Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 49 of 331

23% water, and a trace of fluorides. Performance properties of chromate-phosphate films are generally very close to chromic acid anodizing films and those of chromate-oxide films to be discussed later. Adhesion and corrosion protection increase with coating weight up to a point, then fall off. The best range is between100 and 200 mg/ft2 . Because of its excellent qualities, the American Architectural Manufacturers Association (AAMA) has designated the chromate-phosphate process as a stan- dard prepaint treatment. It also meets U.S. military specifications. Generally, this process is recommended for severe and long-term service conditions. CHROMATE-OXIDE COATINGS Chromate-oxide films are more versatile and widely applied than the chromate- phosphate treatments. They comprise the bulk of treatments for the coil stock and transportation industries. In applications where anodizing is not feasible, for example, where parts are too long or assembled with dissimilar metals, chro- mate treatments of this type have been used in place of anodizing. Typically, a chromate-oxide bath consists of three principal constituents: acid chromates, etchants and accelerators or complexing agents. Application may be by spray, immersion, or brush at 70 to 110°F for 15 to 45 seconds. The aluminum surface is converted to an iridescent golden yellow color. The film is tightly adherent, amorphous, and mixed with metallic oxide products. Film thicknesses range between 0.005 and 0.04 mil. Coating weights are from 15 to 100 mg/ft2 . A freshly formed film can be leached to a nearly colorless appearance. Chromate-oxide films have superior performance ratings compared to chro- mate-phosphate coatings. Unpainted films have almost twice the salt spray re- sistance of a chromate-phosphate coating. Chromate parts are known to have passed 2,000 hours of salt spray. Such performances are partly explained by the fact that these films retain hexavalent chromate ions in their structures. In cor- rosive environments, they trigger a repair mechanism into action. Chromate films are capable of withstanding very severe service conditions. They comply with military specification MIL-C-5541 and AAMA 605.2. ALKALINE CHROMATES These are among the oldest coating processes analogous to the phosphate treat- ments described earlier. One ingredient serves to attack the aluminum substrate to a limited degree, while a second functions to form an oxide coating on the re- active substrate. Alkaline chromates are primarily solutions of 2% to 3% sodium carbonate and 0.5% potassium dichromate. Immersion times range from 10 to 20 minutes at 180 to 190°F. Thin (0.04 to 0.1 mil) gray and porous films made up of aluminum oxide and dispersed chromate oxides are produced. Maximum corrosion resistance is achieved by sealing in hot 5% potassium dichromate. Consistent coating action depends on the correct ratio of carbonate to dichro- mate in the treatment solution. MISCELLANEOUS TREATMENTS No survey of prepaint treatment is complete without mention of numerous oth- er products being successfully used in diverse applications. Some of the older suc- cessful ones include wash primers, chemical oxide films, and mechanical treat- 48

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