Coatings on non-ferrous metals — Al, Zn, Cu, Pb

Coatings on non-ferrous metals — Al, Zn, Cu, Pb

Progress in Organic Coatings, COATINGS ON NON-FERROUS BARBARA BIEGArjSKA 215 10 (1982) 215 - 234 METALS - Al, Zn, Cu, Pb and EDWARD SMIESZEK In...

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Progress in Organic Coatings,




10 (1982) 215 - 234

METALS - Al, Zn, Cu, Pb


Institute of Plastics and Paints Industry, Gliwice (Poland)

Contents 1 Introduction



2 Coatings on ahrminium .............................................. 2.1 Aluminium surface preparation prior to painting. ...................... 2.1.1 Degreasing and pickling ..................................... 2.1.2 Conversion coatings ........................................ 2.1.3 Application of wash primers ................................. 2.2 Paints for aluminium coating ...................................... 2.2.1 Primingpaints ............................................ 2.2.2 Topcoat paints and enamels .................................. 2.2.3 Lacquers ................................................ 2.3 Application of paints ............................................

216 216 217 218 220 221 222 222 223 224

3 Coatingsonzinc ................................................... 3.1 Zinc surface preparation prior to painting ............................ 3.1.1 Degreasing and pickling ..................................... 3.1.2 MechanicaI treatment. ...................................... 3.1.3 Conversion coatings ........................................ 3.1.4 Application of wash primers. .................................. 3.2 Paints for zinc coating ........................................... 3.2.1 Priming paints ............................................ 3.2.2 Topcoat paints ............................................ 3.3 Application of paints ............................................

224 225 225 225 226 226 226 228

4 Coatings on copper ................................................. 4.1 Substrate preparation prior to painting .............................. 4.2 Paints for copper painting ........................................ 5 Coatingsonlead



229 229

230 230 231

6 Conclusions .......................................................





1. Introduction Non-ferrous metals such as aluminium, zinc, copper, lead, their alloys and steel plated with these metals have become widely used in various fields of modern technology. The rate of corrosion of non-ferrous metals is minimum in an atmosphere with low corrosion aggressiveness since the corrosion products of metals such as zinc or aluminium form a protective layer on the metal substrate, as opposed to steel corrosion products. This layer retards 0033-0655/82/010215-20/$05.25

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the access of atmospheric oxygen to the metal surface and protects it against further corrosion. An increase of the concentration of aggressive substances in the environment leads, however, to a failure of these protective layers. To decrease corrosion losses of non-ferrous metals in a more corrosive environment, it is recommended that they are additionally protected with appropriate organic coatings. Non-ferrous metals are also painted for decorative purposes. This paper presents the methods for painting non-ferrous metals, i.e., aluminium, zinc, copper, lead, their alloys and metallized steel by dipping or spraying. The methods of surface preparation prior to painting, that are characteristic for a given metal, have been also discussed. Recommendations and restrictions concerning the selection of paints have been given. The paper does not deal with coil-coating technology and related paint products. The authors are of the opinion that this problem should be discussed in a separate publication since it is specific both in terms of the technology and the special paints required.

2. Coatings on aluminium More than 100 years ago, aluminium belonged to a group of metals rarely used in practice due to its high price. Now aluminium has become of interest as a constructional material owing to its good performance characteristics, low specific gravity, good workability, high dyeability for decorative purposes, etc. Chemically pure aluminium, when exposed to air, becomes coated with a uniform alumina layer the thickness of which can reach as much as 0.1 pm at a higher humidity and elevated temperature. The oxide layer protects the metal against further oxidation, hence, aluminium and its alloys do not corrode under normal atmospheric conditions. In industrial and marine atmospheres containing aggressive gases and salts, the protective oxide layer undergoes dissolution that results in decreased corrosion resistance. In such cases, it is necessary to protect aluminium additionally, e.g. with organic coatings.

2.1 Aluminium surface preparation prior to painting The formation of adhesive and durable organic coatings on aluminium and its alloys depends to a great extent on surface pretreatment prior to painting. Prior to applying the paint, it is necessary to remove any dirt particles, oil stains, metallic filings and residues from mechanical treatment. However, the protective alumina layer, which is a passivator, impedes further treatment. To increase corrosion resistance of the protective paint system and to improve adhesion of the successive layers, conversion coatings or wash primers are usually applied to the carefully cleaned and degreased aluminium surface.


