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Chromate conversion coatings
surface treatments CHROMATE CONVERSION COATINGS BY FREDW. EPPENSTEINER(RETIRED) AND MELVIN R.JENKINS MACDERMID INC,, NEW HUDSON, MICH.; www.macdermid.com Chromate conversion coatings are produced on various metals by chemical or electrochemical treatment with mixtures of hexavalent chromium and certain other compounds. These treatments convert the metal surface to a superficial layer containing a complex mixture of chromium compounds. The coatings are usually applied by immersion, although spraying, brushing, swabbing, or electrolytic methods are also used. A number of metals and their alloys can be treated; notably, aluminum, cadmium, copper, magnesium, silver, and zinc. The appearance of the chromate film can vary, depending on the formulation of the bath, the basis metal used, and the process parameters. The films can be modified from thin, clear-bright and blue-bright, to the thicker, yellow iridescent, to the heaviest brown, olive drab, and black films. A discussion of specific formulations is not included in this article because of the wide variety of solutions used to produce the numerous types of finishes. It is intended to present sufficient general information to permit proper selection and operation of chromating baths. Proprietary products, which are designed for specific applications, are available from suppliers. PROPERTIES AND USES
physical Char~te~rics Most chromate films are soft and gelatinous when freshly formed. Once dried, they slowly harden or "set" with age and become hydrophobic, less soluble, and more abrasion resistant. Although heating below 150°F (66°C) is of benefit in hastening this aging process, prolonged heating above 150°F may produce excessive dehydration of the film, with consequent reduction of its protective value. Coating thickness rarely exceeds 0.00005 in., and often is on the order of several microinches. The amount of metal removed in forming the chromate film will vary with different processes. Variegated colors normally are obtained on chromating, and are due mainly to interference colors of the thinner films and to the presence of chromium compounds in the film. Because the widest range of treatments available is for zinc, coatings for this metal afford an excellent example of how color varies with film thickness. In the case ofelectroplated zinc, clear-bright and blue-bright coatings are the thinnest. The bIue-brights may show interference hues ranging from red, purple, blue, and green, to a trace of yellow, especially when viewed against a white background. Next, in order of increasing thickness, come the iridescent yellows, browns, bronzes, olive drabs, and blacks. Physical variations in the metal surface, such as those produced by polishing, machining, etching, etc., also affect the apparent color of the coated surface. The color of the thinner coatings on zinc can also be affected indirectly by chemical polishing, making the finish appear whiter.
Corrosion Prevention Chromate conversion coatings can provide exceptionally good corrosion resistance, depending upon the basis metal, the treatment used, and the film thick413
ness. Protection is due both to the corrosion-inhibiting effect of hexavalent chromium contained in the film and to the physical barrier presented by the film itself. Even scratched or abraded films retain a greatdeal of their protective value because the hexavalent chromium content is slowly leachable in contact with moisture, providing a self-healing effect. The degree of protection normally is proportional to film thickness; therefore, thin, clear coatings provide the least corrosion protection, the light iridescent coatings form an intermediate group, and the heavy olive drab to brown coatings result in maximum corrosion protection. The coatings are particularly useful in protecting metal against oxidation that is due to highly humid storage conditions, exposure to marine atmospheres, handling or fingerprint marking, and other conditions that normally cause corrosion of metal.
BondingofOrganicFinishes The bonding of paint, lacquer, and organic finishes to chromate conversion coatings is excellent. In addition to promoting good initial adhesion, their protective nature prevents subsequent loss of adhesion that is due to underFflm corrosion. This protection continues even thought he finish has been scratched through to the bare metal. It is necessary that the organic finishes used have good adhesive properties, because bonding must take place on a smooth, chemically clean surface; this is not necessary with phosphate-type conversion coatings, which supply mechanical adhesion that is due to the crystal structure of the coating.
ChemicalPolishing
Certain chromate treatments are designed to remove enough basis metal during the film-forming process to produce a chemical polishing, or brightening, action. Generally used for decorative work, most of these treatments produce very thin, almost colorless films. Being thin, the coatings have little optical covering power to hide irregularities. In fact, they may accentuate large surface imperfections. In some instances, a leaching or "bleaching" step subsequent to chromating is used to remove traces of color from the film. If chemical-polishing chromates are to be used on electroplated articles, consideration must be given to the thickness of the metal deposit. Sufficient thickness is necessary to allow for metal removal during the polishing operation.
Alasorb~ andDyeing When initially formed, many films are capable of absorbing dyes, thus providing a convenient and economical method of color coding. These colors supplement those that can be produced during the chromating operation, and a great variety of dyes is available for this purpose. Dyeing operations must be conducted on fleshly formed coatings. Once the coating is dried, it becomes nonabsorbent and hydrophobic and cannot be dyed. The color obtained with dyes is related to the character and type of chromate film. Pastels are produced with the thinner coatings, ancl the darker colors are produced with the heavier chromates. Some decorative use of dyed finishes has been possible when finished with a clear lacquer topcoat, though caution is required because the dyes may not be lightfast. In a few cases, film colors can be modified by incorporation of other ions or dyes added to the treatment solution.
Hardness Although most coatings are soft and easily damaged while wet, they become reasonably hard and will withstand considerable handling, stamping, and cold forming. They will not, however, withstand continued scratching or harsh abrasion. A 414
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few systems have been developed that possess some degree of "wet-hardness," and these will withstand moderate handling before drying.
Heat Resistance Prolonged heating of chromate films at temperatures substantially above 150°F (66°C) can decrease their protective value dramatically. There are two effects of heating that are believed to be responsible for this phenomenon. One is the insolubilization of the hexavalent chromium, which renders it ineffective as a corrosion inhibitor. The second involves shrinking and cracking of the film, which destroys its physical integrity and its value as a protective barrier. Many factors, such as the type of basis metal, the coating thickness, heating time, temperature, and relative humidity of the heated atmosphere, influence the degree of coating damage. Thus, predictions are difficult to make, and thorough performance testing is recommended if heating of the coating is unavoidable. The heat resistance of many chromates can be improved by certain posttreatments or "sealers." Baking at paint-curing temperatures after an organic finish has been applied is a normal practice and does not appear to affect the properties of the treatment film.
