In the name of ALLAH, The most Gracious, most Compassionate

1. In the name of ALLAH, The most Gracious, most Compassionate

2. CORROSION CONTROL BY METALLIC COATINGS,a review By Engr.Dr.KHURSHEED MAHMOOD, Professor, NED University of Engineering & Technology, KARACHI

3. ABSTRACT Today, coatings of many different types are applied to metal surface, mostly to separate the environment from the metal, but often to control the micro-environment of the metal surface. Coatings can be organic or inorganic, metallic or non-metallic; the range of the possibilities is vast. The present paper specifically discusses the corrosion protection of metallic structures or equipment and explains the salient features of such a protection system.

4. INTRODUCTION • Environmental resistance and service life of metallic structures/equipment are improved by applying some suitable coatings. • The chief function of coatings is to provide an effective barrier between the substrate metal and the environment. • Factors for the selection of a suitable coating system: (i) service conditions (ii) structural material & (iii) the protection system itself.

5. Strong bonding of the coating to the substrate is one of the most important requirements for the successful working of a good protection system.This bonding depends upon the diffusion of the coating material into the substrate metal.

6. Some times inter diffusion of the base metal elements into the coating during and after deposition can lower the coating’s environmental resistance. • In many of the potentially interesting coating-substrate • system the transition from the substrate to the coating • involves abrupt changes in such properties as hardness, • coefficient of thermal expansion, and thermal conductivity • Stresses are inevitably produced at the coating-substrate • interface due to external effects such as loading and • temperature changes. • The concentration of these permanent and transient • stresses leads to cracking and spalling off of the coating • from the substrate as shown in figures 1 and 2.

8. More common methods of reducing stresses in a coating • protection system may be as follows: • Matching the properties of coating and the substrate. • Forming intermediate layer to reduce large gradient • of properties b/w coating & substrate. • Controlling the structure of coating • Increasing the radius of curvature of the coated surface. The last two methods, as mentioned above are too restrictive, since relatively thick coatings are required in many applications and the shape of the component to be coated is often dictated by design and engineering considerations.

9. Consideration for optimum selection of coating Key aspectsConsiderations Project requirementsNature of structure. Life time required. Time available for application. Primer applied in factory or on site. Access for maintenance. Special requirements, e.g contact with food. Exposure conditions Climate Cathodic protection Contact with chemicals Impact and abrasion Surface temperature. Surface preparation Technique and staff available Contamination removal Ability to achieve required grade Acess available Environmental aspects Application and safety Techniques and staff available Expected climate conditions Time and number of coats required Environment and safety Economics Available budget Can economics be made?

10. Keeping economics aside the choice of coating is dictated by environment, base metal and coating itself represented as below: Application Substrate Mechanical properties Physical properties Thermal properties Chemistry Phases-microstructure Environmental resistance Cost Service environment Desired life Component geometryt Protection system Environmental resistance Mechanical properties Physical propertie Thermal propertis Chemistry Phases-microstructure Application processes Cost

11. The discussion to follow shall be specifically focused about metallic coatings. The Metallic Coatings: Many of the commonplace objects arround us are finished with metallic coatings to preserve and give metallic luster to the basic substrate metal, which provides strength, rigidity and formability. e.g. dust bins are galvanized,cars are coated with tin, and bright parts of cars are chrome plated.

12. IDEAL PROPERTIES OF METALLIC COATINGS • A metal coating should resist from the environment better than the substrate metal. • The selected coating should not cause corrosion of the underlying metal at any flaws or deliberately introduced breaks in the coating. • A protection system with any type of coating is not cent per cent perfect. Also coating may develop micro cracks during service giving chances to environmental micro species to enter and corrode the base metal.

13. Mechanical properties of a metallic coating such as elasticity or hardness or strength should be adequate to meet the operational requirements of the structure. • The stresses induced in the coating may cause the coating to crack in a direction perpendicular to the direction of tensile loading as indicated in figure 3. • Therefore, coating should be strained in harmony with the underlying metal due to externally applied loading. • Also matching values of thermal expansion coefficient of both the coating and the substrate metal may preclude the chances of cracking under elevated temperature services.

14. The method of application of a metallic coating must be compatible with the fabrication processes used to produce the completed product. • Some fabrication processes may cause cracking and detachment of applied coating as shown in figure. • SURFACE PREPARATION • A surface preparation is a prerequisite to all coating processes to remove all surface contamination, grease oil, dirt & process debris; to remove surface corrosion products and to control physical characteristics of the surface.