2.1 .l Degreasing and pickling Solvents, emulsions and aqueous solutions of detergents and pickling agents can be used to degrease aluminium surfaces. Among the solventsused for degreasing, non-flammable chlorinated hydrocarbon derivatives, mainly trichloroethylene and tetrachloroethylene, are of great importance. Trichloroethylene used for degreasing should be stabilized to avoid corrosive action of the hydrogen chloride evolved due to ultraviolet radiation. Tetrachloroethylene does not undergo decomposition under ultraviolet radiation, so it seems to be more suitable. Recently, fluoric aliphatic hydrocarbon derivatives, e.g., trichloro-trifluorethane (Freon TF) [ 11 have been introduced to degreasing procedures. Chemically clean surfaces cannot be obtained by simple degreasing with solvents since they do not remove fatty acids, soaps, inorganic salts and other contaminations. Therefore, a consecutive stage of surface treatment should include degreasing, for example, in alkaline solutions. The degreasing power of emulsions is similar to that of solvents, but in addition emulsions remove water-soluble contaminations such as soaps and sulphonated oils. In this case, a chemically pure surface is not prepared, so additional alkaline degreasing is recommended, as in the case of solvents. Aqueous solutions of degreasing agents are also used for degreasing aluminium. They are divided into three groups - alkaline, neutral and acidic depending on their pH values. Alkaline degreasing agents are composed mainly of inorganic salts (sodium metasilicate, sodium phosphate, sodium silicate, sodium carbonate, sodium hexametaphosphate or mixtures thereof), surfaceactive agents and inhibitors preventing an excessive aluminium dissolution. Degreasing agents are considered to be neutral if their aqueous solutions have pH 6.5 to 7.5. They include aqueous solutions of surfactants without inorganic compounds or contain some neutral salts in aqueous solution. Acidic degreasing agents used for cleaning aluminium surfaces contain mostly acid ortho- or pyrophosphates, surfactants and, occasionally, inhibitors. Amchem Products Inc. has patented a method for cleaning the aluminium surface with acidic aqueous solution containing active fluorides and an acid, e.g., sulphuric acid [ 21. The ability of acidic degreasing agents to react with aluminium is lower than that of alkaline agents. Acidic degreasing agents also comprise mixtures of chromic/sulphuric acids and chromic/phosphoric acids. These mixtures react very slowly with aluminium and completely remove fats and corrosion products from its surface. Metallgesellschaft AG [ 31 proposes a variety of both alkaline and acidic agents for aluminium degreasing by dipping or spraying methods, as well as preparations applicable for simultaneous or separate degreasing of articles made of steel, aluminium, zinc and their alloys. Apart from the alkaline and acidic degreasing agents discussed above, alkaline and acidic pickling agents are also used. Pickling solutions are mainly used to remove oxide skins formed during rolling and casting processes and to ensure good adhesion of paint coatings to aluminium. Among alkaline agents for aluminium pickling, sodium hydroxide with additions, e.g., carbonates, sodium phosphate and possibly wetting agents, is


most often used; the sodium hydroxide content varies from 5 to 20%. When aluminium alloys containing copper, zinc or nickel are subjected to pickling in alkaline baths, a black deposit is formed on the pickled surface that is subsequently removed in 30% nitric acid aqueous solution. Hartwood [ 41 proved that hot alkaline pickling gives better results than the application of conversion coatings when aluminium is then painted by electrodeposition. Among acidic pickling agents, phosphoric acid solution with an additive, usually alcohol, is mainly used. Phosphoric acid causes an additional thin layer of aluminium phosphate to be formed on an aluminium substrate. Garnish [6] stated that an addition, for example, of copper ions in an acidic pickling bath influences advantageously the adhesion of paint coatings to the aluminium substrate. 2.1.2 Conversion coatings The most important methods of preparing the aluminium substrate comprise oxidation by chemical and electrochemical procedures, phosphating and chromating. Oxide coatings are prepared by chemical or electrochemical methods. Chemical surface treatments are more economical than electrochemical ones since no complicated equipment and power supply are required and also the formation of oxide coating proceeds very quickly. Oxide coatings formed in oxidation reactions are much thicker than those obtained under natural oxidation conditions. One of the methods for improving the chemical resistance of aluminium at pH 3.5 to 9.0 consists in preparing a Boehmite layer (Al,Oa.HzO). This compound is formed when aluminium is treated with boiling water or steam. Sumitomo Light Metal Industries patented a method of improving the adhesion of thermosetting paint coatings to aluminium substrates and improving their resistance to chemical and mechanical attacks as well as resistance to weathering by formation of a Boehmite layer and organosilicone compounds [ 71. Chemical oxide coatings are prepared in baths containing mainly chromates and carbonates. There is a great variety of processes of preparing the oxide coating that are well established in practice on an industrial scale. They differ in bath composition and data processing. The most important processes are MBV, EW, LW, Pylumin and Alrok [l]. Another method for preparing the oxide coatings is electrolytic oxidation of aluminium. The article to be coated with an oxide layer is suspended on the anode in an aqueous solution of sulphuric, chromic, boric, oxalic, phosphoric or other acids. It is also possible to use alkaline electrolytes. If a 5 wt.% sodium carbonate solution is used, corrosion resistance of the oxide layers obtained is not worse than that of the layers prepared by oxidation in sulphuric acid solutions. In recent years, alkaline electrolytes for anodizing aluminium substrates have become a subject of interest in Japan [ 1, 81. The properties of anodic oxide films depend on bath composition, processing conditions and chemical composition of the metal substrate. Chemi-


