Corrosion Technology

TECHNICAL PUBLICATION SERIES

CORROSION TECHNOLOGY

METALLIC AND INORGANIC COATINGS

Version 1.0

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The information contained in its publications is intended for general guidance only and in no way replaces the services of professional consultants on particular projects.

No legal liability for negligence or otherwise can be accepted by the Association for the information contained in this publication.

No part of this publication may be reproduced in whole or in part, or stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without written permission of the publisher.

This publication is sold subject to the condition that it shall not be lent, resold, hired out, or otherwise circulated without the publisher’s prior consent in any form of binding or cover other than that in which it is published. This condition being imposed on any subsequent purchasers.

For information regarding permission, write to: The Chief Executive Officer The Australasian Corrosion Association Incorporated PO Box 112 Kerrimuir Victoria 3129 Australia Email: [email protected]

ACA 11: Corrosion Technology – Metallic and Inorganic Coatings. First Edition Published 2013 ISBN – 978-0-9875650-0-6.

The Australasian Corrosion Association Incorporated Australasian Office Suites 1 and 3 458 Middleborough Road Blackburn Victoria 3130 Australia

PHONE: +61 3 9890 4833 FACSIMILE: +61 3 9890 7866 EMAIL: [email protected] WEBSITE: www.corrosion.com.au

For contact information on ACA’s Branches and networks in Queensland, New South Wales, Newcastle, Victoria, Tasmania, South Australia, Northern Territory, Western Australia, and New Zealand see the ACA’s website www.corrosion.com.au.

ACA 11 – Corrosion Technology – Metallic and Inorganic Coatings www.corrosion.com.au i © The Australasian Corrosion Association Inc 2013

FOREWORD

The Australasian Corrosion Association has developed the Corrosion Technology technical publication series to provide an understanding of how and why corrosion happens, how it manifests itself and how the relevant methods of corrosion prevention and control operate. This series describes corrosion and its mitigation in general terms, applicable to a wide range of industries. The publication series is suitable for many working in a corrosion-related field and is an integral part of the certification scheme developed by the Association.

The information in this publication series is largely taken from a number of text books and from the notes to other ACA courses and publications.

CORROSION TECHNOLOGY TECHNICAL PUBLICATION SERIES

The following are included in the series:

ACA 2 Corrosion Technology - Introduction 1st Edition - 2013

ACA 3 Corrosion Technology - The Corrosion Process 1st Edition - 2013

ACA 4 Corrosion Technology - Predicting Corrosion Reactions 1st Edition - 2013

ACA 5 Corrosion Technology - Types of Metallic Corrosion 1st Edition - 2013

ACA 6 Corrosion Technology - Corrosion in Natural Environments 1st Edition - 2013

ACA 7 Corrosion Technology - Corrosion Control by Design Improvement 1st Edition - 2013

ACA 8 Corrosion Technology - Corrosion Properties of Metals 1st Edition - 2013

ACA 9 Corrosion Technology - Environmental Modification 1st Edition - 2013

ACA 10 Corrosion Technology - Cathodic and Anodic Protection 1st Edition - 2013

ACA 11 Corrosion Technology - Metallic and Inorganic Coatings 1st Edition - 2013

ACA 12 Corrosion Technology - Organic Coatings and Linings 1st Edition - 2013

ACA 13 Corrosion Technology – Introduction to Metallurgy 1st Edition - 2013

ACKNOWLEDGEMENT

The Australasian Corrosion Association Inc would like to acknowledge Dr Robert Francis for his contributions to this publication series.

ii www.corrosion.com.au ACA 11 – Corrosion Technology – Metalic and Inorganic Coatings © The Australasian Corrosion Association Inc 2013 CONTENTS

CORROSION TECHNOLOGY METALLIC AND INORGANIC COATINGS ...... 1

1. METAL SPRAYING ...... 1

2. HOT DIP COATINGS ...... 3

3. ELECTROPLATING ...... 6

4. OTHER METAL COATING PROCESSES ...... 8

5. ANODIZING ...... 10

6. PHOSPHATING, CHROMATING & ALTERNATIVES ...... 11

7. VITREOUS ENAMEL COATINGS ...... 12

8. CEMENT COATINGS ...... 13

FURTHER INFORMATION ...... 14

WEBSITES ...... 14

BOOKS ...... 15

STANDARDS ...... 15

GLOSSARY OF CORROSION TERMS ...... 16

ACA 11 – Corrosion Technology – Metallic and Inorganic Coatings www.corrosion.com.au iii © The Australasian Corrosion Association Inc 2013

CORROSION TECHNOLOGY

METALLIC AND INORGANIC COATINGS

In this technical publication, coatings used for corrosion prevention other than organic coatings will be covered. These can be divided into metallic coatings, such as galvanizing or electroplated coatings and inorganic coatings, such as phosphate or chromate conversion coatings, anodizing or vitreous enamelling.

Metallic coatings can be divided into those which provide barrier protection and are more noble than the substrate, and sacrificial coatings of base metals which provide protection by galvanic action. While both can provide corrosion protection, the process on breakdown of the protective film will differ greatly. Figure 1 shows the difference in behaviour of a scratch on say, tin plate compared to a scratch on galvanized steel. Tin will protect the iron while the coating is continuous, but when the coating is broken, there will be a small iron anode and a large tin cathode. Very rapid localised corrosion will then occur. On galvanized steel, the exposed steel will act as a small cathode while the zinc coating acts as a large anode. Corrosion of the steel will not occur while zinc is adjacent to the scratch.

Figure 1: Corrosion of (a) scratched tin-plate and (b) scratched galvanized steel.

1. METAL SPRAYING Metal or thermal spraying (sometimes known as metallizing) basically involves the projection of molten or heated particles from a suitably designed torch onto a prepared surface. The metal can be in the form of a wire or powder and the heat can be supplied by either a gas flame or an electric arc. In wire flame spraying [see Figure 2(a)], the oldest spray process, heat from combustion of oxy-acetylene melts the wire and particles are stripped off by means of a stream of compressed air. This atomises the material and propels it onto the work piece. Only materials that can be made into wires can be used, but this includes zinc and aluminium, so the process is widely used. It is portable, easy to use and reasonably inexpensive but coatings exhibit lower bond strengths, higher porosity and tend to heat the substrate more than other methods.

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In electric arc spraying [see Figure 2(b)], two wires of the coating material serve as electrodes which are fed together and an arc struck between them. A gas jet, generally compressed air, blows molten material from the tip of the two wires producing a jet of molten droplets which are deposited on the surface being coated. Because of the high temperatures in the arc, the coatings have excellent adhesion and high cohesive strength and the process is very efficient as two wires are used.

(a) (b) Figure 2: Schematic of (a) wire flame spray and (b) electric arc spray processes.

Thermal spray coatings rely on a purely mechanical bond to the substrate so surface preparation is critical for a successful coating. Grit blasting using a suitable abrasive such as chilled iron grit is required to produce a perfectly clean surface (Class Sa 3) with a rough profile (usually 75µm or more). Metal spray coatings should be applied to clean dry surfaces as soon as possible after grit blasting. Preheating of the substrate is sometimes used before flame spraying to eliminate condensation. Metal spray coatings are porous, containing from 5 to 20 per cent porosity, but oxidised metal and sealer coats tend to fill the pores. There is no Australian standard for metal spraying and they should be applied in accordance with ISO 2063.

