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Pagina principale Shreir's Corrosion VOL IV
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4.01 Surface Pretreatment G. L. Higgins Chemetall Ltd, Aylesbury, UK R. S. Hullcoop Ray Hullcoop and Associates, High Wycombe, UK S. Turgoose CAPCIS-INTETECH, Manchester, UK W. Bullough Corus Research Centre, Rotherham, UK This article is a revision of the Third Edition articles 11.1 and 11.2 by G. L. Higgins and R. S. Hullcoop & S. Turgoose and W. Bullough, volume 2, pp 11:3–11:23, ß 2010 Elsevier B.V. 4.01.1 4.01.2 4.01.2.1 4.01.2.1.1 4.01.2.1.2 4.01.2.1.3 4.01.2.2 4.01.2.3 4.01.2.4 4.01.2.5 4.01.2.5.1 4.01.2.5.2 4.01.2.5.3 4.01.2.5.4 4.01.2.5.5 4.01.2.6 4.01.2.6.1 4.01.2.7 4.01.3 4.01.3.1 4.01.3.2 4.01.3.3 4.01.3.3.1 4.01.3.3.2 4.01.3.4 4.01.3.5 4.01.4 References Introduction Chemical Cleaning Solvent Cleaning Vapor degreasing Emulsifiable cleaners (water rinsable cold solvent cleaning) Emulsion cleaners Neutral Cleaners Acidic Cleaners Alkaline Cleaners Acid Pickling of Ferrous Materials Formation of millscale Mechanism of scale removal Hydrogen embrittlement Pickling of alloy steels Pickling inhibitors Acid Pickling of Nonferrous Metals Electrochemical cleaning Ultrasonic Cleaning Mechanical Cleaning Flame Cleaning Manual Cleaning Abrasive Blast Cleaning Surface finish Surface profile Ultrahigh Pressure Water Jetting Health, Safety, and the Environment Standards Glossary Millscale An oxide that forms on steel during high temperature processing (e.g., hot rolling) and must be effectively removed prior to the application of any subsequent surface protection. Surfactant A substance that lowers the surface tension of a liquid, allowing easier spreading across the surface of a material. 2484 2484 2484 2484 2485 2485 2485 2485 2486 2487 2487 2488 2489 2489 2489 2491 2491 2491 2491 2491 2491 2492 2492 2492 2492 2492 2493 2493 Workpiece The item, component, or material that is undergoing surface pretreatment. Abbreviations BS British Standard EDTA Ethylene diamine tetra-acetic acid 2483 2484 Surface Treatment and Modification EN European Norm ISO International Standards Organisation TLV Thr; eshold limit values VOC Volatile organic compounds 4.01.1 Introduction The attainment of a clean surface prior to the application of any subsequent treatment or coating is essential, whether this subsequent operation is electroplating, anodizing, chemical treatment, or organic coating. The standard of cleanliness which must be achieved has been stated to be ''that which will allow the subsequent process to be carried out satisfactorily''; however, this is an almost meaningless statement. Often it is industry experience that provides a guide to the standard of surface preparation required. As an example, the degree of cleanliness required to satisfactorily zinc plate from an acid solution is somewhat higher than that required prior to zinc plate from a high-cyanide alkali zinc solution. However, this should never be taken as a license to skimp on surface preparation and in another example the arguments over the degree of pretreatment required for 'surface-tolerant' paint coatings abound and will probably continue. It should be remembered that it is to a large extent true that problems of early failure in metal finishing are traceable to incorrect or insufficient surface preparation. 4.01.2 Chemical Cleaning Chemical contamination consists of oils, greases, preservatives, or old paint coatings which must be removed prior to further finishing. Paint coatings can be removed by chemical paint strippers (e.g., solvent or alkali based); however, these methods will not be discussed further here. Sources of most organic contamination are, for example, cutting and machining fluids, preservatives, oils, and greases from, for example, rolling operations, press lubricants, and mechanical or manual handling operations. Various means of removal have been proposed: mechanical action; solvency; detergency; and chemical reaction. In all cleaning operations one or more of these mechanisms will contribute more or less to the overall cleaning procedure, dependent upon the cleaning method and solution employed. With chemical cleaning, performance will be enhanced by the use of mechanical action, such as brushing, air agitation, spraying, electrolysis or ultrasonic agitation. Solvency is where the materials to be removed dissolve in the cleaning medium, for example mineral oil in chlorinated solvents. Detergency is the 'lifting' action attributed to some alkalis and to special surface-active agents commonly referred to as surfactants. Chemical reaction is characterized by, for example, the saponification of some oils in strong alkali, or the reaction of rust with acid solutions. The main types of cleaners used for the removal of organic contaminants are solvent cleaners, neutral cleaners, acid cleaners, and alkali cleaners. 4.01.2.1 Solvent Cleaning The traditional use of flammable hydrocarbon solvents (e.g., white spirit or paraffin) used either by immersion or by manual application is not to be recommended as an effective or particularly safe method of degreasing. When used by immersion, the holding tank can become heavily contaminated and the contaminants will remain on the work after the solvent has evaporated. The use of solvent-soaked rags, although a time-honored procedure, is now being frowned upon on the grounds of operator safety; aqueous based prewipes are available. 4.01.2.1.1 Vapor degreasing The use of hot/boiling solvents, with either immersion of the articles to be cleaned in the bulk solvent or in the overlying vapor using specially designed installations, is an effective use of solvents for cleaning purposes. The solvent, which traditionally was of a halogenated hydrocarbon type, is held in a sump at the base, which is heated by any suitable means and under thermostatic control. Above this may be a wire mesh on which the workpieces are rested. The solvent is condensed near the top of the chamber by cooling coils; thus, between the mesh and the coils is created a region where the solvent is in vapor form. When cold items are introduced, the vapor condenses upon them and liquid solvent flows off, thereby removing contamination. To a large extent, only clean solvent is vaporized, thus ensuring that only fresh solvent is used to clean the workpiece until the sump becomes overcontaminated, when the solvent must be cleaned or replaced. Care should be exercised with some metals, notably aluminum, that solvent with free chloride is not used, as this could lead to pitting of the metal surface. Currently, the use of chlorinated solvents is increasingly deprecated in view of the extent of release of volatile organic compounds (VOC). Therefore, the advice of the manufacturers of the installation and the suppliers of the solvent should always be heeded in the operation of these installations to ensure Surface Pretreatment their trouble-free running. Effective fume control must be available above the installation and the work must be removed slowly enough to ensure that all the solvent has evaporated from the work before it leaves the extracted area. Also, as per governing legislative rules, exposure limits for solvents are increasingly being reduced and current advice must be sought. 4.01.2.1.2 Emulsifiable cleaners (water rinsable cold solvent cleaning) Emulsifiable cleaners (sometimes incorrectly referred to as emulsion cleaners) are blends of organic solvent (often hydrocarbons) with surface active agents and dispersed in an aqueous medium. The work is immersed in the unheated solution for a sufficient time for the cleaner to penetrate the surface thoroughly. The articles are then removed and water rinsed. Additives in the cleaner allow the solvent to emulsify in the water thus removing the contamination. Spray rinsing or agitation in an immersion rinse will aid removal of the residues. This form of cleaning creates disposal problems as the rinse water cannot be merely disposed of into sewerage systems. Thus, the effluent will require storage and the emulsion is broken up before discharge of the water layer and approved disposal of the organic material. As with all solvent-based materials, the need to observe threshold limit values (TLV) for personal exposure and the need for the work to be carried out only under effective fume extraction must be taken into account when considering this type of cleaning product. The cleanliness of the surface produced by emulsifiable cleaners is not of the highest standard, and additional cleaning may well be necessary before further finishing operations. However, in the use of this method prior to some chemical conversion coating processes (e.g., immersion in phosphate baths), the crystal growth can be quite refined because of the absence of the passivation effect often encountered with some heavy-duty alkali cleaners. Needless to say, manufacturers' recommendations should be followed at all times and the suitability of any particular cleaning/pretreatment combination should be confirmed. Another benefit gained from the use of emulsifiable cleaners is that the surface produced is usually hydrophobic and therefore, to an extent, resistant to tarnishing and corrosion in storage. 4.01.2.1.3 Emulsion cleaners These are materials, containing blends of organic solvents and surfactants, which are added to water to form an emulsion. Typical concentrations are in the range 0.5–5%. Such emulsions are normally used 2485 by spray, as either a preclean in a multistage pretreatment line, or as the cleaner in an industrial washing machine. Such washing machines are often used to clean parts which are contaminated with cutting oils and which require inspection before storage. Like the emulsifiable cleaners, the emulsion cleaners, after rinsing, often leave a hydrophobic surface which is resistant to short-term corrosion. Emulsion cleaners can be used hot or cold. Heat generally improves the cleaning action but, in most cases, leads to an objectionable increase in the smell associated with solvent products. 4.01.2.2 Neutral Cleaners These are rapidly replacing emulsion and alkali products for use in industrial washing machines and are generally used at pH 7.5–9, which is considerably lower than that of the corresponding alkali products. Neutral cleaners are on the basis of chemical soaps, with additions of surfactant (to improve cleaning, wetting, penetration, and defoaming), corrosion inhibitors (which may be nitrite or organic), and a bactericide. Neutral cleaners provide the benefits of generally lower operating temperatures, reduced odor, easier effluent treatment, and improved health and safety considerations over the alkali or emulsion products. Because of the inhibited nature of the surface produced, such products are used for inter stage cleaning and prior to assembly. The surface is generally not suitable for immediate painting. 4.01.2.3 Acidic Cleaners The vast majority of acid-based cleaning products are designed for the removal of scale, rust, and other oxide films. These products may also contain solvents and surfactants to degrease and derust simultaneously. There are, however, certain acid-based materials which can primarily be construed as cleaners. One such type of material is used in the cleaning of aluminum cans prior to treating and lacquering. Such cleaners are normally on the basis of sulfuric or phosphoric acid, with, generally, additions of hydrofluoric acid and surfactants. These materials are sprayed on to preformed cans to remove the lubricant used during the can-forming operation. The fluoride is present to enhance the removal of fine metal swarf as well as to remove the oxide film. Fluoride-free acid cleaners are used for general pretreatment cleaning of aluminum as an alternative to strong alkali materials. Although more expensive in terms of initial make-up and plant requirements the rate of material loss through cleaning and etching is reduced. Furthermore, the need for an acid de-smut (required after alkali cleaning) can be eliminated. 2486 Surface Treatment and Modification As the surface smoothing and leveling effects are somewhat limited, the use of acid cleaners prior to anodizing or electropainting, where surface defects can be enhanced, is not common. Care must be taken here not to confuse acid cleaners with high-strength, phosphoric acid-based chemical polishes and chemical brighteners, which are used to obtain specific types of attractive surface finish. Also in the category of acid cleaners could be considered the lightweight alkali– metal phosphating cleaner–coater solutions; a fuller discussion of such materials is best left to specialist publications on metal pretreatment chemicals. 