tally pure aluminium undergoes oxidation more easily than itas alloys. Additives and alloy contaminations such as copper, silicon, iron and manganese unfavourably affect the quality of the coatings obtained. Oxide coatings prepared by anodizing exhibit good protective properties, advantageous mechanical resistance and the ability to adsorb various dyes. However, owing to the porous structure and brittleness of coatings prepared at room temperature and the lightening or darkening of the coatings that occurs when alloys containing iron, copper, silicon etc. are oxidized, additional procedures are used to eliminate these defects. Oxide coatings are subjected to sealing in boiling water, steam, solutions of dichromates, nickel or cobalt salts, etc. Lockheed Ltd. developed a method for double sealing aluminium and its alloys, previously anodized, using sulphuric acid. The process consists of dipping the anodized aluminium in cobalt acetate solution and then in sodium dichromate solution. Double sealing of an oxide layer allows the preparation of coatings of excellent corrosion resistance with improved adhesion to the aluminium substrate. Another method for improving the properties of oxide coatings is to react a hydrated alumina with organic compounds [lo] , yielding coatings with better characteristics. Opinions concerning the advantages of using a system composed of anodic coatings and organic finishes are somewhat divergent. Many authors [ 1, 8,ll - 151 recommend anodizing the aluminium surface prior to painting, while others [ 16,171 are of the opinion that the advantages of anodic coatings are small compared with the costs involved. For this reason, they suggest the use of other methods of aluminium surface preparation prior to painting. The phosphating of aluminium is performed in baths, the composition of which is close to that of baths used for zinc and steel [l, 18,191. From a technical viewpoint, zinc phosphate coatings prepared in baths containing, apart from phosphoric acid, phosphates and nitrates, fluorides and hydrofluoric acid, are of the greatest importance. It should be noted that the A13+ cations behave as an undesirable inhibitor which has to be continuously removed from the bath in order to obtain a phosphate coating as clean and uniform as possible. To remove the A13+ cations, alkaline metal ions are added to the bath causing precipitation of fluoroaluminates [20 - 231. The formation of zinc phosphate coatings on steel, zinc and aluminium substrates has been reviewed by Bender et al. [24]. The formation of phosphate coatings is more complicated and expensive than the chemical oxidation of aluminium surfaces, but phosphate coatings have characteristically improved protective properties and are more appropriate substrates for paints. In a number of cases, particularly in the automotive industry, it is often necessary to treat surfaces of steel and aluminium simultaneously previous to paint application [ 21,25 271. The ratio of steel/aluminium surfaces should be large. If a sufficiently uniform phosphate coating is required, the surface fraction of the aluminium parts should not exceed 10 - 15% of the total area subjected to surface treatment. A higher fraction of aluminium surface can be tolerated in simultaneous aluminium/steel phosphating processes if a bath is composed of phosphates, fluorides, ammonium molybdate and phosphoric acid. This bath


yields an amorphous phosphate coating on steel, zinc and aluminium [ZO] . Warter [ 281 presented flow-sheets for cleaning steel, aluminium and zinc surfaces and applying conversion coatings to them, where particular sections are put into operation in accordance with the optimum method for each metal. Metallgesellschaft AG recommends the phosphating of steel and aluminium articles in the following baths: Bonder 37, when 10% surface area of articles is aluminium and Bonder 170, when up to 100% aluminium parts are processed [ 31. Amchem Products Inc. manufactures Granodine 38 solution for simultaneous phosphating of steel, zinc and aluminium. A variation of a chemical aluminium surface treatment prior to painting, which is considered to be a phosphating process, is chromium phosphating. The method for preparing amorphous phosphate coatings, patented in the U.S.A., uses a bath composed of phosphoric acid or phosphates, chromic acid and its salts and simple and complex fluorides [ 11.The coating formed consists mainly of aluminium and chromium phosphates, but also contains fluorides and hydrated oxides. In chromium phosphating, the coatings become more or less intensively green. Therefore in technical literature this method is called green chromating. Conversion chromate coatings are also of great importance for aluminium surface preparation prior to painting. They are prepared in the baths composed principally of chromic acid, its salts and simple and complex fluorides [ 11. The coating is an amorphous layer of aluminium and chromium oxides, while the coating colour varies from clear through opalescent yellow to yellow-brown. The method of preparing the coatings described above was patented among others by Amchem Products Inc. [29]. Advantages and disadvantages of chromate coatings including those on aluminium were discussed by Wilkowski [30] . With respect to protective characteristics, chromate coatings are better than phosphate coatings, so they find a wider application in aluminium surface preparation prior to painting [ 16, 20, 31 - 37 1. 2.1.3 Application of wash primers Drinberg and collaborators [38] report that the adhesion of organic coatings to metal decreases, depending on the type of the substrate, in the following order: nickel-steel-cast iron-copper-brass-aluminium-tin-lead. It can be concluded from this arrangement that aluminium is a substrate to which the paint coatings do not adhere sufficiently. The adhesion of paint layers to aluminium can be greatly improved by applying a wash primer coating. This method of surface treatment is cheaper compared with others, while the adhesion of a paint film to a treated surface is often equivalent to that obtained with a much more expensive anodizing. It is recommended to use one- or two-component wash primers for aluminium surface treatment prior to coating. Two-component wash primers consist of alkaline zinc chromate dispersed thoroughly in polyvinyl butyral resin and phosphoric acid solution in alcohol as a second component. The need to mix both components just before use is a disadvantage of wash primers.


