DEGREE PROJECT IN CHEMICAL SCIENCE AND ENGINEERING, SECOND CYCLE, 30 CREDITS STOCKHOLM, SWEDEN 2020
Corrosion protection of aluminum coated with a polymer matrix in presence and absence of conductive polymer
MOHAMED HASSAN ABDI
KTH ROYAL INSTITUTE OF TECHNOLOGY SCHOOL OF ENGINEERING SCIENCES IN CHEMISTRY, BIOTECHNOLOGY 1AND HEALTH
DEGREE PROJECT IN CHEMICAL SCIENCE AND ENGINEERING, SECOND CYCLE, 30 CREDITS STOCKHOLM, SWEDEN 2020
Abstract
Aluminum and aluminum alloys have rather good corrosion resistance, but these materials can still corrode. Metal corrosion is never wanted, and it can lead to disastrous outcomes in various industries and applications. There are different methods to protects aluminum and its alloys from corrosion, such as anodization and the use of various coatings techniques. Not a lot of research have been done on aluminum coated with an organic coating containing conductive polymer. Even less is known about aluminum coated with a waterborne polymer matrix containing conductive polymer. Three systems were investigated in my diploma thesis work regarding their anti-corrosion properties. To this end electrochemical impedance spectroscopy, open circuit potential and potentiodynamic polarization were utilized, and also some atomic force microscopy, AFM, measurements were done. Aluminum coated with a waterborne polymer matrix in the absence of PANI had a good corrosion protection at first but reduced barrier properties over time in 1 M NaCl. The shelf-life of the waterborne polymer matrix in the absence of PANI was also briefly investigated. It was shown that a freshly made waterborne polymer matrix exhibited better corrosion protection than a 2 years old waterborne polymer matrix stored at room temperature. Aluminum coated with the waterborne polymer matrix in the presence of PANI showed signs of active corrosion protection initially, but it transitioned to passive corrosion protection with time. Atomic force microscopy was used in various modes to gain insight on the waterborne polymer matrix in the presence of PANI. A conducting network was observed in the AFM measurements and confocal light optical microscopy indeed suggested that this would be the case. The conducting network in the waterborne polymer matrix could explain the active corrosion protection observed initially. More studies are needed to gain insight on the chemical processes at the interface of the aluminum alloy and the waterborne polymer matrix in presence of PANI.
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DEGREE PROJECT IN CHEMICAL SCIENCE AND ENGINEERING, SECOND CYCLE, 30 CREDITS STOCKHOLM, SWEDEN 2020
Sammanfattning Aluminium och aluminiumlegeringar har ganska bra korrosionsbeständighet, men dessa material kan fortfarande korrodera. Metallkorrosion är aldrig önskvärt och det kan leda till katastrofala resultat i olika branscher och applikationer. Det finns olika metoder för att skydda aluminium och dess legeringar från korrosion, såsom anodisering och användning av olika beläggningstekniker. Inte mycket forskning har gjorts på aluminium belagd med en organisk beläggning innehållande ledande polymer. Ännu mindre är känt om aluminium belagt med en vattenbaserad polymer matris innehållande ledande polymer. Tre system undersöktes i mitt examensarbete om deras korrosionsskyddande egenskaper. För detta ändamål användes elektrokemisk impedansspektroskopi, öppen kretspotential och potentiodynamisk polarisering, samt gjordes även en del atomkraftsmikroskopi, AFM, mätningar. Aluminium belagt med en vattenbaserad polymer matris i frånvaro av PANI hade ett bra korrosionsskydd först men reducerade barriäregenskaper över tiden i 1 M NaCl. Hållbarheten för den vattenbaserade polymer matrisen i frånvaro av PANI undersöktes också kort. Det visades att en nygjord vattenbaserad polymer matris uppvisade bättre korrosionsskydd än en 2 år gammal vattenbaserad polymer matris lagrad vid rumstemperatur. Aluminium belagt med den vattenbaserade polymer matrisen i närvaro av PANI visade initialt tecken på aktivt korrosionsskydd men övergick till passivt korrosionsskydd med tiden. Atomkraftmikroskop användes i olika lägen för att få insikt om den vattenbaserade polymer matrisen i närvaro av PANI. Ett ledande nätverk observerades i AFM-mätningarna och konfokalt ljusmikroskop antydde verkligen att detta skulle vara fallet. Det ledande nätverket i den vattenbaserade polymer matrisen kan förklara det aktiva korrosionsskyddet som observerades initialt. Fler studier behövs för att få insikt om de kemiska processerna vid gränssnittet mellan aluminiumlegeringen och den vattenbaserade polymer matrisen i närvaro av PANI.
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DEGREE PROJECT IN CHEMICAL SCIENCE AND ENGINEERING, SECOND CYCLE, 30 CREDITS STOCKHOLM, SWEDEN 2020
Table of contents
1. Introduction ...... 6 1.1. Aluminum and Aluminum alloys ...... 6 1.2. Corrosion mechanisms ...... 6 1.2.1. Motivation and scope ...... 7 2. Literature review ...... 8 2.1. Introduction ...... 8 2.2. Protective methods for aluminum and its alloys ...... 9 2.2.1. Anodization ...... 9 2.2.2. Cathodic protection ...... 9 2.2.3. Coating techniques ...... 10 2.2.3.1. Metallic coating techniques ...... 10 2.2.3.2. Other coating technique ...... 10 2.2.3.3. Organic coatings ...... 12 2.2.3.4. Sol-gel coatings ...... 12 2.2.3.5. Corrosion inhibitors ...... 13 3. Aluminum alloy 6xxx series ...... 13 3.1. Microstructure ...... 14 3.2. Corrosion mechanisms of aluminum alloys ...... 15 4. Conductive polymer PANI ...... 17 5. Polymer matrix ...... 18 6. Materials ...... 18 6.1. Aluminum alloy 6060 ...... 18 6.1.1. Preparation of Aluminum alloy 6060 surfaces ...... 19 6.2. Polymer matrix ...... 19 6.2.1. Preparation of polymer coating ...... 19 6.2.2. Preparation of polymer coating containing conductive polymer ...... 19 7. Experimental Methods ...... 20 7.1. Open Circuit Potential ...... 20 7.2. Electrochemical Impedance Spectroscopy ...... 21 7.3. Potentiodynamic Polarization ...... 25 7.4. Atomic Force Microscopy, AFM ...... 26 5.4.1 Conductive AFM, C-AFM ...... 27 5.4.2 Peak Force Tuna, PF-TUNA AFM ...... 27 5.4.3 Intermodulation AFM, Im-AFM ...... 27 8. Light optical microcopy ...... 28 9. Results and Discussion ...... 29
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DEGREE PROJECT IN CHEMICAL SCIENCE AND ENGINEERING, SECOND CYCLE, 30 CREDITS STOCKHOLM, SWEDEN 2020
9.1. Bare Aluminum ...... 29 9.2. Polymer Matrix ...... 32 9.2.1. Polymer Matrix – new batch ...... 32 9.2.2. Shelf-life issues ...... 34 9.3. Polymer Matrix (new batch) with PANI ...... 36 9.4. AFM measurements ...... 38 10. Conclusion ...... 40 11. Acknowledgement ...... 41 12. References ...... 42 13. Appendix ...... 49 13.1. Polymer Matrix – Old batch ...... 49 13.2. Potentiodynamic Polarization ...... 50
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DEGREE PROJECT IN CHEMICAL SCIENCE AND ENGINEERING, SECOND CYCLE, 30 CREDITS STOCKHOLM, SWEDEN 2020
1. Introduction
Although aluminum is a common metal used in various industries and known for exhibiting good corrosion properties, aluminum and its alloys can still corrode. Aluminum may deteriorate due to different types of corrosion mechanisms such as pitting corrosion, intergranular corrosion, exfoliation corrosion, galvanic corrosion, and crevice corrosion. Aluminum and its alloys have a good natural protection against corrosion in atmosphere and solutions due to the presence of a passive oxide film on the surface. The thickness of the oxide film, which forms in contact with oxygen, is typically in the range 4-10 nm and it rebuilds quickly if damaged. The corrosion resistance is therefore governed by the passive oxide film and when it breaks, the aluminum starts to corrode. Consequently, corrosion protection of aluminum is crucial, and possible methods include anodization, cathodic protection as well as the use of inhibitors, and various coating techniques. [1][2] A good way to protect aluminum from corrosion is to use a passive barrier organic coating. In addition, one may add e.g. polyaniline (PANI), which is a conductive polymer, to also achieve active protection. There are different forms of PANI with different physical and chemical properties. Emeraldine is the most common one, and it is used for coating metals like aluminum. [1][26][54][55] There are doped as well as undoped conductive polymers and the doping process is used for increasing the conductivity of the polymers. [30][54][55]
1.1. Aluminum and Aluminum alloys Aluminum is produced in industry by either a chemical or an electrochemical process. Chemically, aluminum was produced in 1854 by modifying the Wöhler’s process. [4] Aluminum is more commonly produced electrochemically by electrolysis, in most cases from bauxite.[1] Aluminum has advantages over other metals which can explain its prominent usage in various industries. An example is in electrical applications where aluminum has been a good replacement to previously used copper. This is because aluminum have 60 % of copper’s conductivity, but this is achieved with a lower weight. Aluminum are used in wires and it is the main metal used in power lines across France. Aluminum’s excellent corrosion properties give rise to lower maintenance, longer appearance, and longer lifespan. Aluminum alloys contain other elements except aluminum and these alloys are classified into 8 series differing in alloying elements, which in turn leads to different properties of the aluminum alloy. [1] Aluminum and its alloys are used in manufacturing of vehicles, infrastructures, electrical conductors, aircrafts, tanks, and machines. Aluminum used in various applications are based on its strength, toughness, and corrosion resistance. [1][5] [7] Nowadays recycling of aluminum is an important issue since the energy needed to produce pure aluminum from recycled items is considerably less than to produce it from bauxite.
1.2. Corrosion mechanisms Corrosion is a process where usually a metallic material deteriorates due to chemical or electrochemical reactions. During chemical corrosion, transfer of electrons takes place 6
DEGREE PROJECT IN CHEMICAL SCIENCE AND ENGINEERING, SECOND CYCLE, 30 CREDITS STOCKHOLM, SWEDEN 2020 between the oxidant and the metal in absence of current. For electrochemical corrosion, the metal is reacting electrochemically with chemical species. [2][3] Metallic corrosion requires formation of an electrochemical cell that consists of an anodic area and a cathodic area of the metal. Oxidation of the metal takes place at the anodic area whereas in the cathodic area active species in the environment are reduced. Electrons transfer from the anodic area to the cathodic area where it consumed in the reduction reaction. There are usually millions of cathodic and anodic places per cm2 at the metal surface and the anodic and cathodic places are commonly close to each other. [20] 1.2.1. Motivation and scope Not much research has been done on the corrosion behavior of aluminum coated with a polymer matrix containing conductive polymer. Even less research has been made using a waterborne polymer matrix, which makes this thesis report more interesting as it provides a steppingstone for obtaining more knowledge on the topic. The polymer matrix used in this work is a hydroxyacrylatemelamine copolymer from PTE Coatings, Gamleby, Sweden, and in this report it will for simplicity be referred to as the polymer matrix. Aim: To study anti-corrosion properties of aluminum coated with a waterborne polymer matrix in absence and presence of polyaniline, PANI. The work involved investigating and comparing corrosion properties of three systems: i) Bare aluminum alloy 6060, ii) the same alloy coated with the waterborne polymer matrix in absence of PANI, and iii) the same alloy coated with the waterborne polymer matrix in presence of a low amount of PANI doped with p-toluenesulfonic acid, PTSA. Objective: First, conduct a literature review on corrosion protection methods used for aluminum and its alloys. Second, prepare and produce a polymer dispersion and coat it on one aluminum alloy. Third, use electrochemical impedance spectroscopy, EIS, open circuit potential, OCP, and atomic force microscopy, AFM, in various modes to draw conclusions on protection performance and surface properties.