15. As a simplest method, dip the component in solvet like acetone, trichloroethylene, tetrachloromethane or benzine at room temperature. • Use of hot solvent or solvent vapor is more efficient. • Many of the solvents are toxic and carcinogenic and most are highly inflamable. • Alkaline solutions are also used in immersion baths followed by thorough was of the component. The effectiveness of the sokvent and alkaline bath can be improved by ultrasonic agitation during the cleaning process. • Corrosion products are removed by acid or alkaline solutions.

16. Wire brushing and grit blasting is also used to clean the steel surfaces. Some of the types of wire brushes are shown in figure 4. • Spray coating ,as compared to other methods of coating, adheres only to roughened surfaces. A uniform roughness can be achieved by grit blasting with a grit size appropriate to the substrate metal. • On steel surfaces the spray coat is put after blasting to prevent re-oxidation of the metal.

18. METHODS OF METALLIC COATINGS. • Figure 5 illustrates some of the more common methods of applying metallic coatings to substrate metals. They are briefly discussed here. • (i) Electroplating:- The part to be coated is immersed in an electrolyte • containing salt of the plating metal along with a rod or plate • of the plating metal. • A certain required amount of potential is applied so that the Component to be plated becomes the cathode and the rod or Plate becomes the anode. • Using correctly formulated solutions and anode, alloys as • well as pure metals can be plated.

19. Good practice produces coatings of controlled thickness, a fine grain size and relative freedom from porosity. • Throwing power describes the ability of a plating solution to produce an even thickness as the distance between anode and component surface (cathode) varies when plating complicated shapes. • Chromium has poor throwing power and requires complicated anode arrays to give an even plating thickness especially on curved and convoluted durfaces. • Hydrogen embrittlement can occur during the plating process and post-coating heat treatment may be specified to diffuse the hydrogen and prevent cracking of the substrate.

20. (ii) Hot dipping:- • The component to be coated is dipped into a bath of molten coating metal. • A good metallurgical bond is formed with the substrate owing to interfacial alloying. • There is less control over the coating thickness in the dipping process; the coat tends to be thicker on lower surfaces and thinner on the top. • However, all surfaces exposed to the molten metal are coated. • The process is limited to low melting point metals such as zinc and aluminium.

21. (iii) Spray coat :-- • This method uses wires of coating material which are fed fed into a torch where they are melted and blown out under pressure as fine droplets. • The droplets traveling @ 100-150 m/s are flattened on striking the substrate and adhere to it. • For a given thickness the coat will be more porous than dipped or electroplated coats, but thick layers are built up by repeated spraying. • However, it is labor-intensive and requires grit blasting before spraying. The coat is anodic to the substrate, Aluminium and zinc are applied to steel by spray coating.

22. (iv) Clad coating:-- STAINLESS STEEL • They are corrosion resistant metal skins which can be • laminated with other metals. • Skins have been applied by rolling, by explosive welding • and by building up welded coat on the substrate, a process • known as buttering. • Normally diffusion across the interface between the metals • produces an alloy-bonded layer which having good • adhesion. • Rolling should apply sufficient pressure to break up the oxide. • Explosive welding scours the two metal surfaces at the moment they • are forced together. • Cuts. Holes or other breaks require additional protection. • The skin should cope with onward fabrication process to make final • produci. COPPER CLADDING

23. (iv) Diffusion coating :-- • A number of prcesses exist for both metals and non-metals to form an an alloy layer on a component. • A very good bond is present but the process is limited to relatively small objects. • The components after degreasing and cleaning are heated either in contact with powdered mcoating metal in an inert atmosphere (the solid route) or in a gaseous stream of volatile compound of the coating metal (the gaseous route). Figure 6 shows schematic of a gaseous process. • In the solid route the components and the metal powder mixed with the sand and sealed in a drum. They are then heated below the melting point of the coating metal and rotated for several hours. • This technique is used to coat steel with zinc,shreding, or with aluminium, calorizing. • Gaseous phase uses halides to coat steel with Chromium,chromizing or to produce silicon coating.

25. Behaviour of metalliccoatings:-- • Factors influencing behaviour of metallic coatings: • The nature of electrolyte • Oxygen concentrations • Polarization characteristics • relative areas of anode and cathode • Surface deposits on the coatings • If an anodic (sacrificial) coating is used, conductivity and • continuity of the electrolyte will control the size of the • surface defect that can be tolerated before corrosion occurs. • For a zinc coating on steel, defect width as small as 3mm will • produce corrosion in distilled water or soft water, whereas • in sea water the steel will be protected at a defect several • decimeters across (but the rate of loss of zinc will be much • higher.

