Metal Finishing Guide Book

2012-2013

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Table I. Common Uses of Chromate Conversion Coatings General Usage Corrosion Resistance Paint Base Aluminum X X Cadmium X X X X Copper X X X X Magnesium X X Silver X Zinc X X X Metal X Chemical Polish Metal Coloring X Remarks 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. descent 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 appreciably with 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 of chromated zinc surfaces. Mechanical Fastening. The assembly of chromated 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. 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 482

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