Zinc and aluminium are most widely used for metal spray anti-corrosion coatings. The life of a zinc coating depends on its thickness and the atmosphere to which it is exposed. Figure 3 shows the effect of environment and thickness on the life of a zinc coating. In dry, non-polluted atmospheres, very long lifetimes can be obtained. Near the sea or in industrial areas the presence of chlorides or pollutants reduces the coating life and additional protection, such as a paint system, may be required.

Thermal spray coatings are usually 100 to 250 m, so will give very good protection even in severe environments. They can be applied on-site or in-plant. The choice of zinc or aluminium coating depends on the environment. Zinc or zinc- aluminium are usually chosen where corrosion protection is most important, and aluminium when heat resistance is required. Metal sprayed zinc or aluminium, with an organic sealer, will usually provide the longest possible service life for atmospheric exposure. However, it is slow so a very expensive process. Metal spraying cannot be used for coating cavities and other difficult areas and would not be economical for treating large, open structures such as mesh, because of the large loss of metal that would result.

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Figure 3: The effect of environment and thickness on the life of a zinc coating.

2. HOT DIP COATINGS Dipping the item to be protected in a bath of molten zinc is perhaps the most common technique for providing a protective coating to steel. Hot dipping has a number of advantages, including ability to coat edges and internal areas, resistance to mechanical damage because the coating is metallurgically bonded to the substrate and its low cost. However, the item to be coated must not be able to be damaged at the application temperature.

The details of the hot dip galvanizing process depend on the item to be coated, whether it is continuous strip, small items such as nuts and bolts or large fabricated steelwork, but basically the steel is cleaned, fluxed and dipped in a bath of molten zinc (see Figure 4). The material is degreased in a hot caustic solution, washed and pickled in sulphuric or hydrochloric acid. It is then rinsed in water to remove any traces of acid. The item is then fluxed in a zinc-ammonium chloride solution. This will dissolve any oxide films formed on the steel after pickling. The cleaned item is then dipped into the molten zinc at 445 to 465°C and a series of iron-zinc alloys is formed by reaction of the zinc with the surface of the article. The item is removed from the galvanizing bath and excess zinc may be removed by wiping or centrifuging. The work may be quenched in water to avoid staining by white rust. The item is cleaned of any excess zinc and inspected.

The coating reaches a certain thickness fairly rapidly but does not increase greatly if the article is held in the bath for a much longer time. The thickness of the coating depends on a number of factors but the most important are the mass and thickness of the steel being galvanized and the silicon content of the steel. Heavier steel sections over 5 mm thick will normally contain a minimum of 600 grams of zinc per square metre, equivalent to about 84µm while steel under 2 mm thick will have about 350 grams of zinc per square metre (50µm). Coating thickness is slightly greater at corners. Silicon killed steels produce very thick coatings which are dull, rough and brittle, but provide protection as long as the coating is sound

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and continuous. By the use of double dipping techniques, it is possible to treat members longer than the bath size. Double dipping does not increase the thickness of the galvanized coating.

Figure 4: The process of hot dip galvanizing.

AS/NZS 4680 describes batch hot dip galvanized coatings and galvanized threaded fasteners are covered by AS 1214. The Galvanizers Association of Australia (www.gaa.com.au) provides information on all aspects of galvanizing including availability of plants.

A white stain, called white rust or wet storage stain may form on the surface during storage or shipment. It forms especially under moist conditions and consists of basic zinc carbonate. It is relatively innocuous but can be avoided by ensuring dry storage or by dipping in a chromate solution. Under prolonged exposure to moist conditions, the zinc coating may break down completely and the steel underneath rusts.

Hot dip galvanizing often provides an alternative to a paint coating for steelwork. There are a number of advantages and disadvantages that must be considered when selecting which method to use. The specific advantages of galvanized coatings over other types are:  It can give complete protection to an entire fabrication, regardless of its complexity. For example, it can provide inside and outside protection of hollow structures.  The coating is highly abrasion resistant  The coating is metallurgically bonded to the steel giving excellent adhesion  Inspection requirements are simpler, easier and more reliable  Galvanizing is a factory process and can continue under any weather and humidity conditions  No cure time is required  Unlike paint coatings which are thinner at edges, galvanized coatings are as thick at corners and edges  Galvanizing may be the most economic coating system, especially for items of large surface area per tonne.

4 www.corrosion.com.au ACA 11 – Corrosion Technology – Metalic and Inorganic Coatings © The Australasian Corrosion Association Inc 2013 However, there are some disadvantages:  Galvanizing can only be applied in a factory  Only articles smaller than a certain size can be coated  Coating thickness cannot be controlled  Mechanical properties of the steel may be affected, such as hydrogen embrittlement of high strength steels  Drain and vent holes may have to be drilled  The item may distort from heating and cooling during the process  The metallic colour may not be acceptable in some applications, and paint coatings applied to galvanizing often show poor adhesion  Damaged galvanized coatings are not as easily repaired as paint coatings  Galvanized coatings do not resist certain environments such as acids as well as acid-resistant paints  Galvanized coatings may not be as economic, especially for items of small surface area per tonne.

Continuous galvanizing is used to apply a zinc coating at high speeds to steel sheet, pipe and wire. To produce coated sheet, coils of thin gauge steel are degreased and the surface cleaned by gaseous oxidation and reduction. The strip enters the galvanizing bath and passes through fine jets of air or steam to remove excess zinc to provide the required thickness. It is then cut into sheets or coiled. As shown schematically in Figure 5, the thickness of the zinc on continuously galvanized products is thinner than that on hot dipped items (usually 15 to 30 microns) and should not be used in severe environments. Continuous galvanizing of steel sheet is described in AS 1397 and AS 1445 and of wire products in AS 1394, AS 2423 and AS 2841.

Figure 5: Thickness of continuously galvanized coating compared to hot dip (batch) galvanized coating.

Aluminium-zinc coating of steel sheet (Zincalume) provides the durability and high temperature resistance of aluminium coatings along with the sacrificial protection of zinc coatings. The coatings contain 55 per cent aluminium, the remainder zinc with a small amount of silicon and the process is very similar to continuous galvanizing lines. Tests have shown the 55Al-Zn coating provides two to six times the corrosion resistance of galvanized panels (based on equal coating thicknesses). Aluminium-zinc coated steel sheet are described in AS 1397 and AS 1445.

Aluminium and aluminium-zinc can be applied by thermal spray. Aluminium tends to have better performance in industrial (acidic) environments and saltwater than zinc, and also has better high temperature resistance. Zinc provides better performance in cold, fresh water, has better galvanic protection and is easier to apply.

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3. ELECTROPLATING Electroplating is the application of metallic coatings by immersing articles to be coated in a solution (electrolyte) and making them the cathode (connecting them to the negative terminal in a low voltage DC circuit). To complete the circuit, anodes are immersed in the electrolyte and connected to the positive leads (see Figure 6). The electrolyte provides the source of metal ions which deposit on the cathode surface. The anode may be of the metal being deposited, as in the case of nickel, copper and zinc plating, or may be insoluble, for example in chromium plating.

