Protective Coat Surface Preparations

Unit-IV PROTECTIVE COATINGS AND THEIR APPLICATIONS SURFACE PREPARATIONS: SOLVENT CLEANING: Solvent cleaning is used to remove oils, greases, buffing compounds and fatty substances. This involves in cleaning the surfaces by the application of organic solvents like naphtha, chlorinated hydrocarbons (CCl4), toluene, xylene or acetone. This is followed by cleaning with steam and hot water, containing wetting agents and alkalis. This treatment provides a metal surface, readily wetted by aqueous solution, which is particularly required for electroplating. ALKALI CLEANING: Alkali cleaning is particularly well-adapted for the removal of old paint coating from metal surfaces. Alkali cleaning agents are trisodium phosphate along with soaps and wetting agents like caustic soda. An alkali treatment is always to be followed by a very thorough rinsing with water and then immersion in a slightly acidic solution of 0.1% chromic acid or sodium or potassium chromate to remove the last traces of alkalis. Alkali cleaning method can be made more effective by the application of an electric current, and making the “metal cathodic” in alkaline medium. The copious evolution of hydrogen at the cathode metal results in strong agitation, which helps to dislodge the oily substances. SAND-BLASTING: Sand blasting is used or removing oxide scales, particularly when a slightly roughened surface is desired. Paint coatings on sand-blasted surface are, particularly, more durable than those on metal surfaces cleaned by other methods. Sand-blasting is especially suitable for large steel surfaces and should always be used, whenever maximum protection from the coating is required. However, the proves required expensive equipment and the sand-blasting operation is dangerous to health of workers, because of the possibility of disease, silicosis, unless special protective measures are taken. The process consists in introducing the sand an abrasive into an air stream, under pressure of 25 to 100 atmospheres. The blast is impacted on the metal surface to be cleaned. The sand-blast impact removes any scale present and also causes a certain degree of hardening of the cleaned metal surface MECHANICAL CLEANING: Mechanical cleaning removes loose rust and other impurities from the surface. This is, generally, done by hand cleaning with a bristle brush plus some abrasive like sand and detergent like soap. Impact tools like dull chisels, knife scrapers, wire brushes, grinding wheels and cutters are also used for removing strongly adhering scales, etc. the remaining dust and loose particles of dirt are then removed by solvent cleaning, followed by steam or hot water treatment. PICKING AND ETCHING: Acid picking is more convenient method of scale removal in many cases than mechanical cleaning and sand-blasting. It is, usually, accomplished by immersing the metal except Al in an acid-pickling solution. Al is picked in alkaline solution. Acid-pickling of steel is accomplished by dipping in warm dil.H2SO4 or in cold HCl solution to which some “inhibitor” has been added. For cleaning of articles made of copper, brass or nickel, the pickling bath consists of dilute HNO3 or a mixture of dil Nitric acid and dil. Sulphuric acid. If the cleaning operation is carried out in an efficient manner, it provides a clean, smooth surface for electro-deposition. Moreover, the deposit obtained is adherent, tough, and bright in appearance. 1

Generally, there are two types of metallic coatings are protection from corrosion. 1. Anodic coatings 2. Cathodic coatings And some other coatings like 3. Chemical conversion coatings 4. Organic coatings 1. ANODIC COATINGS: Anodic coatings are those in which, the metal used for is more anodic than the metal which is to be protected. For example, coating of Aluminum, Cadmium and Zinc on steel surface are anodic, because their electrode potentials are lower than that of the base metal iron. Therefore sacrificially, anodic coatings protect the underlying base metal, In case of galvanized steel. Zn is used as coating metal, while steel is base metal. Zinc being anodic is attacked, leaving the underlying cathodic metal like iron or steel unattacked. Even if it is exposed when pores, breaks or discontinuities occur in such type of coating. Zinc dissolves anodically, since being anodic to iron while the iron is protected, since being cathodic as shown in the figure. Hence, an anodic coating when properly prepared give high electrical insulation to the base metal and imparts good corrosion resistance and resistance to abrasion and stain.