One-component wash primers differ from two-component ones in that chromic acid is used instead of zinc chromate. The application of one-component wash primers is simpler and less expensive, however, their protective properties and ability to improve adherence of organic coatings seem to be worse. The optimum thickness of wash primer coating is 8 to 10 pm. According to Wernick et al. [l] , the adhesion of a wash primer is a function of phosphoric acid content and is almost independent of substrate material. When aluminium is painted, it is admissible to use less phosphoric acid in the wash primer than in the case of steel painting. Aluminium to which a wash primer layer has been applied must be painted with a complete paint system composed of primer and topcoat, since the thin wash primer layer cannot ensure prolonged corrosion protection [ 51. Andriejewa and Lamaka [39] proved that a 10 wt.% additive of aluminion dust in wash primer improves its anticorrosive properties, while interlayer adhesion is left unchanged. Apart from the formation of conversion coatings and the application of wash primers, there are also other methods for improving the adhesion of paint coatings to aluminium. One of them involves the addition of phosphoric acid, its compounds or hexavalent chromium compounds or their mixtures [40,41] to a binder. Alcan Folien patented a method for increasing the adhesion of paints to aluminium, by which thin coatings of thermosetting resins are applied to the metal substrate in the form of diluted solution which is then subjected to thermal treatment [42] . Another method of improving the adhesion of synthetic resins to aluminium substrates with anodic oxide or Beohmite layers consists in applving an interlayer of an organosilicone compound prior to painting [7] . 2.2 Paints for aluminium coating Painting is the method most often used for final protection of aluminium surfaces. If the aluminium surface is properly treated, strongly adherent and durable coatings can be obtained; however, the best results are achieved with conversion coatings. Practically, the protective effectiveness of a paint coating depends on its durability, i.e., adhesion to the aluminium surface and mechanical properties. A range of paints applicable for aluminium coating does not differ, in general, from that used for protection of steel against corrosion. Therefore, a selection of the appropriate paint system and a suitable application technique depends on: - type, shape and dimensions of the article to be painted; - volume of production; - technical possibilities; - requirements concerning the appearance of the painted article; - conditions under which the article is to be used; - economic factors.


2.2.1 Primingpaints Aluminium should be adequately primed to ensure long-lasting anticorrosion protection. The primer coating should have good adhesion to the substrate and high elasticity. In practice, all known types of resins can be used as binders for primers; however, oil-modified alkyd resins, polyvinyl and phenolic resins are mostly used. The zinc chromate pigmented primers [ 1,43 - 451 have the best properties. Satisfactory results can be also achieved using pigments such as alkaline zinc chromate, iron oxide red, aluminium dust, mixtures of zinc oxide and titanium dioxide and strontium, barium and calcium chromates [l, 46 - 491. Pigments containing lead compounds should not be used since they accelerate pitting corrosion, probably due to a reduction of the pigment to metallic lead [ 1,43, 501. Van Oeteren [ 51, however, considers that these provisions are wrong, at least in the case of minium. According to Silman and collaborators [46] , primers drying at an elevated temperature have better adhesion to an aluminium surface. Ovendrying temperatures must not exceed 200 “C since a higher temperature may destroy the aluminium structure. 2.2.2 Topcoat paints and enamels A proper aluminium surface treatment prior to priming and painting al_ lows free selection of the type of finish, the choice of which depends only on its service conditions. As opposed to primers, both paint type and adhesion to aluminium play a negligible role in topcoat finishes. Air-drying and thermosetting finishes are suitable for the final painting of aluminium. Among air-drying finishes, the following should be mentioned: oil paints, chlorinated rubber, acrylic polyvinyl, alkyd, epoxy, polyurethane and polyester paint products [l, 17, 34, 44, 45, 47, 48, 51- 541. The thermosetting finishes most often used for aluminium finishing are alkyd and epoxy paints and many others including polyvinyl products - polyesters, polyurethanes, silicones, . Binger acrylics, organosols and plastisoles [ 16,35,36,44,45,48,52,53,55] and Rolles [ 561 conducted prolonged service tests of newly developed paints based on fluoric polymers and silicones. They showed that the durability of coatings based on fluoric polymers and silicones and applied to a chromate or phosphate aluminium surface is much higher than that of conventional paints based on acrylics, polyesters or polyvinyls. To improve the properties of some finishes for aluminium finishing, modifications have been developed, e.g., by introducing rosin modified dryingoil based alkyd resin to melamine-formaldehyde resin [ 571 or the polysiloxanes to epoxy resin [58]. Sorokin and co-authors [59] discussed methods of modifying paints in order to increase their adhesion to anodized aluminium and its alloys. Special polymeric compositions designed for aluminium finishing have been patented [60 - 621. Apart from the paints mentioned above, water-soluble paints, particularly those based on acrylic resins, can be used for aluminium painting [63 - 661. From investigations performed by many authors, it is found that aluminium