The rate of deposition is governed by Faraday’s laws which, in simple terms, state that the amount of metal deposited is related to the current which passes. The average thickness can therefore be calculated knowing the current density. However, the current density is generally not uniform over the surface, but varies according to the shape of the article and the way it is positioned relative to the anodes. Solutions are chosen on the basis of their throwing power, referring to their ability to deposit sound metal in recesses. Cyanide solutions have better throwing power than acid solutions and are, therefore, commonly used although they are more dangerous to handle and difficult to dispose of safely.

Figure 6: Schematic diagram of an electroplating bath.

Good plating practice produces coatings of controlled thickness, a fine grain size and relative freedom from porosity. A chemically clean surface is required so that the plating material can adhere. Proper degreasing, cleaning and pickling are essential with water rinses between each stage to ensure solution is not carried forward to the next stage. The plating metal tends to be attracted to, and build up on corners and protrusions. This makes it difficult to obtain uniform coating thickness on parts of irregular shape containing recesses and interior corners. Another problem which can arise during plating is hydrogen embrittlement due to hydrogen forming on the surface and diffusing into the steel substrate. Baking at temperatures of around 200°C will drive off most of the hydrogen from the metal if cracking has not occurred.

For small intricate shapes such as bolts and screws, barrel plating may be used to apply the metal where a rotating barrel containing the parts to be plated is turned slowly in the electrolyte while current is discharged to the tumbling parts inside. Brush or selective plating is a portable process which has an anode

6 www.corrosion.com.au ACA 11 – Corrosion Technology – Metalic and Inorganic Coatings © The Australasian Corrosion Association Inc 2013 mounted on handle covered with a pad soaked in electrolyte. It can be used for repair or build-up of worn components.

Zinc plating is widely used for the production of protective deposits on steel. The coatings are much thinner than can be obtained with hot dip application (usually less than 20µm) and zinc plating would not normally be recommended for outdoor exposure without supplementary coatings. Requirements for electroplated zinc coatings on iron and steel are described in AS 1789 although requirements for fasteners are given in AS 1897.

Copper plating is used as a base coat due to its surface levelling properties. It can fill minor surface irregularities and improve corrosion resistance of subsequent coatings.

Nickel provides good corrosion resistance by barrier protection, but does not retain its lustre and is expensive. Its major application is as the initial deposit before chromium plating.

Chromium has two major applications, as a thick hard deposit for engineering applications and as a much thinner (usually about 1 micron) decorative finish in conjunction with nickel. The chromium provides tarnish resistance and assists the nickel in giving protection against corrosion. Chromium coatings are highly stressed and are difficult to deposit without the coating containing microcracking. However, chromium can be deposited over the two (bright and semi-bright) nickel layers with different electrochemical reactivities resulting in reduced tendency for pitting (Figure 7). The protection given by a composite nickel-chromium coating depends largely on the thickness of the nickel coating. In mild, unpolluted atmospheres a thickness of 10 to 15 microns of nickel is required, while in severe conditions, at least 25 microns of nickel is required for good protection of the steel. Microcracked chromium coatings are used extensively for bright trim. Requirements for nickel plus chromium coatings are described in AS 1192, apart from those on screw threads.

Figure 7: Microcracked chromium over duplex nickel on steel. Corrosion is concentrated in anodic nickel coating.

Tin, gold, silver, lead and other metals can be applied by electroplating for decorative or industrial purposes.

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4. OTHER METAL COATING PROCESSES Chemical nickel coatings, commonly called electroless nickel, use sodium hypophosphite solution to reduce nickel salts to nickel metal, incidentally producing a coating containing between 3 and 15 per cent phosphorus. The bath is operated at 85 to 95°C and the deposit produced is smooth and non-porous. Because no current is employed, coverage can be obtained in deep recesses without difficulty and the coating is uniform in thickness. The coatings are amorphous in structure, unlike crystalline electroplated coatings, and practically non-porous but they are more brittle. However, the phosphorus content makes it possible to restore ductility to the coating by a low temperature heat treatment to 400°C. Electroless nickel has excellent corrosion and wear resistance but high chemical costs, brittleness and a slower plating rate than electrolytic methods. It is used for protection against high temperature oxidation, acid chlorides and against dilute acids. Aircraft components, chemical process equipment, oil drilling equipment and valves have all used electroless nickel. Its resistance to wear, abrasion and galling have led to use in parts such as rotor blades, pump components and pistons. Copper, tin and other certain metals can also be deposited by chemical reduction of metal salts.

Mechanical plating or peen plating is an electroless plating method used to deposit coatings of ductile metals onto a metal substrate. It is used particularly for coating threaded components. Parts are cleaned and tumbled together with metal powder, glass beads, water and a suitable chemical promoter. The glass beads peen and weld the metal powder to the steel surface. The film thickness depends on the amount of metal powder and the time. Only soft metals such as tin, aluminium, lead and cadmium can be applied in this way but the process is mainly used for zinc. The process is slow but useful for plating high-tensile articles where hydrogen embrittlement may be a problem.

Cladding is the application of a corrosion-resistant skin onto another metal which has the required engineering properties, but lacks the necessary corrosion resistance in the working environment. Skins can be applied by rolling, where two metals are mated by heavy rolling in a mill after the surfaces have been thoroughly cleaned and degreased. Aluminium roll bonded to duralumin or other aluminium alloy substrate is marketed as Alclad and stainless steel-clad copper is used for the manufacture of cooking vessels. Pipes and fittings, usually of carbon or low- alloy steel clad with stainless steel, are increasingly used in the refinery and processing industries where they are showing considerable cost savings. Roll bonded cladding accounts for 90 per cent of all clad plate produced world-wide. Explosion bonding uses an explosive to produce shock waves which weld metals together. This process enables virtually any metal to be bonded to any other, for example lead has been explosively bonded to steel. Building up a welded coat on the substrate in a process known as buttering enables relatively thick layers of corrosion-resistant metal to be deposited. Cupro-nickel has been butter welded to steel pipes used in sea water services.

8 www.corrosion.com.au ACA 11 – Corrosion Technology – Metalic and Inorganic Coatings © The Australasian Corrosion Association Inc 2013 Composite tubing is available made from two tube materials, one with good resistance to the external environment and one with good resistance to the internal environment. Such tubes have been used in coolers and condensers handling ammonia, brines, sea water and organic chemicals. Special methods of joining, expanding into header plates and of avoiding galvanic corrosion at the exposed ends have been devised.

Sherardising is a method of coating with zinc by tumbling the articles to be coated in a barrel containing zinc dust at a temperature just below its melting point (about 370°C). The zinc bonds to the steel by a diffusion process which forms a hard, even layer of zinc-iron compounds. The process is characterised by its ability to form a very uniform coating on small articles, such as fasteners, springs and chain links. The sherardising process is not used in Australia or New Zealand. A similar process can be used to coat steel with aluminium, in which case it is called calorising.

Vacuum coating and cathodic sputtering are processes of depositing very thin metal films on various substrates in a high vacuum. Virtually any metal can be deposited by the process – aluminium, chromium, gold, nickel, silver and platinum being common – often to thicknesses as little as 0.025µm. Because of the very thin deposits, the process can be economical for depositing expensive materials but there is a comparatively restricted capacity of the vacuum plant. These two processes are the most common physical vapour depositions (PVD) coatings. Chemical vapour deposition (CVD) coatings use a reactant processing gas in a reaction chamber which decomposes at the workpiece, liberating a material on the surface. For example, nickel is liberated from a nickel carbonyl atmosphere. The process can deposit thick (up to 6 mm), dense, high quality films, but only on substrates that can withstand the high temperature (about 1000°C) necessary for CVD.