2. The soaps produced by saponification may give excessive foam during spray cleaning or react with hard-water salts to form scum and scale. 3. Amphoteric metals, such as zinc and aluminum, can be attacked (i.e., corroded) more than is desirable. 4. Powder products formulated with too much hydroxide can be hygroscopic and thus tend to form solid blocks in storage rather than remaining as free-flowing powders. 5. Spray cleaners based on hydroxide can pick up carbon dioxide from the atmosphere and such carbonated solutions become less effective. 4.01.2.4 Carbonates and bicarbonates are used as lower alkalinity adjuncts or substitutes for hydroxide and are more resistant to carbonation during spraying than hydroxide-only solutions. Powder products blended with light sodium carbonate are much less hygroscopic and the carbonate can be a useful 'carrier' for liquid additives, such as surfactants and solvents. Silicates can offer an almost complete cleaning system on their own. Sodium silicate, the most commonly used of these materials, has a high enough pH value to cause saponification on its own and the anion formed gives a certain degree of detergency and inhibition. Thus, silicates are often found in multimetal cleaners. Light metals, such as zinc and aluminum will generally not be attacked if the silicate level is sufficiently high and the free hydroxide level sufficiently low. However, cleaners containing silicate can cause problems. They should not be used prior to an alkaline process on aluminum, owing to the formation on the surface of alkali-insoluble aluminum silicate. Cleaners containing silicates can also cause problems if used prior to some surface-sensitive treatments, such as use of zinc phosphating solutions. Other common ingredients of alkaline cleaners include phosphates, which have both detergent and dispersive properties; also, the polyphosphates have water softening capabilities. In addition, borates are often the base for light-duty cleaners, because of their inhibiting action and less alkaline pH. They can also be used, to a certain extent, as a substitute for phosphates when a phosphate-free product is required. The organic acid salts, such as EDTA and heptonate, are included for water softening properties and to assist in the removal of solid particles. Gluconate and heptonate, in particular, are effective in the highly alkaline solutions used for etching aluminum and prevent the precipitation of aluminum hydroxide scale and sludge. Surfactants are probably the materials which most affect the performance of alkali cleaners. Surfactants Alkaline Cleaners Alkali cleaners can be used before almost every conceivable metal finishing operation at one stage or another; they can be used by spray, by immersion, by manual application or by all three, or maybe by two out of the three methods. They can come as powders or as made-up liquids. They may be single or multipack, to be used as supplied or at a range of dilutions. They may require high temperatures or work successfully at ambient temperature. They may be suitable for cleaning one metal only or have multimetal capability. The user thus has an immense range from which to choose. Consideration will first be given to the inorganic species used to produce the base material. The pH values of several commonly used materials are shown in Table 1. Hydroxides are the simplest, strongest alkalis and most commonly used. A major effect of hydroxides in cleaning is saponification: the conversion of certain oils and greases to water-soluble soaptype materials. Beside the benefits of hydroxides must be placed certain disadvantages: 1. The possible passivation of iron and steel surfaces; this can be a problem prior to chemical conversion coatings. Table 1 at 50 C pH values of selected alkalis as 1 wt% solution Alkali pH NaOH Na2CO3 Na2SiO3 Na3PO4 Na2P2O7 Na5P3O10 Na2B4O7 NaC7H13O8 12.7 11.3 12.2 11.8 10.6 9.8 9.3 7.8 Surface Pretreatment are chemicals that modify the solubility of various materials in, and their surface affinity for, oil and water. The diverse composite which makes up the surface of a metal object must be fully wetted out if the cleaner is to perform properly. Surfactants lower the surface tension to allow wetting out to occur. Oils and greases must either be dissolved off the surface or lifted from it; surfactants assist in both areas. There are four broad categories of surfactant on the basis of the charge associated with the active part CH3 N+ C16H33 CH3 Cl− CH3 Cetyl trimethyl ammonium chloride (cationic) SO3− C12H25 Na+ Sodium dodecyl benzene sulphonate (anionic) of the molecule (typical examples of each are given in Figure 1): 1. cationic, where the residual charge is positive; 2. anionic, where the residual charge is negative; 3. amphoteric, where there exists both positive and negative charge centers; 4. nonionic, where there is no residual change. Care must be taken with some surfactant-containing cleaners not to exceed certain temperature and concentration limits. Thus, it is not always true that the hotter and more concentrated the solution, the better the cleaning. Many commonly used surfactants have limited solubility in alkali and become less soluble as the alkalinity, ionic strength, and temperature rise. A point can, therefore, be reached when the surfactants come out of solution and, in immersion cleaning especially, performance will suffer drastically. Similarly, some spray cleaners are designed to work above a certain minimum temperature and strength. In this case, a surfactant is designed to come out of solution to act as a defoamer for the system. Examples of typical simple formulations for various types of alkali cleaners are given in Table 2. 4.01.2.5 CH3 C14H29 CH2COO− N+ CH3 Myristyl dimethyl betaine (amphoteric) 2487 Acid Pickling of Ferrous Materials The hot rolling of steel produces a surface layer of complex oxides known as 'millscale.' It is unstable and must be removed prior to painting if reliable performance is to be obtained. Removal of millscale may be accomplished by a variety of methods but it is traditionally removed by pickling in strong acid containing one or more corrosion inhibitors. 4.01.2.5.1 Formation of millscale C9H19 (OCH2CH2) nOH Polyethoxylated nonyl phenol (nonionic) Figure 1 Common surfactants of each main category. Table 2 When mild steel is heated in air an oxide or scale will form on the steel surface. This scale consists of three well defined layers, whose thickness and composition depend on the duration and temperature of heating. In general, the layers, from the steel base outwards, Typical formulations for alkali cleaners Constituent NaOH Na2SiO3 Na2CO3/NaHCO3 Na3PO4 EDTANa4 Na2B4O7 Surfactant Substrate application Composition (%) 20 50 20 – 5 – 5 Steel immersion 60 20 10 – 5 – 5 Steel electrocleaning 20 40 20 12 5 – 3 Zinc electrocleaning 0 60 10 20 5 – 5 Multimetal immersion 0 0 20 20 8 50 2 Aluminium spray 2488 Surface Treatment and Modification comprise a relatively thick layer of wüstite, the composition of which approximates to the formula FeO, a layer of magnetite (Fe3O4), and a thin layer of hematite (Fe2O3). When the steel is rapidly cooled, the thickness and composition of these layers remain more or less unchanged, but when it is slowly cooled through 575 C the scale becomes enriched in oxygen and the remaining wüstite layer breaks down to some extent into an intimate mixture of finely divided iron and magnetite.1 Holding the temperature between 400 and 575 C causes the iron particles to agglomerate and the scale becomes further enriched in oxygen. As wüstite is unstable below 575 C, scales produced at temperatures lower than this contain magnetite and hematite only. In addition, the scales are often cracked and porous. This is due to the difference in contraction between scale and metal on cooling and to the change in volume when the metal is oxidized. 4.01.2.5.2 Mechanism of scale removal When steel that has been slowly cooled through 575 C is immersed in mineral acid, the acid penetrates through the cracks and pores in the upper layers of scale and rapidly attacks the oxide layers (Figure 2). The iron, being anodic, dissolves to form ferrous ions and the magnetite, being cathodic, is reduced, forming more ferrous ions. As the three constituents of these cells are good electrical conductors, their resistance is sufficiently small so that the rate of dissolution is largely governed by the rate at which acid diffuses in through the cracks and the rate at which spent acid diffuses out through them. A similar but slower action occurs between the Acid Fe2O3 Fe3O4 Partially decomposed wüstite Fe (a) Acid Fe2O3 Fe3O4 Fe (b) Figure 2 Mechanism of scale removal with acid: (a) high temperature scale and (b) low-temperature scale. exposed metal and the magnetite and hematite layers which have not been detached. The mechanisms of oxide dissolution and scale removal have been widely studied and have been thoroughly reviewed by Frenier and Growcock,2 who concluded, in agreement with others,3 that oxide removal from the surface of steel occurs predominantly by a process of reductive dissolution, rather than by chemical dissolution, which is slow in mineral acids. In this process the reduction of the ferric components of the scale is coupled to oxidation of the base metal, both reactions yielding ferrous species readily soluble in the acid. For magnetite the processes are: 1. Fe3 O4 þ 8Hþ þ 2e ¼ 3Fe2þ þ 4H2 O 2. Fe ¼ Fe2þ þ 2e Scale removal is also assisted by the dissolution of the underlying metal by normal acid corrosion processes, which undermines the scale, and by the physical effect of hydrogen gas evolved in this latter reaction. In general there does not appear to be any direct correlation between the rate of the chemical dissolution of oxides and the rate of scale removal, although most work on oxide dissolution has concentrated on magnetite. For example, Gorichev and coworkers have studied the kinetics and mechanisms of dissolution of magnetite in acids4,5 and found that it is faster in phosphoric acid than in hydrochloric, while scale removal is slower in the former. Also, ferrous ions accelerate the dissolution of magnetite in sulfuric, phosphoric and hydrochloric acids, whereas the scale removal rate is reduced by the addition of ferrous ions. These observations appear to emphasize the importance of reductive dissolution and undermining in scale removal, as opposed to direct chemical dissolution. As further confirmation of this Rozenfeld3 has reviewed Russian work on this subject and reports that in pickling with sulfuric acid the amount of acid used in scale dissolution is only about one-tenth that consumed by the dissolution (corrosion) of the underlying metal. However, in hydrochloric acid the direct scale dissolution occurs to a much greater degree, and is responsible for about 40% of the acid consumption. A mechanism such as that given above provides explanations for the known effects of many process variables. The reductive dissolution and undermining processes require access of the acid to the metal surface, hence the benefits obtained by the deliberate introduction of cracks in the oxide by cold-working prior to pickling. Also the increase in pickling rate with agitation or strip velocity can be explained in Surface Pretreatment terms of the avoidance of acid depletion at the oxide– solution interface. 4.01.2.5.3 Hydrogen embrittlement Although the majority of the hydrogen produced on the cathodic areas is evolved as gas and assists the removal of scale, some of it diffuses into the steel in the atomic form and can render it brittle. Hardened and higher carbon ferritic steels are particularly susceptible to this hydrogen embrittlement and cracks may appear during pickling. Austenitic steels, however, are much more resistant to embrittlement because of the lower diffusivity of hydrogen in the face-centered cubic lattice. Certain impurities (e.g., arsenic, sulfur, antimony, etc.) promote the dissolution of hydrogen into the metal (or inhibit the hydrogen recombination reaction generating hydrogen gas, which amounts to the same thing). Consequently, this increases the quantity of hydrogen that diffuses into the steel and the consequent risk of embrittlement. Several theories of the mechanism of embrittlement have been put forward6,7 and these are discussed in more detail elsewhere in this volume. As well as causing brittleness, absorbed gas may combine to form molecular hydrogen on the surface of inclusions and voids within the steel. Thus a gas pressure is set up in the voids and this may be sufficient to cause blisters to appear either during pickling or during subsequent processing such as hot-dip galvanizing. The embrittlement effect can be largely removed by heat treatment of the steel at 150–200 C, but even then the original ductility may not be entirely restored. In estimating the degree of embrittlement, the temperature and rate of testing have an important effect. Thus the embrittlement tends to disappear at very low and very high temperatures and is reduced at high strain rates. 