E/ectr/ca/Res/~e The contact resistance of articles that have been protected with a chromate conversion coating is generally much lower than that of an unprotected article that has developed corroded or oxidized surfaces. As would be expected, the thinner the coating, the lower the contact resistance, i.e., clear coatings have the least resistance, iridescent yellow coatings have slightly more, and the heavy, olive drab coatings have the greatest. If exposure of an article to corrosive conditions is anticipated, the choice of a coating thickness normally involves a compromise between a very thin film-which, although having very low initial contact resistance, is likely to allow early development of high electrical resistance corrosion products-and a heavier film, with somewhat higher initial contact resistance, but which is likely to remain relatively constant for a longer period under corrosive conditions.
Fabrication
Resistance Welding. Thin chromate films do not interfere appreciablywith spot, seam, or other resistance-welding operations. Aluminum coated with a thin, nearly colorless film, for example, can be spot welded successfully with no increase in welding machine settings over those required for bare metal. Metal coated with thicker, colored films also can be resistance welded. The increased contact resistance of thicker coatings, however, necessitates using slightly higher machine settings. Fusion Welding. These operations, likewise, are not hampered by the presence of chromate films. It has been reported, in fact, that chromate treatments on aluminum actually facilitate inert gas welding of this metal and its alloys, producing contamination-free welds. Soldering. Cadmium and silver surfaces coated with thin chromate films can be soldered without difficulty using a mild organic flux. Conflicting reports exist regarding the solderabilty ofchromated zinc surfaces. MechanicalFastening. The assembly ofchromated parts using bolts, rivets, and other mechanical fastening devices usually results in local damage to the chromate film. Corrosion protection in these areas will depend upon the effectiveness of the self-healing properties of the surrounding coating. Summary of Common Uses Table I summarizes the most common applications of chromate conversion coatings. 416
TaMe I. Common Uses of Chromate Conversion Coatings General Usage Metal
Corrosion Resistance
Paint Base
Chemical Metal Polish Coloring Remarks X
Aluminum
X
X
Cadmium
X
X
X
X
Copper
X
X
X
X
Magnesium
X
X
Silver
X
Zinc
X
X
X
X
Economical replacement for anodizing if abrasion resistance is not required. Used to "touch-up" damaged areas on anodized surfaces. Thin coatings prevent "spotting out"of brass and copper electrodeposits. No fumes generated during chemical polishing.
MATERIALS OF CONSTRUCTION Generally, suppliers of proprietaries recommend materials for use with their products, which are resistant to oxidants, fluorides, chlorides, and acids. Materials that have been found to be satisfactory for most chromating applications are stainless steels and plastics. Stainless steels such as 304, 316, 317, and 347 are suitable for tanks and heaters where chlorides are absent. Containers and tank linings can be made from plastics such as polyvinyl chloride (PVC), polyvinylidinechloride (PVDC), polyethylene, and polypropylene. Acid-resistant brick or chemical stoneware is satisfactory for some applications, but is subject to attacks by fluorides. Parts-handling equipment is made of stainless steel, plastisol-coated mild steel, or plastic. Mild steel can be used for leaching tanks because the solutions are generally alkaline, whereas tanks for dyeing solutions, which are slightly acid, should be of acid-resistant material. Usually, ventilation is not necessary because most chromate solutions are operated at room temperature and are nonfuming. Where chromating processes are heated, they should be ventilated.
FILM FORMATION
Mechanism The films in most common use are formed by the chemical reaction of hexavalent chromium with a metal surface in the presence of other components, or "activators," in an acid solution. The hexavalent chromium is partially reduced to trivalent chromium during the reaction, with a concurrent rise in pH, forming a complex mixture consisting largely of hydrated basic chromium chromate and hydrous oxides of both chromium and the basis metal. The composition of the film is rather indefinite, because it contains varying quantities of the reactants, reaction products, and water of hydration, as well as the associated ions of the particular systems. There are a number of factors that affect both the quality and the rate of formation of chromate coatings. Of the following items, some are peculiar to chromating; many derive simply From good shop practice. A working understanding of these factors will be helpful in obtaining high-quality, consistent results. Different formulations are required to produce satisfactory chromate films on various met417
als and alloys. Similarly, the characteristics of the chromate film produced by any given solution can vary with minor changes in the metal or alloy surface. Commonly encountered examples of this follow.
EA~umCtof Basis Metals inureAlloys.The ease with which coatings on aluminum can be produced, and the degree of protection afforded by them, can vary significantlywith the alloyingconstituents and/or the heat treatment of the part being processed. In general, low alloying constituent metals that are not heat treated are easiest to chromate and provide the maximum resistance to corrosion. Conversely, wrought aluminum, which is high in alloying elements (especially silicon, copper, or zinc) or which has undergone severe heat treatment, is more difficult to coat uniformly and is more susceptible to corrosive attack. High silicon casting alloys present similar problems. The effect of these metal differences, however, can be minimized by proper attention to the cleaning and pretreatment steps. Most proprietary treatment instructions contain detailed information regarding cleaning, desmutting, etc., of the various alloys. Magnesium Alloys. As in the case of aluminum, the alloying element content and the type 0fheat treatment affect the chromating of magnesium. With the exception of the dichromate treatments listed as Type III in Military Specification MIL-M-3171, all of the treatments available can be used on all the magnesium alloys. Zinc Alloys. Chromate conversion coatings on zinc electroplate are affected by impurities codeposited with the zinc. For example, dissolved cadmium, copper, and lead in zinc plating solutions can ultimately cause dark chromated films. Similarly, dissolved iron in noncyanide zinc plating solutions can create chromating problems. Furthermore, the activity of zinc deposits from cyanide and noncyanide solutions can differ sufficiently to produce variations in the chromate film character.
Variations in the composition of zinc die casting alloys and hot-clipped galvanized surfaces can also affect chromate film formation; however, in the latter case, the result is usually difficult to predict, due to the wide variations encountered in spelter composition, cooling rates, etc. Large differences in the chromate coating from spangle to spangle on a galvanized surface are not uncommon. This is especiallyevident in the heavier films. CopperAlloys. Since chromate treatments for copper and its alloys can be used to polish chemically as well as to form protective films, the grain structure of the part becomes important, in addition to its alloying content. Whereas fine-grained, homogeneous material responds well to chromate polishing, alloys such as phosphor bronze and heavily leaded brass usually will acquire a pleasing but matte finish. In addition, treatment of copper alloys, which contain lead in appreciable amounts, may result in the formation of a surface layer of powdery, yellow lead chromae.
of
One of the more important factors in controlling the formation of the chromate film is the pH of the treatment solution. For any given metal/chromate solution system, there will exist a pH at which the rate of coating formation is at a maximum. As the pH'is lowered from this point, the reaction products increasingly become more soluble, tending to remain in solution rather than deposit as a coating on the metal surface. Even though the rate of metal dissolution increases, the coating thickness will remain low. Chemical-polishing chromates for zinc, cadmium, and copper are purposely operated in this low pH range to take advantage of the increased rate of metal removal. The chromate films produced in these cases can be so thin 418
that they are nearly invisible. Beyond this point, further lowering of the pH is sufficient to convert most chromate treatments into simple acid etchants. Increasing the pH beyond the maximum noted above will gradually lower the rate of metal dissolution and coating formation to the point at which the reaction, for all practical purposes, ceases.