Table 1 shows some features of the most important zinc and aluminium coating systems available for corrosion protection in Australia and New Zealand.

Table 1: Some characteristics of commonly used metallic coatings. Metal Method of Standard Typical Major use Coating Deposition thickness (microns) Zinc Electroplating AS 1789 5 – 25 Fasteners Mechanical plating – 5 – 42 Fasteners Hot dip galvanize AS 4680 42 – 85+ Fasteners, structural steel Continuous galvanize AS 1397 15 – 50 Sheet (various) 15 – 30 Tube Metal spray ISO 2063 50 – 200 Structural steel Aluminium/ Continuous process AS 1397 50 – 60 Sheet zinc Aluminium Metal spray ISO 2063 50 – 150 Structural steel

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5. ANODIZING Anodizing or anodic oxidation is a widely used process that provides corrosion- resistant and decorative finishes to aluminium. The process is similar to electroplating in that current is passed through the metal to be coated, but in the case of anodizing the work is made the anode with an inert cathode, such as lead. The current causes the aluminium to corrode slightly thickening the existing protective aluminium oxide coating on the aluminium. The anodic film is hard and abrasion resistant as well as being chemically stable offering increased corrosion resistance. It also has appreciable electrical breakdown resistance, valuable for insulation purposes. The film consists of two parts; a thin barrier layer next to the metal and a much thicker, porous columnar layer (see Figure 8). The pores can take up dyes to give a range of coloured decorative effects. A range of electrolytes can be used but the most common is sulphuric acid. Requirements for anodized coatings are given in AS 1231.

Figure 8: Schematic diagram of pore structure on anodized aluminium.

Sulphuric acid anodizing typically uses a solution of 15% sulphuric acid at a temperature of 21°C at 15 to 22 volts, depending on the alloy. The object is held in the bath for 30 to 60 minutes with a current density of 100 to 200 amperes per square metre. This produces coatings from 8 to 30 µm thick. The coatings are fairly hard, and sulphuric anodize is the cheapest and most widely used type. It can be used in applications requiring light-to-moderate wear resistance, such as sliding assemblies. It is aesthetically pleasing and can be dyed almost any colour and has good corrosion resistance.

Hardcoat anodize also uses sulphuric acid but the process is operated at a lower temperature, as low as 0°C, with higher electrolyte concentration and current density. This results in an oxide layer that is 45 to 55µm thick and extremely hard. It has excellent wear resistance and corrosion resistance but can only be dyed with a dark colour and is more expensive.

10 www.corrosion.com.au ACA 11 – Corrosion Technology – Metalic and Inorganic Coatings © The Australasian Corrosion Association Inc 2013 Because of the porous nature of the oxide layer, it is necessary to seal the coating and close the pores if corrosion resistance is required. Sealing involves immersing the coating in hot water or steam; this hydrates the film causing it to swell. Sealing is generally done at 95°C for not less than 15 minutes. ‘Cold’ sealing processes have been developed. Sulphuric anodizes are almost always sealed but, because sealing softens the oxide, hardcoat anodizing is not usually sealed.

6. PHOSPHATING, CHROMATING & ALTERNATIVES Phosphating, sometimes known as phosphatising, has been a widely used chemical pretreatment for steel for items such as white goods and car bodies. It is carried out by dipping or spraying articles with a phosphoric acid solution containing zinc or manganese phosphate. Such coatings are usually applied to enhance corrosion resistance or improve paint adherence, or both, but can also be used to enhance electrical insulation properties and for lubricity (for example, to increase the formability of sheet metals). Phosphate treatments can also be applied to galvanized steel or aluminium. Parkerizing and Bonderizing are two proprietary phosphate treatments.

The steel is pickled (or occasionally abrasively blasted) and immediately treated with hot phosphoric acid containing accelerators (nitrates or chlorates which reduce treatment times) as well as zinc or manganese phosphates. The steel is attacked by the acid with the formation of iron phosphates which are strongly bonded to the metal surface and act as nucleation sites for the formation of sparingly soluble manganese or zinc phosphates. To afford better protection against corrosion, all phosphate films are given a final rinse in a dilute solution of chromic acid or a phosphoric/chromic acid mixture and then dried. Phosphate coatings range in thickness from 3 to 50 µm but coating weight in grams per square metre rather than coating thickness is used to express amount deposited. Phosphating processes are divided into five classes depending on the composition and weight of the coating according to AS 1627 Part 6.

Chromating is a process that resembles phosphating, and can be used as a passivating treatment after phosphating. It is not used alone on steel but rather on magnesium, aluminium, zinc and cadmium where the films have a distinctive yellow colour, although thin films may be colourless and thicker films may be brown. Chromating involves treatment of the metal surface in a bath containing chromic acid or chromates and accelerators. The films are usually very thin, often less than 0.5µm, dense and adherent. However, they generally provide a poor surface for paints on zinc and have little abrasion resistance. Chromate coatings are applied to aluminium for such applications as domestic appliances, aircraft and electronic equipment and continuous coil coating of architectural aluminium. They are widely used as a cheaper substitute for anodizing and provide excellent paint adhesion. Most hot dipped galvanized and Zincalume sheet products are chromated as part of the hot dip process. Chromate conversion coatings on non- ferrous metals such as aluminium are covered by AS 1627 Part 6.

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Phosphate-free and chromate-free techniques providing similar properties have been developed in recent years, based on zirconium, silanes or similar chemistries. These have lower environmental impact than phosphate and chromate, and lower energy and water consumption.

7. VITREOUS ENAMEL COATINGS Vitreous enamel is a coating of glass which has been fused to the metal, providing both decoration and corrosion protection. It is used on many items of household equipment, such as cookers, sinks and baths, but also for architectural applications (e.g. building panels, tiles and signs) and industrial products such as chemical reactors, food processing vessels and tanks and silos. It has poor resistance to alkalis, but good resistance to acids (except hydrofluoric acid), boiling water, organic solvents and heat (up to 600°C). It shows good colour retention, is electrically insulating, free from taste and odour and has good resistance to wear. Its major weakness is its brittleness and poor heat transfer. Vitreous enamel, known as porcelain enamel in the USA, should not be confused with the general term ‘enamel’ which is used to describe some paint coatings. AS 1914 defines terms relating to vitreous enamelling.

The raw material for vitreous enamelling (known as frit) is produced from a combination of glass forming materials such as silica, feldspar, soda and borax with colour pigments and stabilizers. The chemicals are smelted together at 1200 to 1300°C and the molten mass is quenched in water or crushed between rollers. The frit is ground and mixed with water, clay and sodium silicate to a viscous dispersion which is applied by dipping or spraying to a pickled or otherwise well- prepared surface. It is heated in a furnace until the particles fuse and flow forming a shiny surface strongly bonded to the metal. Two coatings are usually applied to sheet steel or cast iron, a ground coat to promote adherence and a cover coat to improve the appearance and properties of the coating. However, single fire processes have been developed to reduce costs. A variation in the composition of frits enables different properties to be built into enamel to comply with the required service conditions. For example, acid-resistant enamels have additions of titania and zirconia and a higher silica content.