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DEGREE PROJECT IN CHEMICAL SCIENCE AND ENGINEERING, SECOND CYCLE, 30 CREDITS STOCKHOLM, SWEDEN 2020
2. Literature review As a background for this master thesis work, a review has been made om corrosion protection strategies for aluminum. Although aluminum has a protective oxide layer, there is still risk of corrosion. This can lead to disastrous events in various industries which uses aluminum as a backbone for their products and instruments. The most common protection methods are anodization, cathodic protection, use of corrosion inhibitors and various coating techniques. The most used methods are anodization and chromate conversion coating, especially in aircraft and automotive industries. Due to environmental and health risks of chromate conversion coating, there have been a significant amount of research regarding greener choices for corrosion protection such as organic coatings and sol-gel coatings. The corrosion resistance of aluminum can be improved by sealing or coating the aluminum as a pre- treatment. 2.1. Introduction Corrosion is a natural process which deteriorates metallic materials due to interaction with its environment. Corrosion can be slow or rapid and can negatively affect the surface of the material, properties, and structure. Corrosion occurs due to chemical or electrochemical reactions and the latter will be briefly discussed because the cause of metallic corrosion are often electrochemical reactions. [1] The criteria for an electrochemical corrosion is a corrosion cell which consist of; cathode, anode, electrolyte, and a metallic passage which allows electron transport from the anode to the cathode. The anode is the area of the surface of the metal where the oxidation occurs, and the cathode is the area of the surface of the metal where the reduction occurs. The electrolyte is the conductive solution containing ions, and an electrochemical reaction is defined by electron transfer between the electrolyte and the surface of the metal.[1][3] There are several types of corrosion such as pitting corrosion, uniform corrosion, crevice corrosion, galvanic corrosion etc. [1]. Aluminum is a common metal used in several mechanical and chemical processes. It is used in tubing, heat-exchangers, cars, and airplanes. Failure in some of these instruments and processes can lead to disastrous events. [12] Although aluminum and its alloys are known for having relatively good corrosion resistance, they can corrode by means of the different corrosion types mentioned above. [1][2] How aluminum alloys corrode is determined by the microstructure, especially the intermetallic particles and their size and chemical composition.[16][5] The reason for aluminum to have good corrosion resistance properties is the formation of an inert passive protective oxide film when oxygen is present. Thus, on the surface of aluminum, there is a thin protective oxide layer that strongly reduces corrosion due to dissolution resistance and insulating properties. This reduces the ability of electrons formed at the anode to move to and partake in reactions at cathodic sites. [2] On the other hand, alloyed aluminum has less corrosion resistance due to the lesser amount of aluminum. [5] The ability to corrode increases if the aluminum and its alloys encounters another metal. In such scenarios, crevices can be formed between the two metal surfaces or metal- polymer surfaces. This is called crevice corrosion and it commonly occurs for aluminum alloys. [3-4]
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DEGREE PROJECT IN CHEMICAL SCIENCE AND ENGINEERING, SECOND CYCLE, 30 CREDITS STOCKHOLM, SWEDEN 2020
Corrosion is irreversible, making it important to prevent, or at least significantly retard, it in the first place, which is why there are several types of corrosion protection methods that will be shortly explained in this survey. [1] 2.2. Protective methods for aluminum and its alloys 2.2.1. Anodization The theory behind anodization was first documented in 1857 by Buff [6]. Later, in 1911, the first patent was filed for an anodizing method. Anodization has as purpose to increase the thickness of a metal’s oxide layer. There are many different anodization processes developed since 1857 such as, oxalic anodizing, sulphuric and chromic anodizing. The latter two anodizing processes are specific for aluminum and improves the anticorrosion properties. [1] The aluminum oxide film that develops during anodizing is formed by an electrolytic oxidation method. The surface of the aluminum turns into the anode and water from the electrolyte solution degrades into oxygen that combines with aluminum to convert the surface into aluminum oxide, Al2O3, in the presence of a current passing through the electrolytic bath. [7-8] Pre-treatment methods and the design of the anodization process for aluminum can be found, for instance, in the book “Anodic Oxidation of Aluminum and its Alloys” [8]. Bensalah et al, inspected the oxide film formed from sulphuric anodizing and observed that the film was thick and dense, [11] which both are important characteristics for providing corrosion protection. According to Rashid et al, the aluminum alloy ASA 6061, which was anodized with chromic acid, had a high corrosion resistance. [12] There are two forms of protective films that forms on the aluminum: barrier type and porous type. In the barrier type, the structure is amorphous and there is a correlation between the thickness of the film and the anodizing potential. In the porous type, the structure consists of two layers: an inner barrier layer with an outer layer which is porous. The thickness of the porous layer has a correlation with both anodizing time and current density. [8-9] [14] Nanoparticles merged with the anodized film have been investigated and it was discussed whether it could improve the corrosion protection.[10] The influence of intermetallic particles on anodized alloy AA7075 has also been investigated. [13] The aluminum and its alloys can be further improved with respect to corrosion resistance by a process called sealing. This additional treatment is performed either in cobalt and nickel salt solution or in boiling water. The sealing process reduces the porosity of the aluminum oxide film according to Biestek and Weber. [2] Different types of sealing methods such as hot water sealing, dichromate sealing, physical and chemical sealing and many more methods have been described in detail.[8] In summary one can conclude that anodization, particularly in combination with sealing, is an industrially very relevant method for protection of metals, including aluminum and its alloys. It provides good corrosion protection, and several processes are well-established and applicable to many industrial situations. 2.2.2. Cathodic protection In cathodic protection, the anodic parts of the surface are converted to cathodes which stops the oxidation reactions that lead to aluminum dissolution. This method prevents both localized
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DEGREE PROJECT IN CHEMICAL SCIENCE AND ENGINEERING, SECOND CYCLE, 30 CREDITS STOCKHOLM, SWEDEN 2020 and general corrosion. Cathodic protection includes two methods: the use of a sacrificial anodes or, alternatively, the use of a forced current. For aluminum, sacrificial anodes are commonly used, and, as the name implies, a metal is coupled to another metal which will corrode instead of the metal being protected. In the forced current method, the goal is to decrease the cathode potential and it is achieved by having a current between the anode and cathode. [1][15] 2.2.3. Coating techniques Another method to counteract corrosion of metals such as aluminum is by coating the aluminum surface. This can be done either by metallic or other coatings. 2.2.3.1. Metallic coating techniques Electro or electroless plating and spraying techniques are examples of metallic coating processes. Below I focus on one of the spraying techniques and the plating processes that are commonly used for aluminum and aluminum alloys. Thermal spraying has been used frequently for coating aluminum to increase the anti- corrosion properties.[3] In the thermal spray technique solid powder is melted at high temperatures and the melted powder is deposited onto the surface of the metal. [42-43] Uozato et al. (2003) concluded that the corrosion resistance of aluminum alloys increased when this method was used. Thermal spray including nickel was used and nickel was judged to be the most significant reason for the improved corrosion resistance of the aluminum alloy [41]. Electroplating is a coating technique which consists of an electrolyte solution with metal salts and a cathode which in this case is the substrate going to be plated. The anode is the material which provides the plating. The most significant difference between electroplating and electroless plating is that the electroplating method is an electrolysis process and needs an external current supply. [18] Electroplating significantly counteracts corrosion but it is difficult to apply for aluminum due to the passive oxide layer and the risk of having bimetallic corrosion. To run a successful plating process, chemical and mechanical treatments of the surface need to be made, where one step is to remove the oxide layer. [7][3][17][19] The efficiency of anti-corrosion properties of electroless plating with nickel deposits are dependent on surface state of the metal, pre-treatment methods, coating structure and the condition of the bath. Court et al. monitored the pre-treatment of aluminum with zincate before use of electroless plating with nickel deposits, and demonstrated the importance of pre- treating the aluminum.[40] Electroless plating by using copper and nickel on aluminum is widely used in the electronics industry and on parts in airplanes. [7] To summarize, metallic coating techniques consists of plating and spraying techniques on the aluminum surface to improve corrosion resistance. Thermal spraying and electroless plating are two relevant processes for protecting aluminum from corrosion. 2.2.3.2. Other coating technique A boehmite, g-AlO(OH), coating forms at around 60-70 °C when the aluminum oxide layer reacts with water. This occurs in the anodic reaction and take place at the whole surface, meanwhile the cathodic reaction occurs at the grain boundaries. The boehmite coating have
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DEGREE PROJECT IN CHEMICAL SCIENCE AND ENGINEERING, SECOND CYCLE, 30 CREDITS STOCKHOLM, SWEDEN 2020 good anti corrosion properties. The formation of boehmite can either be done at 120 °C using water vapor which is under pressured, or in boiling water at air pressure. The latter method results in the thickest film and the best corrosion resistance when the solution is alkaline.[1] Industrial applications for boehmite coatings are limited compared to other coating techniques such as conversion coating (see below). The reason is that the latter method is uncomplicated and requires less time to form thick films. Boehmite are used for sealing anodized layers on aluminum alloys to improve corrosion resistance.[1] Fernandes et al, studied whether a boehmite film on aluminum used in spent fuel storage could provide efficient corrosion protection. It was shown that the boehmite film could prevent pitting corrosion on aluminum alloy AA6061. However, a better resistance could be obtained by adding rare earth metals such as cerium to the boehmite film. [21] Another coating technique is the chemical conversion coating, and the most common subcategory of chemical conversion is chemical oxidation. By having a metal surface immersed in a solution with oxidizing agents, where oxidation occurs at a predetermined temperature, chemical oxidation can take place. This conversion treatment is widely used for metals such as aluminum. A thick film of aluminum oxide on aluminum can be generated by chemical oxidation if the solution has sufficient oxidizing power. The solution should include a film forming agent with oxidizing properties and a cosolvent.[3] The risk of having a dense film, meaning absence of pores, should be considered when producing oxide films. Oxide films without pores make it hard for the aluminum substrate to be in direct contact with the solution. This reduces the possibility to form a thick oxide film. The role of the cosolvent is to dissolve, to a certain degree, the oxide film for the pores to be created. This provides a new pathway between the solution and aluminum through the pores. Chemical oxide films have great anti-corrosion properties and compared to the anodic oxide films, the adhesion between the substrate and the film is better. Pores of anodized films are in a second step sealed for obtaining even better protection against corrosion.[3][5] A widely used treatment in the chemical conversion family is the chromate conversion coating. The layer is formed at the metal substrate when in contact with trivalent and/or hexavalent chromium, several oxides, water, and other elements such as fluorides and phosphates. The process of creating the film can be done by either immersion or spraying the metal substrate with the solution.[22] [24] An advantage with this method is that hexavalent chromium has self-healing properties and is a very good corrosion inhibitor. [2, 22-23] Chromate conversion is a common treatment for metals in aircraft industries. This method allows formation of the protective layer on the metal substrate without applying any potential, which is an advantage compared to the anodic oxidation method.[23] A disadvantage is, however, the hazard which comes with use of the hexavalent chromium which is carcinogenic and toxic.[3] To recapitulate the main points of this chapter, coating techniques which improves the corrosion protection for aluminum are boehmite coatings and chemical conversion treatments. The advantages of using conversion treatments with chromate compared to boehmite coatings are less complicated process steps and shorter time to form thick protective films.