is an excellent substrate for electrodeposition of paint [67 - 721. The absence of aluminium ions in solution prevents a premature curing or a change of coating colour and maintains good contact of paint with the substrate metal [4,69] . Other authors note that during electrodeposition the substrate metal undergoes oxidation with oxygen which is evolved during the reaction, resulting in a resin Coagulation. The presence of the thin alumina layer formed improves corrosion resistance of the coatings prepared; however, the high electric resistance of the oxide coating affects undesirably the coating thickness [68,73,74]. There are accurate correlations between the chemical composition of aluminium alloys, surface treatment procedures, electrodeposition parameters and the properties of the coatings obtained [73,75 - 771. Czupryna and collaborators [74] found that corrosion resistance of the electrodeposits applied to aluminium and its alloys increases, depending on the type of conversion coating, in the following order: - amorphous phosphating; - degreasing in organic solvents; - chromating; - chromium phosphating. Recently, there has been a great deal of interest in powder coatings for aluminium finishing. Powder coatings have properties that in many cases are better than those of conventional paint coatings. Different powder coatings can be applied to aluminium, such as epoxies, polyesters, polyurethanes and acrylics [ 31, 78 - 811. Since the high curing temperature of powder coatings makes their application difficult, modifications aiming at a decrease of stoving temperature are currently under investigation [ 801. Another difficult problem to be solved is the development of paints having antifouling properties for the shipbuilding industry. The best antifouling properties of paints to be applied to an aluminium substrate can be achieved with additives of copper and mercury salts. However, it is not recommended to apply paints containing these antifouling agents directly to the aluminium surface as such additives accelerate the corrosion of aluminium. Zinc oxide and lo-chloro-5,10-dihydroxyphenantrene are also used as antifouling agents, but their toxicity is much weaker compared with the Cu/Hg salts. Hence, paints containing copper and mercury salts are more often used, but they are applied on aluminium surface previously coated with zinc chromate pigmented primer. At present this method is used successfully in the U.S.A. [l] . Many authors discuss the application of aluminium in the automotive industry where it is used as a constructional material for car bodies or parts [23, 26, 27, 82, 831. The use of aluminium for car bodies is not, so far, widespread in Europe. Steuer et al. [25] describe a process for painting the Porsche 928 body, which is made of aluminium alloy. 2.2.3 Lacquers Aluminium can also be coated with lacquers, the binder of which is composed of alkyd, acrylic, polyvinyl or epoxy resins. The most durable


coatings are obtained when acrylic lacquers are applied to anodized aluminium surface. To obtain durable coatings, the aluminium substrate must be carefully cleaned prior to painting, since alkaline residues left on the aluminium surface can react with a resinous binder to form soaps that in turn may cause a loss of adhesion of the coating to aluminium. A small amount of polyvinyl acetate added to cellulose nitrate lacquers improves significantly their adhesion to the oil-contaminated surface, while an addition of polyamide resin increases the adhesion to the abrasive contaminated surface [l] . When acrylic lacquers are modified with an addition of dispersed fluorocarbon polymers, the coatings obtained therefrom have a low gloss, excellent resistance to weathering and good adhesion to aluminium [ 841. Vandamme [ 851 used a silicone resin-based lacquer for car parts made of aluminium. 2.3 Application of paints Any known painting technique is suitable for applying paints to aluminium surfaces, i.e., manual painting with brush or rollers, air and airless spraying methods, electrostatic spraying, dipping, electrodeposition and roller coating. The thickness of a paint film depends on the conditions and range of application. In the case of aluminium-coated steel, it depends also on the Al layer thickness. When the equipment has to serve indoors or in a non-polluted environment, one coating is sufficient for protection, otherwise, two or more coats are needed. 3. Coatings on zinc

Zinc is considered to be a rather unstable metal, however, when compared with steel it has the important advantage that it becomes covered with a protective layer of its corrosion products if exposed to air. It is assumed that this protective layer is principally composed of alkaline zinc carbonate, zinc oxide and hydroxide. The composition of zinc corrosion products is dependent on the corrosion aggressiveness of the environment [ 861. In a slightly polluted environment, the loss of a zinc layer per year due to corrosion is small and reaches 10 - 50 g/m2 of galvanized steel. It was assumed that a loss of about 70 g/m2 corresponds to the loss of a zinc layer of 0.01 mm in thickness [87], Zinc is unstable in acidic and strongly alkaline environments. Sulphur dioxide present in the air is also corrosive. It reacts with moisture to form sulphuric acid which in turn reacts with zinc to form water-soluble salts. The effect of air pollutants on the rate of zinc corrosion is discussed by Van Oeteren [ 51 who reports that the annual corrosion losses of zinc films are 1 pm in rural environments, while they increase up to 10 pm in strongly polluted industrial environments. A method of steel protection against corrosion that consists in applying successively zinc and paint coatings to the steel substrate is known as the Duplex system. It is assumed that the durability of the metallic/paint coating system is about 1.5 times greater than that derived from the sum of durabilities of each coating used separately [ 51. The application of metallic/paint


coatings is often the cheapest way to ensure many years’ steel anticorrosion protection. Furthermore, it decreases considerably the costs of repainting sin since the surface preparation is not so expensive and labourconsuming as in the case of steel repainting. The Duplex system is also used when repainting is difficult or impossible, i.e., for high structures bridges, masts, etc. Galvanized steel is also painted for decorative purposes. The metallic/paint coating systems, in spite of high production costs, are almost ideal for protecting steel against corrosion, and therefore they should be widely used in industry. 3.1 Zinc surface preparation prior to painting The durability and resistance of an organic coating applied to zinc or galvanized steel depend on a proper selection of methods for surface preparation and an appropriate paint system. The method of cleaning and surface preparation prior to painting is selected bearing in mind both the appearance and condition of the surface to be painted, which depend on the zinc plating technique (presence and type of zinc bloom) and period of zinc layer conditioning prior to painting [88]. When galvanized steel is conditioned for a sufficient time, the Fe-Zn alloy layer becomes exposed. A mechanical treatment of the surface to remove zinc corrosion products on the substrate improves the adhesion satisfactorily for most paints [ 50,891. However, this method for improving the adhesion of paints to the zinc substrate is not recommended, since it is not possible to determine precisely the period of conditioning. Furthermore, the conditioned zinc coating contains soluble sulphates that cause premature zinc corrosion when they are not completely removed. Surface preparation of zinc and its alloys is intended not only to remove dirt particles, oil stains and corrosion products, but also to form a uniform and active substrate for organic coatings. 3.1.1 Degreasing and pickling