Steels used for porcelain enamelling should be low carbon steel free from surface blemishes, often especially fabricated for vitreous enamelling. Good quality grey cast iron is most suitable for enamelling. Vitreous enamel can also be applied to aluminium, normally using a single coat system. Enamelled aluminium is used particularly in the building industry where a wide range of permanent colours can be obtained.

Ceramics may be used in much the same way as vitreous enamels, but they are usually applied to provide resistance against heat or hot, high velocity gases or both. They are used in exhausts, furnace linings and other similar applications. Unlike amorphous enamels, they are formed from crystalline particles cemented together by a glassy or vitreous material. Ceramic coatings can be based on silicates, oxides, silicides, carbides and other inorganic materials. They can be applied by spraying, dipping or flame spraying.

12 www.corrosion.com.au ACA 11 – Corrosion Technology – Metalic and Inorganic Coatings © The Australasian Corrosion Association Inc 2013 8. CEMENT COATINGS Cement or concrete is mainly used as a coating on pipes. Cement coatings on pipelines perform a number of functions including protection against mechanical damage and corrosion inside and outside pipes as well as ballasting (weight coatings) in sea water. The coatings are low cost, have the advantage of a coefficient of expansion similar to steel (so unlikely to crack due to temperature changes) and can easily be applied and repaired. Coatings can be applied by spray, trowelling, mould casting or centrifugal casting (for pipe interiors). Thicknesses range from 5 to 25 mm and over, with thicker coatings being reinforced with wire mesh or cages. The usual material is Portland cement, but other cements such as pozzolanic cements, can be applied.

Protection of the interiors of steel or cast iron pipes by Portland cement mortar is well established, having been carried out for over 70 years. The cement maintains an alkaline environment keeping the steel in a passive state. Furthermore, cracks tend to fill up will material from the cement or rust. Both these factors contribute to prevention of internal corrosion. Soft waters, especially desalinated water, may leach the alkaline material from the lining and a bituminous seal coat may be required under such conditions. Cement mortar lining may be applied centrifugally where mortar slurry is fed into the bore of a rapidly rotating pipe, by a spray process where a rapidly rotating lance traverses the bore of the pipe or by hand. AS 1516 describes lining of pipelines in situ and AS 1281 deals with the lining of steel pipes in the factory.

Cement coatings for external use are mainly weight coatings for submerged pipelines to ensure they remain on the bed, even when empty. They are applied on top of a normal anti-corrosive treatment and the thickness depends on the buoyancy of the empty pipe. The coating must be able to withstand the severe treatment associated with the pipe laying operation.

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FURTHER INFORMATION

Corrosion is an enormous subject, and this series of technical publications can only touch on some topics. Rather than give specific books or other references that may be difficult or expensive to obtain, this section gives general guidance of where to look for further information.

WEBSITES a) Google Search The Internet has rapidly become perhaps the most important source of all sorts of information, good and bad. You can search on-line whenever you are looking for products, trying to find answers to questions, trying to contact some expert, or many other types of research. You simply type in your keywords into your favourite search engine, probably Google and with any luck, are provided with a list of web sites or other items that may be of interest. This is so simple, cheap and quick that even if you think it highly unlikely you will find the information you require, it is worth doing for the smallest job. Even if the actual items found directly are of little worth, they may have links to others that are valuable.

There is a mine of information out there on the web, and the amount is increasing rapidly. As with any other source of information, it needs careful management. Much is informative and honest, but distortions, half-truths and the straight-up misinformation will sneak in. Keep alert when using the web so that you can recognise the reliable information.

b) Some Websites It is not the intention of this section to provide a full list of websites. They come and go all the time. Furthermore, the better ones will have their own list of links to other sites, so you don’t even have to type them in. Following are some starting points:  www.corrosion.com.au: The Australasian Corrosion Association web site which includes full details on ACA technical activities, seminars, conferences, other activities and publications.  www.corrosion-doctors.org: Lots of educational information, including modules on corrosion science and engineering.  www.corrosionsource.com: Lots of news, information and links on all aspects of corrosion and its control.  www.nace.org: The US association of corrosion engineers. Organise conferences, produce standards, publish books, etc.  www.sspc.org: The US organisation involved in protective coatings. Organise conferences, produce standards, publish books, produce Journal “JPCL”, etc.  www.nickelinstitute.org: The Nickel Institute (formerly NiDI) – produce large number of free papers on all aspects of stainless steels and other nickel alloys.

14 www.corrosion.com.au ACA 11 – Corrosion Technology – Metalic and Inorganic Coatings © The Australasian Corrosion Association Inc 2013 BOOKS Books are still the most important source of background or basic knowledge, although even this area is being taken over by the Internet. The following are books that contain useful information on corrosion and its control. This list is not exhaustive and there are many others available. Try the section in your Technical Library with the Dewey Number 620.1. The NACE journal Materials Performance is a good means of keeping up with latest developments in corrosion and its control.

 Metals Handbook - Volume 13A, 13B, 13C, Corrosion, 9th Ed, ASM International, Metals Park, Ohio, (2006). These three volumes contain an enormous amount of useful practical material, and many references, on forms of corrosion and corrosion properties of metals. It is weaker on subjects such as coatings and cathodic protection. However, certainly a good starting point.  M.G. Fontana, Corrosion Engineering, 3rd Edition, McGraw-Hill, New York, (1986).  Denny A Jones, Principles and Prevention of Corrosion, 2nd Ed, Prentice Hall, Upper Saddle River, NJ, USA, (1996).  Shrier’s Corrosion, 4 Volumes (ed T J A Richardson), Elsevier Science, (2009).  P R Roberge, Corrosion Basics - An Introduction, 2nd Ed, NACE, Houston, Texas, (2005).  Joseph R Davis, Corrosion: Understanding the Basics, ASM International, (2000) The above five books are perhaps the best corrosion textbooks.  C.G. Munger & L Vincent, Corrosion Prevention by Protective Coatings, 2nd Ed, NACE, Houston, Texas, (1999).  A.W. Peabody, Control of Pipeline Corrosion, 2nd Ed, NACE, Houston, Texas, (2001).  Betz Laboratories, Handbook of Industrial Water Conditioning, 9th Ed, Betz, (1991). The above three books are classics in coatings, CP and water treatment.  P.A. Schweitzer, Corrosion Resistance Tables, CRC Press, (2004). This publication gives the performance of metals and non-metals in a wide range of different chemical environments.

