Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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ing. They will not, however, withstand continued scratching or harsh abrasion. A 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 150O (66O F C) can decrease their protective value dramatically. There are two effects of heat- ing that are believed to be responsible for this phenomenon. One is the insolubi- lization 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 perfor- mance testing is recommended if heating of the coating is unavoidable. The heat resistance of many chromates can be improved by certain posttreat- ments 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. Electrical Resistance The contact resistance of articles that have been protected with a chromate con- version 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 coat- ing, the lower the contact resistance, i.e., clear coatings have the least resistance, iri- Replacement for Aluminum Conversion Coatings. RoHS Compliant Aluvert NC (NO CHROME ) Aluvert TC (Trivalent Chrome) Environmentally Responsible Replacement for Hexavalent Aluminum Conversion Coatings SIMPLE MAKE-UP USING TAP WATER AT ROOM TEMPERATURE SURPASSES UP TO 400 HOURS NEUTRAL SALT SPRAY with NO SEAL WORKS ON ALL ALUMINUM AND ALLOYS INEXPENSIVE (Consult your salesman about a free make-up) LOW CONCENTRATION <2% MAKE-UP SUPERIOR ADHESION (CLASS 1 COATING UNDER MIL-C-5541) LOW ELECTRICAL RESISTANCE (CLASS 3 COATING UNDER MIL-C-5541) COMPLIES "ELV," "RoHS," and "WEEE" directives PRODUCES CLEAR TO VERY LIGHT IRIDESCENT FINISH DIRECT REPLACEMENT FOR HEXAVALENT ALUMINUM CHROMATE FINISHES IN COMPLIANCE WITH MIL-DTL-81706 American Chemical and Equipment Manufacturing in Northlake, IL • Distribution in New York, California Phone: 1-800-734-CHEM (708-531-9199) • FAX: 708-531-1978 Email: amcheq@aol.com www.amerchem.com www.metalfinishing.com/advertisers 419 Alu ve rt

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