Hexavalent Chromium Concentration Although the presence ofhexavalent chromium is essential, its concentration in many treatment solutions can vary widely with limited effect, compared with that of pH. For example, the chromium concentration in a typical aluminum treatment solution can vary as much as 100% without substantially affecting the film-formation rate, as long as the pH is held constant. In chromating solutions for zinc or cadmium, the hexavalent chromium can vary fairly widely from its optimum concentration if the activator component is in the proper ratio and the pH is constant.
Actiuators Chromate films normally will not form without the presence of certain anions in regulated amounts. They are commonly referred to as "activators' and include acetate, formate, sulfate, chloride, fluoride, nitrate, phosphate, and sulfamate ions. The character, rate of formation, and properties of the chromate film vary with the particular activator and its concentration. Consequently, many proprietary formulations have been developed for specific applications and they are the subject of numerous patents. Usually, these proprietary processes contain the optimum concentrations of the activator and other components; therefore, the user need not be concerned with the selection, separate addition, or control of the activator.
OPERATING CONDITIONS In addition to the chemical make-up of the chromating solution, the following factors also govern film formation. Once established for a given operation, these parameters should be held constant. Treatment Time. Immersion time, or contact time of the metal surface and the solution, can vary from as little as 1 second to as much as 1 hour, depending on the solution being used and metal being treated. If prolonged treatment times are required to obtain desired results, a fault in the system is indicated and should be corrected. Solution Temperature. Chromating temperatures vary from ambient to boiling, depending on the particular solution and metal being processed. For a given system, an increase in the solution temperature will accelerate both the film-forming rate and the rate of attack on the metal surface. This can result in a change in the character of the chromate film. Thus, temperatures should be adequately maintained to ensure consistent results. Solution A~tation. Agitation of the working solution, or movement of the work in the solution, generally speeds the reaction and provides more uniform film formation. Air agitation and spraying have been used for this purpose. There are, however, a few exceptions where excessive agitation will produce unsatisfactory films.
Solution Contamination Although the presence of an activator in most treatment solutions is vital, an excessive concentration of this component, or the presence of the wrong activator, can be very detrimental. Most metal-finishing operations include sources of potential activator contamination in the form of cleaners, pickles, deoxidizers, and
419
desmutters. Unless proper precautions are taken, the chromate solution can easily become contaminated through drag-in of inadequately rinsed parts, drippage from racks carried over the solution, etc. A common source of contamination is that resulting from improperly cleaned work. If allowed to go unchecked, soils can build on the surface of the solution to the point at which even clean work becomes resoiled on entering the treatment tank, resulting in blotchy, uneven coatings. Other contaminants to be considered are those produced by the reactions occurring in the treatment solution itself. With very few exceptions, part of the trivalent chromium formed and part of the basis metal dissolved during the coating reaction remain in the solution. Small amounts of these contaminants can be beneficial, and"broken-in" solutions often produce more consistent results. As the concentration of these metal contaminants increases, effective film formation will be inhibited. For a certain period, this effect can be counteracted by adjustments, such as lowered pH and increased hexavalent chromium concentration. Eventually, even these techniques become ineffective, at which point the solution must be discarded or a portion withdrawn and replaced with fresh solution.
P~nsingand Drfing Once a chromate film has been formed satisfactorily, the surface should be rinsed as soon as possible. Transfer times from the chromating stage to the rinsing stage should be short in order to minimize the continuing reaction that takes place on the part. Although rinsing should be thorough, this step can also affect the final character of the chromate film and should be controlled with respect to time and temperature, for consistent results. Prolonged rinsing or the use of very hot rinsewater can dissolve, or leach, the more soluble hexavalent chromium compounds from a freshly formed coating, resulting in a decrease in protective value. If a hot rinse is used to aid drying, avoid temperatures over about 150°F (66°C) for more than a few seconds. This leaching effect sometimes is used to advantage. In instances in which a highly colored or iridescent coating may be objectionable, a prolonged rinse in hot water can be used as a"bleaching" step to bring the color to an acceptable level. Instead of hot water leaching, some systems incorporate dilute acids and alkalis to accelerate this step.
SolutionControl Because most chromate processes are proprietary, it is suggested that the suppliers' instructions be followed for solution make-up and control. Even though specific formulations will not be discussed, certain general principles can be outlined, which apply generally to chromate solutions. The combination ofhexavalent chromium concentration, activator type and concentration, and pH, i.e., the "chemistry" of the solution, largely determines the type of coating that will be obtained, or whether a coating can be obtained at all, at given temperatures and immersion times. It is important that these factors making up the "chemistry" of the solution be properly controlled. As the solution is depleted through use, it is replenished by maintenance additions, as indicated by control tests or the appearance of the work. Fortunately, analysis for each separate ingredient in a chromate bath is not necessary for proper control. A very effective control method uses pH and hexavalent chromium analysis. The pH is determined with a pH meter and the chromium is 420
Table II. Typical Salt Spray Data for Electroplated Zinc Treatment
Hours to White Corrosion
Untreated Clear chromate Iridescent yellow Olivedrab
<8 24-100 100-200 100400
determined by a simple titration. Indicators and pH papers are not recommended because of discoloration by the chromate solution. Additions are made to the solution to keep these two factors within operating limits. The amount of control actually required for a given treatment depends on how wide its operating limits are, and on the degree of uniformity of results desired. Control by pH alone is adequate in some cases. COATING EVALUATION Chromate conversion coatings are covered by many internal company standards and/or U.S. government and American Society for Testing and Materials (ASTM) specifications. These standards usually contain sections on the following methods of evaluation. Visual
Inspection
The easiest way to evaluate chromate conversion coatings is to observe the color, uniformity of appearance, smoothness, and adhesion. Type of color and iridescence is a guide to film thickness, which is considered proportional to protective value; however, visual inspection by itself is not sufficient to indicate the protective value of the coating, especially if the film has been overheated during drying.