4.01.2.5.4 Pickling of alloy steels The furnace scales which form on alloy steels are thin, adherent, complex in composition, and more difficult to remove than from nonalloy steels. Several mixed acid pickles have been recommended for stainless steel, the type of pickle depending on the composition and thickness of the scale.8 For lightlyscaled stainless steel, a nitric–hydrofluoric acid mixture is suitable, the ratio of the acids being varied to suit the type of scale. Strict chemical control of this mixture is necessary, as it tends to pit the steel when the acid is nearing exhaustion. For heavy scale, two separate pickles are often used. The first conditions the scale and the second removes it. For example, a sulfuric–hydrochloric acid mixture is recommended 2489 as a scale conditioner on heavily scaled chromium steels and a nitric–hydrochloric acid mixture for scale removal. A ferric sulfate–hydrofluoric acid mixture has advantages over a nitric–hydrofluoric acid mixture in that the loss of metal is reduced and the pickling time is shorter, but strict chemical control of the bath is necessary. Electrolytic pickling of stainless steel in 5–10% (w/v) sulfuric acid at 50 C can be used for removing the majority of the scale. The strip is first made anodic, when a little metal dissolves, and then cathodic, when the evolved hydrogen removes the loosened scale. To complete the pickling, a nitric plus hydrofluoric acid dip is given for austenitic steels and a nitric acid dip for ferritic steels. The chromium-containing oxides on stainless steels are more resistant to reductive dissolution and harder to remove than oxides on mild steel. Typically mixed acids and multistage treatments are used and many formulations have been reported.9 Scale conditioning can be carried out in acids, in molten salts (e.g., sodium hydroxide plus sodium nitrate) or in alkaline solutions (e.g., alkaline permanganate). Scale removal can be obtained with a variety of acids, the commonest being a nitric/hydrofluoric mixture. Rozenfeld3 also reports effective pickling with ferric sulfate plus sulfuric acid mixtures and considers that the effect of the ferric ions is to speed up the dissolution of the underlying metal. Organic acids, such as citric acid, also have a role in the cleaning of lightly corroded alloy steels.10 4.01.2.5.5 Pickling inhibitors Organic inhibitors During the pickling of scaled steel, the thinner and more soluble scale is removed before the thicker and less soluble scale. Consequently, some exposed base metal is attacked before the pickling operation is complete. In order to reduce this acid attack to a minimum, corrosion inhibitors are used. Although an effective inhibitor reduces acid attack, it does not prevent the attack of oxidizing agents on the exposed base metal. Thus, the ferric ions resulting from the gradual dissolution of the detached magnetite and hematite may, even in the presence of an inhibitor, attack the exposed steel causing pitting. Clearly, the selected inhibitor should not decompose during the life of the pickle nor decrease the rate of scale removal appreciably. Some highly efficient inhibitors, however, do reduce pickling speed a little. It would be expected that as the hydrogen evolution is reduced the amount of hydrogen absorption and Surface Treatment and Modification embrittlement would also be reduced. This is not always the case; thiocyanate inhibitors, for example, actually increase the absorption of hydrogen. Many organic substances soluble in acid or colloidally dispersible have been shown to have inhibiting properties. The most effective types contain a nonpolar group such as a hydrocarbon chain and a polar group such as an amine. They contain oxygen, nitrogen, sulfur, or other elements of the fifth and sixth groups of the Periodic Table. They include alcohols, aldehydes, ketones, amines, proteins, amino acids, heterocyclic nitrogen compounds, mercaptans, sulfoxides, sulfides, substituted ureas, thioureas, and thioazoles. The efficiency of an inhibitor (I ) under a given set of conditions can be expressed by the ratio of the decreased corrosion rate as a function of the original uninhibited corrosion rate (as a percentage): ðUnhibited corrosion rate Inhibited corrosion rateÞ I ¼ 100% Inhibited corrosion rate In general the efficiency increases with an increase in inhibitor concentration; typically an effective inhibitor gives >95% inhibition at a concentration of 0.01 wt% and 90% at 0.005 wt%. Provided the inhibitor is stable, increase in temperature usually increases the efficiency although the actual acid attack may be greater. A change in acid concentration, or in type of steel, may also alter the efficiency. Thus, the conditions of a laboratory determination of efficiency should closely simulate the conditions expected in commercial practice. Figure 3 shows the relationship between efficiency and concentration of some thiourea derivatives and gelatin in the pickling of cold-reduced and annealed strip in 6 wt% sulfuric acid at 85 C. The thiourea derivatives, diluted with sodium chloride, gelatin and a wetting agent, are used commercially. Mono- and di-O-tolyl thioureas are stable in this pickle for at least 50 h, but diphenyl thiourea and gelatin decompose after 4 or 5 h. The principles behind the selection of effective inhibitors for steel in the various acids have been reviewed by Schmitt11 and Gardner.12 The selection of an inhibitor is dependent on both the metal and the acid. For steel, in general, nitrogen-based inhibitors (e.g., amines and heterocylic compounds) are used in hydrochloric acid, whereas sulfur-containing ones (e.g., thiourea and its derivatives) find more favor in sulfuric acid. Given the reductive dissolution process involved and the contributions from undermining and 100 % Inhibition efficiency 2490 a b c d 95 e 90 85 0 0.01 0.02 0.03 % Inhibitor concentration in pickle 0.04 Figure 3 Relationship between % inhibitor efficiency and inhibitor concentration in 6 wt% H2SO4: (a) di-o-tolyl thiourea; (b) mono-o-tolyl thiourea; (c) commercial inhibitor containing 20% di-o-tolyl thiourea; (d) commercial inhibitor containing 20% diphenyl thiourea; and (e) gelatin. hydrogen evolution in scale removal, inhibitors might be expected to affect the rate of this removal. Also, if the inhibitor adsorbs on the oxide surface then the rate of chemical dissolution of the oxide may be affected. Experimental evidence suggests that these effects may occur, depending on the acid and the inhibitor. Cumper13 has shown that pyrrole and indole can increase the rate of dissolution of magnetite in hydrochloric acid. It has been reported14 that commercial amine-based inhibitors can either increase or decrease the rate of scale removal in the same acid. Other reports suggest that the presence of inhibitor has little effect on scale removal rate in hydrochloric acid but markedly decreases it in sulfuric acid. One area that has not been widely studied is the effectiveness of inhibitors on scaled surfaces, but there is experimental evidence that the presence of magnetite scales can significantly affect the performance of nitrogen-based inhibitors in alkaline solutions used for chemical cleaning. Inorganic inhibitors Inorganic inhibitors are salts of metals having a high hydrogen overvoltage, for example, antimony and arsenic salts. The inhibiting action is associated with the formation of a coating of the metal, which, being cathodic to the steel and having a high hydrogen overvoltage, prevents the discharge of hydrogen ions and so stops the dissolution of the steel. These inhibitors are seldom used in commercial practice, but antimony chloride dissolved in concentrated hydrochloric acid is used in the laboratory for stripping Surface Pretreatment deposits of zinc, cadmium, tin, and chromium from steel, and with the addition of stannous chloride for removing scale and rust.15 4.01.2.6 workpiece. Care must also be taken during cathodic cleaning of steel alloys as hydrogen embrittlement, which must always be considered a risk during cathodic cleaning, must be either avoided or allowed for. Acid Pickling of Nonferrous Metals Although less common, acids are also used for the removal of oxide and scale from nonferrous metals and alloys. Table 3 summarizes a number of pickling solutions for selected materials. 4.01.2.6.1 Electrochemical cleaning A significant increase in immersion cleaning performance can be achieved by local agitation caused by vigorous gas evolution on the surface of the workpiece. Thus, the use of an applied cathodic voltage evolves hydrogen, which provides a 'scrubbing' action, greatly enhancing the cleaning process. However, during cathodic cleaning any dissolved metal ions in solution will tend to plate-out on the metal surface, so the work will normally be given a short anodic cycle at the end of the cleaning time to dissolve this contamination. Also, periodic reverse cathodic–anodic cycling is another way of achieving the same effect. Anodic cleaning can also be performed as a small fraction of the metal is dissolved, thereby simultaneously removing any contamination; however, this should be done with care, as otherwise the workpiece may be destroyed. For electrocleaning, care must be taken that a solution of sufficiently high conductivity is used to prevent solution voltage drops and 'burning' of workpieces in high current density areas. Clearly, excessively high anode current densities should be avoided as these will cause excess corrosion/dissolution of the Table 3 2491 4.01.2.7 Ultrasonic Cleaning Another method for introducing mechanical action into immersion cleaning is by the use of ultrasonic agitation. Here, a high frequency vibration is imparted to the solution. At the surface to be cleaned, minute bubbles are formed by local cavitation of the fluid. When they collapse, they impart a high degree of local agitation, thereby efficiently removing any surface contamination. 4.01.3 Mechanical Cleaning 4.01.3.1 Flame Cleaning Now little used as a preparatory method, flame cleaning is a process whereby an intensely hot flame is played on the surface of the steel. In theory, differential expansion causes millscale to detach. In practice, there is evidence that the treatment may not remove thin, tightly adhering millscale. Also, steel less than 5 mm thick can buckle. Finally, the process can 'burn in' chemicals deposited on the surface, causing premature paint failure. 4.01.3.2 Manual Cleaning The term encompasses all manual and mechanical methods of cleaning other than blast-cleaning, for example abrasive discs, wire brushes, scrapers, vibratory needle guns and chipping hammers are all Pickling solutions for nonferrous metals Metal Copper and brass (60–90% Cu, 10–40% Zn) Aluminimum bronze (82–95% Cu, 5–10% Al, 0–5% Fe, 0–5% Ni) Copper–silicon alloys (96–97% Cu, 1–3% Si) Nickel–copper alloys (55–90% Cu, 10–30% Ni, 0–27% Zn) Nickel–chromium alloys (35–80% Ni, 16–20% Cr, 0–45% Fe, 0–2% Si) Aluminium alloys (0–10% Cu, 0–30% Mg, 0–6% Zn, 0–12% Si) Magnesium alloys (0–10% Al, 0–3% Zn. 0–0.2% Mn) Acid 7–25% (w/w) H2SO4 or 15–25% (w/w) HCl Temperature Time 15–60 C 15 C 75 C 1–10 min 1–3 min 2–5 min 15 C 1–5 min 10% (w/w) HCl þ 1.5% (w/w) CuCl2 80 C 30 min Scale conditioned with 20% NaOH þ 5% (w/w) KMnO4 followed by 20% (w/w) HNO3 þ 4% (w/w) HF 25% (w/w) H2SO4 þ 5% (w/w) CrO3 40% (w/w) HNO3 þ 1–5% (w/w) HF 10–20% (w/w) CrO3 þ 3% (w/w) H2SO4 100 C 50 C 65 C 15 C 100 C 25 C 1–2 h 5–30 min 20 min 1–5 min 1–30 min 15 s Scale conditioned with 10% (w/w) NaOH followed by H2SO4 or HCl as above 7–25% (w/w) H2SO4 þ 1–3% (w/w) HF 2492 Surface Treatment and Modification available. Manual cleaning removes neither tightly adhering millscale nor deep-seated rust from pits. None the less, it is often used for maintenance work or for the preparation of new steelwork to be exposed in nonaggressive conditions. Manual cleaning is rarely used in conjunction with, as it is inappropriate for, high-performance long-life systems, for example, two-pack chemically curing coatings which require a high standard of blast cleaning. 