STANDARDS Standards keep the wheels of industry turning. Whatever field you are in, there will be some relevant standards. With access to the Internet, these can be obtained rapidly (although they are still expensive) by downloading. Standards Australia often provides well-priced, quality documents as good as any others, if not better. Two very useful ones are AS/NZS 2312 (Guide to the protection of structural steel against atmospheric corrosion by the use of protective coatings) and the AS/NZS 2832 series (Cathodic protection of metals). ASTM in the USA has perhaps the greatest number of standards related to all aspects of corrosion, but NACE and SSPC (see above), as well as ISO and other national standards organisations have useful documents.  www.standards.com.au  www.astm.org  www.iso.org

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GLOSSARY OF CORROSION TERMS

Acid dew point corrosion See Dew point Aqueous Relating to water; an aqueous corrosion. solution is a water solution. Active The state in which a metal tends to Austenitic The name given to a specific corrode (opposite of passive). atomic arrangement (face centred cubic) Additive A substance added, usually to a of alloying elements in iron. Ordinary fluid, in a small amount to change steel has this structure at high properties, such as corrosion, friction, temperatures; certain stainless steels etc. have this structure at room temperature. Adsorption Concentration of a substance on Auxiliary electrode An electrode commonly a surface. used in corrosion studies to pass current to or from a test electrode. Aging (or ageing) Changing the properties of an alloy by passage of time at ambient Backfill Low resistance, moisture holding or higher temperatures. surrounding a buried cathodic protection anode. Aliphatic Organic compounds containing open chains of carbon atoms, as Bimetallic corrosion Corrosion largely opposed to the closed rings of carbon caused by two dissimilar metals in atoms in aromatic compounds. electrical contact with one another in a common electrolyte. Preferred term for Alkyd Resin used in coatings made from Galvanic corrosion. reaction between an alcohol and an acid. Blowdown In connection with boilers or cooling towers, the discharging of a Anaerobic An absence of free oxygen. large amount of the cooling water in Anion An ion with a negative charge (e.g. Cl-, order to remove impurities. OH-) Blueing A treatment of ferrous alloys by Anode (Corrosion) The electrode at which action of air, steam or molten salts, to oxidation or corrosion takes places. form a thin, blue oxide film on the (Cathodic protection) The electrode surface. which applies cathodic protection to a Bonderizing A process for treating steel with structure. (Electroplating) Part to be phosphate. plated. Brightener Additive that results in a bright Anodic inhibitor A chemical substance that electroplated finish, or improves the reduces the rate of the anodic reaction. brightness of such a deposit. Anodic polarisation Change of the potential Brush Plating Electroplating in which the of the anode in the noble (positive) anode is in the form of a brush or a pad. direction due to current flow at or near Buttering One or more layers of deposited the anode surface. weld metal on a surface. Anodic protection A technique to reduce Calomel electrode See Saturated calomel corrosion by polarising a metal into its electrode. passive region. Calorizing Impregnation of a steel surface Anodising Forming an oxide film on a metal with aluminium. surface by making it the anode in an Cathode In a corrosion cell, the electrode electrolytic bath. where reduction, and no corrosion, Anolyte Electrolyte adjacent to anode. takes place. Anti-fouling Prevention of marine organism Cathodic disbondment Destruction of adhe- attachment or growth on a submerged sion between a coating and a substrate structure, usually by a toxic chemical in because of cathodic reaction products. the metal or coating.

16 www.corrosion.com.au ACA 11 – Corrosion Technology – Metalic and Inorganic Coatings © The Australasian Corrosion Association Inc 2013 Cathodic inhibitor A chemical substance that used for electrodeposits and anodic reduces the rate of the cathodic films on aluminium. reaction. Copper-copper sulphate electrode A refer- Cathodic polarisation Change of potential ence electrode made from a copper rod (more negative) of the cathode resulting in a saturated solution of copper from current flow at or near the cathode sulphate. Often used for determining surface. potentials in soil. Cathodic protection Reduction or elimination Corrodkote test An accelerated corrosion of corrosion by making the metal test for electrodeposits. structure a cathode by means of Corrosion Destruction of a material (usually a impressed current or attachment of a metal) because of its reaction with the sacrificial anode. environment. Catholyte Electrolyte adjacent to cathode. Corrosion-erosion See Erosion corrosion. Cation A positively charged ion (e.g. H+ or Corrosion fatigue Fracture of a metal Fe++) which migrates towards the because of the combined action of cathode. corrosion and cyclic stressing. Caustic embrittlement or cracking Cracking Corrosion potential The potential that a of a metal (usually steel) as a result of corroding metal exhibits relative to a the combined action of tensile stress reference electrode under given condi- and an alkaline environment. An tions. Also called the rest potential, obsolete term denoting a form of stress open-circuit potential or freely corroding corrosion cracking. potential. Cavitation corrosion (or damage) Deterio- Corrosion rate The speed at which corrosion ration of a surface by pressure progresses. Often expressed in terms of differences arising from sudden an average rate such a millimetres per formation and collapse of bubbles in a year liquid. Cracking Fracture in a brittle manner along a Cell A circuit consisting of an anode and cath- single or branched path. ode in an electrolyte. Crevice corrosion Localised corrosion Cementite A compound of iron and carbon because of the formation of a concen- found in steels. tration cell in a crevice. Cementation See Deposition corrosion. Current density Current per unit area, expressed as amps per square metre, Chromizing Impregnation of the surface of etc. steel with chromium. Deaeration Removal of air and other gases Cladding A process for covering one metal from an environment, usually water. with another. Dealloying The selective removal of a metal- Cold end Corrosion See Dew point lic constituent from an alloy. Examples corrosion. are dealuminification (removal of Concentration cell A cell formed from two aluminium), denickelification (removal identical electrodes where the potential of nickel), dezincification (removal of difference arises by differences in zinc). electrolyte composition at each of the Decarburization Partial or complete loss of electrodes. carbon from the surface layers of steel. Concentration polarisation Polarisation of Dehumidification Reducing the amount of an electrode due to changes in concen- water vapour in a given space. tration in the electrolyte near the metal surface. Delamination Splitting into layers or leaves. Conversion coating A surface coating Demetallification see Dealloying. produced by chemical or electro- Demineralisation Removal of dissolved chemical treatment of the metal. mineral matter, usually from water. Examples are chromate, phosphate and Deoxidation Removal of oxygen from molten oxide coatings. metal. Copper-accelerated salt spray test (CASS Depolarisation Elimination or reduction of test) An accelerated corrosion test polarisation, usually by mechanical or

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chemical means, resulting in increased Electrodeposition See Electroplating. corrosion. Electroless plating Deposition of a metal Deposit attack (or deposit corrosion) Pitting coating by immersion in a bath corrosion resulting from deposits on a containing reducing agents. metal surface which cause Electrolysis The use of an electrical current concentration cells. to cause chemical changes in an Deposition corrosion Form of bimetallic electrolyte. The term is commonly corrosion in which a corroding upstream misused to describe stray current metal produces ions which deposit on a corrosion or, occasionally, bimetallic more active downstream metal causing corrosion. corrosion. Electrolyte A liquid, usually an aqueous Dew point The temperature at which water or solution, in which ions conduct electrical other liquid vapours present in the air current. begins to condense. Electrolytic corrosion See Stray current Dew-point corrosion Attack in the low corrosion. temperature section of combustion Electroplating The deposition of a substance equipment due to condensation of acidic by passing electric current through an flue gas vapour. electrolyte. Dezincification See dealloying. EMF (Electromotive Force) Series See Differential aeration cell An oxygen concen- Electrochemical series. tration cell resulting from different Embrittlement Severe loss of ductility of a amounts of oxygen at different sites on metal or alloy. the metal. Environment The surroundings or conditions Diffusion coating A coating produced by in which a material exists. heating a metal or alloy in a suitable Erosion Deterioration of a surface by environment causing diffusion of a abrasive action of fluids. coating into the base metal. Erosion-corrosion Deterioration of a surface Dislocation Linear imperfection in an array of by combined action of erosion and atoms. corrosion. Dissimilar metal corrosion see Bimetallic Evans diagram A diagram showing electrode corrosion. potential - current density relationship, Drainage bond Metallic conductor used to used to explain polarisation behaviour. conduct electrical current from an Exchange current The rate at which positive underground structure. or negative charges enter or leave the Duplex stainless steel Stainless steel with a surface when the rate of anodic disso- metallurgical structure of austenite and lution equals the rate of the cathodic ferrite. reaction. Electrical current Flow of electrons in a wire Exfoliation Corrosion that proceeds along or ions in an electrolyte. Conventional planes parallel to the surface giving rise current is the direction that positive to a layered appearance. charges would flow (opposite to the flow Fatigue Fracture of a metal due to repeated of electrons). stress cycles well below its normal Electrochemical Series A listing of elements tensile strength. according to their Standard Electrode Ferritic The name given to a specific atomic Potentials. arrangement (body centred cubic) in Electrochemical Impedance Spectroscopy many iron-based alloys. Steels have this (EIS) Electrochemical test based on the structure at room temperature. response of a corroding electrode to Filiform corrosion Random small threads of small amplitude alternating potential or corrosion that develop beneath thin current signals at various frequencies. lacquers and similar films. Electrode A metal in contact with an Film A surface layer usually providing electrolyte where electrical current can protection. Often invisible. enter the metal or leave the metal. Flaking See Hydrogen cracking. Electrode potential The potential of an electrode when measured against a reference electrode.