Accelerated Corrosion Test The salt spray test, ASTM B 117, is the most c o m m o n accelerated test developed in specification form. Although some disagreement exists as to the correlation of salt spray tests to actual performance, it remains in many specifications. Variations in results are often obtained when tested in different salt spray cabinets, and even Table III. Typical Salt Spray Data for Copper and Brass Treatment
Hours to Green Corrosion
Copper, untreated Copper, bright chromate Copper, heavychromate Brass, untreated Brass, bright chromate Brass, heavychromate
<24 24 50 24 100 150
Table IV. Typical Salt Spray Data for Aluminum Hours to White Corrosion
Alloy
No Treatment
3003 24 2024~ <24 413.0 <24 ~Heattreatment willaffect the final results.
Clear
Yellow-Brown
60-120 40-80 12-24
250-800 150-600 50-250
421
in different locations within the same cabinet. Coatings should be aged for at least 24 hours before testing, for consistent results. Generally, specifications require a m i n i m u m exposure time before visible corrosion forms. Typical salt spray test data are provided in Tables II to IV.
Humidity Tests There appears to be no standard specification covering humidity tests for unpainted chromate conversion coatings. Evaluations are conducted under various conditions and cycles. Humidity tests may be more useful than salt spray tests, as they correspond to the normal environment better than the salt spray, except in marine atmospheres.
Water Te~ Immersion tests in distilled or deionized water have proven valuable in simulating such conditions as water accumulation in chromated zinc die castings, e.g., carburetors and fuel pumps. Coatings applied on hot-dipped galvanized surfaces in strip mills are often tested by stacking wet sheets and weighing the top sheet. Periodic checks are made to determine when corrosion products first develop. The tests should be conducted at relatively constant temperatures to ensure consistent results.
Cbemical and Spot Tests The a m o u n t of hexavalent chromium in the film can be an indication of the corrosion protection afforded by the coating. Analytical procedures for small amounts of chromium on treated surfaces are comparatively rapid, quantitative, and reproducible. Consequently, chemical analysis for the chromium content of the film appears to be a valuable tool. It would not be suitable, however, for predicting the performance of bleached, overheated, excessively dehydrated coatings. Total coating weight is sometimes used as an indication of corrosion resistance. It is derived by weighing a part having a known surface area before and after chemically stripping only the chromate film. Spot tests are used to test corrosion resistance by dissolving the chromate coating and reacting with the basis metal. The time required to produce a characteristic spot determines empirically the film thickness or degree of corrosion protection. It is advisable to use these tests as comparative tests only, always spotting an untreated and treated surface at the same time. Frequently, the spot tests are sufficient only to indicate differences between treated and untreated surfaces. Reproducibility is not good because aging affects the results.
PerfinmanceTestsfor OrganicFinishes Paint, lacquer, and other organic finishes on chromate conversion coatings are tested in numerous ways to evaluate bonding and corrosion protection. These include pencil-hardness, cross-hatch, bending, impact, and tape tests with or without prior exposure to water or salt spray.
SPECIFICATIONS A list of the more commonly used specifications covering chromate conversion coatings on different basis metals follows. Only the basic specification or standard number is listed, and reference should be made only to the appropriate revision of any particular document.
Aluminum AIvlS 2473-Chemical Treatment for Aluminum Base Alloys-General Purpose 422
Coating AMS 2474-Chemical Treatment for Aluminum Base Alloys-Low Electrical Resistance Coating ASTM D 1730--Preparation of Aluminum and Aluminum Alloy Surfaces for Painting MIL-C-5541-Chemical Films and Chemical Film Material for Aluminum and Aluminum Alloys MIL-C-81706-Chemical Conversion Materials for Coating Aluminum and Aluminum Alloys MIL-W-6858-Welding, Resistance: Aluminum, Magnesium, etc.; Spot and Seam
Cadmium AMS 2400 Cadmium Plating AMS 2416-Nickel-Cadmium Plating, Diffused AMS 2426 Cadmium Plating, Vacuum Deposition ASTM B 201--Testing Chromate Coatings on Zinc and Cadmium Surfaces MIL-C-8837--Cadmium Coating (Vacuum Deposited) QQ-P-416-Plating, Cadmium (Electrodeposited) Magnes/um AMS 247S-Protective Treatments, Magnesium Base Alloys MIL-M-3171-Magnesium Alloy, Process for Pretreatment and Prevention of Corrosion on MIL-W-6858--Welding, Resistance: Aluminum, Magnesium, etc.; Spot and Seam
Silver QQ-S-365-Silver Plating, Electrodeposited, General Requirements for
Zinc AMS 2402--Zinc Plating ASTM B 201-Testing Chromate Coatings on Zinc and Cadmium Surfaces ASTM D 2092--Preparation of Zinc-Coated Steel Surfaces for Painting MIL-A-81801-Anodic Coatings for Zinc and Zinc Alloys MIL-C-17711-Coatings, Chromate, for Zinc Alloy Castings and Hot-Dip Galvanized Surface MIL-T-12879--Treatments, Chemical, Prepaint and Corrosion Inhibitive, for Zinc Surfaces MIL-Z- 17871-Zinc, Hot-Dip Galvanizing QQ-Z-325-Zinc Coating, Electrodeposited, Requirements for SPECIAL TREATMENTS
Solutions containing chromium compounds are used in some processes where disagreement exists as to whether these form "true" chromate conversion coatings or combination coatings, or act as passivating processes.
Electrolylic Processes Although early chromate conversion coatings for zinc were electrolytically applied, this method has been largely replaced by immersion processes. More recently, the use of electric current has reappeared with solutions containing mixtures of chromates, phosphates, fluorides, etc., to produce "anodic coatings." The 423
coatings, however, are not similar to anodic coatings such as those produced on aluminum. The coatings on zinc surfaces are complex combinations of chromates, phosphates, oxides, etc. They are formed with 100-200 V AC or DC, and the flitted coating will withstand more than 1,000 hr of salt spray. The process is used where outstanding corrosion resistance is needed. The coating also exhibits superior hardness, heat resistance, thickness, and dielectric strength when compared with normal chromate conversion coatings. Colors range from dark green to charcoal for different processes. Electrolytic treatments using chromium compounds are also applied to steel strip, where chromium along with oxides, etc., are deposited in a very thin, discontinuous film. These processes, which promote lacquer and paint adhesion, may be more chromium plate than chromate coating.