4.01.3.3 Abrasive Blast Cleaning Abrasive blast cleaning uses the action of abrasive particles suspended in a fluid (e.g., air or water) in order to remove surface contamination by mechanical action. Thus, abrasive particles are directed at high velocity against the blasted surface by means of compressed air, high-pressure water, or centrifugal force from an impeller wheel. As the surface removal rate can be controlled by choice of equipment, abrasive grit and blast parameters, all kinds of surfaces may be prepared including heavily rusted steel and concrete, as well as lightly contaminated surfaces. 4.01.3.3.1 Surface finish Surface 'finish' commonly means surface cleanliness. It is important to remember that cleanliness refers not only to the degree of residual rust present, but also to the level of surface chemical contamination from soluble salts and from condensed moisture layers, all of which will detrimentally affect coating adhesion and ultimate performance. 4.01.3.3.2 Surface profile Blast-cleaning produces a roughened surface, the size and nature of which is largely determined by the type and size of the abrasive used. The nature of the profile is important, especially for application of organic coatings (paints) and for thermally sprayed metal coatings. This is because the profile acts to aid the mechanical 'keying' of the coating to the surface and, hence, greatly increases coating adhesion. Likewise, the peak-to-trough profile roughness should not be excessive as this causes rogue peaks from the substrate to penetrate beyond the coating surface. Finally, profiles should be relatively open as excessively compressed surfaces will contain pores and defects underneath the coating that can act as reservoirs for chemical contamination and certainly will prevent coating penetration to the complete surface. 4.01.3.4 Ultrahigh Pressure Water Jetting Water jetting is a convenient method for light cleaning and washing of surfaces. However, if the pressure of the water is increased significantly, the fluid becomes capable of removing, with high efficiency, relatively loosely adherent materials on a surface, including organic coatings. Water jetting also leaves surfaces relatively free of soluble salt contamination, which is a considerable advantage. Because of the kinetic energy of the water droplets on hitting the blasted surface, ultrahigh pressure water jetting leaves a dry, slightly warm surface that is free of dust and which dries relatively quickly; therefore, repainting can be carried out without significant delay. The recent trend is to increase pressures as less volume of water is used. Thus, ultrahigh pressure (>170 MPa) water jetting equipment is becoming preferred as disposal costs for contaminated water are reduced. Water jetting cannot change an existing surface profile; hence if this is inadequate, abrasive blasting is to be preferred. However, for surfaces that have an adequate abrasive blast profile, it is sufficient to merely remove the defective coating and water jetting can be a highly economical option to achieve this end. 4.01.3.5 Health, Safety, and the Environment In all surface preparation methods, including chemical and mechanical cleaning options, health, safety and environmental concerns are essential. Organic solvents are toxic and clearly have high VOC levels while aqueous alternatives may still possess significant organic content; in both cases operator protection and chemical disposal need to addressed adequately. Organic-free chemical cleaners are frequently highly corrosive (acidic or alkaline), and also require correct neutralization and disposal procedures. Mechanical cleaning operations contain their own danger, not the least of which is the safety of the operator; to this end correct operator training, especially for abrasive and water jetting operations, is essential. Finally, waste abrasive grit and paint are frequently contaminated by toxic heavy metals previously used as corrosion inhibitive pigments (i.e., lead, chromium, etc). Removal and disposal of such materials often require a special license and certainly require great care and attention to detail. Surface Pretreatment 4.01.4 Standards Generally standards for surface preparation, especially for surfaces subsequently to be painted, use visual comparisons. In all such standards, some caution should be exercised because the color reproduction may vary from one copy to another. Standards for degrees of surface rusting are described in ISO 8501-1 as follows: (a) Steel surface largely covered with adhering mill scale, but little if any rust. (b) Steel surface which has begun to rust and from which mill scale has begun to flake. (c) Steel surface on which the mill scale has rusted away or from which it can be scraped, but with slight pitting visible under normal vision. (d) Steel surface on which the mill scale has rusted away and on which general pitting is visible under normal vision. Surface rust and millscale must be removed before surface protection is applied. Where this is done by abrasive blasting, standards of surface cleanliness are also described in ISO 8501-1 and are on the basis of the original Swedish SIS 05 5900: Sa 1: Light blast cleaning. Sa 2: Thorough blast cleaning. Sa 2½: Very thorough blast cleaning. Sa 3: Blast cleaning to visually clean steel. The minimum standard that is normally accepted for organic coating of steel structures is the Sa 2½ grade, which is generally achievable in the field using handheld equipment. Sa 3, which is effectively blasting to white metal, can be achieved using modern automated shot-blast plant under factory conditions. References 1. 2. 3. 4. 5. 6. 7. 8. 9. Pfeil, L. B. J. Iron Steel Inst. 1931, 123, 237. Freiner, W. W.; Growcock, F. B. Corrosion 1984, 40, 663. Rozenfeld, I. L. Corrosion Inhibitors; McGraw-Hill, 1981. Gorichev, I. G.; Klyuchnikov, N. G.; Bibikova, Z. P.; Boltovskaya, I. G. Russ. J. Phys. Chem. 1976, 50. Gorichev, I. G.; Gorsheneva, V. F.; Boltovskaya, I. G. Russ. J. Phys. Chem. 1979, 53, 1293. Petch, N. J.; Stables, P. Nature (London) 1952, 169, 842. Morlet, J. G.; Johnson, H. H.; Troiano, A. R. J. Iron Steel Inst. 1958, 189, 37. Spencer, L. F. Metal Finish. 1954, 52(2), 54. Roberts, W. J. In Cleaning Stainless Steel; ASTM Special publ. 538, 1972; p 77. 10. 11. 12. 13. 14. 15. 2493 Blume, W. J. In Cleaning Stainless Steel; ASTM Special publ. 538, 1972; p 43. Schmitt, G. Br. Corros. J. 1984, 19, 165. Gardner, G. In Corrosion Inhibitors; Nathan, C. C., Ed.; NACE, 1973; p 156. Cumper, C. W. N.; Grzeskowiak, R.; Newton, P. Corros. Sci. 1982, 22, 551. Riggs, O. L.; Hurd, R. M. Corrosion 1968, 24, 45. Clarke, S. G. Trans. Electrochem. Soc. 1936, 69, 131. Further Reading Hudson, R. Guide to Good Practice in Corrosion Control: Surface Preparation for Coatings; National Corrosion Service, National Physical Laboratory: UK, 2000. Freeman, D. B. Phosphating and Metal Pre-treatment; Woodhead, 1986. Plaster, H. S. Blast Cleaning and Allied Processes; Industrial Newspapers, 1972/1973; Vol. I/II. Relevant Websites www.waterjetting.org.uk – Water Jetting Association. www.sspc.org – Steel Structures Painting Council/Society for Protective Coatings (SSPC). Standards Swedish Standard SIS 05 5900: ''Pictorial surface preparation standards for painting steel surfaces'' BS 7079:2009: ''General introduction to standards for preparation of steel substrates before application of paints and related products'' BS EN ISO 8501: ''Preparation of steel substrates before application of paints and related products: Visual assessment of surface cleanliness'' BS EN ISO 8502: ''Preparation of steel substrates before application of paints and related products: Tests for the assessment of surface cleanliness'' BS EN ISO 8503: ''Preparation of steel substrates before application of paints and related products: Surface roughness characteristics of blast-cleaned steel substrates'' BS EN ISO 8504: ''Preparation of steel substrates before application of paints and related products: Surface preparation methods'' BS EN ISO 11124: ''Preparation of steel substrates before application of paints and related products: Specifications for metallic blast-cleaning abrasives'' BS EN ISO 11125: ''Preparation of steel substrates before application of paints and related products: Test methods for metallic blast-cleaning abrasives'' BS EN ISO 11126: ''Preparation of steel substrates before application of paints and related products: Specification for nonmetallic blast-cleaning abrasives'' BS EN ISO 11127: ''Preparation of steel substrates before application of paints and related products: Test methods for nonmetallic blast-cleaning abrasives'' BS ISO 27831: ''Metallic and other inorganic coatings: Cleaning and preparation of metal surfaces'' 4.02 Phosphate Coatings M. O. W. Richardson University of Loughborough, Loughborough, Leicestershire, UK R. E. Shaw ICI Paints, UK This article is a revision of the Third Edition article 15.2 by M. O. W. Richardson and R. E. Shaw, volume 2, pp 15:22–15:37, ß 2010 Elsevier B.V. 4.02.1 4.02.2 4.02.2.1 4.02.2.2 4.02.3 4.02.3.1 4.02.3.2 4.02.3.3 4.02.3.4 4.02.3.5 4.02.4 4.02.4.1 4.02.4.2 4.02.4.3 References Introduction Phosphate Coating Formation Deposition Mechanism Accelerators Nature of Coatings Types of Coating Effect of Metal Surface Effect of Phosphate Solution Composition and Structure Post-phosphating Treatment: Rinses Performance Corrosion Protection Testing Phosphophyllite–Hopeite Ratio Glossary Diacetone alcohol 4-Hydroxy-4-methyl-2pentanone: (CH3)2C(OH)CH2COCH3 Hopeite Hydrated zinc phosphate of composition: Zn3(PO4)24H2O Hureaulite Hydrated manganese iron phosphate of composition: (Mn,Fe)5(PO4)44H2O Phosphophyllite Hydrated iron–zinc phosphate of composition: FeZn2(PO4)24H2O Scholzite Hydrated zinc calcium phosphate of composition: CaZn2(PO4)22H2O Vivianite Hydrated iron phosphate of composition: Fe3(PO4)28H2O Abbreviations BS British Standard BSTSA British Surface Treatment Suppliers Association EN European Norm ISO International Standards Organisation Me In a chemical equation, represents the metal ion (e.g., Fe, Zn, Mn, etc.). 2494 2494 2495 2495 2496 2496 2496 2497 2497 2498 2499 2499 2499 2500 2501 2501 4.02.1 Introduction Phosphate treatments are readily adaptable to production requirements for articles of all sizes, and for large or small numbers. Mild steel sheet is the material most frequently subjected to phosphate treatment, but a great variety of other ferrous surfaces is also processed. Examples include cast-iron plates and piston rings, alloy steel gears, high-carbon steel cutting tools, case-hardened components, steel springs and wire, powdered iron bushes and gears, etc. Phosphate treatments designed for steel can also be used for the simultaneous treatment of zinc die-castings, hot-dipped zinc, and zinc-plated articles. However, if there is a large quantity of these nonferrous articles, it is often more economical to phosphate them without the steel. Aluminum articles may also be phosphated; however, these require somewhat different bath chemistries. The usual method of applying phosphate coatings is by immersion, using a sequence of tanks that includes degreasing and phosphating stages, with their respective rinses. The treatment time ranges from 3 to 5 min for thin zinc phosphate coatings and up to 30–60 min for thick zinc, iron, or manganese phosphate coatings. The accelerated zinc phosphate processes lend themselves to application by power Phosphate Coatings spray, and the processing time may then be reduced to 1 min or less. Power spray application is particularly advantageous for mass-production articles such as motorcars and refrigerators, as the conveyor can run straight through the spray tunnel, which incorporates degreasing, rinsing, phosphating, rinsing, and drying stages. Flow-coating and hand spray-gun application are sometimes employed when a relatively small number of large articles have to be phosphated. 