomes thermodynamically more stable with time.

The distinguishing feature of the anodic behavior of the titanium boronitride in sea-water and soil-water in comparison with the acid media 1M HCl is the high resistance to the oxidation and pitting processes; the passivation range of the coating exceeds 4 VAg/AgCl. The presence of long passivation regions on the anodic polarization curves indicates that the coating examined is in the passive state in SSS and SSW. Thus the corrosion behavior of TiBN-3 in these solutions can be compared with the behavior of the transition metals Ti, Ta, not susceptible to pitting corrosion.

157

Chapter 5: Discussion

Generally, the pH value of a solution essentially influences the kinetics of the anode and cathode processes and the corrosion rate. The corrosion process in the SSS and SSW solutions proceeds with the oxygen depolarization. It means that the corrosion rate is controlled by the oxygen concentration in the solution and by the rate of the reaction. The generation of oxygen at higher anodic potential is prevented by the oxide film formed on the coating surface; this is due to its low electron conductivity.

Additionally, it is confirmed that the anodic polarization curves practically do not change at different pH values, but the onsets of oxidation reaction [47]. Only at pH=0 for 1M HCl solution the oxidation peak at +2.0 V was shifted to higher potential. Nevertheless in acidic medium at open circuit potential conditions, the formed titanium oxides and due to their strong basic character dissolve exposing the underlying coating directly to the acidic solution; the EIS measurements and XPS characterization supports the dissolution argument.

The concentration of major ions present in simulated natural water solutions, Na+, + + + 2- K , Ca , Mg , Cl‾, SO4 , and HCO3‾, influences strongly the corrosion behavior, since the water aggressiveness is closely related to the concentration of either of these ions. HCO3‾ has the same concentration in both solutions (SSS and SSW) and thus both of them have the same pH value. The screening of pH changes while experiments did not show any changes over test periods indicating that no increase in solution acidity took place. The different concentration of Cl‾ in test solutions, i.e. SSW, was found not to play a role in the corrosion resistance of the applied coating layer. This can be attributed to the superior corrosion resistance of titanium oxides to chlorides. The cations Ca+, Mg+ might deposit from corrosion as a calco-magnesium which has a barrier protection effect, traces of Ca was detected by XPS but it was difficult to define the chemical bonding.

158

Chapter 5: Discussion

5.2.2.2 The effect of test temperature

Generally, the increase in temperature is favorable for diffusivity of oxygen and the various interfacial reactions. The slight increase in cathodic current density observed in Figure 4.32, indicates that the oxygen diffusion was slightly enhanced when the test temperature increases from 15 to 45 °C, and consequently enhanced the oxidation of the TiBN-3 coating. The anodic current densities were also slightly stimulated by the increased temperature but the corrosion potentials did not show a remarkable change. The stimulation of the cathodic and anodic processes in the vicinity of the corrosion potential, suggests a stimulation of the whole electrochemical process. Nevertheless, increasing temperature to 45°C was not sufficient to offset the effect resulting from the enhanced reactivity of the coating with increased temperature. A previous study carried out on titanium diboride showed that the coating behaves like a passive metal in NaCl solution due to the formation of a surface oxide film, whose protectivity decreases with the temperature and disappears at 65°C [69]. No signs of corrosion could be observed. XPS analysis after electrochemical measurements showed that the oxidation rate increases with increasing test temperature, and that the oxidation is mainly of TiN phase where the peak intensity of TiO2 increased with temperature.

5.2.2.3 Passivity and localized corrosion

Titanium based ceramic films having nobler potential than the steel substrates are classified as cathodic coatings. These coatings will provide significant corrosion protection when they are free of pinholes and cracks. Thus, in the presence of defect; the substrate is subject to galvanic corrosion in the coating defect as shown in Figure 5.1. The corrosion is rather intensive because the ratio between the cathodic coating and the anodic spot of bare substrate is very high.

159

Chapter 5: Discussion

Cathode Coating Anode Steel substrate

Figure 5.1: Localization of corrosion at a defect in cathodic coating on steel

A part from the galvanic action, small pores can also increase the corrosion by the mechanism of crevice corrosion or pitting corrosion. The area of a pore will form an occluded corrosion cell, where the electrolyte will become more aggressive because of the increased acidity and increased concentration of aggressive ions, e.g. Cl‾.