Galvanization: Galvanization is a process in which the iron article is protected from corrosion by coating it with a thin layer of zinc. In this process, at first iron or steel is cleaned by pickling with dilute sulphuric acid solution (H2SO4) at a temperature range of 60-900C for 15 to20 minutes. Therefore, it removes scale, rust and other impurities present if any and then washed well and dried. Then after, dipped in the bath molten zinc which is at 425-4500C. To prevent it from oxide formation, the surface of bath is covered with a flux (NH4Cl). When the iron piece is taken out it is coated with a thin layer of zinc. And to remove excess zinc, it is passed through a pair of hot rollers; lastly, it is annealed at a temperature of 4500C and then cooled slowly. For the protection of iron and steel products, it is most widely used in the form of wires, pipes, nails, bolts, screws, buckets, tubes, roofing sheets etc., CATHODIC COATINGS: TINNING: The process of coating tin over the iron or steel articles to protect it from corrosion is known as tinning. Tin is a nobler metal than iron, therefore, it is more resistance to chemical attack. In this process, at first Iron sheet is treated in dilute sulphuric acid (pickling) to remove any oxide film if present. A cleaned Iron sheet is passed through a bath molten flux. Like Zinc chloride, then through molten tin and finally through a suitable vegetable oil.

METAL CLADING: The surface to be protected is sandwiched between two thin layers of coat metal and pressed between rollers. The finished product may be welded at the edges or riveted at some points. The coat metal has to be anodic to the base metal and only plain surfaces can be cladded. This method is used for coating Al, Cr, Ni, Duraluminium, etc.

ELECTOPLATING; Electroplating is the process or method of coating metals and non-metals, to change their surface properties such as to improve the appearance, to properties such as to improve the appearance to corrosion and wear or chemical attack. Electroplating is the electro-deposition of metal, by means electrolysis over surface of metals, alloys or non-metals. 2

The wear resistance of a metal part can be improved by electroplating a harder metal on its surface. The metals most often plated on base metals or materials are chromium, Nickel and Rhodium. For example metals like Iron which are easily corroded by atmospheric air, moisture and CO2 are coated electrolytically with base metals such as nickel or chromium which are more resistant to wear or chemical attack. Some of the applications of electroplating are, 1. Plating for protection from corrosion and chemical attack. 2. Plating for decoration. 3. Plating for special for special surface and engineering effects. 4. Electroforming 5. Plating on non-metallic materials. Therefore, this process is widely used in automobiles, aircrafts, refrigerators, jewellery, radios, cameras, type-writers, umbrellas, watches etc. METAL SPRAYING: In this process, the coating metal in the molten state is sprayed on the roughened surface of the base metal. The metal thus sprayed adheres t be the base metal surface. The sprayed-coatings are continuous, but some what porous. Consequently, a sealeroil or paint is applied on such a coating to provide a smooth surface. The process offers certain advantages over the other methods, due to its: 1. greater speed of work 2. Applicability to large surfaces and 3. Ease of application, even of thick coatings to restricted area. However, adhesion strength of such coatings is, usually, lesser than those obtained by hot-dipping or electroplating. It is, therefore, essential to have a clean and roughened metal surface on which a proper adhesion is to be secured. The surface is, generally, roughened by sandblasting. Sprayed-coatings can be applied by the following 2 techniques. 1. wire-gun method is more widely used for common metals. In this, the wire of the coating metal is melted by an oxy-acetylene flame and atomized by a blast of compressed air. 2. Powder metal method: in this, finely divided powdered metal is sucked from the powder chamber and then heated as it passes through the flame of the blow-pipe. The blow-pipe disintegrates the metal into a cloud of molten globules, which are then adsorbed on the base metal surface. This method is, however, limited to low-melting metals like Zn, Pb, Sn, etc.