4.02.2 Phosphate Coating Formation 4.02.2.1 Deposition Mechanism All conventional phosphate coating processes are based on dilute phosphoric acid solutions of iron, manganese, and zinc dihydrogen phosphates either separately or in combination. The free phosphoric acid in these solutions reacts with the iron surface undergoing treatment in the following manner1: Fe þ H3 PO4 ! FeðH2 PO4 Þ2 þ HHhH2 ½1 Thus, ferrous dihydrogen phosphate is produced and hydrogen is liberated. Local depletion of phosphoric acid occurs at the metal–solution interface. As the dihydrogen phosphates of iron, manganese, and zinc dissociate readily (i.e., are soluble) in aqueous solution, the following reactions take place: MeðH2 PO4 Þ2 ! MeHPO4 þ H3 PO4 ½2 3MeHPO4 ! Me3 ðPO4 Þ2 þ H3 PO4 ½3 3MeðH2 PO4 Þ2 ! Me3 ðPO4 Þ2 þ 4H3 PO4 ½4 The neutralization of free phosphoric acid by reaction [1] alters the position of equilibrium of equations [2]–[4] towards the right and thereby leads to the deposition of the sparingly soluble hydrogen phosphates and insoluble phosphates on the metal surface. As reaction [1] takes place even when the phosphating solution contains zinc or manganese phosphate with little or no dissolved iron, it will be seen that the simple (or nonaccelerated) phosphate treatment gives coatings that always contain iron phosphate derived from the ferrous parts being processed. After prolonged use, a manganese phosphate bath often contains more iron in solution than manganese and produces coatings with an iron content 2–3 times that of manganese. The relation between free phosphoric acid content and total phosphate content in a processing bath, whether based on iron, manganese, or zinc, is very 2495 important; this relation is generally referred to as the acid ratio. An excess of free acid will retard the dissociation of the dihydrogen and hydrogen phosphates and hinder the deposition of the insoluble phosphate coating; sometimes excessive loss of metal takes place and the coating is loose and powdery. When the free acid content is too low, dissociation of phosphates takes place in the solution as well as at the metal/solution interface and leads to precipitation of insoluble phosphates as sludge. Nonaccelerated processes operated near boiling generally work best with a free-acid titration between 12.5% and 15% of the total acid titration. A zinc phosphate solution tends to produce coatings more quickly than iron or manganese phosphate solutions, and dissociation of zinc dihydrogen phosphate proceeds rapidly through reaction [2] to [3] or directly to zinc phosphate via reaction [4]. Even so, a processing time of 30 min is usual with the solution near boiling. Another factor in the initiation of the phosphate coating reaction is the presence in the processing solution of phosphate, either as a colloidal suspension or as fine particles.2 This effect is most apparent in zinc phosphate solutions, which produce good coatings only when turbid. The zinc phosphate particles can be present to a greater extent in cold processing solutions and may act as nuclei for the growth of many small crystals on the metal surface, thereby promoting the formation of smoother coatings. Similarly, the ferric phosphate sludge formed during the processing of steel in a zinc phosphate solution can play a useful part in coating formation.3 The solubility of ferric phosphate is greater at room temperature than at elevated temperatures, and is increased by the presence of nitrate accelerators. To allow for saturation at all temperatures, it is desirable always to retain some sludge in the processing bath. Coatings with optimum corrosion resistance are produced when the temperature of the bath is rising and causing supersaturation of ferric phosphate. With zinc/iron/phosphate/nitrate baths, the iron content of the coating comes predominantly from the processing solution and very little from the surface being treated.4 This greatly diminished attack on the metal surface by accelerated baths has a slight disadvantage in practice in that rust is not removed, whereas the vigorous reaction of the nonaccelerated processes does remove light rust deposits. The solution of iron represented in eqn [1] takes place at local anodes of the steel being processed, while discharge of hydrogen ions with simultaneous dissociation and deposition of the metal phosphate takes place at the 2496 Surface Treatment and Modification local cathodes.1 Thus, factors that favor the cathode process will accelerate coating formation and, conversely, factors favoring the dissolution of iron will hinder the process. Application of a cathodic potential to the treated surface exerts an accelerating action, as the reaction at all cathodic areas is assisted and the formation of a phosphate layer is speeded accordingly. Conversely, an anodic potential results in the dissolution of iron at local anodes and hinders phosphate coating formation. An oxidizing agent acts as an accelerator by depolarization of the cathodes, raising the density of local currents so that rapid anodic passivation of active iron in the pores takes place. This inactivation of local anodes favors the progression of the cathodic process. The accelerating effect of alternating current is explained by the practical observation that the cathodic impulse acting protectively greatly exceeds in its effect the anodic impulse which dissolves iron. In a similar manner, the electrolytic pickling of iron with alternating current can dissolve iron at a slower rate than when no current is used. Reducing agents have the same ultimate effect as cathodic depolarization in that they convert anodic regions into cathodic regions and increase the ratio of cathodic to anodic areas. Nitrogenous organic components such as toluidine, quinoline, aniline, etc. all act as inhibitors to the anodic reaction between metal and acid and thereby favor the cathodic reaction and accelerate the process. 4.02.2.2 Accelerators þ 3þ 2Fe2þ þ NO þ NO 3 þ 2H ! 2Fe 2 þ H2 O In acid solution, the nitrite ion acts as an oxidizing agent. A slight degree of acceleration can be obtained by introducing traces of metals which are more noble than iron, for example nickel, copper, cobalt, and silver. These metals are deposited electrochemically over the iron surface undergoing treatment, thereby providing more active cathodic centers and promoting phosphate deposition. This method of acceleration has the disadvantage of leaving minute particles of the noble metal in the coating, and, in the case of copper, this can seriously inhibit the drying of some types of paint coatings. 4.02.3 Nature of Coatings The majority of phosphate processes in use are 'accelerated' to obtain shorter treatment times and lower processing temperatures. The most common mode of acceleration is by the addition of oxidizing Table 1 agents such as nitrate, nitrite, chlorate, and hydrogen peroxide. By this means, a processing time of 1–5 min can be obtained at temperatures of 45–70 C. The resultant coatings are much smoother and thinner than those from nonaccelerated processes, and, while the corrosion resistance is lower, they cause less reduction of paint gloss and are more suited to massproduction requirements. The presence of nitrate as accelerator has a pronounced effect on the amount and composition of gas evolved from the work being treated1 (Table 1). It will be observed that hydrogen evolution drops to a very low figure with the zinc/nitrate baths. The formation of nitrite arises from the oxidation of ferrous ion by nitrate leading to ferric ions, which will then deposit out as ferric phosphate: 4.02.3.1 Types of Coating Although there is a limited number of phosphate coating types, there are many specific bath compositions Amount and composition of the gases evolved on phosphating of 1 m2 of sheet metal for deep drawing Phosphate solution Pointagea Time (min) Amount of gas (cm3m2) H2 (%) N2 (%) O2 (%) N2O þ NO (%) Manganese phosphate Zinc phosphate Manganese phosphate (accelerated with nitrate) Zinc phosphate (accelerated with nitrate) Zinc phosphate containing 1.5–2 g l1 iron (accelerated with nitrate) 30 40 30 60 30 15 7000 2540 3500 87.5 92.7 84.6 11.4b 6.4b 9.1 1.1 0.9 1.3 – – 3.0 70 5 78 16.7 75.3 8.0 – 70 5 85 32.1 57.0 1.6 9.3 A measure of the total of a phosphatine solution, as indicated by the number of ml of 0.1 s sodium hydroxide (4.0 g l1) needed to neutralise 10 ml of the phosphating solution to phenolphthalcin. b Presumably from anrides present in the steel. a Phosphate Coatings that have been formulated for optimization of particular applications and processes.5 Lightweight amorphous iron phosphates provide excellent paint adhesion and are of low cost; however, the corrosion resistance is relatively poor. Lightweight (microcrystalline) zinc phosphate coatings offer higher corrosion resistance while still retaining very good adhesion for organic coatings. Heavy macrocrystalline zinc phosphates are used in applications such as fastener coatings or wire and tube drawing aids. Where used in this fashion, the coating provides excellent adhesion and a slightly porous surface that can act as a reservoir for drawing lubricants such as soaps and/or oils. Likewise, heavy-weight manganese–iron phosphates and calcium–zinc phosphates may be used to reduce galling on mating surfaces such as valve train components or hypoid gears. These coatings have good oil absorbing properties and provide good corrosion resistance when used with a rust-preventing oil. Military ordnance components are often finished by this method. Table 2 provides a summary of some types and applications of phosphate coatings. 4.02.3.2 Effect of Metal Surface The state of the metal surface has a pronounced effect on the texture and nature of phosphate coating produced by orthodox processes. Heavily worked surfaces tend to be less reactive and lead to patchy coatings. Surface abrasive blasting greatly simplifies treatment and gives uniform phosphate coatings. Accidental contamination of sheet steel with lead has been shown to have an adverse effect on the corrosion resistance and durability of phosphate coatings and paint.6 Cleaning operations that make use of strong acids or strong alkalis tend to lead to the formation of Table 2 2497 excessively large phosphate crystals which do not completely cover the metal surface and therefore show inferior corrosion resistance; this is particularly serious if rinsing is inadequate between the preparatory treatment and the phosphating. Adherent dust particles can also lead to the formation of relatively large phosphate crystals, whereas surfaces that have been wiped beforehand show much smoother and more uniform phosphate coatings. On the other hand, the provision of vast numbers of minute nuclei assists the phosphate coating reaction to start at a multitude of centers, resulting in a finely crystalline coating. This effect can be obtained chemically by a predip in a solution of sodium phosphate containing minutely dispersed traces of titanium or zirconium salts7 or in weak solution of oxalic acid. This type of predip entirely eliminates any coarsening effect due to previous treatment in strong alkalis or acids. 4.02.3.3 Effect of Phosphate Solution Improved nucleation within the phosphate solution itself can produce smoother coatings without the necessity of recourse to preliminary chemical treatment. This may be accomplished by introducing into the phosphating bath the sparingly soluble phosphates of the alkaline-earth metals or condensed phosphates such as sodium hexametaphosphate or sodium tripolyphosphate. Such modified phosphating baths produce smoother coatings than orthodox baths and are very much less sensitive to cleaning procedures. Very thin coatings of iron phosphate can be produced by treatment with solutions of alkali-metal phosphate. These serve a useful purpose for the treatment of office furniture, toys, etc. where a high Types of phosphate coating Type Mineral name Formula Coating weight (g m2) Appearance Lightweight iron phosphate Microcystalline zinc phosphate Macrocrystalline zinc phosphate Manganese iron phosphate Calcium zinc phosphate Vivianite Fe3(PO4)28H2O 0.3–0.9 Hopeite or phosphophyllite Hopeite or phosphophyllite Hureaulite Zn3(PO4)24H2O; FeZn2(PO4)24H2O 1.4–2.7 Iridescent pink/yellow/ blue to light gray Light gray Zn3(PO4)24H2O; FeZn2(PO4)4H2O 7.5–30 Light gray (Mn,Fe)5(PO4)44H2O 7.5–30 Dark gray to black Scholzite CaZn2(PO4)22H2O 1.6–6.5 Light gray Source: British Surface Treatment Suppliers Association, Technical Brochure: ''Metal pre-treatment.'' Available for download from http:// www.bstsa.org.uk. Surface Treatment and Modification degree of protection is not required, and also as a base for phenolic varnishes or resin varnishes requiring high temperature curing, for example, at over 200 C. The coating is of heterogeneous nature and contains less than 35% iron phosphate (FePO48H2O) with the remainder probably g-Fe2O3.8 Thin phosphate coatings can be formed by application of phosphoric acid solution alone, that is, not containing metallic phosphates, to a steel surface, sufficient time being allowed after application to enable complete reaction to take place. In this way, a thin film of iron phosphate can be formed. In practice, it is difficult to obtain complete conversion, and the remaining traces of phosphoric acid can cause blistering of paint coatings. This effect may be insignificant on rough, absorbent steel surfaces, for example, ship's plating, where heavy coats of absorbent paint are applied, and under these circumstances the treatment can enhance the corrosion resistance of the finishing system. 