There are several reasons for the defect formation in the coatings, i.e. columnar growth, pores, microscopic cracks. In the experiments of the present study, it has been shown that when pitting corrosion takes place on coated low carbon steel, a very high dissolution rate of the substrate material was observed followed with pit propagation, the coating was not able to sustain the substrate from further dissolution. On the other hand, on the coated Ta, repassivation was observed when the applied potential was reduced in the reverse scan.

Hence, each of the analyzed coated materials, i.e. low carbon steel and Ta, show a different passivity breakdown, the chemical stability of the substrate seems to play a basic role in the pitting propagation. Galvanic coupling which is supposed to be the reason for the pitting corrosion in TiBN-3/LCS system could be the reason as well for the pitting corrosion of TiBN-3/Ta, owing to the fact Ta is more stable than the coating at the applied anodic potential.

160

Chapter 5: Discussion

5.2.2.4 The effect of interrupted cathodic polarization

SEM micrographs of the cathodically polarized samples showed that the coating underwent blistering and deterioration. This result was also proved by XPS analysis since Fe was detected in a small amount on the surface. This lead to the conclusion that the coating system TiBN-3/LCS is not stable under interrupted cathodic polarization.

Two different mechanisms can explain this damage of the coating:

a. The effect of blistering caused by hydrogen absorption. As can be seen from cyclic voltammogram of TiBN-3 in SSS (Figure 5.2), hydrogen

evolution takes place at cathodic potentials higher than 0.9 V vs Ag/AgCl. It is well known that in aerated neutral solutions the oxygen reduction is stronger than the water reduction as cathodic reaction:

½ O2 + H2O + 2e‾ → 2OH‾ (5-1)

When lowering the potential to achieve cathodic protection, hydrogen evolution takes place by water reduction reaction:

2H2O+ 2e‾ → H2 + 2OH‾ (5-2)

When hydrogen atoms meet in a trap and combine, they form hydrogen molecules in the trap. The accumulated hydrogen gas inside the extremely small cracks will lead to build up of excessive internal hydrogen pressure. At cretin times, the internal hydrogen pressure will become sufficient to cause the coating to blister.

161

Chapter 5: Discussion

b. The reduction and repassivation of the oxide layer on film surface. At the most negative potential, the oxide layer from the TiBN-3 deposit is reduced, and the film is directly exposed to the solution. After cutting the applied potential, and when the coated steel electrode is at OCP, the anodic reactions occurring at the coating surface and coating microcracks

are the oxidation of titanium to TiO2 and of iron to iron oxides and hydroxide, in this way, the volume of resulting oxides and corrosion products will increase, consequently, the size of defects and microcracks will increase further. Thus and obviously the frequent repeat of on/off cathodic potential will lead to enlarging the defects in the coating and the exposed substrate to test solution, increasing the corrosion rate.

120,0µ

)

-2 100,0µ 80,0µ TiBN-3 in SSS 60,0µ 40,0µ 1 20,0µ 2 0,0 -20,0µ -40,0µ

Current Density (A.cm Density Current -60,0µ -80,0µ -1 0 1 2 3 Potential (V ) vs Ag/AgCl Figure 5.2: Cyclic voltammetry of TiBN-3 coated LCS in SSS

162

Chapter 6: Conclusion

6 Conclusion Different titanium based ceramic films with different chemical compositions were deposited on two different steels by chemical vapor deposition (CVD).

The electrochemical and corrosion behaviors characterized in different solutions with different measuring techniques.

The results clearly demonstrate that the corrosion resistivity of a good coating is related to the impermeability of the protective coating which is in turn depends on: a) its thickness; b) the absence of defects; c) the adhesion of the layer to the substrate. In these terms, TiB2 (Titanium diboride) and TiBN-2 (Titanium Boronitrude) showed the best corrosion behavior on stainless steel, while on low carbon steel the best corrosion resistivity was obtained by TiBN-2. Generally, the deposited titanium based ceramic coatings are electrochemically noble compared with most structural materials and their corrosion is negligible in most of the ordinary used electrolytes but the existing growth defects in the coating are detrimental to the corrosion resistance of the coated metal: these defects are particularly dangerous as they provide direct paths for corrosive electrolytes to reach the coating/substrate interface, where the localized corrosion can be initiated due to the potential difference between the coating and the metallic substrate [218].