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DEGREE PROJECT IN CHEMICAL SCIENCE AND ENGINEERING, SECOND CYCLE, 30 CREDITS STOCKHOLM, SWEDEN 2020
2.2.3.3. Organic coatings Organic coatings typically consist of filler, resin, solvent, pigment, and additives. Application of an organic coating results in a homogenous layer which protects not only from corrosion but also from physical aging and breakdown of the structure from stress. This type of coating is defined by the chemical structure of the resin. The role of the resin is to form the coating matrix, where all the components are integrated. The role of the pigment is either esthetical, i.e. to give the coating a desired color, or functional, i.e. to increase the corrosion resistance of the coating. The increasing corrosion resistance is due to that the pigments are preventing imbuing of aggressive species by extending their diffusion path. [27] The organic coating can act as a passive barrier in order to increase corrosion resistance of the coated substrate. Alternatively, it can function as an active barrier containing inhibitors or electroactive compounds. [26] The most common reasons why organic coatings eventually fail are due to i) poor adhesion between the coating and the substrate, and ii) coating defects.[23] Basak Dogru Mert studied the corrosion protection of aluminum coated electrochemically with doped TiO2- polypyrrole, a conductive polymer, and found that the doped organic coating reduced the corrosion.[25] Polymeric coatings are an important type of organic coatings, commonly used for protecting metals such as aluminum from corrosion. They are widely used together with anodic oxidation and conversion coatings. One main reason for polymer coatings anti-corrosion properties on aluminum is that it provides obstacles for the diffusion process of chloride ions to the surface, which often is the rate determining step for the corrosion reaction.[23] The most common conductive polymers used for corrosion protection are based on polyaniline and polypyrrole. [26] An approach for corrosion protection of metals such as aluminum is to coat it with conductive polymer. [1] The conductivity is derived from the doping with counter-ions. According to Paloumpa et al., doping a polypyrrole coating with ZrFe6, TiFe6 and ZnO resulted in great corrosion protection for aluminum alloys.[30] Improvement of the corrosion resistance for polyaniline coatings could be achieved by adding hydrophobic functional groups to the coating. The reason for this is reduced water penetration in hydrophobic coatings. [1] Kamaraj et al. (2008) investigated how well a polyaniline coating on aluminum alloy 7075 protected the metal from corrosion. They concluded that the corrosion resistance improved further when the aluminum alloy coated with polyaniline was post-treated with cerium. The reason was that the corrosion rate decreased due to the lowering of the rate of the oxygen reduction reaction.[29] In summary, organic coatings are commonly used for protecting metals like aluminum and its alloys from aggressive species. The organic coatings act as a barrier and can provide a good protection. 2.2.3.4. Sol-gel coatings Due to the hazardous risk involved in applying conversion coatings such as chromate conversion coatings, significant research has been made on the sol-gel coating process. Sol-
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DEGREE PROJECT IN CHEMICAL SCIENCE AND ENGINEERING, SECOND CYCLE, 30 CREDITS STOCKHOLM, SWEDEN 2020 gel coatings have been produced not only to promote adhesion between topcoat and metal but also to improve corrosion protection.[22] Sol-gel coatings can be made either by using inorganic or organic materials, but the organic ones are the more common. Preparation of sol- gels can be done using either hydrolytic (i.e. involving a hydrolysis step) or non-hydrolytic processes. The formed sol-gels will obtain different properties depending on the process parameters such as pH, temperature, chemical structure of reactive species, and time. Hence, changing these factors will affect the corrosion protection performance of sol-gel coatings. [22][31-34] 2.2.3.5. Corrosion inhibitors Corrosion inhibitors may be formulated using a single compound or a mixture of compounds with the aim to reduce the corrosion rate. The choice of inhibitors matters due to efficiency and toxicity. Chromate usually is used due to high efficiency but have the drawback of being highly toxic. Additionally, the inhibitors should be chemically stable and have no unnecessary interaction with other substances. The inhibitor can protect the anodic or cathodic sites or both depending on choice. The inhibitor can also be classified as film forming or adsorptive. The inhibitors should reduce the rate of the cathodic or anodic reactions, or both. Their use may lead to changed surface conditions and formation of a protective layer. Examples of inhibitors for aluminum are hexamine, iodate, benzotriazole, sodium silicate, etc. [1] Rare earth metal salts as inhibitors are promising alternatives for chromate coating on aluminum and its alloys, several reviews are going deeply into this topic. [35-39]
3. Aluminum alloy 6xxx series In my study I used aluminum alloy 6060 so I pay special attention to the aluminum alloy 6xxx series in this section. The structure of aluminum and its alloys is face-centered cubic, FCC, and the stability of the structure for pure aluminum is maintained under the melting point at 657 °C. [59] 6xxx alloys are categorized into three subcategories depending on their silicon to magnesium ratio. The first group to which 6060 belongs, has roughly the ratio of Mg to Si as in Mg2Si, the second group have a little excess of silicon which results in better mechanical properties. The third group have more excess of silicon, and thus even better mechanical characteristics.[1][52] Aluminum alloys have different properties depending on the alloying elements. The alloying elements are added in a small quantity but significantly affect the properties of the alloy. Commonly used elements are silicon, magnesium, zinc, and copper. Pure aluminum are usually too soft to be useful in engineering applications where material strength and shape stability are important. Properties like corrosion resistance, mechanical properties, castability and others can generally be influenced by alloying. Aluminum alloy can be made either through casting or wrought, and grouped into casting alloys, heat, or non-heat-treatable wrought alloys. [1][44][59] Aluminum series 6xxx is of the age-hardened type, which is a subcategory of wrought. Best mechanical properties of alloys in this series can be acquired by heat treatment which consist of three steps i) Formation of a solid solution of the elements at high temperature, ii) quenching which is a prompt cooling process and iii) ageing in artificial or natural way.[1] Wrought aluminum alloys consists of these main alloying elements: magnesium, zinc, copper,
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DEGREE PROJECT IN CHEMICAL SCIENCE AND ENGINEERING, SECOND CYCLE, 30 CREDITS STOCKHOLM, SWEDEN 2020 silicon and manganese. The 6xxx alloy series has tensile strength between 125-400 MPa, and this type of material is often used in manufacturing of aircrafts.[59] 3.1. Microstructure Aluminum and its alloys have a good corrosion resistance due to the formation of a protective spontaneously formed oxide layer on the surface. The aluminum oxide film forms quickly because aluminum has high affinity to oxygen. The oxide film can reform in a few milliseconds, giving it self-healing properties. The oxide film often consists of two layers, an amorphous outer layer, and a crystalline inner layer. The reasons why the formed oxide layer on aluminum is protective are due to i) alumina, Al2O3 which is formed when aluminum is in contact with air and this is a stable oxide, ii) the melting point for alumina is above 2000 °C, iii) the film acts as a semiconductor, and iv) the Pilling-Bedworth ratio, RPB, is 1.28. This value pf RPB means that the oxide film is denser than aluminum and this is an important reason why aluminum is protected by the oxide film.
The RPB ratio is defined in Eq. 1, where � and � are the densities for the metal and the oxide, AM and AO are the molecular mass of the metal and the oxide. The molar volumes of the metal and the oxide film are associated to how efficient the oxide film can shield from oxidation at the metal surface. If an oxide layer is formed at the metal - oxide interface, the RPB ratio can describe the volume changes due to the oxide formed at the interface.