Oils and greases present on a galvanized steel surface are usually removed with organic solvents, degreasing emulsions or alkaline degreasing solutions. The use of alkaline degreasing substances in the form of a salt, e.g. sodium carbonate, is not recommended because it is not possible to remove them completely from a zinc surface, and on the other hand their residues increase the susceptibility of zinc to corrosion. 2 - 10% ammonia solution including an additive of 0.5 - 1% surfactants [87,90, 911 is recommended for degreasing. Since zinc reacts easily with alkalis, the pH value of degreasing and washing solutions must be strictly controlled. 3.1.2 Mechanical treatment

A smooth and glossy zinc surface is a bad substrate for organic coatings, since it is hardly wetted by paints. To obtain a rough surface, and therefore, to improve the adhesion of paints to the substrate, this surface is subjected to abrasive-blast treatment associated with rinsing, mostly with ammonia solutions of detergents [91]. Mechanical treatment is also recommended for partly corroded zinc surfaces to remove corrosion products.


3.1.3 Conversion coatings

One of the methods for improving the corrosion resistance of zinc and the adhesion of paints is to prepare conversion phosphate, chromate or oxide coatings on the zinc substrate. The best adhesion of paints to zinc can be achieved with phosphate coatings and then yellow/clear chromate coats. From the work of Szaniewski and collaborators it can be concluded that good results are obtained with a zinc phosphate coating applied to a freshly galvanized steel substrate [ 921. The experiments proved that it is necessary to use a bath of increased nickel content, containing a fluoride additive [9] when articles made c,f galvanized steel and bare steel, e.g., in car bodies, are simultaneously subjected to phosphating. If the phosphating process cannot be carried out, Van Eijnsbergen [93] proposes cold etching with phosphoric acid. The etching time recommended should be strictly followed and when the process is completed, the zinc surface should be thoroughly rinsed with water. Zinc chromating is carried out in baths containing mainly chromic acid, acid chromates and sulphates. The corrosion resistance of a chromate pretreatment greatly decreases when the layer is heated above 160 “C [89]. The properties of chromate layers on zinc were discussed by Rijmer et al. [94]. Apart from many patented chemical conversion processes, it is worthwhile to recommend the non-proprietary pretreatment material, known as British Rail “T-wash”. It is a blue solution that makes the properly prepared zinc surface black. The composition of this bath, given in a British Standard [95] , is as follows: 9 wt.% phosphoric acid (d = 1.7); 57 wt.% water; 16.5 wt.% ethyl cellosolve; 1 wt.% copper carbonate; 16.5 wt.% methylated spirit, 3.1.4 Application of wash primers

If it is impossible to apply conversion coatings, one- or two-component wash primers are used. Porter [ 891 states that wash primers based both on unmodified polyvinyl butyral resin and resins modified with phenolics have good adhesion to most types of zinc layers. Some authors suggest the use of a modified wash primer due to its lower sensibility to steam. However, others are of the opinion that en unmodified primer of greater reactivity should be used. The authors’ opinions concerning the expediency of using wash primers for zinc coating are divergent. The latest investigations show that one- and two-component wash primers are useless for painting galvanized steel to be used in an environment of high humidity [50, 90,961. 3.2 Paints for zinc coating The coatings that adhere well to steel are often unsuitable for zinc painting. Paints show bad adhesion most often to steel plated with zinc by galvanizing and electrogalvanizing. Such difficulties are not usually observed for


coatings prepared by flame spraying, zinc alloys, die castings, etc. owing to a rougher surface. Van Oeteren [ 51 presents the differences in adhesion of paints to a zinc substrate , where the zinc has been applied by flame spraying or galvanizing. The same paint applied to a flame-sprayed zinc substrate retains its initial adhesion for one year, while the paint applied to galvanized steel with no pretreatment loses its initial good adhesion after 3 months. Contrary to unconditioned galvanized steel, zinc sprayed coatings do not need any additional treatment prior to painting since at a boundary of zinc/paint films the zinc substrate is covered with a thin oxide layer. This oxide layer prevents a reaction of metallic zinc with the binder, The layer of sprayed zinc is porous, and for this reason, anticorrosion coatings with good penetration characteristics are recommended to be applied, if possible, directly after a metallized coating is formed. In many cases, the paints that demonstrate initially a good adhesion to zinc substrate lose this property after some time. The causes most often cited in the literature of bad adhesion of the paint coating to the zinc surface are as follows [89, 91,971 : zinc

- great difference

between thermal expansion coefficients and galvanized steel; - smoothness of zinc surface; - insufficient degreasing and cleaning of zinc surface; - improper selection of paint system.