18 www.corrosion.com.au ACA 11 – Corrosion Technology – Metalic and Inorganic Coatings © The Australasian Corrosion Association Inc 2013 Fouling Accumulation of deposits, especially structure which affect mechanical, and in reference to heat exchanger tubing often corrosion, behaviour. and growth of marine organisms. Holiday A hole or gap in a protective coating. Fretting Deterioration of a material by Hot corrosion Accelerated high temperature repetitive rubbing between two surfaces. corrosion resulting from combined effect The term fretting corrosion is used of reaction with sulphur compounds, when deterioration is increased by metal chlorides, etc, to form a molten salt corrosion. which destroys the oxide film. Galvanic cell A cell consisting of two different Hydrogen attack Reaction of hydrogen at metals in contact in an electrolyte. high temperatures with carbides or Galvanic corrosion Common term for oxides within the metal to form methane Bimetallic corrosion. or steam, usually causing cracking. Galvanic Series A list of metals arranged Hydrogen blistering Formation of blister-like according to their relative corrosion bulges on a ductile metal caused by potentials in a given environment; hydrogen gas at high pressures. seawater is often used. Hydrogen cracking or hydrogen induced Galvanize To coat a metal surface with zinc, cracking (HIC) Internal cracks in a usually by immersion in a bath of molten metal caused by build-up of hydrogen zinc. gas. Galvanostatic A constant current technique Hydrogen damage General term for various of applying current to a specimen in an types of damage to a metal caused by electrolyte. the presence of hydrogen within the General corrosion Often called Uniform metal structure. corrosion. Corrosion proceeding over Hydrogen embrittlement Loss of ductility of the whole metal surface. a metal due to the presence of atomic Grain An individual crystal of metal (usually hydrogen within the metal structure. The microscopic) in which atoms are term is often used to include HIC. arranged in an orderly manner. Hydrophilic Having an affinity for water. Grain boundaries The junctions between Hydrophobic Repelling water. adjacent grains. Hygroscopic Having a tendency to absorb Graphitization A specific form of dealloying moisture. of cast iron in which the metallic constit- Immunity A state of resistance to corrosion uents are corroded leaving the graphite due to the fact that the electrode flakes intact. Also called graphitic potential of the surface is below the corrosion. potential required for anodic dissolution. Green rot A form of high temperature corro- Impingement attack Localised erosion- sion of chromium-bearing alloys in corrosion caused by high velocity which green chromium oxide forms. flowing fluid. Grooving corrosion Localised corrosion in Impressed current cathodic protection the form of a groove in the weld of (ICCP) A cathodic protection system electric resistance welded carbon steel utilising an external source of power. pipe exposed to aggressive waters, Inclusions See Non-metallic inclusions. caused by redistribution of sulphide inclusions during welding. Inhibitor A substance which, when added to a corrosive liquid in small amounts, Half cell A pure metal in contact with a reduces to the corrosion rate. solution of its own ions of known concentration. At a given temperature it Instant Off Potential Potential reading taken develops a characteristic and repro- immediately after switching off the ducible potential. The term is sometimes cathodic protection current. Removes used to mean reference electrode. errors due to current flows. Hardcoat anodise Thicker, harder anodised Intergranular cracking Cracking or fracture coating for wear and corrosion which occurs preferentially at grain resistance. boundaries. Heat affected zone (HAZ) The area adjacent to a weld where the heating and cooling have caused changes to the metal

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Ion An electrically charged atom (e.g. Na +, Nernst equation An equation that expresses potential of a cell in terms of the Cl-, Al3+) or group of atoms (e.g. NH +, 4 activities of the products and reactants - 2- OH , SO4 ). of the cell. IR Drop Potential difference between two Noble metal A metal such as gold, silver or electrodes due to product of the platinum which is not very reactive. resistance and current flow. In cathodic Strongly cathodic in the Galvanic series protection, compensated by or Electrochemical series. measurement of Instant Off Potential. Non-metallic inclusions Impurities such as Knife line attack A form of weld decay where sulphides and silicates distributed as the zone of attack is very deep and small discrete particles throughout a narrow, close to or in the weld. solid metal matrix. Lamellar Material arranged in thin plates. Open circuit potential The measured potential of a cell when no current flows. Langelier index A calculated figure that is useful in predicting scaling or corrosion Overpotential See Overvoltage. behaviour of water containing calcium Overprotection In cathodic protection, carbonate. generation of higher protective current Layer corrosion see Exfoliation. than necessary. Liquid metal embrittlement (LME) Cracking Overvoltage The difference in electrode of a normally ductile metal in tension potential when a current is flowing caused by contact with a liquid metal. compared to when no current flows. Also known as polarisation. Local cell A small cell formed by small differences in composition in the metal Oxidation Loss of electrons, as when a metal or electrolyte. corrodes. Opposite of Reduction. Localised corrosion Corrosion at discrete Oxidising agent Substance causing oxidation sites, e.g. crevice corrosion, pitting, by accepting electrons. Substance is stress corrosion cracking. reduced during oxidation. Low temperature corrosion See Dew-point Oxygen concentration cell A cell caused by corrosion. a difference in oxygen concentration at two points on a metal surface. Also Magnetite Naturally occurring black oxide of called a differential aeration cell. iron, Fe O . 3 4 Parkerizing A proprietary phosphating Martensitic The name given to a specific treatment. atomic structure obtained most Parting Obsolete term for dealloying. commonly in ferrous alloys by quenching to form a very hard material. Passivation Decrease in corrosion rate by the formation of a protective oxide or Mechanical plating Producing a metallic similar film on the surface. coating by tumbling the item with the metal powder, glass beads and Passivator An chemical agent which forms a appropriate chemicals. passive film on the metal surface. Metal dusting A form of high-temperature Passive State of a metal surface where the corrosion which forms a dust-like corrosion rate is low due to formation of corrosion product. a protective film through passivation. Metallizing See Thermal Spraying. Passivity The phenomenon of an active metal becoming passive. Mild steel Carbon steel containing a maximum of about 0.25% carbon and Patina A green coating of copper oxide and no other significant additions of alloying other copper salts formed by exposure elements. of copper and some copper alloys to the atmosphere. Mill scale The heavy oxide layer formed on steel as a result of hot working or heat Peen plating see Mechanical plating. treatment. pH A measure of the acidity or alkalinity of a Mixed potential A potential resulting from two solution. A value of seven is neutral, a or more electrochemical reactions value less than seven is acid, more than occurring simultaneously on one metal seven is alkaline. In chemical terms, pH surface. is the negative logarithm of the hydrogen ion concentration.