Coatings on Beryllium It has been reported that chromate conversion coatings can be applied to beryllium to retard high-temperature oxidation in humid air.
Chromate-Phosphate Treatments Chromate-phosphate treatments are based on chromate-phosphate mixtures and form a combination conversion coating on aluminum. The coating can appear practically colorless to a light-green hue. These treatments have been used to impact color for decorative purposes or to provide an imposed base for subsequent lacquer or paint operations.
424
physical Char~te~rics Most chromate films are soft and gelatinous when freshly formed. Once dried, they slowly harden or "set" with age and become hydrophobic, less soluble, and more abrasion resistant. Although heating below 150°F (66°C) is of benefit in hastening this aging process, prolonged heating above 150°F may produce excessive dehydration of the film, with consequent reduction of its protective value. Coating thickness rarely exceeds 0.00005 in., and often is on the order of several microinches. The amount of metal removed in forming the chromate film will vary with different processes. Variegated colors normally are obtained on chromating, and are due mainly to interference colors of the thinner films and to the presence of chromium compounds in the film. Because the widest range of treatments available is for zinc, coatings for this metal afford an excellent example of how color varies with film thickness. In the case ofelectroplated zinc, clear-bright and blue-bright coatings are the thinnest. The bIue-brights may show interference hues ranging from red, purple, blue, and green, to a trace of yellow, especially when viewed against a white background. Next, in order of increasing thickness, come the iridescent yellows, browns, bronzes, olive drabs, and blacks. Physical variations in the metal surface, such as those produced by polishing, machining, etching, etc., also affect the apparent color of the coated surface. The color of the thinner coatings on zinc can also be affected indirectly by chemical polishing, making the finish appear whiter.
Corrosion Prevention Chromate conversion coatings can provide exceptionally good corrosion resistance, depending upon the basis metal, the treatment used, and the film thick413
ness. Protection is due both to the corrosion-inhibiting effect of hexavalent chromium contained in the film and to the physical barrier presented by the film itself. Even scratched or abraded films retain a greatdeal of their protective value because the hexavalent chromium content is slowly leachable in contact with moisture, providing a self-healing effect. The degree of protection normally is proportional to film thickness; therefore, thin, clear coatings provide the least corrosion protection, the light iridescent coatings form an intermediate group, and the heavy olive drab to brown coatings result in maximum corrosion protection. The coatings are particularly useful in protecting metal against oxidation that is due to highly humid storage conditions, exposure to marine atmospheres, handling or fingerprint marking, and other conditions that normally cause corrosion of metal.
BondingofOrganicFinishes The bonding of paint, lacquer, and organic finishes to chromate conversion coatings is excellent. In addition to promoting good initial adhesion, their protective nature prevents subsequent loss of adhesion that is due to underFflm corrosion. This protection continues even thought he finish has been scratched through to the bare metal. It is necessary that the organic finishes used have good adhesive properties, because bonding must take place on a smooth, chemically clean surface; this is not necessary with phosphate-type conversion coatings, which supply mechanical adhesion that is due to the crystal structure of the coating.
ChemicalPolishing
Certain chromate treatments are designed to remove enough basis metal during the film-forming process to produce a chemical polishing, or brightening, action. Generally used for decorative work, most of these treatments produce very thin, almost colorless films. Being thin, the coatings have little optical covering power to hide irregularities. In fact, they may accentuate large surface imperfections. In some instances, a leaching or "bleaching" step subsequent to chromating is used to remove traces of color from the film. If chemical-polishing chromates are to be used on electroplated articles, consideration must be given to the thickness of the metal deposit. Sufficient thickness is necessary to allow for metal removal during the polishing operation.
Alasorb~ andDyeing When initially formed, many films are capable of absorbing dyes, thus providing a convenient and economical method of color coding. These colors supplement those that can be produced during the chromating operation, and a great variety of dyes is available for this purpose. Dyeing operations must be conducted on fleshly formed coatings. Once the coating is dried, it becomes nonabsorbent and hydrophobic and cannot be dyed. The color obtained with dyes is related to the character and type of chromate film. Pastels are produced with the thinner coatings, ancl the darker colors are produced with the heavier chromates. Some decorative use of dyed finishes has been possible when finished with a clear lacquer topcoat, though caution is required because the dyes may not be lightfast. In a few cases, film colors can be modified by incorporation of other ions or dyes added to the treatment solution.
Hardness Although most coatings are soft and easily damaged while wet, they become reasonably hard and will withstand considerable handling, stamping, and cold forming. They will not, however, withstand continued scratching or harsh abrasion. A 414
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few systems have been developed that possess some degree of "wet-hardness," and these will withstand moderate handling before drying.
Heat Resistance Prolonged heating of chromate films at temperatures substantially above 150°F (66°C) can decrease their protective value dramatically. There are two effects of heating that are believed to be responsible for this phenomenon. One is the insolubilization of the hexavalent chromium, which renders it ineffective as a corrosion inhibitor. The second involves shrinking and cracking of the film, which destroys its physical integrity and its value as a protective barrier. Many factors, such as the type of basis metal, the coating thickness, heating time, temperature, and relative humidity of the heated atmosphere, influence the degree of coating damage. Thus, predictions are difficult to make, and thorough performance testing is recommended if heating of the coating is unavoidable. The heat resistance of many chromates can be improved by certain posttreatments or "sealers." Baking at paint-curing temperatures after an organic finish has been applied is a normal practice and does not appear to affect the properties of the treatment film.
E/ectr/ca/Res/~e The contact resistance of articles that have been protected with a chromate conversion coating is generally much lower than that of an unprotected article that has developed corroded or oxidized surfaces. As would be expected, the thinner the coating, the lower the contact resistance, i.e., clear coatings have the least resistance, iridescent yellow coatings have slightly more, and the heavy, olive drab coatings have the greatest. If exposure of an article to corrosive conditions is anticipated, the choice of a coating thickness normally involves a compromise between a very thin film-which, although having very low initial contact resistance, is likely to allow early development of high electrical resistance corrosion products-and a heavier film, with somewhat higher initial contact resistance, but which is likely to remain relatively constant for a longer period under corrosive conditions.