(1) ��� = In case RPB <1 it means that the oxide film is not protecting the metal because the oxide film is less dense, and holes can be found in the oxide film. This is due to formation of tensile stress in the oxide layer that are relieved when the oxide thickness increases. The second scenario is when 1
Second-phase particles, like Mg2Si, form in the aluminum alloy matrix [45], which gives rise to improved heat treatability. [58] The intermetallic particles are either cathodic or anodic compared to the aluminum matrix. This indicates that two types of pits can be found on corroded aluminum alloys. For cathodic intermetallic particles, circumferential pits are
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DEGREE PROJECT IN CHEMICAL SCIENCE AND ENGINEERING, SECOND CYCLE, 30 CREDITS STOCKHOLM, SWEDEN 2020 formed, and such pits can be seen as circles around the particles in SEM images. Deep pits are formed for anodic intermetallic particles. [52][76] The intermetallic particles influence important properties such as corrosion resistance of the alloy. Intermetallic particles have a large influence on corrosion properties because of the difference in electrochemical properties compared to the aluminum matrix. Localized corrosion of aluminum alloys increases when the difference in potential between the intermetallic phases and the matrix increases. When an aluminum alloy is in contact with a corrosive environment, a micro galvanic cell is created. The alloy matrix has phases with difference in electrochemical activity, the anodic processes take place on the phases with lower electrochemical potential, whereas the cathodic processes take place on the phases with higher potential.[45][63] 3.2. Corrosion mechanisms of aluminum alloys In real life, the oxide film on aluminum alloys have defects because the underlying microstructure is diverse. These defects occupy places close to the grain boundaries and intermetallic particles. The oxide film can disintegrate in those weak areas which makes it easier for localized corrosion to develop. The most common types of localized corrosion to occur at aluminum alloy surfaces are intergranular, exfoliation, pitting and crevice. [44][46] Generally, localized corrosion consists of these steps in chronology: i) Reactive species adsorb onto the aluminum oxide film, ii) the adsorbed species participate in a chemical reaction with precipitated aluminum hydroxide, Al(OH)3 or aluminum ions, iii) dissolution resulting in a thinner oxide film and penetration of reactive species into the oxide film, and iv) pitting propagation which consists of reactive species directly attacking the aluminum. In the second step, which is also called the competitive process, aggressive species like chloride ions compete with water molecules or hydroxyl ions to adsorb onto the aluminum surface. The surface would become less active if the aggressive species lost the adsorption competition. [64] Pitting corrosion occurs when holes form from localized corrosion attacks. This takes place when the passive oxide layer disintegrates. The first step of pitting corrosion is passivity breakdown (pitting initiation) which includes three mechanisms: i) adsorption and mechanisms which are adsorption-induced in which corrosive species, such as chloride ions, actively try to settle on the positions of the hydroxyl groups on the surface, ii) ions move across the metal oxide interface, iii) the aggressive species break down the film and replace the preexisting ions, resulting in a new film containing the aggressive ions. [44][46-49] After pitting initiation, propagation takes place which means that the pit grows autocatalytically. Cavities are formed on the metal surface from the autocatalytically reaction. [50]
The pitting potential, Epit is a measure of the propensity for pitting to occur, and from Fig. 1, having lesser amount of chloride ions results in increasing Epit. This means more resistance against pitting corrosion. The larger the difference between the corrosion potential, Ecor, and Epit the higher is the pitting corrosion resistance. [44][53] At the pit, there are a cathodic and an anodic site. At the anodic site, oxidation of aluminum and electron insertion into the aluminum matrix take place. Simultaneously, at the cathodic part of the pit, a reduction reaction of water reacting with aqueous oxygen to form hydroxyl
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DEGREE PROJECT IN CHEMICAL SCIENCE AND ENGINEERING, SECOND CYCLE, 30 CREDITS STOCKHOLM, SWEDEN 2020 ions takes place. Another reaction that can take place at the cathodic site is hydrogen evolution. The bulk alloy facilitates the transfer of electrons between the anodic and cathodic sites. The reactions occurring at the anodic and cathodic sites are shown in the reactions (2-5).[44] (2) �� → �� + 3� (3) 2� � + � ( ) + 4� → 4�� (4) 2� + 2� → � (5) �� + 3�� + 3� � → ��(��) + 3��� At the pit, reaction 5 shows that hydrochloric acid, HCl is formed as a result of Al3+ reacting with H2O which makes the solution acidic. [44] Precipitation of Al(OH)3 takes place and reduction of hydrogen cations makes micro bubbles that will bring the Al(OH)3 to the top of the pit. This will result in deposits that can be seen as white bumps. The corrosion products will accumulate at the opening of the pit and prevent chloride ions to move into the pits, and thus reduce the pitting rate and even stop it completely. [1]
Fig. 1. Examples of an anodic polarization curve and how the concentration of Cl- affects Epit.[44]. Crevice corrosion occurs in crevices which are confined parts of the metal consisting of an electrolyte, like in a gap, and is another type of localized corrosion, which occurs due to similar chemical reactions as pitting corrosion.[1] Filiform corrosion takes place when a network of filaments appears at the oxide layer or under a coating. This type of corrosion can lead to a disastrous outcome such as coating deterioration. The filament consists of tail and head parts. Reaction (2) and (5) takes place at the head where anodic reactions occur, resulting in dissolution of Al3+. Filaments develop as 3+ the pH of the surrounding decreases due to H2O reacting with Al . At the tail part, cathodic reaction takes place, reactions (3-4). Filiform corrosion often takes place when coatings are used. It starts at defects of the coating and propagates at the interface. [44] [61] Intergranular corrosion, IGC, is a localized attack which takes place at grain boundaries. Phases including intermetallic particles deposited at the grain boundaries are the cause for IGC when the phases are galvanically coupled. At the grain boundary if aluminum dissolves can lead to cracks being propagated at the grains. [44] IGC is not likely to take place at the 6xxx alloys when the magnesium-silicon ratio is in balance. If the ratio moves toward excess of silicon the risk for IGC to take place is increased. [61]
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How easily aluminum alloys corrode is attributed to the diverse microstructure, especially the concentration and type of alloying elements. Intermetallic particles, such as precipitates and constituents, can give rise to localized attacks on the alloy. The sizes of the intermetallic particles differ from nano to micrometers, where the largest ones are the intermetallic constituents. The risk of localized corrosion attacks decrease with decreasing intermetallic particle size. There is big variation of intermetallic particles in each alloy series of aluminum, which leads to different types of localized corrosion. There is also variation on the electrochemical properties, like corrosion potential, depending on the alloying elements.[44]
4. Conductive polymer PANI One interest in conductive polymers originates from the likelihood of developing smart coatings for counteracting corrosion of metals. Smart coatings means coatings which recognize and respond to the threats from the environment in an accordant approach. [22] [54] For the case of corrosion protection it means that the conducting polymer should facilitate reactions that counteract or inhibit the corrosion reactions. A conductive polymer can be synthesized in a chemical or electrochemical manner. An oxidizing agent is added to the chemical process for the purpose of oxidizing the monomer. In the electrochemical process the same reaction takes place but in the presence of an applied potential in an electrochemical cell with the monomer, and with a dopant inside the electrolyte. The polymer chain grows from the monomers in the conductive electrolyte until it deposits onto the electrode. [55] There are four theories that may explain why conductive polymers exhibit anti-corrosion properties: First, a coating of conductive polymer may form layers of oxide films which protects the substrate from aggressive species. Second, the dopant may behave like a corrosion inhibitor. Metal coated with conductive polymer can form a galvanic cell between two regions if a scratch is formed. Dopant anions are released when the conductive polymer is reduced in the cathodic reaction. In the meantime, the metal is oxidized as anodic reactions takes place. Reoxidation takes place on the conductive polymer due to O2 reduction on the metal and the conductive polymer. A metal-dopant complex is formed which inhibits aggressive species to enter the scratch that were made on the metal coated with conductive polymer. PANI may also release dopant anions by eliminating acid-dopant if dispersible in water. This process is illustrated in Fig. 2.
Fig. 2. Shows formation of a complex consisting of aluminum and dopant. [78]
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DEGREE PROJECT IN CHEMICAL SCIENCE AND ENGINEERING, SECOND CYCLE, 30 CREDITS STOCKHOLM, SWEDEN 2020
Third, a conducting network is produced when a conducting polymer is attached to a metal. The conducting network decreases charge transfer between metal and conductive polymer, which in turn leads to lower corrosion rate. Fourth, the conducting polymer creates a dense film which inhibits oxidation to take place at the metal and limits the presence of oxidizing agents. The barrier effect of the coating is inversely proportional to the porosity of the film. [54] PANI is used frequently as a protection against corrosion because it is easy to manufacture electrochemically or chemically, it is environmentally friendly, and provides the ability to control properties due to the presence of different redox states. [28][54] The redox states for PANI are: Emeraldine, Leucoemeraldine and pernigraniline. The bases of these three forms are stable and exhibit insulating properties. Protonating the emeraldine base, Eb, results in emeraldine salt, Es, which is the formed used for corrosion protection and it is the conductive form of PANI. Different colors are observed for the three bases and their salts. Eb has blue color and Es has green color. The electrical conductivity of the polymer is due to polarons or radical cations that are transported across the backbone due to oxidation or reduction. This process leads to a delocalized charge across the backbone. Doping of the polymer makes it possible to either increase or decrease the conductivity of the polymer. [54-55] PANI can be added to a polymer matrix to make the composite conductive. A conductive PANI network is formed in the polymer matrix leading to conducting pathways in the polymer matrix. [65][92]
5. Polymer matrix Organic polymer coatings act as a barrier from exterior threats like aggressive species, and are discussed more in the literature survey. [1] They counteract the charge transfers taking place between local cathodic and anodic sites, primarily by reducing the rate of oxygen, water, and ion transport to the metal surface. [77] A typical polymer matrix is a rather complex commercial product, where the different elements like fillers, solvent, and other additives are not publicly known but a trade secret of the manufacturer. In my case I used a polymer matrix from PTE Coatings, Gamleby, Sweden, with a solid content of 38±3 wt.%. The polymer matrix consists mainly of a resin and a cross- linker. The resin is hydroxyacrylate, HEA, and the cross-linker is hexakis(methoxymethyl)melamine, HMMM. The cross-linker gives rise to a transesterification reaction between HEA and HMMM. A coating which is cross-linked is formed when dimethyl ether groups of HMMM reacts with hydroxyl groups of HEA, and this results in release of methanol. [57]
6. Materials 6.1. Aluminum alloy 6060 Aluminum 6060 was provided by the supervisor. It is a recycled aluminum alloy 6060. It has the following chemical composition: 0.2 % Fe, 0.37 % Mg, and 0.49 % Si with the rest being aluminum. [62]
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6.1.1. Preparation of Aluminum alloy 6060 surfaces The aluminum alloy had a white paint coating on the surface which made it crucial for a surface preparation. The aluminum alloy was first cut to 15 mm x 15 mm pieces, and the surface was polished with the grinding/polishing machine Struers LaboPol-5, to remove solid inorganic particles. The alloy was grinded first with 800-grit size and then further with 1200- grit size in the presence of water. The alloy surface was then cleaned from rest particles from the grinding/polishing machine with help of a VWR Ultrasonic cleaner USC – TH system. The samples were put in a beaker with Milli-Q® water and the beaker was put in the ultrasonic cleaner which was filled with water. The process time for the Ultrasonic cleaner was approximately 8 minutes ant the temperature was about 27° C. 6.2. Polymer matrix Two batches of the polymer matrix were used. One was freshly prepared by the manufacturer and the other was approximately 2 years old. This allowed me to draw conclusions related to the shelf life of the polymer matrix. 6.2.1. Preparation of polymer coating The preparation of the polymer matrix hydroxyacrylate-melamine coating began with magnetic stirring of the matrix dispersion for 5 minutes. The polymer matrix was then applied on the aluminum surface in a fume hood by using a cube applicator with 60 μm gap size. Then the polymer matrix was dried for 1 hour in a fume hood. The reason why these steps took place in the fume hood was to avoid exposure to volatile organic compounds. In the next step the polymer matrix on the aluminum surface was cured for 10 min at 180 °C in a Thermolyne 47900 furnace. The substrate coated with the polymer matrix was thereafter ready to be used by the experimental instruments. The thickness of the cured coating is around 17 µm as determined in a previous study. [57] 6.2.2. Preparation of polymer coating containing conductive polymer The waterborne polymer matrix was mixed with PANI doped with p-toluenesulfonic acid. This is not straight forward since PANI is insoluble in water. Thus, the PANI sample was dispersed in water using a commercial anionic surfactant recommended by Dr. Ahniyaz, RISE. Next, this mixture was added to the polymer matrix under stirring with the help of an ultrasonic probe, which is a sonicator that makes the mixture homogeneous by ultrasonic waves. The coating procedure was done in the same way as for the polymer matrix without PANI. PANI had a dark green color when mixed with water and surfactant. The pH of the PANI solution was 2.7 and it was added to give 1 wt% of PANI in the cured coating. The samples with this weight percent PANI were analyzed by electrochemical measurements and various AFM modes. Coatings with 3 % PANI were also prepared and used for additional AFM measurements.