CEMENTATION: Cementation is the process in which base metal is packed in the powdered coating metal and heated to the temperature just below the melting point. Resulting in the formation of layers of alloys of varying composition. Generally an inert or reducing atmosphere is usually maintained during this process. Due to the diffusion of coating metal into base metal, an alloy of two different metals is formed. It is also suitable for coating small articles of uneven surfaces and shapes such as screws, bolts, valves and gauge tools. The coating metals used in this process such as zinc, chromium and aluminum are those which can alloy even with iron. COLORISING: Colorizing is carried out by first sand-blasting the metal objects and then heating then in a tightly-packed drum with a mixture of aluminium powder and aluminium oxide, together with a trace of ammonium chloride as a flux. Air is excluded and in some process, a reducing atmosphere of hydrogen is used. The layer formed has an approximate composition of Al3Fe2, corresponding to about 25% al by weight. Colorizing is frequently applied, especially for the protection of furnace parts.

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CHROMIZING: Chromizing is carried out by heating together a mixture of 55% chromium powder and 45% alumina, together with the base metal parts at about 1300 to 14000C for 3 to 4 hours. The use of alumina prevents the coalescence of chromium particles. Chromizing is also produced by the interaction of a mixture of volatile chromous chloride and hydrogen with steel parts at about 10500C. The diffusion of chromium into iron surface is more rapid than the above powder methods. The process is believed to occur in three stages: 1. Fe + CrCl2 FeCl2 + Cr (Displacement) 2. CrCl2 + H2 Cr + 2HCl (Reduction) 3. CrCl2 Cr + Cl2 (Thermal decomposition) The corrosion-resistance of chromized coatings corresponds to that of ferrite stainless steels. The chromium content in the diffusion layers varies, generally, from 10 to 20%. The layers are supposed to be solid solutions of iron and chromium. Chromizing process is fairly extensively applied for the protection of gas turbine blades. SHERADIZING: Sheradizing is the process of cementation, using zinc powder as coating metal. The iron articles to be coated are first cleaned and then packed with “zinc dust” in a drum. The drum is then sealed tightly so that oxidation of zinc is minimum. The drum is slowly rotated for 2 to 3 hours and its temperature is kept between 350 to 3700C, either by gas heating or electricity. During this process, zinc gets diffused into iron forming Fe-Zn alloy at the surface. At the end of the process, the surface of the articles becomes covered with a thin film of iron-Zinc alloy. Application: sherardizing is used especially for small steel articles like bolts, screws, nuts, threaded-parts, washers, valves and gauge tools. The main advantage of sherardizing is that coating is quite uniform, even if the surface has crevices or depression. Moreover, there is practically no change in the dimension of articles. CHEMICAL CONVERSION COATINGS: PHOSPHATE COATING: Phosphate coating is obtained by the chemical reaction of aqueous solution of phosphate of iron, manganese and Zinc with base metals such as Iron, Steel and Zinc. The chemical reaction produces a surface film consisting of manganese iron phosphates or Zinciron phosphates. For increasing the reaction rate, accelerators are also added such as nitriles, chlorates or nitrates. Application of phosphate coating on the base metal surface can be done by brushing or spraying. This coating so not offers corrosion resistance but offer excellent bases for painting, impregnation with oils etc. Iron and Manganese coatings are generally used to reduce wear caused by friction. Phosphate coating can be done on other metals such as Aluminum, Cadmium and Tin. The colour of phosphate coating is usually grey.

