Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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surface treatments 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 elec- trochemical 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 usual- ly 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 for- mulations 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 suffi- cient general information to permit proper selection and operation of chro- mating baths. Proprietary products, which are designed for specific applica- tions, 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 abra- sion resistant. Although heating below 150O aging process, prolonged heating above 150O F (66O C) is of benefit in hastening this 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 com- pounds in the film. Because the widest range of treatments available is for zinc, coat- ings for this metal afford an excellent example of how color varies with film thick- ness. 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 back- ground. 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 resis- 417

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