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DEGREE PROJECT IN CHEMICAL SCIENCE AND ENGINEERING, SECOND CYCLE, 30 CREDITS STOCKHOLM, SWEDEN 2020
Fig. 3. Polymer matrix consisting of PANI, A) two concentrations of PANI and B) color of the polymer matrix with PANI.
7. Experimental Methods 7.1. Open Circuit Potential Open circuit potential, OCP, also called the corrosion potential, was measured to determine the nobility and stability of the uncoated and coated aluminum substrate. The OCP is the potential at which the electrochemical interactions are in equilibrium, i.e. the total anode current is equal to the total cathode current. The OCP equals the electric potential difference between the reference electrode (Ag/AgCl) and the working electrode (the aluminum sample) in absence of a current. [1][55][60] A plot of OCP as a function of time provides information on the stability of the metal. From a corrosion perspective it is often the case that a higher OCP signifies a lower propensity for corrosion. [55] OCP was measured by connecting the reference electrode to the working electrode through an electrochemical cell. The working electrode was immersed in 1 M NaCl solution. The measurement was done with a potentiostat and the results were obtained from the software Versastudio and analyzed using the program Corrview. Alloying elements can influence OCP as seen in Fig. 4. Magnesium and silicon have little effect on the potential and it is thus expected to have a rather small effect on the protective power of the oxide film.[1]
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DEGREE PROJECT IN CHEMICAL SCIENCE AND ENGINEERING, SECOND CYCLE, 30 CREDITS STOCKHOLM, SWEDEN 2020
Fig. 4. Shows how alloying elements changes the open circuit potential. [1] Aluminum alloy 6060 has been reported to have an OCP of -710 mV relative to the standard saturated calomel electrode, SCE. [1] I find similar values in my work. 7.2. Electrochemical Impedance Spectroscopy Electrochemical Impedance spectroscopy, EIS, is a technique used for obtaining frequency- depended electrical information. It provides useful information on corrosion properties without destroying the sample in the process. [55] EIS was measured with an Autolab potentiostat which was connected to an electrochemical cell containing 3 electrodes. The same electrodes as were used to measure OCP were used for EIS measurements. The studied samples all had an exposed area of 1 cm2. The results were collected using the software Versastudio and the data was interpreted using the software Zview and Corrview. With EIS one measures the relationship between applied potential, V, and current, I. At zero frequency this reduces to the well-known Ohm’s law (Eq. 6), where the resistance, R, is the ratio between voltage and current. For an applied voltage which changes with time, with the radial frequency w, the impedance, Z, take the role of the resistance as shown in (Eq. 7). The impedance depends on the frequency and is influenced by the systems resistive and capacitive (and inductive) properties. In case the response is purely resistive the phase angle between voltage and current is zero, whereas for a purely capacitive response the phase angle difference between these two properties is -90°. [66-67]
(6) � = (7) �(�) = ( ) ( ) In my experiment I used a sinusoidal excitation with a small amplitude (10 mV) in order to be in the linear response regime. Thus, the voltage varies with both time and frequency as illustrated in (Eq. 8). Where Vt is the potential of the excitation signal at time t, Vm is the excitation signal magnitude and ω is the radial frequency, which is related to the frequency, ƒ as shown in (Eq. 9).
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DEGREE PROJECT IN CHEMICAL SCIENCE AND ENGINEERING, SECOND CYCLE, 30 CREDITS STOCKHOLM, SWEDEN 2020
(8) � = � sin �� (9) � = 2�ƒ
The response current, It, is also sinusoidal as shown in (Eq. 10). Here, the phase angle between voltage and current is denoted by ∅. Impedance can be described in system by (Eq. 11).
(10) � = � sin(�� + ∅) (11) �(�) = = = � ( ∅) ( ∅) (12) �(�) = = � exp(�∅) = � (cos(∅) + jsin(∅))
The impedance is described by its magnitude (or modulus), � and by the phase angle, ∅. The impedance can also be written as a complex function through Euler’s relationship as seen in (Eq. 12). The last equation consists of an imaginary and a real part which when plotted against each other results in the so called Nyquist plot which is illustrated in Fig. 5. The semicircle in Fig. 5 represents one time constant, t, as would be seen for a resistor coupled in parallel to a capacitor. [66][68][71] For this system t = RC. It is a characteristic relaxation time due to a perturbation of a steady state. [73]
Fig. 5. A) Resistor coupled in parallel with capacitor, which is a simple equivalent circuit. B) A Nyquist plot representing this electrical circuit. Note that the frequency decreases in clockwise direction in this plot.[69] Lower frequencies are shown at the right side in the Nyquist plot and high frequency area are correspondingly represented at the left side. [66][68] The downside with this type of plot is that the frequency is not shown directly in the plot but can only be acquired from the data. For that reason, the Bode plot is of importance as it directly contains information on the frequency. In the Bode plot the impedance characteristics are visualized by plotting the phase angle, ∅, and the impedance modulus, � , as a function of frequency. Normally one uses a log(Zm) – log(w) plot since both quantities vary orders of magnitude, see Fig. 6. Several quantities in the Bode plot are of interest from a corrosion protection perspective. For a barrier coating, the impedance modulus at low frequency equals the coating resistance (really the total polarization resistance, but this is dominated by the coating resistance) and the
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DEGREE PROJECT IN CHEMICAL SCIENCE AND ENGINEERING, SECOND CYCLE, 30 CREDITS STOCKHOLM, SWEDEN 2020 modulus at high frequency equals the solution resistance. The slope of the curve at intermediate frequencies can be used for estimating the coating capacitance. If several capacitors are needed to describe the systems, several slopes are seen, each corresponding to a given capacitor with a given time constant.
Fig. 6. A Bode plot, with the logarithm of frequency at the x-axis, and the logarithm of the impedance modulus on the left y-axis and the phase angle on the right y-axis. [72] In order to interpret impedance data from coated or uncoated metals one usually applies equivalent electronic circuits, which can contain one or more resistors, capacitors or inductors. In my case I only used resistors and capacitors when interpreting my data. The more electrical elements one adds, the better fit one obtains. However, this is not helpful since one would like to assign the properties of these circuit elements to properties of the system under study. Thus, a good rule, which I applied, is to use as few circuit elements as possible to give a good description of the experimental data. The impedance of a resistor is simply the resistance as shown in Eq. 13.
(13) � = � + �� = � + � ∗ 0 = � (14) � = � (15) � = j � L (16) � = In my work I used one resistor to model the resistive properties of the coating and another resistor was used to model the solution resistance. The relation between potential and current for an inductor is shown in Eq. 13, where L stands for the inductance. For an inductor, the impedance increases with frequency (Eq. 15). In my analysis I did not need to use any inductor. A capacitor is created when a dielectric separates two conducting plates. When the frequency is increased, the impedance of the capacitors decreases as seen in Eq. 16, where the
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DEGREE PROJECT IN CHEMICAL SCIENCE AND ENGINEERING, SECOND CYCLE, 30 CREDITS STOCKHOLM, SWEDEN 2020 capacitive response gives rise to a phase shift of -90°. I needed a capacitor to model the capacitive properties of the dielectric coating separated by the conductive metal and conductive solution. To describe the impedance of the coating, I thus utilized a capacitor and a resistor in parallel. In series with this element, I used a resistor to describe the solution resistance. This is the Randles cell shown in Fig. 7. [66-68]
Fig. 7. A Randles cell consisting of two resistors and one capacitor. In my case, when I studied the coated aluminum alloy, R1 is the solution resistance, R2 the coating resistance and C the coating capacitance. Under other conditions, R2 and C can have other physical meanings. R.E and W.E stands for reference electrode and working electrode, respectively. [67] The capacitance can be calculated from the Bode plot by acquiring the slope of the curve in the linear part of the log(Zm) – log(w) plot. [69] The coating capacitance is given by Eq. 17:
(17) � = where εr is the dielectric constant of the coating, ε0 is the vacuum permittivity, D is the thickness of the coating, and A stands for the exposed samples area. The dielectric constant for organic coatings is typically between 4-8, which is much smaller than that of water (around 80). Thus, if water absorbs in the coating material the capacitance will increase and this can be used to estimate the water content of the coating. [57] [66- 68] For comparison, the dielectric constant for alumina, Al2O3, is about 10. [13] The electrolyte resistance, also called the solution resistance, is influenced by the temperature, ion concentration, and ion type. From the EIS data, the solution resistance can be calculated by fitting the data with a suitable model. [67] The solution resistance can be determined from the Nyquist plot from the position where the semicircle intercepts the high frequency part of the x-axis (left part). [69] There are also other contributions to the impedance that most often can be ignored for a polymer-coated metal, but may be of importance for an uncoated metal, such as the aluminum alloy in my experiments. One of these is the double layer capacitance that arises from accumulation of counterions next to the charged metal surface, and the separation of these charges from the charges on the metal surface. The double layer capacitance is influenced by parameters like ion concentration, temperature, presence of an oxide film, surface state, adsorption of impurities, electrode potential and many more. [55][68] There is also a resistance related to the transport of electrons between the metal and the solution and this resistance is called the charge transfer resistance. Typically, this resistance
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DEGREE PROJECT IN CHEMICAL SCIENCE AND ENGINEERING, SECOND CYCLE, 30 CREDITS STOCKHOLM, SWEDEN 2020 is not treated separately, but included in the resistance of the polymer coating or in the oxide layer. The reason for this is that one strives to use as few parameters as possible when describing the EIS spectra. The total resistance of the system, as determined at low frequency should be called the polarization resistance as its value is given by the resistance of all structural elements coupled in series. For a barrier coated metal, the resistance of the coating is dominating so in this case the polarization resistance is often just called the coating resistance. A problem arises since the layers formed at the metal – solution interface are not ideal, but inhomogeneous in terms of density, chemical composition and thickness. Thus, their capacitive properties are not accurately described by that of an ideal capacitor. In order to take this non-ideality into account one often uses a constant phase element, CPE, instead of an ideal capacitor. The expression for the impedance of a constant phase element is shown in Eq. 18, where Y0 is the non-ideal coating capacitance, and n is a constant. When the n equals 1 the CPE becomes an ideal capacitor and � equals C. [55] [68] [90] For a non-ideal capacitor the relation between � and C is given by Eq. 19. [13] (18) � ( ) ∗ (19) � =
7.3. Potentiodynamic Polarization The relationship between the current and potential of the electrochemical cell in the presence of two kinetically controlled reactions is given by Eq. (17), where � is the corrosion current, � is the potential at OCP, also called the corrosion potential, the β coefficients are the Tafel constants for the anodic and cathodic reaction, respectively. Derivation with respect to the potential, simplification, and rearrangement of (Eq. 20) results in (Eq. 21) which includes Rp, the polarization resistance. High polarization resistance implies a high corrosion resistance. [70]
. ( ) . ( ) ( (20) � = � � − �
(21) � = ∗ . ( )
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DEGREE PROJECT IN CHEMICAL SCIENCE AND ENGINEERING, SECOND CYCLE, 30 CREDITS STOCKHOLM, SWEDEN 2020
Fig. 8. Typical data obtained from a potentiodynamic polarization experiment, with anodic and cathodic branches (curves). The slopes of the linear part of these branches are called Tafel slopes. [75] In a potentiodynamic polarization experiment one measures the current at different applied potentials, above and below the � . From these measurements one can determine the � , � and the Tafel constants as illustrated in Fig. 8. � is the potential at which the extrapolation of the linear part of the anodic and cathodic branches intersect, and the � is the current at this point. A disadvantage with this method is that it affects the sample since you drive the electrochemical reactions with the applied potentials. [74] I used this method as a complement to the EIS measurements.