Fabrication
Resistance Welding. Thin chromate films do not interfere appreciablywith spot, seam, or other resistance-welding operations. Aluminum coated with a thin, nearly colorless film, for example, can be spot welded successfully with no increase in welding machine settings over those required for bare metal. Metal coated with thicker, colored films also can be resistance welded. The increased contact resistance of thicker coatings, however, necessitates using slightly higher machine settings. Fusion Welding. These operations, likewise, are not hampered by the presence of chromate films. It has been reported, in fact, that chromate treatments on aluminum actually facilitate inert gas welding of this metal and its alloys, producing contamination-free welds. Soldering. Cadmium and silver surfaces coated with thin chromate films can be soldered without difficulty using a mild organic flux. Conflicting reports exist regarding the solderabilty ofchromated zinc surfaces. MechanicalFastening. The assembly ofchromated parts using bolts, rivets, and other mechanical fastening devices usually results in local damage to the chromate film. Corrosion protection in these areas will depend upon the effectiveness of the self-healing properties of the surrounding coating. Summary of Common Uses Table I summarizes the most common applications of chromate conversion coatings. 416
TaMe I. Common Uses of Chromate Conversion Coatings General Usage Metal
Corrosion Resistance
Paint Base
Chemical Metal Polish Coloring Remarks X
Aluminum
X
X
Cadmium
X
X
X
X
Copper
X
X
X
X
Magnesium
X
X
Silver
X
Zinc
X
X
X
X
Economical replacement for anodizing if abrasion resistance is not required. Used to "touch-up" damaged areas on anodized surfaces. Thin coatings prevent "spotting out"of brass and copper electrodeposits. No fumes generated during chemical polishing.
MATERIALS OF CONSTRUCTION Generally, suppliers of proprietaries recommend materials for use with their products, which are resistant to oxidants, fluorides, chlorides, and acids. Materials that have been found to be satisfactory for most chromating applications are stainless steels and plastics. Stainless steels such as 304, 316, 317, and 347 are suitable for tanks and heaters where chlorides are absent. Containers and tank linings can be made from plastics such as polyvinyl chloride (PVC), polyvinylidinechloride (PVDC), polyethylene, and polypropylene. Acid-resistant brick or chemical stoneware is satisfactory for some applications, but is subject to attacks by fluorides. Parts-handling equipment is made of stainless steel, plastisol-coated mild steel, or plastic. Mild steel can be used for leaching tanks because the solutions are generally alkaline, whereas tanks for dyeing solutions, which are slightly acid, should be of acid-resistant material. Usually, ventilation is not necessary because most chromate solutions are operated at room temperature and are nonfuming. Where chromating processes are heated, they should be ventilated.
FILM FORMATION
Mechanism The films in most common use are formed by the chemical reaction of hexavalent chromium with a metal surface in the presence of other components, or "activators," in an acid solution. The hexavalent chromium is partially reduced to trivalent chromium during the reaction, with a concurrent rise in pH, forming a complex mixture consisting largely of hydrated basic chromium chromate and hydrous oxides of both chromium and the basis metal. The composition of the film is rather indefinite, because it contains varying quantities of the reactants, reaction products, and water of hydration, as well as the associated ions of the particular systems. There are a number of factors that affect both the quality and the rate of formation of chromate coatings. Of the following items, some are peculiar to chromating; many derive simply From good shop practice. A working understanding of these factors will be helpful in obtaining high-quality, consistent results. Different formulations are required to produce satisfactory chromate films on various met417
als and alloys. Similarly, the characteristics of the chromate film produced by any given solution can vary with minor changes in the metal or alloy surface. Commonly encountered examples of this follow.
EA~umCtof Basis Metals inureAlloys.The ease with which coatings on aluminum can be produced, and the degree of protection afforded by them, can vary significantlywith the alloyingconstituents and/or the heat treatment of the part being processed. In general, low alloying constituent metals that are not heat treated are easiest to chromate and provide the maximum resistance to corrosion. Conversely, wrought aluminum, which is high in alloying elements (especially silicon, copper, or zinc) or which has undergone severe heat treatment, is more difficult to coat uniformly and is more susceptible to corrosive attack. High silicon casting alloys present similar problems. The effect of these metal differences, however, can be minimized by proper attention to the cleaning and pretreatment steps. Most proprietary treatment instructions contain detailed information regarding cleaning, desmutting, etc., of the various alloys. Magnesium Alloys. As in the case of aluminum, the alloying element content and the type 0fheat treatment affect the chromating of magnesium. With the exception of the dichromate treatments listed as Type III in Military Specification MIL-M-3171, all of the treatments available can be used on all the magnesium alloys. Zinc Alloys. Chromate conversion coatings on zinc electroplate are affected by impurities codeposited with the zinc. For example, dissolved cadmium, copper, and lead in zinc plating solutions can ultimately cause dark chromated films. Similarly, dissolved iron in noncyanide zinc plating solutions can create chromating problems. Furthermore, the activity of zinc deposits from cyanide and noncyanide solutions can differ sufficiently to produce variations in the chromate film character.
Variations in the composition of zinc die casting alloys and hot-clipped galvanized surfaces can also affect chromate film formation; however, in the latter case, the result is usually difficult to predict, due to the wide variations encountered in spelter composition, cooling rates, etc. Large differences in the chromate coating from spangle to spangle on a galvanized surface are not uncommon. This is especiallyevident in the heavier films. CopperAlloys. Since chromate treatments for copper and its alloys can be used to polish chemically as well as to form protective films, the grain structure of the part becomes important, in addition to its alloying content. Whereas fine-grained, homogeneous material responds well to chromate polishing, alloys such as phosphor bronze and heavily leaded brass usually will acquire a pleasing but matte finish. In addition, treatment of copper alloys, which contain lead in appreciable amounts, may result in the formation of a surface layer of powdery, yellow lead chromae.
of
One of the more important factors in controlling the formation of the chromate film is the pH of the treatment solution. For any given metal/chromate solution system, there will exist a pH at which the rate of coating formation is at a maximum. As the pH'is lowered from this point, the reaction products increasingly become more soluble, tending to remain in solution rather than deposit as a coating on the metal surface. Even though the rate of metal dissolution increases, the coating thickness will remain low. Chemical-polishing chromates for zinc, cadmium, and copper are purposely operated in this low pH range to take advantage of the increased rate of metal removal. The chromate films produced in these cases can be so thin 418
that they are nearly invisible. Beyond this point, further lowering of the pH is sufficient to convert most chromate treatments into simple acid etchants. Increasing the pH beyond the maximum noted above will gradually lower the rate of metal dissolution and coating formation to the point at which the reaction, for all practical purposes, ceases.