4.02.3.4 Composition and Structure The simplest phosphate coating that is formed from a solution containing only ferrous phosphate and phosphoric acid consists of dark gray to black crystals of ferrous phosphate, Fe3(PO4)2 and ferrous hydrogen phosphate, FeHPO4, with a small proportion of ferric phosphate, FePO4. Coatings formed from manganese phosphate solutions consist of Mn3(PO4)2, and those from zinc phosphating solutions consist of Zn3(PO4)2. With both the manganese and zinc type of coating, insoluble iron phosphates, derived from iron present in the bath, may also be present in solid solution. Iron from the surface being treated can also be present in the coating, particularly at the metal– phosphate interface. The PO3 4 content of coatings may vary from 33% to 50%, whereas the theoretical PO3 4 content is lowest, at 41%, in Zn3(PO4)24H2O and highest, at 63%, in FePO4. X-ray diffraction studies have given very good correlation between the mineral hopeite, Zn3(PO4)24H2O, and the zinc phosphate coatings that form on steel.9 Zn3(PO4)24H2O appears in three crystal forms: a-hopeite (rhombic plates), b-hopeite (rhombic crystals), and d-hopeite (triclinic crystals). Their transition points are at 105, 140, and 163 C, respectively. It has been observed10 that zinc phosphate coatings heated in the absence of air lose their corrosion resistance at between 150 and 163 C probably due to disruption in coating integrity due to the volume changes arising from the phase change. Likewise, manganese phosphate coatings lose their corrosion resistance at between 200 and 218 C, where they lose between 75 and 80% of their water of hydration. It is assumed that this results in a volume decrease of the coating which causes voids and thereby lowers the corrosion resistance. Figure 1 shows the loss of water of hydration from zinc, iron, and iron– manganese phosphate coatings. The heating of phosphate coatings in the absence of air provides conditions similar to those prevailing during the stoving of paint on phosphated articles, but in general the paint stoving temperatures and times are well below those at which damage to zinc phosphate coatings takes place. The loss of water from conventional zinc and manganese phosphate coatings heated in air is from 10 to 20% higher than the loss on heating in the absence of air. It is thought that this greater loss may be due to oxidation of the iron phosphate present in the coatings. The most important uses for phosphate coatings entail sealing with oil or paint and it is therefore of interest to study absorption values. Table 3 compares the absorption of diacetone alcohol into coatings of widely differing thicknesses and composition; despite these differences, values of 10.5–13 g m2 are obtained throughout. It is therefore evident that absorption is predominantly a surface effect and not appreciably influenced by coating thickness. 10 8 Loss in coating weight (%) 2498 A 6 4 2 B C 0 100 120 140 160 Heating temperature (⬚C) 180 Figure 1 Effect of heating on phosphate coatings for 16 h at various temperatures, showing loss of water of hydration: (a) zinc phosphate; (b) iron phosphate, and (c) iron manganese phosphate. Reproduced from B.S.I. Phosphate Coatings (Drafting) Panel, J. Iron Steel Inst. 1952, 170, 13. Phosphate Coatings 2499 Table 3 Analytical tests on industrial phosphate coatings Processa P S T Q V R Main cation in phosphate bath Fe Mn Zn Zn Zn Zn Method of application Immersion Immersion Spraying Immersion Immersion Immersion Duration of treatment (min) 15 30 1.5 4 5 12 Change in weight on phosphating (g m2) Coating weight (g m2) 2 PO3– 4 (g m ) Moisture (mg m2) PO3– 4 content of coating (%) Moisture content of coating (%) Hygroscopicity of coating (%) Absorption value (diacetone alcohol (g m2) 26.1 26.4 2.61 3.37 1.63 5.87 14.2 7.0 81.5 49.0 0.6 0.3 11.4 21.2 8.9 76.1 42.0 0.4 0.2 10.9 4.46 1.96 152.2 44.0 3.4 1.2 10.9 5.43 2.07 396.6 38.0 6.9 1.3 13.04 3.48 1.20 173.9 34.0 5.0 1.0 10.87 12.28 4.46 771.7 36.0 6.4 1.5 11.96 a The letters used for designation indicate proprietory process. Data reproduced courtesy of the Journal of the Iron and Steel Institute, 1952, 170, 11. 4.02.3.5 Rinses Post-phosphating Treatment: After phosphating, thorough rinsing with water is necessary in order to remove soluble salts which would otherwise tend to promote blistering under a paint film. Care should also be taken to ensure that the water supply itself is sufficiently free from harmful salts. Experience has shown that a water supply is potentially injurious if it exceeds any one of the three following limits: 1. 70 ppm total chlorides and sulfates (calculated as Cl þ SO2 4 ); 2. 200 ppm total alkalinity (calculated as CaCO3); and 3. a maximum of 225 ppm of (1) and (2) together. Improved corrosion resistance and reduced tendency to blistering was traditionally obtained by treating the final rinse with chromic acid, or preferably with phosphoric and chromic acids combined. Normally, a total acid content of 0.05% was used, with higher concentrations of chromate increasing corrosion resistance. However, legislation restricting the use of chromates has now eliminated this practice. The use of demineralized water for rinsing purposes is recommended, especially before electrophoretic painting, as this practice reduces residual surface contamination. The demineralized water is generally applied by misting jets at the end of all other pretreatment stages and allowed to flow back into the last rinse tank. In certain cases, rinsing may be dispensed with after nonaccelerated phosphate treatment, but blistering of paint due to local concentration of solution in seams and crevices may occur. Thus, it is good practice to employ rinsing, regardless of the type of phosphate process.11 4.02.4 Performance 4.02.4.1 Corrosion Protection The corrosion protection provided by phosphate coatings without a sealing treatment is of a low order; their value when sealed is considerably greater. Unsealed corrosion tests are therefore of little value except perhaps for studying porosity or efficiency of coatings destined to be sealed only with oil. It is usually necessary to control the acid ratio of phosphating baths, particularly those of iron, manganese, and zinc operating near boiling point that produce heavy coatings. The best performing coatings are obtained when the free acidity is maintained between 12.5% and 15%; above this figure, coatings with progressively lower corrosion resistance are obtained. Note that heavy, that is, thicker phosphate coatings do not necessarily have better corrosion resistance than lighter coatings. Even with a single process, for example, zinc/iron/phosphate/nitrate, no consistent relationship has been found between corrosion resistance and either coating weight or weight of metal dissolved. Jaudon12 tested phosphate coatings and found the salt-spray resistance, as judged by the first appearance of rust, while only a few minutes for base steel, was around a few hours for phosphated steel. 2500 Table 4 Surface Treatment and Modification Typical phosphate coating processes Phosphate coating solution Accelerator Immersion time (min) Type of coating Coating weight (g m2) Iron Iron/manganese Manganese Zinc Zinc None None Nitrate Nitrate Nitrate/nitrite or chlorate None 30 30 15 15 3 Heavy Heavy Heavy Medium Light 10.87–32.61 10.87–32.61 8.70–32.61 3.26–32.61 1.09–6.52 1–2 (spray) Very light 0.22–0.65 Sodium/ammonium Phosphating prior to painting doubled the observed time to first rusting. Phosphate processes containing little or no oxidizing agent and based on manganese or zinc tend to accumulate iron in solution from the work being processed. With a manganese content from 0.2% to 0.5%, it is best to control the iron between 0.2% and 0.4%; a higher iron content reduces the corrosion resistance and may lead to the formation of thin powdery coatings, while a lower iron content gives soft coatings. Similarly, a zinc process operates best with 0.15–0.5% zinc and 0.4–0.5% iron. Again, with a higher iron content corrosion resistance falls off and powdery coatings may be formed, and soft coatings result from lower iron content. Within broad limits, phosphate processes can be classified according to the main metallic radical of the processing solution and the type of accelerator used; typical processes are given in Table 4. The selection of process and of coating weight is mainly dependent on the end-use of the article being processed; the general requirements for corrosion resistance and wear prevention are given in Table 5. 4.02.4.2 Table 5 Typical uses of phosphate coatings on steel Coating weight (g m2) For corrosion resistance For wear prevention and metal forming 21.74–32.61 – 10.87–21.74 Military equipment, etc. requiring oil or grease finish 5.43–10.87 Nuts, bolts, clips, brackets 2.17–2.72 Cars, refrigerators, washing machines Steel drums, bicycles, office machinery Toys, office furniture Strip steel, for painting and forming Critical cold extrusion Normal cold extrusion 'Running in' treatment for piston rings, gears and tappets Wire and tube drawing Sheet steel pressing Light metal pressing 1.63–2.17 0.22–0.65 Testing Heavy phosphate coatings are generally used as protection against corrosion in conjunction with a sealing film of oil or grease. The porosity or free pore area of these coatings should be kept to a minimum. Machu13 devised a method of examination based on the quantity of electricity necessary to effect passivation of the bare steel and used this to determine the free pore area which, in the phosphate coatings tested, varied from 0.27% to 63%; however, this method is insufficiently sensitive to evaluate thin zinc phosphate coatings. A method for making rapid measurements of the electrical resistance of phosphate coatings has been described by Scott and Shreir.14 The most widely used accelerated tests are based on salt spray, whose main value is in the evaluation of the effectiveness of phosphate coatings in restricting the spread of rust from scratches or other points of damage in a paint film.15 This feature is of particular interest to the automotive industry, as vehicles are often exposed to marine atmospheres and to moisture and salt when the latter is used to disperse ice and frost from road surfaces. Great care is needed in the interpretation of a salt-spray test, as it has been found to favor thin iron phosphate coatings more than is justified by experience with natural weathering. Humidity tests are generally of more practical use than salt-spray tests, particularly where painting is employed, as the thoroughness of rinsing may be checked by this means. The use of contaminated water can leave water-soluble salts in the phosphate Phosphate Coatings Table 6 Weights of phosphate coatings Class of phosphate process A 1. Heavyweight (Mn or Fe) A 2. Heavyweight (Zn) B Medium weight (Zn, etc.) C Lightweight (Zn, etc.) D Extra lightweight (Fe) Coating weight (g m2) Min. Max. 7.61 7.61 4.34 1.09 0.33 – – – 4.34 1.09 Source: BS EN 12476: Phosphate conversion coatings of metals. Method of specifying requirements. coating and lead to blistering of the paint film under humid conditions, as paint films are permeable to water vapor. Immersion in water, or subjection to high humidity in a closed cabinet, will generally show any defects of this kind within a few days. Phosphate coating weight determinations are generally performed by dissolving the coating from weighed panels using inhibited acid (e.g., by immersion in a solution of 20 g dm3 of antimony trioxide in concentrated hydrochloric acid at a temperature of 13–21 C. Thin iron or zinc phosphate coatings can be removed for weight determination by immersion in 5% chromic acid solution at 70 C, but this solution should also be used only once, as the presence of more than a trace of phosphate leads to pitting of the steel and false results. Zinc phosphate coatings can be removed by immersion in 10% sodium hydroxide at boiling temperature, aided by rubbing during rinsing. BS EN 12476 contains information on the operation of phosphate processes to obtain optimum results and on the testing of phosphate coatings.16 The classification of coatings according to composition and weight is shown in Table 6. The inspection and testing includes determination of coating weight, freedom from corrosive residues as shown by a humidity test, and resistance to corrosion by salt spray. BS EN ISO 12944-1 is also a valuable source