7.4. Atomic Force Microscopy, AFM AFM is best known as an imaging technique used to study surfaces of materials. The imaging of the sample often takes place in air. Depending on the imaging mode used one does not only obtain topography images but also other images such as phase images (tapping mode) and nanomechanical images (PeakForce QNM, QI-mode). AFM generally consists of a cantilever with a tip, a piezo electric stage to move the sample in 3D, a detector and feed-back system and a computer, as seen in Fig. 9. The tip touches the surface of the material which bends the cantilever horizontally. As a result, the laser beams that is reflected at the backside of the cantilever shift its position at the photodiode, and this registers how much the cantilever is bent. A feedback loop is utilized to maintain the force between cantilever and sample constant, i.e., to maintain the cantilever bending steady. In contact mode, the tip is in constant contact with the surface. In contrast, in tapping mode the cantilever vibrates close to the resonance frequency of the cantilever. The surface of the material may be deformed in contact mode by the tip which is a disadvantage. [79] Electrochemical and surface properties for the samples were studies with the help of conductive AFM, Peak Force tuna AFM and Intermodulation AFM.
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DEGREE PROJECT IN CHEMICAL SCIENCE AND ENGINEERING, SECOND CYCLE, 30 CREDITS STOCKHOLM, SWEDEN 2020
Fig. 9. General setup of an AFM. The chip is connected to a piezo electric crystal. The cantilever bending is monitored by a split photodiode detector and the data is collected by a computer. [79] 5.4.1 Conductive AFM, C-AFM Conductive AFM, C-AFM, is a mode of AFM with a similar setup as seen in Fig. 9. In this case, the tip is conducting and set to be in contact mode. In this mode a bias voltage is applied between the tip and the material. If a conductive network can be established current will flow through the material, and this can be measured at the same time as the topography. [80] The conductive tip has a metallic coating usually less than 20 nm thick and the interior of the conductive tip is typically made of silicon. Electrochemical information is obtained by using a preamplifier coupled to the conductive tip. A disadvantage with this mode is that there may be current irregularities due to breakdown of the thin conductive layer on the tip, or due to the existence of water molecules at the sample surface which may interfere when the tip is in contact with the surface. [81] 5.4.2 Peak Force Tuna, PF-TUNA AFM Peak Force Tuna is an AFM mode which provides images of the sample topography as well as mechanical and electrochemical properties. This mode is a development of the Peak Force tapping mode which produces topography and material property images. The lateral forces are reduced due to tapping, and this mode does have lesser risk of destroying the sample and impairing the tip. This is an advantage over the C-AFM. This mode makes it possible to maintain the tip’s peak force, which is the maximum force used to tap the surface, resulting in reduced damage of sample and tip. It is somewhat similar to tapping mode, where the amplitude of the probe oscillation is maintained by the feedback loop. [82] 5.4.3 Intermodulation AFM, Im-AFM Im-AFM is a mode which uses two drive frequencies near the resonance of the cantilever for obtaining intermodulation products. These products emerge from the non-linear response of
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DEGREE PROJECT IN CHEMICAL SCIENCE AND ENGINEERING, SECOND CYCLE, 30 CREDITS STOCKHOLM, SWEDEN 2020 the tip to the presence of the sample surface. [83] As the cantilever vibrates and the tip approaches the surface, the non-linear forces give rise to frequency mixing. [84] By using a multi frequency lock-in amplifier with the AFM, the phases and the magnitude of the intermodulation products can be determined. [85] One advantage with this mode is that elastic and viscous contributions to the force can be separated, but the underlying physics is quite complex. Using a special set-up, Im-AFM can also determine the potential at the sample with high resolution. [86]
8. Light optical microcopy Light optical microscopy was used to obtain magnified images of the sample surface. This was done by applying lenses of different magnification power and visible light. [93]
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9. Results and Discussion 9.1. Bare Aluminum The Randles cell was used for fitting of the EIS data. A CPE was used instead of a capacitor because of the non-ideal nature of the system. The parameters � , Rp, Rs and � were obtained with Zview by fitting the curves of the Nyquist plot. The large value of n shows that the capacitive response is dominating for the CPE. Table. 1. Fitting data of EIS spectra of bare aluminum alloy 6060, mean value and standard deviation of three samples.
-1 α Time Rs [Ω] Rp [Ω] Y0[Ω s ] n C [F] elapsed 1 Hour 8.8±2.3 (9.7±2.0) x103 (1.9±2.3) x10-5 0.8±0.007 1.5 x10-5
1 Day 11.2±0.5 (5.4±2.3) x103 (2.4±0.3) x10-5 0.8±0.001 1.7 x10-5
3 Day 10.2±1.4 (1.9±0.5) x103 (3,03±0.190 x10-5 0,9±0.01 2.2 x10-5 6 Day 10.0±1.9 (1:9±0.5) x103 (3,87±0.41) x10-5 0.8±0,007 2.7x10-5
8 Day 5.5±2.7 (2.2±0.9) x103 (4,24±0.26) x10-5 0.9±0-01 3.1x10-5 10 Day 5,8±1.5 (2.1±1.2) x103 (4.83±0.15) x10-5 0.9±0.02 3,7 x10-5
13 Day 9.4±0.4 (1.8±0.9) x103 (5.97± 0.45) x10-5 0.9±0.02 4.7 x10-5
15 Day 9.6±0.9 (1.9±0.9) x103 (6,61±0.62) x10-5 0,9±0,02 5.3 x10-5
The polarization resistance decreased significantly during the first three days and then became relatively stable, as seen in Table 1. A possible explanation may be that the thin oxide film partly broke down due to attack by chloride ions. One possible mechanism for the plateau is that Al(OH)3 at the surface reduces the ability of chloride ions to reach the surface and a new steady state is reached. At the same time the capacitance increased with exposure time. If we regard that it primarily is due to the presence of an oxide film with er = 10, as for Al2O3, the data suggests that the oxide film thickness initially is about 0.6 nm, decreasing to about 0.16 nm at the end of the experiment. This is significantly lower than reported for aluminum alloy 6060 in water [60]. One reason for this may be that in the 1 M NaCl solution used in my study, chloride ions disrupt the oxide layer and decrease the oxide layer thickness. Another complementary reason may be that water penetrates the oxide layer and thus increases the dielectric constant of the layer, which would result in a larger calculated layer thickness.
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104 Al_1_3DAY.dat.txt 103 Al_2_3DAY.dat.txt Al_3_3DAY.dat.txt 102 |Z|
101
100 10-1 100 101 102 103 104 105 Frequency (Hz)
-75
-50
-25 theta 0
25 10-1 100 101 102 103 104 105 Frequency (Hz) Fig. 10. Bode plots for three bare aluminum samples. The upper plot shows the impedance modulus as a function of frequency and the lower plot the phase angle as a function of frequency. The measurements were done in 1 M NaCl. Fig. 10 shows that there is only one time constant for bare aluminum as there is only one peak in the phase vs. frequency plot, and only one slope in the plot of impedance modulus vs. frequency. This means that the Randles cell is appropriate for interpreting the data for bare aluminum. A sign of reproducibility is also shown, as the curves are in the same range.
-10000 Al_2_1DAY.dat.txt Al_2_1HOUR.dat.txt Al_2_3DAY.dat.txt
-5000 Z''
0
5000 0 5000 10000 15000 Z' Fig. 11. Nyquist plot for one bare aluminum sample measured after 1 hour, 1 day and 3 days of exposure to 1 M NaCl. The decreasing corrosion resistance of bare aluminum over time can be seen in the Nyqvist plot in Fig. 11 as the semicircle decreases with increasing exposure time.
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Mean and standard deviation of impedance modulus over time for Bare Al 14000 12000 10000 ]
Ω 8000 6000 |Z| [ 4000 2000 0 1 hour 1 day 3 day 6 day 8 day 10 day 13 day 15 day Time
Fig. 12. Impedance modulus as a function of exposure time for bare aluminum in 1 M NaCl. The mean value and the standard deviation of the impedance modulus at 0.01 Hz for the three bare aluminum samples were extracted from Bode plots for each day of the measurements, and the data is plotted in Fig. 12. It is clearly seen that the impedance modulus initially decreases with time and then reaches a plateau. The decreasing impedance modulus over time may be due to that the oxide layer, which acts as a protection from corrosive species, partly broke down. The standard deviation is high in the first hour and first day of measurements, which could be attributed to initial differences of the samples arising from slightly different preparations, despite attempts to make them the same.
Mean and standard deviation of OCP over time for Bare Al -0,69 -0,695 -0,7 -0,705 -0,71 [V] -0,715 OCP
P -0,72 -0,725 -0,73 -0,735 -0,74 1 hour 1 day 3 day 6 day 8 day 10 day 13 day 15 day Time
Fig. 13. Open circuit potential as a function of time for bare aluminum in 1 M NaCl. From the OCP in Fig. 13, it is clearly seen that OCP is stable, within ± 1 mV, with time in 1 M NaCl. Thus, the OCP data does not allow us to draw any conclusions related to corrosion of aluminum, but such information was obtained from the EIS data.
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9.2. Polymer Matrix In this section I first report data for a new batch of the polymer matrix. I then compare these data with an old batch of the same matrix in order to obtain information on the shelf life of the matrix. 9.2.1. Polymer Matrix – new batch Table 2 shows that the polarization resistance initially is in the order of 109 Ω, but with time it decreases. This shows that the coating initially has good barrier properties and blocks the corrosive species from reaching the aluminum-coating interface. However, the barrier is not sustainable for prolonged times in 1 M NaCl. It is worth noting that the same coating had good barrier properties for carbon steel in 0.1 M NaCl for up to more than a month. [57] Clearly, the higher NaCl concentration used in my study results in more rapid loss of barrier properties. Just as for uncoated aluminum the solution resistance is small. It should be noted that the exact value is difficult to determine from the EIS data for aluminum coated with the matrix polymer since it hardly affects the EIS data even at high frequencies (no plateau is observed at high frequencies as seen in Fig.14) Table. 2. Fitting data of EIS spectra of polymer matrix coated aluminum, mean value and standard deviation of three samples.