Hexavalent Chromium Concentration Although the presence ofhexavalent chromium is essential, its concentration in many treatment solutions can vary widely with limited effect, compared with that of pH. For example, the chromium concentration in a typical aluminum treatment solution can vary as much as 100% without substantially affecting the film-formation rate, as long as the pH is held constant. In chromating solutions for zinc or cadmium, the hexavalent chromium can vary fairly widely from its optimum concentration if the activator component is in the proper ratio and the pH is constant.
Actiuators Chromate films normally will not form without the presence of certain anions in regulated amounts. They are commonly referred to as "activators' and include acetate, formate, sulfate, chloride, fluoride, nitrate, phosphate, and sulfamate ions. The character, rate of formation, and properties of the chromate film vary with the particular activator and its concentration. Consequently, many proprietary formulations have been developed for specific applications and they are the subject of numerous patents. Usually, these proprietary processes contain the optimum concentrations of the activator and other components; therefore, the user need not be concerned with the selection, separate addition, or control of the activator.
OPERATING CONDITIONS In addition to the chemical make-up of the chromating solution, the following factors also govern film formation. Once established for a given operation, these parameters should be held constant. Treatment Time. Immersion time, or contact time of the metal surface and the solution, can vary from as little as 1 second to as much as 1 hour, depending on the solution being used and metal being treated. If prolonged treatment times are required to obtain desired results, a fault in the system is indicated and should be corrected. Solution Temperature. Chromating temperatures vary from ambient to boiling, depending on the particular solution and metal being processed. For a given system, an increase in the solution temperature will accelerate both the film-forming rate and the rate of attack on the metal surface. This can result in a change in the character of the chromate film. Thus, temperatures should be adequately maintained to ensure consistent results. Solution A~tation. Agitation of the working solution, or movement of the work in the solution, generally speeds the reaction and provides more uniform film formation. Air agitation and spraying have been used for this purpose. There are, however, a few exceptions where excessive agitation will produce unsatisfactory films.
Solution Contamination Although the presence of an activator in most treatment solutions is vital, an excessive concentration of this component, or the presence of the wrong activator, can be very detrimental. Most metal-finishing operations include sources of potential activator contamination in the form of cleaners, pickles, deoxidizers, and
419
desmutters. Unless proper precautions are taken, the chromate solution can easily become contaminated through drag-in of inadequately rinsed parts, drippage from racks carried over the solution, etc. A common source of contamination is that resulting from improperly cleaned work. If allowed to go unchecked, soils can build on the surface of the solution to the point at which even clean work becomes resoiled on entering the treatment tank, resulting in blotchy, uneven coatings. Other contaminants to be considered are those produced by the reactions occurring in the treatment solution itself. With very few exceptions, part of the trivalent chromium formed and part of the basis metal dissolved during the coating reaction remain in the solution. Small amounts of these contaminants can be beneficial, and"broken-in" solutions often produce more consistent results. As the concentration of these metal contaminants increases, effective film formation will be inhibited. For a certain period, this effect can be counteracted by adjustments, such as lowered pH and increased hexavalent chromium concentration. Eventually, even these techniques become ineffective, at which point the solution must be discarded or a portion withdrawn and replaced with fresh solution.
P~nsingand Drfing Once a chromate film has been formed satisfactorily, the surface should be rinsed as soon as possible. Transfer times from the chromating stage to the rinsing stage should be short in order to minimize the continuing reaction that takes place on the part. Although rinsing should be thorough, this step can also affect the final character of the chromate film and should be controlled with respect to time and temperature, for consistent results. Prolonged rinsing or the use of very hot rinsewater can dissolve, or leach, the more soluble hexavalent chromium compounds from a freshly formed coating, resulting in a decrease in protective value. If a hot rinse is used to aid drying, avoid temperatures over about 150°F (66°C) for more than a few seconds. This leaching effect sometimes is used to advantage. In instances in which a highly colored or iridescent coating may be objectionable, a prolonged rinse in hot water can be used as a"bleaching" step to bring the color to an acceptable level. Instead of hot water leaching, some systems incorporate dilute acids and alkalis to accelerate this step.
SolutionControl Because most chromate processes are proprietary, it is suggested that the suppliers' instructions be followed for solution make-up and control. Even though specific formulations will not be discussed, certain general principles can be outlined, which apply generally to chromate solutions. The combination ofhexavalent chromium concentration, activator type and concentration, and pH, i.e., the "chemistry" of the solution, largely determines the type of coating that will be obtained, or whether a coating can be obtained at all, at given temperatures and immersion times. It is important that these factors making up the "chemistry" of the solution be properly controlled. As the solution is depleted through use, it is replenished by maintenance additions, as indicated by control tests or the appearance of the work. Fortunately, analysis for each separate ingredient in a chromate bath is not necessary for proper control. A very effective control method uses pH and hexavalent chromium analysis. The pH is determined with a pH meter and the chromium is 420
Table II. Typical Salt Spray Data for Electroplated Zinc Treatment
Hours to White Corrosion
Untreated Clear chromate Iridescent yellow Olivedrab
<8 24-100 100-200 100400
determined by a simple titration. Indicators and pH papers are not recommended because of discoloration by the chromate solution. Additions are made to the solution to keep these two factors within operating limits. The amount of control actually required for a given treatment depends on how wide its operating limits are, and on the degree of uniformity of results desired. Control by pH alone is adequate in some cases. COATING EVALUATION Chromate conversion coatings are covered by many internal company standards and/or U.S. government and American Society for Testing and Materials (ASTM) specifications. These standards usually contain sections on the following methods of evaluation. Visual
Inspection
The easiest way to evaluate chromate conversion coatings is to observe the color, uniformity of appearance, smoothness, and adhesion. Type of color and iridescence is a guide to film thickness, which is considered proportional to protective value; however, visual inspection by itself is not sufficient to indicate the protective value of the coating, especially if the film has been overheated during drying.
Accelerated Corrosion Test The salt spray test, ASTM B 117, is the most c o m m o n accelerated test developed in specification form. Although some disagreement exists as to the correlation of salt spray tests to actual performance, it remains in many specifications. Variations in results are often obtained when tested in different salt spray cabinets, and even Table III. Typical Salt Spray Data for Copper and Brass Treatment
Hours to Green Corrosion
Copper, untreated Copper, bright chromate Copper, heavychromate Brass, untreated Brass, bright chromate Brass, heavychromate
<24 24 50 24 100 150
Table IV. Typical Salt Spray Data for Aluminum Hours to White Corrosion
Alloy
No Treatment
3003 24 2024~ <24 413.0 <24 ~Heattreatment willaffect the final results.