of information.17 4.02.4.3 Phosphophyllite–Hopeite Ratio There is evidence that the ratio of phosphophyllite to hopeite (the so-called P/(P þ H) ratio) can be used as a predictive measure of the performance of phosphated steel when coated with cathodic electroprimer and paint.18,19,20,21 In this context, P is defined as the intensity of X-rays diffracted from the (100) plane of phosphophyllite, FeZn2(PO4)24H2O, and H is defined as the intensity of X-rays diffracted from the (020) plane of hopeite, Zn3(PO4)24H2O, 2501 and it has been suggested that higher values of this ratio (i.e., a low volume fraction of hopeite) are synonymous with good corrosion performance.18 However, other factors to be considered include the need for homogeneous phosphate layers of controlled thickness, the direct attachment of the primer to a coherent layer, and the level of interlayer cohesion within the coating. Some papers21 indicate that adhesion failure results from internal fracture of the phosphate coating and that it is concentrated at the junction between a primary microcrystalline or even amorphous layer close to the metal substrate and a secondary layer exhibiting relatively coarse crystallinity. The primary layer is comprised essentially of a zinc phosphate material, and the Zn/P ratio in the retained primary layer after fracture is lower than that in the detached material, though close enough to be considered essentially similar. Finally, it is worth noting that the quality of the steel substrate can have an effect on the corrosion resistance promoted by any subsequent treatment by phosphating and painting. Indeed, it has been reported22 that interesting results are obtained when cold-rolled steel panels, with different amounts of surface contamination, are zinc phosphated, then coated with anionic or cationic electrocoat primers, and followed by a conventional filler-topcoat system. In salt-spray, scab, and filiform corrosion tests, it is apparently possible to distinguish between different surface contamination levels and primer coatings. Carbonaceous residues on the steel can have a detrimental effect, and this can be confirmed in the case of anionic primer during salt-spray tests. In the scab corrosion and filiform corrosion tests, however, anionic primer performance actually increases with surface contamination. It can be concluded that the steel condition and the type of coating affect the corrosion resistance of the entire system by inducing changes in the phosphate layer. With the current low level of surface contamination of commercial steels and the highly resistant modern coating formulations, it is suggested22 that the phosphate layer is the weakest link in the entire system. References 1. 2. 3. Machu, W. Die Phosphatierung — Wissenschaftliche Grundlagen und Technik; Verlag Chemie: Weinheim, 1950. Wusterfeld, H. Arch. Metallk 1949, 3, 233. Determination of the solubility of ferric phosphate in phosphating solutions using radioiron, US Department of Commerce, Office of Technical Services, Rep. no. PB 111, 1953; p 399. 2502 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. Surface Treatment and Modification Radiometric study of phosphating problems, US Department of Commerce, Office of Technical Services, Rep. no. PB 111, 1951; p 396. British Surface Treatment Suppliers Association, Technical Brochure: ''Metal pre-treatment.'' Available for download from http://www.bstsa.org.uk. Wirshing, R. J.; McMaster, W. D. Can. Paint Varn. Mag. 1956, 30(5), 42, 55. Jernstedt, G. Chem. Eng. News 1943, 21, 710; Trans. Electrochem. Soc. 1943, 83, 361. A radiometric study of the iron phosphating process, US Department of Commerce, Office of Technical Services, Rep. no. PB 111, 1953; p 400. X-ray diffraction study of zinc phosphate coatings on steel, US Department of Commerce, Office of Technical Services, Rep. no. PB 111, 1954, p 486. Doss, J. Org. Finish. 1956, 17(8), 6. B.S.I. Phosphate Coatings (Drafting) Panel. J. Iron Steel Inst. 1952, 170, 13. Jaudon, E. Peint.-Pigm.-Vernis. 1949, 25, 224. Machu, W. Korros, Metallsch. 1944, 20, 1. Scott, J. W.; Shreir, L. L. Chem. Ind. (Rev.) 1957, 807. BS EN ISO 9227: Corrosion tests in artificial atmospheres: Salt spray tests. 16. 17. 18. 19. 20. 21. 22. BS EN 12476: Phosphate conversion coatings of metals. Method of specifying requirements. BS EN ISO 12944-1: Paints and varnishes. Corrosion protection of steel structures by protective paint systems. Miyawaki, T.; Okita, H.; Umehara, S.; Okabe, M. Proc. Interfinish. 1980, 80, 303. Richardson, M. O. W.; Freeman, D. B.; Brown, K.; Djaroud, N. Trans. I.M.F. 1983, 61, 155. Freeman, D. B.; Brown, K.; Richardson, M. O. W.; Tiong, H. Finishing 1984, 27–28. Cooke, B. A. Proceedings of the IX International Conference on Organic Science and Technology, Athens; 1983; pp 29–46. Soepenberg, E. N.; Vrijburg, H. G.; van Ooij, W. J.; Vries, O. T. The Influence of Steel Quality, Pretreatment and Coating Systems on the Corrosion of Automotive Steel, 1984; pp 381–393. Relevant Websites http://www.bstsa.org.uk – British Surface Treatment Suppliers Association, Birmingham, UK. 4.03 Coatings Produced by Anodic Oxidation X. Zhou University of Manchester, Manchester, M13 9PL, UK P. G. Sheasby Alcan International, Banbury, UK B. A. Scott British Aluminium Company, UK This article is a revision of the Third Edition article 15.1 by P. G. Sheasby and B. A. Scott, volume 2, pp 15:3–15:21, ß 2010 Elsevier B.V. 4.03.1 4.03.2 4.03.3 4.03.3.1 4.03.3.2 4.03.3.3 4.03.3.3.1 4.03.3.3.2 4.03.3.4 4.03.3.5 4.03.3.6 4.03.3.7 4.03.3.8 4.03.3.9 4.03.4 4.03.4.1 4.03.4.2 4.03.5 4.03.5.1 4.03.5.2 4.03.5.3 4.03.5.4 References Practice of Anodizing Mechanism of Formation of Porous Oxide Coatings Properties of Coatings Composition Density Mechanical Properties Hardness Flexibility Electrical Properties Physical Properties Thermal Properties Optical Properties Effect of Anodizing on Mechanical Properties Measurement of Film Thickness Corrosion Resistance Atmospheric Exposure Maintenance Recent Developments Practice of Anodizing Mechanism of Anodizing Properties of Coatings Test Methods Glossary Barrier anodic film An oxide coating deliberately formed on a valve metal (commonly aluminum) that is generally relatively featureless and provides an electrical and physical barrier to the bare metal surface. Porous anodic film An oxide coating deliberately formed on a valve metal (commonly aluminum) that generally contains self-organized directional porosity; all porous anodic films are terminated by a form of barrier anodic film adjacent to the base metal surface. Plasma electrolytic oxidation A process for anodizing that uses relatively high voltages that deliberately induce dielectric breakdown, 2504 2505 2507 2507 2508 2508 2508 2509 2509 2509 2510 2510 2511 2511 2511 2513 2513 2514 2514 2515 2516 2516 2516 with associated sparking, on the treated component; also called micro-arc oxidation. Valve metal An element from the group: aluminum, titanium, zirconium, hafnium, tungsten, niobium and tantalum. Such elements will develop protective oxide films when polarized anodically in a suitable electrolyte. Symbols MAO Micro-arc oxidation PEO Plasma electrolytic oxidation SHE Standard Hydrogen Electrode 2503 2504 Surface Treatment and Modification 4.03.1 Practice of Anodizing Anodic oxidation or anodizing, as applied to metallic surfaces, is the production of a coating, generally of oxide, on the surface by electrolytic treatment in a suitable solution, the metal being the anode. Although a number of metals,1 including aluminum, magnesium, tantalum, titanium, vanadium and zirconium, can form such anodic films, only aluminum and its alloys, and to a lesser extent magnesium, are anodized on a significant commercial scale for corrosion protection. The anodic oxidation of magnesium does not normally produce a film that has sufficient corrosion resistance to withstand exposure without further protection by painting, and the solutions used are complex mixtures containing phosphates, fluorides and chromates. In the case of aluminum, a relatively simple treatment produces a hard, compact, strongly adherent film of oxide, which affords considerably increased protection against corrosive attack.2,3 A further advantage of this process lies in the decorative possibilities of the oxide film, which may be almost completely transparent on very high purity aluminum (99.99% Al) and certain alloys based on this purity, and thus protects the surface without obscuring its polish or texture. On metal of lower purity, and other alloys, the oxide layer may become slightly milky, or colored grey or yellowish, although the deterioration is hardly apparent with purities down to 99.7–99.8% Al. The appearance and character of the film may also be influenced by the type of anodizing treatment, and the oxide film may be dyed to produce a wide range of colored finishes. Anodizing characteristics of a number of aluminum alloys are listed by Wernick and Pinner.3 The anodizing procedures in general use are shown in Table 1, sulfuric acid being the most commonly used electrolyte. Treatment time is 15 min to 1 h. The articles to be anodized should be free from crevices where the acid electrolyte can be trapped. They may be given a variety of mechanical and chemical pretreatments, including polishing, satin-finishing, etching, etc. but before anodizing, the surface must be clean and free from grease and polishing compound. After the anodic treatment, the work is removed from the tank and carefully swilled with cold water to remove all traces of acid. At this stage, the anodic film is absorptive, and care should be taken to avoid contamination with oil or grease, particularly if the work is to be dyed. Dyeing may be carried out by immersion for about 20 min in an aqueous solution of the dyestuff at a temperature of 50–60 C. Inorganic pigments may also be incorporated in the oxide layer by a process involving double decomposition. Finally, both dyed and undyed work are sealed by treatment in boiling water (distilled or deionized) or steam, which enhances the corrosion resistance and prevents further staining or leaching of dye. Solutions of metal salts, for example nickel or cobalt acetates, are often used to seal work after dyeing. Traditionally sealing in 5–10% dichromate solution, which gives the coating a yellow color, was used to impart the highest degree of corrosion resistance. However, the use of chromates is being increasingly phased out due to environmental concerns.4 In the architectural field, use is being made of integral color anodizing which is capable of producing self-colored films in a number of fade-resistant tints ranging from gray, through bronze and brown, to a warm black. The electrolytes are developments of the oxalic acid solution and consist of various dibasic organic acids, such as oxalic, malonic or maleic, or sulfonated organic acids such as sulfosalicylic acid, together with a small proportion of sulfuric acid. For constant and reproducible results, a close analytical control of the electrolyte must be maintained, particularly with respect to aluminum which dissolves as Table 1 Traditional anodizing processes for aluminum (the chromic acid treatment is historic and is being increasingly phased out) Electrolyte Temperature ( C) Applied voltage (V) Current density (A m2) 5–10% sulfuric acid 17–22 12–24 110–160 3–25 3–10% chromic acid 2–5% oxalic acid 30–45 20–35 30–45 30–60 30–35 110–215 2–8 10–60 Film thickness (mm) Appearance Transparent: colorless to milky Opaque: light to dark gray Transparent: light yellow to brown Coatings Produced by Anodic Oxidation treatment proceeds, and ion-exchange resins are frequently used to regenerate the relatively expensive electrolyte and keep the aluminum in solution between controlled limits. Some typical color anodizing treatments are summarized in Table 2. Alloys are generally of the Al–Mg–Si type with additions of copper and chromium or manganese. Color varies with the particular alloy and the film thickness. For optimum control of color, the alloy must be carefully produced with strict attention to composition, homogenization and heat-treatment, where appropriate, and the anodizing conditions must be maintained within narrow limits. It is usual to arrange matters, preferably with automatic control, such that current density is held constant with rising voltage up to a selected maximum, after which voltage is held steady; the whole cycle being for a fixed time. Refrigeration of the electrolyte may be necessary to maintain the temperature at the working level, owing to the relatively high wattage dissipation. Hard anodic films, 50–100 mm thick, for resistance to abrasion and wear under conditions of slow-speed sliding, can be produced in sulfuric acid electrolytes at high current density and low temperature.5–7 Current densities range from 250 to 1000 A m2, with or without superposed alternating current in 20–100 g l1 sulfuric acid at 4 to 10 C. Under these conditions, special attention must be paid to the contact points to the article under treatment, in order to avoid local overheating. The films are generally dark in color and often show a fine network of cracks due to differential expansion of oxide and metal on warming to ambient temperature. They are generally left unsealed, since sealing markedly reduces abrasion resistance, but may be impregnated with silicone oils8 to improve the frictional properties. Applications include movable instrument parts, pump bodies and plungers, etc. Decorative self-colored films9 can also be produced in sulfuric acid under conditions intermediate between normal and hard anodizing. Table 2 2505 Continuously anodized sheet, foil and wire, which may be given a dyed finish, are produced through a series of cleaning, etching and anodizing tanks, with a film thickness up to about 6 mm. Uses include electrical windings for transformers and motors, where the light weight of aluminum and the insulating and heat-resistant properties of the film are of value, and production of small or light-section articles by stamping or roll-forming. 