Metal Finishing Guide Book

2013

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actually required for a given treatment depends on how wide its operating limits are, and on the degree of uniformity of results desired. Control by pH alone is adequate in some cases. COATING EVALUATION Chromate conversion coatings are covered by many internal company standards and/or U.S. government and American Society for Testing and Materials (ASTM) specifications. These standards usually contain sections on the following methods of evaluation. Visual Inspection The easiest way to evaluate chromate conversion coatings is to observe the color, uniformity of appearance, smoothness, and adhesion. Type of color and iridescence is a guide to film thickness, which is considered proportional to protective value; however, visual inspection by itself is not sufficient to indicate the protective value of the coating, especially if the film has been overheated during drying. Accelerated Corrosion Test The salt spray test, ASTM B 117, is the most common accelerated test developed in specification form. Although some disagreement exists as to the correlation of salt spray tests to actual performance, it remains in many specifications. Variations in results are often obtained when tested in different salt spray cabinets, and even in different locations within the same cabinet. Coatings should be aged for at least 24 hours before testing, for consistent results. Generally, specifications require a minimum exposure time before visible corrosion forms. Typical salt spray test data are provided in Tables II to IV. Humidity Tests There appears to be no standard specification covering humidity tests for unpainted chromate conversion coatings. Evaluations are conducted under various conditions and cycles. Humidity tests may be more useful than salt spray tests, as they correspond to the normal environment better than the salt spray, except in marine atmospheres. Water Tests Immersion tests in distilled or deionized water have proven valuable in simulating such conditions as water accumulation in chromated zinc die castings, e.g., carburetors and fuel pumps. Coatings applied on hot-dipped galvanized surfaces in strip mills are often tested by stacking wet sheets and weighing the top sheet. Periodic checks are made to determine when corrosion products first develop. The tests should be conducted at relatively constant temperatures to ensure consistent results. Chemical and Spot Tests The amount of hexavalent chromium in the film can be an indication of the corrosion protection afforded by the coating. Analytical procedures for small amounts of chromium on treated surfaces are comparatively rapid, quantitative, and reproducible. Consequently, chemical analysis for the chromium content of the film appears to be a valuable tool. It would not be suitable, however, for predicting the performance of bleached, overheated, excessively dehydrated coatings. Total coating weight is sometimes used as an indication of corrosion resistance. It is derived by weighing a part having a known surface area before and after chemically stripping only the chromate film. 433

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