CHROMATING: Chromating is the process of coating a surface film with a mixture of trivalent and hexavalent chromium for protection from corrosion. Chromating is generally used for the protection of Zinc, Aluminum, Magnesium and Cadmiumplated parts. They are produced by immersion of the article in a bath of acid potassium chromate, followed by immersion in a bath of metal chromate solution. The properties of chromate coatings are amorphous, non-porous and more corrosion resistant than phosphate coatings. These are also sometimes used as base for paints, lacquers and enamels. Anodizing: The process of formation of conversion coating on a metal surface by anodic oxidation is known as Anodizing. This formation of an oxide coating by anodizing may be used to improve the wear resistance of certain metals. This process is usually applied to aluminum, magnesium, zinc and their alloys. And in anodizing, the work is the anode, and oxide layers are built up on the base metal. 4

ORGANIC COATINGS PAINTS: Paint may be defined as the mechanical dispersion mixture of pigments and fillers which are in a liquid medium and later becomes film forming oil. The volatile liquids such as thinners are again added to these liquids. CONSTITUENTS OF PAINT: a. Pigment b. Vehicle or drying oil c. Thinners d. Driers d. Fillers or extenders e. Plasticizers f.. Antiskinning agents a. Pigment: It is solid constituent present in paint which provides a decorative colour effect to protect it from ultraviolet rays. Pigment is one of the essential constituents of paint. The essential functions of pigments are• To provide desired color, opacity and strength to the paint. • To give aesthetical appeal to the paint film, • To give protection the paint film by reflecting harmful ultraviolet light, • To provide resistance to paint film to moisture and • To increase the weather-resistance of the film. b. Vehicle or drying oil: It is a film-forming constituent of the paint. The liquid portion of the paint in which the pigment is dispersed is called as vehicle or drying oil. The important functions of vehicle oil are: • They hold the pigment on the metal surface • They form the protective film, • They impart water-repellency, durability and toughness to the film, and • They give better adhesion to the metal surface. c. Thinners: Viscosity (or consistency) of the paints are reduced by the addition of thinners. So that the paints can be easily applied on the metal surface. The important functions of thinners are: • Thinners reduce the viscosity of the paint to suitable consistency, so that it can be easily handled and applied to the metal surface. • They dissolve the film-forming material and also the other desirable additives in the vehicle. • They evaporate rapidly and help the drying of the paint film. • They suspend the pigments in the paint film. • They increase the elasticity of the paint film, • They also increase the penetration power of the vehicle.

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d. Driers: The drying of the oil is accelerated or catalyzed by driers. They do this by oxidation, polymerization and condensation. In fact, driers are oxygen carrier catalysts. The important functions of the driers areLinileates, borates, naphthalene’s, resonates and tungstates of heavy metals like Pb, Zn, Co and Mn. • Surface driers: Cobalt substances, • Bottom- driers: Lead substances, • Through driers: Manganese substances. e. Fillers or extenders: Fillers are inert materials which are used to improve the properties and reduce the cost of the paint. The important functions of fillers are: • They reducing the cost of the paint.(Expensive pigments which have excellent hiding power (like TiO2 and ZnSO4) are used in a admixture with cheap extenders for reducing the cost without reducing the efficiency), • They serve to fill the voids in the film, • They increase random arrangement of the primary pigment particles, and act as carriers for the pigment color, • They improve the durability of the film by reducing the cracking of the paints after drying. f. Plasticizers: Plasticizers are added to the paint film to give elasticity to the paint film and to prevent cracking of the film. Ex: Tri cresyl phosphate, triphenyl phosphate, dibutyl tartarate, and tributyl phthalate. g.. Antiskinning agents: antiskinning agents prevent the gelling and skinning of the paint film. E.g. Polyhydroxy phenols

Requirements of a Paint: 1. The adhesion capacity of the paint should be high to the material on which it is to be used. 2. The paint should spread easily over the surface to be protected. 3. On drying, the paint film should not be cracked. 4. The paint film should have high corrosion resistance property so as to protect the painted surface from the corrosion environment. 5. The paint film should be stable. 6. The paint film should be prepared such a way as to be applicable easily by spraying or brushing. 7. The paint film should yield a smooth and uniform surface. 8. The paint film obtained on the surface should be tough, uniform and adherent. 9. The colour of the film should be stable and should not get affected by the environment conditions. 10. The covering power of the paint should be high.

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