20 www.corrosion.com.au ACA 11 – Corrosion Technology – Metalic and Inorganic Coatings © The Australasian Corrosion Association Inc 2013 Pickle A solution, usually acid, used to Reference electrode An electrode with a remove mill scale or corrosion products known, stable and highly reproducible from a metal. potential and used as a reference in Pitting Highly localised corrosion resulting in measurement of electrode potentials. cavities. Relative humidity The ratio of the amount of Pitting factor Ratio of the depth of the moisture in the air compared to what it deepest pit to the average penetration could hold if saturated at the temper- as calculated from weight loss. ature involved. Polarisation The shift in potential resulting Residual stress Stresses that remain in a from the effects of current flow. body as a result of metal working Generally taken to mean retardation of processes. corrosion due to build-up of corrosion Rest potential See Open circuit potential. products or consumption of oxygen or Rust A reddish-brown product, primarily water or both at the metal surface. hydrated ferric oxide, formed as a result Porcelain enamel See vitreous enamel. of corrosion of steel. Post Weld Heat Treatment (PWHT) Heating Sacrificial protection Reduction of corrosion of weld regions immediately after of a metal by galvanically coupling in to welding to prevent formation of a hard or a more anodic metal. A form of cathodic brittle structure. protection. Potential-pH diagram See Pourbaix diagram Salt spray (or salt fog) test An accelerated Potentiostat An electronic device which corrosion test in which specimens are maintains an electrode at a constant exposed to a fine mist of a sodium potential with respect to a suitable chloride solution. reference electrode. Saturated calomel electrode A reference Poultice corrosion Corrosion due to electrode consisting of mercury, collection of dirt and other debris in mercurous chloride (calomel) and a crevices and ledges that are kept moist saturated chloride solution. Usually used by weather and washing. A specific type in the laboratory rather than field work. of deposit attack. Scaling (1) High temperature corrosion Pourbaix diagram A plot of potential versus resulting in the formation of thick layers pH of a corroding system compiled of corrosion products on the metal using thermodynamic data and the surface. (2) Deposition of insoluble Nernst equation. The diagram shows materials such as calcium carbonate on the regions in which the metal is active, the walls of boilers or heat exchanger passive or corroding. Also known as a tubes potential-pH diagram. Sealing (Anodising) A process for closing the Primer The first coat of paint applied to inhibit pores of an anodised film. corrosion or improve adherence of the Season cracking Obsolete term describing next coat. the stress corrosion cracking of brass. Profile The anchor pattern produced on a Selective corrosion (or leaching) See metal surface by abrasive blasting. dealloying. Recrystallisation Formation of new grains in Sensitization Precipitation of chromium a cold worked metal, usually accom- carbides in grain boundaries in plished by heating. austenitic stainless steels at tempera- Redox potential The potential of a reversible tures of 550 to 850oC leaving grain reduction-oxidation electrode measured boundaries depleted in chromium with respect to the standard hydrogen making them susceptible to corrosion. electrode in a given electrolyte. Sheradizing Diffusion of a zinc coating into Reducing agent Substance causing steel by tumbling steel parts with zinc reduction by donating electrons. dust at high temperatures. Substance is oxidised during reduction. Silver-silver chloride electrode A reference See also oxidising agent. electrode consisting of silver chloride Reduction Gain of electrons as when a metal plated on silver. Useful for potential plates out from an electrolyte. Opposite measurements in sea water. of oxidation. Slushing compound An obsolete term describing an oil or grease coating

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applied to provide temporary corrosion Tarnish Surface discolouration of a metal protection. caused by a thin film of corrosion Spalling Spontaneous separation of a surface product. or surface coating. Temper In heat treatment, to reheat a Stabilised stainless steel A grade of hardened metal to decrease the austenitic stainless steel which has hardness slightly but greatly improve the been alloyed with a carbide-forming toughness. element such as titanium which makes it Terne An alloy of lead containing 3 to 15 per less susceptible to sensitization. cent tin. Standard electrode potential The reversible Thermal Spraying A process of coating a potential of an electrode process when surface by spraying finely divided all the reactants and products are at unit particles of melted or heated material. activity on a scale on which the potential Thermogalvanic corrosion Corrosion of the standard hydrogen electrode is resulting from the formation of a cell due unity. to temperature differences at two points. Standard hydrogen electrode (SHE) A Throwing power In electroplating, the ability reference electrode consisting of a of a plating solution to produce a platinum strip exposed to hydrogen gas uniform metal distribution over an at a pressure of 1 atmosphere irregularly-shaped cathode. immersed in an acid solution of unit Transgranular cracking Cracking or fracture molarity. It has been assigned by through or across a metal grain. convention the value of 0 volts in the Compare to intergranular cracking. Electrochemical series. Transpassive The noble region of potential Strain age embrittlement A loss in ductility where an electrode exhibits a higher when a low carbon steel is subjected to than passive current density. aging after plastic deformation. Trilaminate Three layer pipeline coating Stray current corrosion Corrosion that is consisting of fusion bonded epoxy, an caused by stray DC currents from some adhesive and extruded polyethylene or external source. polypropylene outer layer. Also called Stress corrosion cracking (SCC) Cracking Three layer. resulting from the simultaneous action of Tuberculation Localised corrosion at a corrodent and sustained tensile stress. scattered locations on the surface in the Stress raiser A change in the contour of a form of knob-like mounds known as structure, or the presence of a discon- tubercules. tinuity, that causes a local increase in Underfilm corrosion Corrosion which occurs stress. under organic coatings at exposed Sulphidation Oxidation by sulphur. edges or by filiform corrosion. Sulphide stress cracking Cracking resulting Uniform corrosion Often called general from the combined action of tensile corrosion. stress and corrosion by hydrogen Vitreous enamel Thin layer of glass fused sulphide. onto a metal surface at a high Surfactant or surface active agent A temperature (Porcelain enamel in the substance introduced into a liquid to US) improve wetting properties. Many Voids A term generally applying to paints to detergents are surfactants. describe holidays, holes and skips in the Tafel line, Tafel slope, Tafel diagram When film. an electrode is polarised, it frequently Waterline attack Attack of metals partially yields a current-potential relationship immersed in water because of presence where the change in potential is directly of a differential aeration cell. proportional to the logarithm of the current density. If such behaviour is Wash primer A thin, inhibiting primer to observed, the line is known as the Tafel improve surface adhesion of the line, the slope of the line is the Tafel subsequent coat. slope, and the overall diagram is termed Water Jetting Surface cleaning using very a Tafel diagram. high pressure water only directed through a nozzle onto a surface.

22 www.corrosion.com.au ACA 11 – Corrosion Technology – Metalic and Inorganic Coatings © The Australasian Corrosion Association Inc 2013 Weld decay Intergranular corrosion, usually White rust The white powdery corrosion of stainless steels, as a result of product on zinc or zinc-coated surfaces. sensitization in the heat affected zone Working electrode The test or specimen during the welding operation. electrode in an electrochemical cell.

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