26. Commonly applied Coating Metals: :-- ZINC: • The e.m.f series indicates that zinc is anodic than iron. • Zinc corrosion products, such as oxide, hydroxide and basic • carbonates form a protective film on the metal surfaces. • A zinc coating has a long life but provides sacrificial protection • at any breaks In the coating. • Corrosion resistance of coat is independent of the method of • application. Coating containing 5.8% Fe are less prone to pitting • attack than pure iron. • The iron content of the zinc coating can be improved by diffusion • through post-coatHeat treatment. • Useful life of a coating depends upon its thickness and the • Environment to which it is exposed.

27. In general, a coating thickness of0.03 mm exposed to atmosphere will last 11-12 year in rural areas or 8 years In marine location, while sulpur oxide pollution in industrial areas will reduce the life to 4 years. Figure 7 shows data regarding the life to first maintenance for specified minimum thickness of zinc coating in different environments. • Cadmium :-- Like zinc Cadmium is cathodic to iron promoting its dissolution in a corrosion cell. Unlike zinc, the nature of the film which develops on the surface of cadmium is strongly dependent upon the atmospheric conditions.

29. In rural or industrial locations soluble salts form part of the film; rain water washes them away and corrosion of cadmium remains high. • Insoluble carbonates and chlorides form in marine areas and the corrosion rate is much lower. • For a given steel thickness, cadmium offers more protection than zinc in marine environments, but situation is reversed in rural and industrial sites. • Cadmiun is normally applied in thin coats by electroplating. • It has a bright finish, but is costlier than zinc and is normally used to coat small articles such as fasteners.

30. Cadmium is preferred to zinc as a coating by electronics industry because it iseasier to solder. • Cadmiun is also suitable to coat high strength steels owing to the lower risk of hydrognn embrittlement frpm the plating bath. • Unfortunately, cadmium salts are highly toxic and its use can form a significant pollution in some cases. ALUMINIUM :- • Coatings of aluminium are normally produced by spraying or hot dipping, although some small articles are diffusion coated.

31. Silicon is added to hot dipsretard the formation of inter- metallics at the interface between coating and the substrate.Figure 8 shows the brittleness of hard inter- metallic layer developed in an aluminized coating under tensile strains. • Sprayed coating contain higher level of oxide then dipped coatings and they are more porous. • However,corrosion products rapidly fills the micro-holes in the coating and form an adherent and impervious coating. • Some corrosion of of the substrate may occur during sealing process giving unsightly rust stains on the outer surface of the coat.

32. This can be avoided by sealing the coat with a lacquer during the early stages of its exposure, although rusting is not harmful. • Both dipped and diffusion coatings of aluminium are used to protect steel exposed to temperatures upto 1100oC. • The coating also offers good protection from attack by sulphur compounds and are used in chemical industry, on some gas turbine blades and for car exausts.

33. Nickel and Chromium:- • Both of these metals are cathodic tom steel and act as barrier coating to protect the substrate. • Figure 9 shows two of the many different types of coating arrangements. • The addition of sulphur compound to nickel produces a layer with a bright finish which is anodic to pure nickel. • During prolonged exposure out doors, nickel coatings tend to lose their surface brighness. • Aplication of thin chromium coating retains brightness which is the traditional chromium plating used on articles.

35. Chromium coatings are highly stressed and the coatimg does contain micro-cracks and porosity. • When water enters micro-cells are generated. • The chromium is cathodic in the micro-cells; it has a large surface area compared to the anode (the nickel plate at the base of the crack) and therefore, corrosion rate of nickel is very high. • The normal cracking of chromium coating is enhanced by the sulphur compounds in the nickel coat. • The bright nickel is anodic to both the pure nickel and the chromium; therefore corrosion proceeds sacrificially in this layer as indicated in figure 9c.

36. However the larger anode/cathode ratio produced by the increased cracking density reduces the tendency for pitting to occur. • Gradually flakes of the over lying chromium drop off, dulling the surface. TIN :- • Large quantities of tin-plate are used for cans in the food industry. • The coating is cathodic to steel on the out side of the container and promotes corrosion if damaged. • However, cell reversal occurs with many organic acids such as citric acid found in fruit juices and sacrificial protection occurs on the inside of the can.

37. CONCLUSIONS: The working of a corrosion protection system usimg any type of coating depends highly on the surface preparations of the substrate material. Selection of coating type from amongst the organic, inorganic, metallic or non-metallic should be on the consideration of the working environment. Keeping economics aside, the choice of a particular coating system depends upon the type of metal and the method of applying coatings on its surface. Metallic coatings in many respects are superior to many other types of coating systems Micro-cracks in metallic coatings may assist in generating micro-cells to fail the system through galvanic corrosion.

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