Depending on the previous results, CVD process parameters were optimized to permit the deposition of dense TiBN coating without mechanical defects, such as micro-porosity, localized cracks or poor local adhesion to the metallic substrate. Thicker TiBN layers were deposited on low carbon steel substrate, these coatings showed a superior corrosion resistance in neutral solutions and a moderate corrosion protection in acidic medium. Increasing test temperature did not significantly affect the corrosion behavior of the deposited TiBN-3 film.

163

Chapter 6: Conclusion

The high corrosion protection of TiBN-3 is related to the fact that they are composed of several phases (TiN, TiB2, and BN). Multiphase systems display many similarities with composite materials and often display better corrosion properties than single-phase materials. The coating can also be thought as a barrier that, by preventing the contact of electrochemically active species with the metal surface, hinders the occurrence of corrosion processes. In neutral solutions, TiBN-3 coating behaves like a passive metal because of the formation of rather protective surface film composed of titanium oxides and oxynitrides. This passive film was less resistive in acidic medium and dissolved exposing the coating to the aggressive medium. Interrupted cathodic polarization in artificial seawater and soil water induces cracks and leads to coating spalling due to hydrogen evolution and the changes in oxides volume due to oxidation- reduction processes, where the volume of the oxide filling the micro-cracks increases and leads to crack broadening and spalling. Finally, this study demonstrated that CVD was successfully used to produce a smooth pore-free coating; giving reliable protection against corrosion to low carbon (pipeline) steel.

164

Chapter 7: Outlook (Future work)

7 Outlook (future work) There are several areas where additional investigations can be an extension of this thesis and provide valuable information. The recommendations for future work include:  Investigations of the different microstructures and different adhesion properties of the different coatings deposited on different steels would be of a great interest.

 A detailed study of the electrochemical oxidation of TiBN and the nature of formed oxides/oxynitrides should also be very interesting as they influence the corrosion protection of the film.

 Applying multiple layers to enhance the corrosion performance of CVD deposited Ti-B-N coatings on low carbon steel might also be studied.

 Evaluation of the wear-resistance and coating-hardness of the coating systems would also be useful to develop the optimum balance between mechanical and corrosion properties for specific applications areas.

 The behavior in acidic environments with different pH values should also be further studied.

 To understand the behavior of the different coatings under applied cathodic protection more experiments should be carried out at different potentials to define the potential window where cathodic protection could be applied without damaging the coating and affecting its corrosion performance.

 More analysis techniques, in particular FIB and TEM, should be used to have better understanding of the formation of cracks and the peeling-off of the coating.