2 2 -1 2 α Time Rs [Ω cm ] Rp [Ω] cm ] Y0[Ω cm s ] n C [F] elapsed 1 Hour (1.5±1.1) x10-6 (3.6±1.7) (4.0±2.6) x10-10 0.9±0.04 4.0 x10-10 x109 1 Day (1.4±1.1) x10-3 (4.2±2.4) (5.5±0.1) x10-10 0.9±0.04 5.8 x10-10 x109 3 Day 67±49 (3.8±0.9) (9.2±2.9) x10-10 0.9±0.06 8.5 x10-10 x108 6 Day 252±190 (8.2±0.8) (7.0±4.3) x10-10 0.9±0.04 6.8 x10-10 x108 8 Day 229±20 (4.4 ±0.6) (1.1±0.6) x10-9 0,9±0.06 1.0 x10-9 x108 10 day 203±9.5 (2.9 ±0.9) (1.3±0.9) x10-9 0,9±0.04 1.2 x10-9 x108 13 day 314±226 (2.5 ±0.5) (1.2±0.6) x10-9 0.9±0.02 1.0 x10-9 x107 15 day 191±113 (5.7±2.2) (1.1±0.9) x10-9 0.9±0.06 1.1 x10-9 x107
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109 108 AL&POL_1_3DAYB.dat.txt AL&POL_2_3DAYB.dat.txt 107 AL&POL_3_3DAYB.dat.txt 106 |Z| 105 104 103 10-1 100 101 102 103 104 105 Frequency (Hz)
Fig. 14. Impedance-150 modulus as a function of frequency for the three polymer matrix samples. The measurements were done in 1 M NaCl. -100 In Fig. 14, the three samples show fair reproducibility and one time constant is also seen.
theta -50 Mean and standard deviation 0 Mean and standard deviation of impedance10-1 modulus100 over101 102 of OCP103 over10 time4 for10 Polymer5 time for polymer Matrix Frequency (Hz) Matrix 1E+10 0
1E+09 -0,2
Ω] -0,4 1E+08 [V] OCP
|Z| [ -0,6 P 1E+07 -0,8
1E+06 -1 1 1 3 6 8 10 13 15 1 1 day 3 day 6 day 8 day 10 13 15 hour day day day day day day day hour day day day Time Time
Fig. 15. A) Mean value and standard deviation of the impedance modulus as a function of time for the polymer matrix. B) OCP as a function of time. The impedance modulus at low frequencies of aluminum coated with the polymer matrix has a value of the order of 109 Ω and decreases with time as seen in Fig. 15. This, just like the polarization resistance, indicates that the polymer matrix initially provides good barrier properties. The OCP value approaches that of bare aluminum which is surprising considering the high impedance modulus. More studies are needed to elucidate the reason for this.
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(Y0) as a function of exposure time for polymer matrix 1,6E-09 1,4E-09 1,2E-09
] 1E-09 a s 1 - 8E-10 Ω [ 0 Y 6E-10 4E-10 2E-10 0 1 hour 1 day 3 day 6 day 8 day 10 day 13 day 15 day Time
Fig. 16. A) Y0 as a function of time. The non-ideal capacitance increased with time as shown in Fig. 16, and at the same time the polarization resistance decreased. The increase in Y0 with time is a clear sign of water absorption. Thus, I conclude that the reduced barrier properties with time is due to water absorption and chloride ions that follow the water into the coating. Thus, the barrier properties decrease with time and when water and chloride ions reach the aluminum surface corrosion will start.
9.2.2. Shelf-life issues An interesting observation was that the use of an old matrix dispersion (approximately 2 years) resulted in a matrix coating that had significantly less good barrier properties than coatings prepared using a newly made matrix dispersion. This is clearly seen in Figure 17, where the impedance modulus decreases much more rapidly with time for the old batch. Clearly, the coating matrix formulation has undergone changes during the 2 year storage time at room temperature.
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Shelf Life of Polymer matrix 1E+11 1E+10 1E+09 1E+08 1E+07
Ω] 1E+06 1E+05 New Batch |Z| [ 1E+04 Old Batch 1E+03 1E+02 1E+01 1E+00 1 hour 1 day 3 day 6 day 8 day 10 day 13 day 15 day Time
Fig. 17. Mean value and standard deviation of the impedance modulus as a function of time coatings prepared using the two batches of the matrix polymer dispersion.
0,00001 Shelf-life of both polymer matrix
0,000001
0,0000001 sa]
1 New batch - Ω 1E-08 old batch Y0[
1E-09
1E-10 1 hour 1 day 3 day 6 day 8 day 10 day 13 day 15 day Time
Fig. 18. Y0 as a function of time for coatings prepared using the two batches of the matrix polymer dispersion.
The Y0 parameter for the old batch changed more rapidly with time compared to that of the new batch, which indicates that the old matrix absorbed more water during the exposure time.
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9.3. Polymer Matrix (new batch) with PANI Curve fitting was not ideal for polymer matrix with PANI because the equivalent circuits used in the model for fitting this system are not justifiable, but Bode plots are used to compare with the bare aluminum and polymer matrix coating without PANI. Only one sample of the polymer matrix with PANI was used to interpret the EIS data, due to difficulty with dispersion of PANI particles in the matrix. In fact, in future studies more attention has to be given to achieving a good dispersion of the PANI particles prior to mixing them with the matrix dispersion.
impedance modulus over time for polymer Matrix+ PANI 100E+08
010E+08
001E+08 Ω]
|Z| [ 1 000E+04
100E+04
010E+04 1 hour 1 day 3 day 6 day 8 day 10 day 13 day 15 day Time
Fig. 19. Impedance modulus as a function of time for one of the samples of the polymer matrix with 1 wt% PANI. The impedance modulus at 0.01 Hz of aluminum coated with the polymer matrix with 1 wt% PANI is about 106 Ω initially. The low impedance modulus suggests that there is a conductive network during the first hour because impedance, which is a measure of the electrical resistance, is inversely proportional to conductivity. Hence, low impedance modulus means presence of conductive properties. The impedance modulus increases with time and reaches a semi-plateau after 3 days of immersion. This suggest that the coating have an active protection that turns into a passive protection at day 3 after which the performance is similar with and without PANI in the matrix coating. Just for the coating without PANI, a low value of the OCP was a surprising finding and it remains elusive why this is the case despite the high impedance modulus.
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OCP over time for polymer Matrix+ PANI 0
-0,2
-0,4 [V]
OCP -0,6 P
-0,8
-1 1 hour 1 day 3 day 6 day 8 day 10 day 13 day 15 day Time
Fig.20. OCP as a function of time. The polymer matrix in absence of PANI exhibited good barrier properties initially which can be seen in Fig. 21. The polymer matrix in presence of PANI showed proof of conducting network initially due to its lower modulus impedance. At day three, both coated systems approach similar values, which indicates that both systems cannot sustain aggressive species for prolonged times. Impedance modulus over time for the three systems
100E+08 010E+08 001E+08 1 000E+04 100E+04 Ω] 010E+04
|Z| [ 001E+04 1 000E+00 100E+00 Polymer matrix 010E+00 Polymer matrix+PANI 001E+00 Bare Al 1 hour 1 day 3 day 6 day 8 day 10 day 13 day 15 day Time
Fig.21. Impedance modulus as a function of time for the new batch of polymer matrix with 1 wt% PANI.
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Table. 3. Obtained values for the parameters in potentiodynamic polarization.
ICORR [μA] E0 [mV] Βa [mV] Βc [ mV] Bare Al 5.4±0.7 -725±10 34±8.9 120±29
Polymer matrix (2.3±1.7) x10-4 -711±11 81±38 202±37 Polymer 9.1 x10-6 -694 137 311 matrix+Pani
The data obtained from the potentiodynamic polarization measurements are reported in Table 3. The data suggests that bare aluminum had a higher corrosion current compared to the two other systems, which indicates that it had a higher corrosion rate. This agrees with the EIS data, where the impedance modulus was lower at low frequency compared to the other systems. Both coated systems have very low corrosion current, but more polarization runs need to be done in order to draw firm conclusions between the surprising large difference between the coating with and without PANI. Figures showing the raw data for the three systems can be found in the appendix, and as can be seen from these there may be a non- negligible error when performing the extrapolation to extract the corrosion current. 9.4. AFM measurements AFM measurements were performed to obtain topographical, current and potential maps on aluminum coated with the polymer matrix in presence of PANI, and some results are shown in Figure 22.
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Fig. 22. A, C) Topography of the polymer matrix containing PANI for two different samples B) Im-AFM potential image for the same sample and the same area as in panel A. D, F) Im- AFM potential image (D) and capacitance gradient image (F) over the same area as shown in panel C. The height difference is relatively small in the topographical images, indicating that the samples are very smooth and that no PANI particles extend from the sample surface. Potential variations were, however, observed with Im-AFM (Figure 22, panels B and D). This suggests that some PANI particles are located sufficiently close to the surface to affect the measured voltage. The potential and capacitance gradient variations seen suggest that there are difference in electrical properties, and this suggests that more studies combined with C-AFM could provide valuable information. Im-AFM should also be utilized on the polymer matrix in absence of PANI to allow firm conclusions to be drawn.
Fig. 23. A, C) Topography of the polymer matrix with PANI. In these areas large height variations are observed. B, D) The corresponding current maps measured at a bias voltage of 9V. The topography images obtained from PF-TUNA AFM measurements demonstrated that on some areas significant height differences were observed, suggesting PANI particles extending above the matrix surface. On these areas a small current was also observed while no such areas could be found in absence of PANI. These data suggests that on some spots there is network of PANI particles that extend from the coating surface down to the aluminum alloy surface. The current is very small which suggests blocking by a thin layer of insulating polymer matrix between the particles and/or by the alumina oxide layer of the aluminum alloy surface. The main aim of succeeding in getting conductive polymer matrix network seems to have been achieved as there are promising indications in the current maps. However, as shown in the optical images (Figure 24) the PANI particles are rather strongly aggregated.
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Thus, to achieve a more conducting network would require a better dispersion of the PANI particles.
Fig. 24. A) Optical images at the upper surface of the polymer matrix with PANI, with the magnification 10X. B) Optical image for aluminum surface underneath the polymer matrix with PANI, with the magnification 20X In Fig. 24 we first see that there are more PANI particles close to the aluminum surface than at the top surface of the coating. One reason for this is that the surface energy of the matrix is lower than that of the PANI particles. It can also be seen that some PANI particles at the upper surface are superimposed on top of PANI particles underneath the polymer matrix, as was observed by changing the focus of the lens. Some PANI particles at the upper surface are thus connected to the PANI particles deeper in the coating near the aluminum substrate, which is a prerequisite for obtaining a conducting network.