Clear
Yellow-Brown
60-120 40-80 12-24
250-800 150-600 50-250
421
in different locations within the same cabinet. Coatings should be aged for at least 24 hours before testing, for consistent results. Generally, specifications require a m i n i m u m exposure time before visible corrosion forms. Typical salt spray test data are provided in Tables II to IV.
Humidity Tests There appears to be no standard specification covering humidity tests for unpainted chromate conversion coatings. Evaluations are conducted under various conditions and cycles. Humidity tests may be more useful than salt spray tests, as they correspond to the normal environment better than the salt spray, except in marine atmospheres.
Water Te~ Immersion tests in distilled or deionized water have proven valuable in simulating such conditions as water accumulation in chromated zinc die castings, e.g., carburetors and fuel pumps. Coatings applied on hot-dipped galvanized surfaces in strip mills are often tested by stacking wet sheets and weighing the top sheet. Periodic checks are made to determine when corrosion products first develop. The tests should be conducted at relatively constant temperatures to ensure consistent results.
Cbemical and Spot Tests The a m o u n t of hexavalent chromium in the film can be an indication of the corrosion protection afforded by the coating. Analytical procedures for small amounts of chromium on treated surfaces are comparatively rapid, quantitative, and reproducible. Consequently, chemical analysis for the chromium content of the film appears to be a valuable tool. It would not be suitable, however, for predicting the performance of bleached, overheated, excessively dehydrated coatings. Total coating weight is sometimes used as an indication of corrosion resistance. It is derived by weighing a part having a known surface area before and after chemically stripping only the chromate film. Spot tests are used to test corrosion resistance by dissolving the chromate coating and reacting with the basis metal. The time required to produce a characteristic spot determines empirically the film thickness or degree of corrosion protection. It is advisable to use these tests as comparative tests only, always spotting an untreated and treated surface at the same time. Frequently, the spot tests are sufficient only to indicate differences between treated and untreated surfaces. Reproducibility is not good because aging affects the results.
PerfinmanceTestsfor OrganicFinishes Paint, lacquer, and other organic finishes on chromate conversion coatings are tested in numerous ways to evaluate bonding and corrosion protection. These include pencil-hardness, cross-hatch, bending, impact, and tape tests with or without prior exposure to water or salt spray.
SPECIFICATIONS A list of the more commonly used specifications covering chromate conversion coatings on different basis metals follows. Only the basic specification or standard number is listed, and reference should be made only to the appropriate revision of any particular document.
Aluminum AIvlS 2473-Chemical Treatment for Aluminum Base Alloys-General Purpose 422
Coating AMS 2474-Chemical Treatment for Aluminum Base Alloys-Low Electrical Resistance Coating ASTM D 1730--Preparation of Aluminum and Aluminum Alloy Surfaces for Painting MIL-C-5541-Chemical Films and Chemical Film Material for Aluminum and Aluminum Alloys MIL-C-81706-Chemical Conversion Materials for Coating Aluminum and Aluminum Alloys MIL-W-6858-Welding, Resistance: Aluminum, Magnesium, etc.; Spot and Seam
Cadmium AMS 2400 Cadmium Plating AMS 2416-Nickel-Cadmium Plating, Diffused AMS 2426 Cadmium Plating, Vacuum Deposition ASTM B 201--Testing Chromate Coatings on Zinc and Cadmium Surfaces MIL-C-8837--Cadmium Coating (Vacuum Deposited) QQ-P-416-Plating, Cadmium (Electrodeposited) Magnes/um AMS 247S-Protective Treatments, Magnesium Base Alloys MIL-M-3171-Magnesium Alloy, Process for Pretreatment and Prevention of Corrosion on MIL-W-6858--Welding, Resistance: Aluminum, Magnesium, etc.; Spot and Seam
Silver QQ-S-365-Silver Plating, Electrodeposited, General Requirements for
Zinc AMS 2402--Zinc Plating ASTM B 201-Testing Chromate Coatings on Zinc and Cadmium Surfaces ASTM D 2092--Preparation of Zinc-Coated Steel Surfaces for Painting MIL-A-81801-Anodic Coatings for Zinc and Zinc Alloys MIL-C-17711-Coatings, Chromate, for Zinc Alloy Castings and Hot-Dip Galvanized Surface MIL-T-12879--Treatments, Chemical, Prepaint and Corrosion Inhibitive, for Zinc Surfaces MIL-Z- 17871-Zinc, Hot-Dip Galvanizing QQ-Z-325-Zinc Coating, Electrodeposited, Requirements for SPECIAL TREATMENTS
Solutions containing chromium compounds are used in some processes where disagreement exists as to whether these form "true" chromate conversion coatings or combination coatings, or act as passivating processes.
Electrolylic Processes Although early chromate conversion coatings for zinc were electrolytically applied, this method has been largely replaced by immersion processes. More recently, the use of electric current has reappeared with solutions containing mixtures of chromates, phosphates, fluorides, etc., to produce "anodic coatings." The 423
coatings, however, are not similar to anodic coatings such as those produced on aluminum. The coatings on zinc surfaces are complex combinations of chromates, phosphates, oxides, etc. They are formed with 100-200 V AC or DC, and the flitted coating will withstand more than 1,000 hr of salt spray. The process is used where outstanding corrosion resistance is needed. The coating also exhibits superior hardness, heat resistance, thickness, and dielectric strength when compared with normal chromate conversion coatings. Colors range from dark green to charcoal for different processes. Electrolytic treatments using chromium compounds are also applied to steel strip, where chromium along with oxides, etc., are deposited in a very thin, discontinuous film. These processes, which promote lacquer and paint adhesion, may be more chromium plate than chromate coating.
Coatings on Beryllium It has been reported that chromate conversion coatings can be applied to beryllium to retard high-temperature oxidation in humid air.
Chromate-Phosphate Treatments Chromate-phosphate treatments are based on chromate-phosphate mixtures and form a combination conversion coating on aluminum. The coating can appear practically colorless to a light-green hue. These treatments have been used to impact color for decorative purposes or to provide an imposed base for subsequent lacquer or paint operations.
424