165

Chapter 8: Bibliography

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200. Stock, H.-R., Jarms, C., Berndt, H., Wielage, B., Hofmann, A., In-situ and ex-situ examination of the early stages of chemical vapor deposition. Fresenius' Journal of Analytical Chemistry, 1998. 361(6-7): p. 645-646. 201. Kessler, O.H., Hoffmann, F.T., Mayr, P., Microstructure and property changes caused by diffusion during CVD coating of steels. Surface and Coatings Technology, 1999. 120-121: p. 366-372. 202. Holzschuh, H., TiBN Coating. 2004: USA. 203. Jehn, H.A., Baumgärtner, M.E., Corrosion studies with hard coating- substrate systems. Surface and Coatings Technology, 1992. 54-55(C): p. 108-114. 204. Brown, R., Alias, M.N., Fontana, R., Effect of composition and thickness on corrosion behavior of TiN and ZrN thin films. Surface and Coatings Technology, 1993. 62(1-3): p. 467-473. 205. He, J.L., Chu, C.H., Wang, H.L., Hon, M.H., Corrosion protection by PECVD-SiOx as a top coating on TiN-coated steel. Surface and Coatings Technology, 1994. 63(1-2): p. 15-23. 206. Liu, C., Leyland, A., Lyon, S., Matthews, A., Electrochemical impedance spectroscopy of PVD-TiN coatings on mild steel and AISI316 substrates. Surface and Coatings Technology, 1995. 76-77(PART 2): p. 615-622. 207. Park, M.J., Leyland, A., Matthews, A., Corrosion performance of layered coatings produced by physical vapor deposition. Surface and Coatings Technology, 1990. 43-44(PART 1): p. 481-492. 208. Townsend, H.E., Cleary, H.J., Allegra, L., Breakdown of oxide films on steel exposed to chloride solutions. Corrosion, 1981. 37(7): p. 384-391. 209. Holleck, H., Material selection for hard coatings. J. VAC. SCI. & TECHNOL. A, 1986. 4(6 , Nov.-Dec. 1986): p. 2661-2669. 210. Liu, C., Leyland, A., Bi, Q., Matthews, A., Corrosion resistance of multi- layered plasma-assisted physical vapor deposition TiN and CrN coatings. Surface and Coatings Technology, 2001. 141(2-3): p. 164-173. 211. Mantyla, T.A., Helevirta, P.J., Lepisto, T.T., Corrosion behavior and protective quality of tin coatings. Thin Solid Films, 1985. 126(3-4): p. 275- 281. 212. Gorbachev, A.K., Thermodynamics of oxidation-reduction equilibria in the TiN-H//2O System. Protection of Metals (English translation of Zaschita Metallov), 1983. 19(2): p. 212-214. 213. Veprek, S., Plasma-induced and plasma-assisted chemical vapor deposition. Thin Solid Films, 1985. 130(1-2): p. 135-154. 214. Musil, J., Kadlec, S., Vyskocil, J., New results in d.c. reactive magnetron deposition of TiNx films. Thin Solid Films, 1988. 167(1-2): p. 107-120. 215. Schmickler, W. Schultze, J.W., Model for the electrochemical behavior of TiN and TiC. Berichte der Bunsengesellschaft/Physical Chemistry Chemical Physics, 1992. 96(6): p. 760-764.

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216. Wiberg, N., Holleman, A.F., Wiberg, F., Inorganic Chemistry. 34th ed, ed. F. Wiberg. 2001, New York: Academic Press. 217. Sarkar, R., General & Inorganic Chemistry. 1st ed. 2001, Kolkata: New Central Book Agency (P) Ltd. 218. Costa, J.M., Trends in electrochemistry and corrosion at the beginning of the 21 st century. 2004, Barcelona.

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List of publications

a) E. Alkhateeb, R. Ali, S. Virtanen, N. Popovska, Electrochemical evaluation of the corrosion behavior of steel coated with titanium-based ceramic layers. Surface and Coating Technology 205 (2011) 3006-3011

b) R. Ali, E. Alkhateeb, F. Kellner, S. Virtanen, N. Popovska-Leipertz, Chemical vapor deposition of titanium based ceramic coatings on low carbon steel: Characterization and electrochemical evaluation. Surface and Coating Technology 205 (2011) 5454-5463

c) E. Alkhateeb, R. Ali, N. Popovska- Leipertz, S. Virtanen, Long-term corrosion study of low carbon steel coated with titanium boronitride in simulated soil solution. Electrochimica Acta, 76 (2012) 312-319.

d) Abolfazl Motalebi, Mojtaba Nasr-Esfahani, Rania Ali, Mehdi Pourriahi, Improvement of corrosion performance of 316L stainless steel via PVTMS/henna thin film. Progress in Natural Science: Materials International, 22(2) (2012) 392-400

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Conference Presentations

 Kurt-Schwabe Symposiom 2009, Erlangen, Deutschland- R.Ali, S.Virtanen, “Corrosion behavior of low carbon steel coated with polyaniline” - P

 EUROCORR 2011, Stockholm, Schweden- R.Ali, E.Alkhateeb, S.Virtanen, N.Popovska, “Electrochemical evaluation of the corrosion behavior of steel coated with titanium-based ceramic layers” - O

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