10. Conclusion The corrosion properties of three systems, bare aluminum alloy 6060, and this alloy coated with a barrier coating and with the same matrix containing 1 wt % PANI. The systems were analyzed by using EIS, OCP, polarization and AFM. As expected, the bare aluminum alloy had a much lower impedance modulus at low frequency compared to the other systems. This was expected because the only protection is the thin oxide layer. The corrosion current density was also higher for the bare aluminum alloy demonstrating a higher corrosion rate compared to the two other coated systems. The polymer matrix in the absence of PANI initially had a good barrier type protection, that blocked the aggressive species from reaching the interface between the aluminum alloy and the coating. However, the polymer matrix is not sustainable for long-time protection in 1 M NaCl solution as water absorbed in the coating and reduced the barrier properties. The importance of using a newly made polymer matrix dispersion when preparing the coating was demonstrated, as the use of a 2-year old coating dispersion resulted in lower impedance modulus and larger water absorption compared to when a newly made matrix dispersion was utilized. The polymer matrix in presence of PANI was initially found to exhibit an active corrosion protection. However, a transition to a passive protection was found with increasing exposure time. This conclusion was drawn since a higher impedance modulus corresponds to lower conductivity. The reason for the active corrosion protection of this system could be due to formation of a conducting network as indicated by AFM measurements. The reason why the
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DEGREE PROJECT IN CHEMICAL SCIENCE AND ENGINEERING, SECOND CYCLE, 30 CREDITS STOCKHOLM, SWEDEN 2020 conducting network reduced in strength may be due to inefficient dispersibility of the PANI particles. AFM was used in various modes and indicated the presence of a conducting network, which was also plausible based on confocal light optical microscopy images. I have showed that it is possible to produce a polymer dispersion with PANI particles, hence the conducting network seen from EIS and AFM results. For further research, more work needs to be done on the dispersibility of the PANI particles in the polymer matrix, to obtain a more homogenous and reproducible system. Differences in electrical properties for the polymer matrix in presence of PANI were also observed with help of Im-AFM, but more studies to correlate these findings with conductive AFM need to be done.
11. Acknowledgement Dr. Arindam Adhikari is thanked for preparing the PANI sample, Dr. Anwar Ahniyaz at RISE for suggestions for how to disperse the PANI particles in water, and Dr. Tomas Deltin, PTE Coatings, for providing the polymer matrix.
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[63] Szklarska-Smialowska, Z. 1999. Pitting corrosion of alumnium. Corrosion Science. 41(9).1743-1767. DOI: 10.1016/S0010-938X(99)00012-8 [64] R. T. Foley, R. T. Localized corrosion of aluminum alloys—a review. Corrosion. CORROSION.42(5).277-28 .DOI: 10.5006/1.3584905 [65] Jafarzadeh, S., Claesson, P.M., Sundell, P.E. et al..2014. Nanoscale Electrical and Mechanical Characteristics of Conductive Polyaniline Network in Polymer Composite Films. ACS Applied Materials & Interfaces 2014, 6, 19168−19175. DOI: 10.1021/am505161z [66] Orazem, M.E. & Tribollet, B.2017. Electrochemical Impedance Spectroscopy. Second Edition. New jersey, United states of America: John Wiley & Sons, Inc. https://onlinelibrary- wiley-com.focus.lib.kth.se/doi/pdfdirect/10.1002/9781119363682 (Accessed 2020-10-15) [67] Loveday, D., Peterson, P & Rodgers, B.2004.Evaluation of Organic Coatings with Electrochemical Impedance Spectroscopy. Part 1: Fundamentals of Electrochemical Impedance Spectroscopy. JCT CoatingsTech., 46-52, Aug 2004. http://www.consultrsr.net/files/jct/JCT200408.pdf (Accessed 2020-11-15) [68] Barsoukov, E. & Macdonald, J.R.2005. Impedance Spectroscopy Theory, Experiment, and Applications.Second Edition. New jersey, United states of America: John Wiley & Sons, Inc.https://cdn.preterhuman.net/texts/science_and_technology/physics/spectroscopy/Impedanc e%20Spectroscopy,%20Theory%20Experiment%20and%20Applications%20- %20Macdonald.pdf (Accessed 2020-11-15) [69] Walter, G.W.1986. A review of impedance plot methods used for corrosion performance analysis of painted metals. Corrosion Science. 26(9).681-703.DOI: 10.1016/0010- 938X(86)90033-8 [70] Jones, D.E. .1996. Principles and Prevention of Corrosion. Second edition. Upper Saddle River, New Jersey, United states of America. Prentice hall. ISBN: 9780133599930 [71] Grassini, S.2013. Electrochemical impedance spectroscopy (EIS) for the in-situ analysis of metallic heritage artefacts. In: Dillmann, P., Watkinson, D., Angelini, E. & Adriaens, A. (eds). Corrosion and Conservation of Cultural Heritage Metallic Artefacts. 347-367 Cambridge, United kingdom: Woodhead publishing. DOI: 10.1533/9781782421573.4.347 [72] Mandeep. 2018. A study on electrochemical impedance spectroscopy. International Journal of Chemical Studies. 6(2): 403-408. https://www.chemijournal.com/archives/2018/vol6issue2/PartF/6-4-281-630.pdf (Accessed 2020-11-15) [73] Amirudin, A & Thierry, D.1995. Application of electrochemical impedance spectroscopy to study the degradation of polymer-coated metals. Progress in Organic Coatings.26(1). Aug 1995.1-28. DOI: 10.1016/0300-9440(95)00581-1 [74] Lillard, S.2012. Corrosion and Compatibility. In: Konings , R.J.M.(ed). Comprehensive Nuclear Materials. Amsterdam, Netherlands: Elsevier science. DOI: 10.1016/B978-0-08- 056033-5.00024-0
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[75] Berradja, A.2019. Electrochemical Techniques for Corrosion and Tribocorrosion Monitoring: Methods for the Assessment of Corrosion Rates, In: Singh, A. (ed). Corrosion Inhibitors, IntechOpen, DOI: 10.5772/intechopen.86743. [76] Birbilis, N, Musler, T.H. & Buchheit, R.G. 2012. Corrosion of Aluminum Alloys. In: Marcus, P. Corrosion Mechanisms in Theory and Practice. Third Edition. London, United kingdom. Taylor & Francis Group. [77] Yunjuan, H., Dobryden, I., Pan, J. et al. 2018. Nano-scale mechanical and wear properties of a waterborne hydroxyacrylic-melamine anti-corrosion coating. Applied Surface Science. 457(1). 548-558. DOI: 10.1016/j.apsusc.2018.06.284 [78] Gupta, G., Birbilis, N., Cook, A.B. et al. 2013. Polyaniline-lignosulfonate/epoxy coating for corrosion protection of AA2024-T3. Corrosion Science.67 (2013).256-267. DOI: 10.1016/j.corsci.2012.10.022 [79] Haugstad, G. 2012. ATOMIC FORCE MICROSCOPY Understanding Basic Modes and Advanced Applications. New jersey, United states of America: John Wiley & Sons, inc. DOI: 10.1002/9781118360668 [80] Rodenbücher, C., Wojtyniak, M. & Szot, K. 2019.Conductive AFM for Nanoscale Analysis of High-k Dielectric Metal Oxides. In: Celano, U. (ed). Electrical Atomic Force Microscopy for Nanoelectronics. 303-350.Cham, Switzerland. Springer, Cham. DOI: 10.1007/978-3-030-15612-1 [81] Jiang L, Weber J, Puglisi FM, et al. 2019.Understanding Current Instabilities in Conductive Atomic Force Microscopy. Materials (Basel). 2019;12(3):459. Published 2019 Feb 1. DOI:10.3390/ma12030459 [82] Pittenger, B.; Erina, N.; Su, C. Quantitative Mechanical Property Mapping at the Nanoscale with PeakForce QNM; Bruker Application Note #128; Bruker Nano Surfaces Division: Santa Barbara, CA, 2012. http://www.bruker.com/fileadmin/user_upload/8-PDF- Docs/ SurfaceAnalysis/AFM/ApplicationNotes/AN128-RevB0- Quantitative_Mechanical_Property_Mapping_at_the_Nanoscale_ with_PeakForceQNM- AppNote.pdf. (Accessed 2020-11-15) [83] Huang, H., Dobryden, I., Thorén, P-A. et al. 2017. Local surface mechanical properties of PDMS-silica nanocomposite probed with Intermodulation AFM. In: Composites Science and Technology.150(2017).111-119. DOI: 10.1016/j.compscitech.2017.07.013 [84] Platz, D., Tu0pel E.A., Hutter, C. et al. 2010. Phase imaging with intermodulation atomic force microscopy. Ultramicroscopy.110(6).573-577. DOI: 10.1016/j.ultramic.2010.02.012 [85] ] Platz, D., Silbernagl, D., Sturm, H. et al. 2019. Insights into Nano-Scale Physical and Mechanical Properties of Epoxy/Boehmite Nanocomposite Using Different AFM Modes. Basel, Switzerland. Polymers. DOI: 10.3390/polym11020235 [86] Kharitonov, D.S., Dobryden, i., Sefer, B. et al.2020. Surface and corrosion properties of AA6063-T5 aluminum alloy in molybdate-containing sodium chloride solutions. Corrosion science.171 (2020).DOI: 10.1016/j.corsci.2020.108658
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13. Appendix 13.1. Polymer Matrix – Old batch
-4e6 AL&P_1_1DAY.dat.txt AL&P_1_3DAY.dat.txt AL&P_1_6DAY.dat.txt -3e6
-2e6 Z''
-1e6
0
1e6 0 1e6 2e6 3e6 4e6 5e6 Z' Fig. 25. Nyquist plots for one polymer matrix sample measured after 1 day, 3 days and 6 days of exposure to 1 M NaCl. Table. 4. Fitting data of EIS spectra of polymer matrix coated aluminum, mean value and standard deviation of two samples.
2 2 -1 2 α Time elapsed Rs [Ω cm ] Rp [Ω cm ] Y0[Ω cm s ] α C [F] 1 Hour 490±140 (6.43±6.42) (5.23±4.65) x10- 2.20 x1017 10 0.84±0.04 x10-8 1 Day 968±483 (2.44±2.36) x108 (5.13±5.05) x10-8 1.31 0.73±0.11 x10-7 3 Day 7346.9±6799.1 (2.92±1.26) x106 (2.16±1.13) x10-7 1.67 0.64±0.06 x10-7 6 Day 9158±5079 (1.10±0.28) x106 (3.16±0.14) x10-7 1.95 0.69±0.04 x10-7
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8 Day 6002±1883 (4.28 ±0.91) (5.71±0.62) x10-7 3.00 x105 0,69±0.01 x10-7 10 day 3321.5±992.5 (4.30 ±0.29) (7.07±1.78) x10-7 3.50 x105 0,63±0.01 x10-7 13 day 2389±1180 (2.31 ±0.17) (1.73±0.46) x10-6 9.18 x105 0.59±0.16 x10-7 15 day 2686.5±688.5 (2.91 ±0.81) (1.74±0.13) x10-6 1.08 x105 0.59±0.16 x10-6
13.2. Potentiodynamic Polarization
Fig. 26. Polarization curve with Tafel slopes for bare aluminum.
Fig. 27. Polarization curve with Tafel slopes the polymer matrix.
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Fig. 28. Polarization curve with Tafel slopes for polymer matrix with 1 wt% PANI.
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