Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 427 of 707

ducible. 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 resis- tance. It is derived by weighing a part having a known surface area before and after chemically stripping only the chromate film. Spot tests are used to test corrosion resistance by dissolving the chromate coating and reacting with the basis metal. The time required to produce a char- acteristic spot determines empirically the film thickness or degree of corrosion pro- tection. It is advisable to use these tests as comparative tests only, always spotting an untreated and treated surface at the same time. Frequently, the spot tests are suf- ficient only to indicate differences between treated and untreated surfaces. Reproducibility is not good because aging affects the results. Performance Tests for Organic Finishes Paint, lacquer, and other organic finishes on chromate conversion coatings are test- ed in numerous ways to evaluate bonding and corrosion protection. These include pencil-hardness, cross-hatch, bending, impact, and tape tests with or without pri- or exposure to water or salt spray. SPECIFICATIONS A list of the more commonly used specifications covering chromate conversion coat- ings on different basis metals follows. Only the basic specification or standard num- ber is listed, and reference should be made only to the appropriate revision of any particular document. Aluminum AMS 2473—Chemical Treatment for Aluminum Base Alloys—General Purpose Coating AMS 2474—Chemical Treatment for Aluminum Base Alloys—Low Electrical Resistance Coating ASTM D 1730—Preparation of Aluminum and Aluminum Alloy Surfaces for Painting MIL-C-5541—Chemical Films and Chemical Film Material for Aluminum and Aluminum Alloys MIL-C-81706—Chemical Conversion Materials for Coating Aluminum and Aluminum Alloys MIL-W-6858—Welding, Resistance: Aluminum, Magnesium, etc.; Spot and Seam Cadmium AMS 2400—Cadmium Plating AMS 2416—Nickel-Cadmium Plating, Diffused AMS 2426—Cadmium Plating, Vacuum Deposition ASTM B 201—Testing Chromate Coatings on Zinc and Cadmium Surfaces MIL-C-8837—Cadmium Coating (Vacuum Deposited) QQ-P-416—Plating, Cadmium (Electrodeposited) Magnesium AMS 2475—Protective Treatments, Magnesium Base Alloys MIL-M-3171—Magnesium Alloy, Process for Pretreatment and Prevention of Corrosion on 426

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