4.03.2 Mechanism of Formation of Porous Oxide Coatings It has been established that in the case of electrolytes, such as boric acid or ammonium phosphate solutions, in which aluminum oxide is insoluble, this anodic passivity is due to the formation of a thin compact layer of aluminum oxide whose thickness is proportional to the applied voltage. In neutral phosphate solutions, for example, film growth practically ceases when the thickness corresponds to about 1.4 nm V1, and a similar value has been found for many other electrolytes of this type. These thin films have a high electrical resistance, and can withstand several hundred volts under favorable conditions. In electrolytes in which the film has a moderate solubility, film growth is possible at lower voltages, for example, in the range 12–60 V, since the rate of formation of the oxide exceeds its rate of solution and current flow continues owing to the different structure of the oxide layer. Electron microscopy has revealed the characteristic porous structure of these films.10 The pore diameter appears to be a function of the nature and concentration of the electrolyte and of its temperature, being greatest in a solution of high solvent activity, while the number of pores per unit area varies inversely with the formation voltage. In any given electrolyte, the lower the temperature and concentration, and the higher the voltage, the denser will be the coating, as the number of pores per unit Traditional integral color anodizing processes Electrolyte Temperature ( C) Current density (A m2) Voltage (V) Time (min) Sulfosalicylic acid (100 g l1) þ sulfuric acid (50 g l1) 4- or 5-Sulfophthaic acid (75–100 g l1) þ sulfuric acid (8–10 g l1) Oxalic acid (80 g l1 to saturation) 22–25 215–320 25–70 20–45 15–30 130–370 >70 30 15–25 130–160 35–70 50–90 2506 Surface Treatment and Modification area are reduced under these conditions; Table 3 illustrates these points. In order to account for the relatively high potential required to maintain the current it was suggested by Setoh and Miyata11 that a thin barrier-layer, similar to that formed in nonsolvent electrolytes, is present below the porous layer. This view has been supported by later work involving capacity and voltage– current measurements, which have allowed the thickness of the barrier-layer to be computed.12 As in the case of electrolytes which produce barrier films, the thickness has been found to be proportional to the anodizing voltage, but is lower than the limiting growth rate of 1 nm V1 and varies with the anodizing conditions (Table 4). The morphology of the anodic film is shown in Figure 1 and diagrammatically in Figure 2. The more or less regular pattern of pores imposes a cellular structure on the film, with the cells approximating in plan to hexagons, each with a central pore.13–15 A barrier layer separates the pores from the aluminum, the alumina cells can self-organize to a close-packed arrangement,13,16 with groups of cells having a potential for organization into regular patterns.13,16–18 The pore diameter and barrier layer thickness are related to the anodizing voltage, with Table 3 Number of pores in anodic oxide coatings Electrolyte Temperature ( C) Applied voltage (V) Pores (cm2) 15% sulfuric acid 10 10 10 49 49 49 24 24 24 15 20 30 20 40 60 20 40 60 77 109 51 109 28 109 22 109 8 109 4 109 36 109 12 109 6 109 3% chromic acid 2% oxalic acid Data reproduced from Keller, F.; Hunter, M. S.; Robinson, D. L. J. Electrochem. Soc. 1953, 100, 411. Table 4 Barrier-layer thickness in various electrolytes Electrolyte Temperature ( C) Barrier layer growth ratio 15% sulfuric acid 3% chromic acid 2% oxalic acid 10 38 24 1.00 1.25 1.18 Data reproduced from Hunter, M. S.; Fowle, P. J. Electrochem. Soc. 1954, 101, 481. ratios of about 1.0 nm V1, with an increased ratio of about 2.8 nm V1 for the cell width.19 The metal surface underlying the film, therefore, consists of a close-packed regular array of nearly hemispherical depressions which increase in size with the anodizing voltage. The thickness of the individual cell walls is approximately equal to that of the barrier-layer.20 In view of its position in the electrochemical series (E 0 Al3þ/Al ¼ 1.66 V (SHE)), aluminum would be expected to be rapidly attacked even by dilute solutions of relatively weak acids. In fact, the rate of chemical attack is slow, owing to the presence on the aluminum of a thin compact film of air-formed oxide. When a voltage is applied to an aluminum anode there is a sudden initial surge of current, followed by a rapid fall to a lower, fairly steady value. It appears that this is due to the formation of a barrier-layer. Before the limiting thickness is reached; however, the solvent action of the electrolyte initiates a system of pores at weak points or discontinuities in the oxide barrier-layer. Pore initiation may be associated with irregularities in original air-formed film on the metal. The development of locally thicker film regions with subsequent pore initiation in the thinner film region is related to the substrate texture. Initial uniform film thickening is apparent, followed by development of locally thicker film material above the distinct metal ridges upon the aluminum surface after pretreatment, and eventual pore development in preferred regions of thinner film between the protuberances, as shown in Figure 3.21–23 Growth of oxide continues on a series of hemispherical fronts centered on the pore bases, provided that the effective barrier-layer thickness between the metal surface and the electrolyte within the pores, represented by the hemisphere radius, is approximately 1 nm V1. As anodic oxidation proceeds at a uniform rate, a close-packed hexagonal cell-pattern is produced, the inward extension of the pore due to solution of oxide keeping pace with the inward movement of the oxide–metal interface, as shown by the arrows in Figure 2. Radiochemical and electron microscopy studies21–25 indicate that the pore base is the actual site of formation of aluminum oxide. The high electric field that exists across the barrier layer drives the transport of Al3+ and O2 ions through the barrier layer, which is necessary for film growth.23,26 The formation of porosity has mainly been attributed to field-accelerated dissolution of anodic alumina at the base of each pore,22,23 which is also enhanced by the heating Coatings Produced by Anodic Oxidation 2507 AI 0.3 µm Anodic film Porous layer Figure 1 Transmission electron micrographs of ultramicrotomed cross section of porous anodic film, revealing pores at the centers of cells that pass perpendicular to metal surface and the barrier layer present at the base of pore. Metal Barrier layer Figure 2 Diagrammatic cross-section of porous anodic oxide film. effect of the current, but will also be limited by diffusion. In a process that is separate to field-assisted dissolution, the Al3þ ions that migrate to the pore base are ejected directly to the electrolyte,23 that is, without formation of anodic alumina, such that new film is formed due to the inward migration of O2 ions. It has been shown that the freshly formed oxide, a-Al2O3, is amorphous.27 The very early stages of pore growth have been extensively studied, and the importance of surface topography and flaw sites in the preexisting oxide established by O'Sullivan, Wood24 and Thompson and Wood28 using electron microscopy. Prolonged action of the acid electrolyte on thick films may cause the pores to become conical in section, widening towards the upper surface of the film. This will impose an upper limit on film thickness in solvent electrolytes, as found in practice. For color anodizing, although it might seem at first sight that dyestuffs are merely held mechanically within the pores, and this view is probably correct in the case of inorganic pigments, there is some support for the opinion that only those dyestuffs which form aluminum–metal complexes produce really light-fast colorations. The effect of hot water sealing is to convert anhydrous g-Al2O3 into the crystalline monohydrate, Al2O3H2O, which occupies a greater volume and blocks up the pores, thus preventing further absorption of dyes or contaminants. The monohydrate is also less reactive. 4.03.3 Properties of Coatings 4.03.3.1 Composition The main constituent of the film is aluminum oxide, in a form which varies in constitution between amorphous Al2O3 and g-Al2O3, together with some monohydrate, Al2O3H2O. In the presence of moisture, both the anhydrous forms are gradually transformed into the monohydrate, and the water content of asformed films is, therefore, somewhat variable. After sealing in boiling water, the composition of the completely hydrated film obtained when using sulfuric acid approximates to: 70% Al2O3, 17% H2O, 13% SO3. It is probable that the SO3 is combined with the aluminum as a basic sulfate. Sealed films show the electron diffraction pattern of the monohydrate, beohmite. Anion incorporation in the film is a further important factor affecting film characteristics. It has been 2508 Surface Treatment and Modification AI 0.1 µm (a) 0.1 µm (b) 0.1 µm (c) Figure 3 Transmission electron micrographs of ultramicrotomed cross-section of porous anodic film in the early stages of development. The arrow in (b) highlights a local crack/repair event. found that anion species are incorporated into the films produced in the main acids, including oxalic acid, sulfuric acid, phosphoric acid and boric acid– formamide electrolytes.29 The levels of acid anion incorporation are generally in the order: sulfuric acid > phosphoric acid > oxalic acid > chromic acid. Using a radiotracer technique and electron probe microanalysis, sulfur incorporation in films formed in sulfuric acid were determined. A typical level of 4–18 wt% sulfur was found, distributed uniformly throughout film thickness, by different researchers.30–33 Anodizing in chromic acid produces contrasting results from films formed in sulfuric, oxalic, phosphoric acids; the films contain relatively low levels of chromate incorporation. The distribution of the anion present after anodizing in oxalic acid has been studied by Rutherford back scattering (RBS) and Auger electron spectroscopy (AES).34 Carbon is present in the oxide layer in measurable concentrations. The carbon concentration shows a maximum at the surface, a slow decrease through the oxide, and with no signal within a thin oxide region adjacent to the metal. An oxalate content of about 2.4 wt% in films formed in oxalic acid was reported.35 4.03.3.2 Density Owing to the variable degree of porosity of the anodic film, it is only possible to determine the apparent density, which varies with the anodizing conditions and also with the film thickness. Figure 4 shows the variation in density with thickness for steam-sealed anodic films produced in sulfuric acid on aluminum of 99.99% and 99.5% purity. A mean figure of 2.7 g cm3 for sealed, and 2.5 g cm3 for unsealed films is accepted by the British Standard for anodized aluminum.36 4.03.3.3 Mechanical Properties 4.03.3.3.1 Hardness It is not possible to obtain a reliable figure for the hardness of anodic coatings with either the indentation or scratch methods, because of the influ