of paint coating

The basic criterion in selecting the appropriate paint system is its fastness to water. If this condition is not fulfilled, the. adhesion of the paint coating to zinc decreases after some time in service and the coating delaminates. Under the coat, there are spots covered with a salt bloom of alkaline reaction under which pitting occurs in the zinc film. The adhesion of paints to a zinc substrate deteriorates also due to formation of metallic soaps and other products of reaction of zinc with binder components. These reactions proceed under an organic coating. Metallic soaps are formed mainly when drying oils and alkyds are used as binders. These soaps are usually formed 3 - 12 months after painting. The formation of soaps causes the adhesion of organic coatings to the zinc substrate to be completely lost. The presence of metallic soaps is observed even when a priming paint does not contain drying oils and alkyds. This phenomenon is caused by permeation of the fatty components from a primer through certain priming coats. For example, fatty acids permeate through a polyvinyl primer coating 80 100 pm in thickness towards the substrate, through plasticizers contained in the binder [91]. Another reaction which affects disadvantageously the adhesion of a paint film to zinc is that of zinc with formic acid yielding zinc formate [89]. Formic acid is the product of decomposition of binders containing the drying oils. Alkyd-, epoxyester- and oil-based paints are particularly susceptible to reaction with zinc.


A proper paint formulation is very important, since as supposed by some authors, the loss of adhesion of epoxy paints cured with polyamides and polyamines can be caused by excess curing agent that reacts with zinc to form water-sensitive reaction products [ 891. 3.2.1 Priming paints In the literature, a wide range of resins exists which are suitable as binders for paints on zinc layers. Primers based on epoxy, acrylic and polyur- 102 1. The German ethane resins are most often proposed [ 80, 86,90,97 Standard [99] recommends also the use of resins such as modified alkyd resins, polyvinyl butyral resins and various paints including bituminous-oil, coal tar pitch, bituminous-epoxy and bituminous polyurethane organic coatings. From the practical tests carried out by Friehe and Schwenke [ 1031 it can, however, be concluded that bituminous-epoxy paints applied to galvanized steel do not show good protective properties in sea water due to their poor adhesion to the zinc surface. The polyvinyl copolymer-based paints and dispersion paint have also found a wide application [90, 1001. Porter [89] considers that the best adhesion to a bare zinc surface is shown by paints based on chlorinated rubber, vinyl copolymers, epoxy and polyurethane resins. Varga [ 1041 proposes to use organophosphoric compounds that improve the adhesion of paints to phosphated galvanized steel. The adhesion of paints to zinc depends on a proper choice of binders as well as pigments. Pigments that are able to react with fatty acids when used in primers also permit the use of alkyds, epoxyesters and oils as binders. The following pigments can be used in primers: zinc chromate, zinc dust, zinc phosphate, barium metaborate, calcium plumbate, metallic lead and minium [89, 97, 101, 1051. Primers pigmented with zinc chromate can be applied to a conditioned galvanized steel or a wash primer coating. Paints of high zinc dust content are used successfully in repainting constructions of galvanized steel where a zinc film has been removed or damaged. Zinc phosphate contained in the paint for priming galvanized steel does not increase the adhesion of paint to substrate, but the addition of alkaline pigments to zinc phosphate pigmented paint greatly improves its adhesion to zinc. When a paint is pigmented with barium metaborate, barium soaps are formed in the coating. Hence, fatty acids do not react with a zinc substrate. The soaps are formed throughout the whole film and not only at a zinc surface and consequently, the adhesion of the coating does not decrease. Calcium plumbate is used as a pigment in many paints. Its content should be at least 50 wt.% of the total amount of pigments used in the formulation. It is advantageous that calcium plumbate reacts with fatty acids. Paints pigmented with metallic lead have a prolonged good adhesion to zinc and no sensibility to moisture. The authors’ opinions on the application of minium for zinc priming differ greatly. Most authors do not recommend the use .of this pigment in paints for galvanized steel while Schatz [105] believes that the application of


minium-pigmented paints is advantageous, particularly when zinc corrosion products cannot be removed completely from a zinc surface. Van Oeteren [ 51 also states that any objections to the application of minium in primers are unjustified. 3.2.2 Topcoat paints In general, the choice of interlayers and topcoat paints for zinc coating is unlimited, However, many authors do not recommend the application of paints based on resins of insufficient adhesion directly to a zinc surface as finishes [50, 891. A top layer should be of low permeability to moisture, so pigments of flaky structure are recommended to be used in the outer coats, since they protect a coating against moisture. Examples of such pigments are micaceous iron oxide and aluminium dust. To improve the properties of acrylic and polyurethane lacquers used for zinc painting, Christie and Carter [102] propose the addition of chelating agents (rubeanic acid) and U.V.absorbers to these lacquers. A galvanized sheet may show a zinc bloom pattern though the complete paint system has been applied on the metallic substrate. This is a serious disadvantage, particularly when these sheets are to be processed in the automotive industry. To obtain hot-dip galvanized sheets with a small zinc bloom pattern, the sheets have to be processed under different conditions or additionally treated (by rolling) [106,107], 3.3 Application of paints Paints can be applied to a zinc substrate by brushing, air and airless spraying techniques as well as by electrodeposition [76, 79,107,108]. The latter method is used, for example, for painting Porsche 911 bodies made of galvanized sheet. The number of layers, and finally, the total thickness of the paint coats on galvanized sheet can be smaller than that on bare steel. Mayfarth and Natschka [97] report that the thickness of air-drying coating on galvanized steel varies from 60 to 100 pm, while that on an untreated steel should be not less than 120 pm or even not less than about 250 E.trnin a highly aggressive environment. In the case of paints drying at an elevated temperature, e.g., acrylics, the coating thickness can be less, i.e., 30 - 50 pm. Van Oeteren [ 51 is of the opinion that a maximum of two coats should be applied to galvanized steel (4 coats for bare steel). One-layer coating is sufficient when glavanized steel is painted for decorative purposes or high-buildup paint is applied. 4. Coatings on copper

The opinion of architects on the expediency of painting the copper used in the building industry are rather divergent. Some of them prefer a natural copper colour preserved by means of clear lacquers while others like the natural beauty of the patina which is formed under atmospheric conditions.


