Metal Finishing Guide Book

2013

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plating processes, procedures & solutions CHROMATE CONVERSION COATINGS BY FRED W. 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 Characteristics 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 150OF (66OC) is of benefit in hastening this aging process, prolonged heating above 150OF 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 of electroplated zinc, clear-bright and blue-bright coatings are the thinnest. The blue-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 thickness. Protection is due both to the corrosion-inhibiting effect of hexavalent 425

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