Metal Finishing Guide Book

2013

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these will withstand moderate handling before drying. Heat Resistance Prolonged heating of chromate films at temperatures substantially above 150OF (66OC) 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. Electrical Resistance 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 RESERVE YOUR SPACE NOW FOR 2013/14 Call for all the details. Arnie Hoffman 847-559-0909 arnie@edmancompany.com 427

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