However, copper, its alloys and copper plated steel, which have to serve in a highly corrosive environment should be protected additionally with organic coatings. The paint coatings improve the resistance of copper to matting, abrasion and aging. 4.1 Substrate preparation prior to painting The preparation of a copper surface prior to painting consists of removing the contaminations and increasing the adhesion of organic coatings to the copper surface by a proper blast-cleaning procedure. Dirt and oil particles can be removed by organic solvents or aqueous alkaline solutions. Adhesion of the organic coatings to copper can be improved by grinding the copper surface with a fine abrasive paper, blast-cleaning or pickling in acidic solutions, e.g., in a mixture of sodium dichromate and sulphuric acid [ 1091. Other methods for improving the adhesion of organic coatings to copper are also known, e.g., dipping copper in solutions of nitrogen compounds such as ammonia, ammonium hydroxide, etc., for 5 to 10 minutes [llO] or by adding polymerizable phosphorous compounds to a binder. 4.2 Paints for copper painting The choice of binder for paints to be used on copper plays a very important role, in the same way as that for zinc painting. If improper binder is used, the reactions between its components e.g., drying oils, and a substrate decrease the adhesion of the organic coatings. According to Van Oeteren [ 51, copper should not be painted with oil-based paints, long-oil alkyds, epoxybituminous resins, chlorinated rubber and epoxies cured with polyamides. Instead, paints based on polyvinyl resins, epoxies cured with amines and amine adducts and polyurethanes are recommended for this purpose. Copper can also be painted with a great variety of lacquers including acrylics, polyurethanes,, ethyl- and cellulose nitrate-based lacquers, nitroalkyds, modified polyesters, stoving and air-drying silicones [43,102,111]. Acrylic and polyurethane lacquers used for painting copper and its alloys often show improper protective and decorative properties. According to Mock [43] and Christie and Carter [ 1021, these disadvantages can be eliminated by adding chelating agents, e.g., benzotriazole, U.V. absorbers and levelling agents to lacquers. According to Philbert [ 50)) the paints containing or evolving acids, water or even sulphur traces should not be used. Recommendations concerning the painting of copper and its alloys are also given in the British Standard [112]. The effect of substrate metal, including copper, on the parameters of anaphoretic deposition was investigated by Makarewicz [ 761. He reports that the behaviour of copper is similar to that of steel during electrodeposition and the coatings obtained have satisfactory anticorrosive properties. However, they are less hard than the same coatings applied to steel. An improved electrodeposition process for copper and its alloys has been patented. The improvement consisted in a higher bath temperature enabling the coatings to achieve better resistance [ 701.


Matuszewski and Pelikan [ 1131 investigated the effect of surface preparation of copper and brass on the properties of coatings from epoxy powder paints. They found that irrespective of the surface preparation, the mechanical properties of the powder paint coatings considerably deteriorated. Decorative quality also deteriorates after 2 months of conditioning, e.g., a change in colour of the coating occurred. The best resistance properties were obtained when the copper surface was prepared by blast-cleaning with steel abrasive or degreasing with organic solvents, the latter being used in the case of moderate protective requirements. The application of conversion coatings did not improve the properties of epoxy powder paint coatings. 5. Coatings on lead Lead coatings of sufficient thickness are impermeable, and hence, they protect steel satisfactorily against corrosion. As in the cases of zinc and copper, the adhesion of many organic coatings to a non-conditioned lead substrate is insufficient. A British Standard [ 1091 recommends degreasing lead prior to painting in organic solvents, followed by painting with a wash primer. Van Oeteren [ 51 states that the lead surface can be well prepared when treated with a diluted ortho-phosphoric acid solution. Primers pigmented with metallic lead or minium show a good adhesion to lead. Graphite pigmented paint should not be used for lead. Because of the corrosive influence of concrete on lead, it is recommended to protect lead with bituminous paints [ 1131. 6. Conclusions From the literature reviewed in this paper, the following conclusions can be drawn: 1. Organic coatings on non-ferrous metals such as aluminium, zinc, copper and lead are useful for both protective and decorative purposes. 2. A proper surface preparation is essential to obtain durable coatings. This preparation is performed to improve the adhesion of the coatings applied to the specified non-ferrous metals. 3. The choice of the proper paint system depends upon the type of metal. Aluminium can be painted with almost all types of paints, while in the cases of zinc, copper and lead, a limited number of paints is suitable. 4. The paints can be practically applied by any known technique and the choice of the appropriate method depends mainly on the size and shape of the article to be painted.

References 1 S. Wernick, R. Pinner, E. Zurbriigg and R. Weiner, Die Oberfliichenbehondlung van Aluminium, Eugen G. Leuze, Saulgau, 1977.


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