Metal Finishing Guide Book

2013

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nearly colorless appearance. Chromate-oxide films have superior performance ratings compared to chromate-phosphate coatings. Unpainted films have almost twice the salt spray resistance 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 corrosive 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 treatments 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 reactive 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 dichromate in the treatment solution. MISCELLANEOUS TREATMENTS No survey of prepaint treatment is complete without mention of numerous other products being successfully used in diverse applications. Some of the older successful ones include wash primers, chemical oxide films, and mechanical treatments such as wire brushing and sandblasting. Wash primers are primarily pigmented polymeric organic chromate compounds similar to paint. They have excellent corrosion protection and adhesion properties. Humidity resistance is poor. Environmental restrictions of the last decade have generated exciting interest in chromium-free products. A substantial number of these have met acceptable requirements for the beverage can industry. Testing is in progress in coil coating and architectural segments. There is a strong indication that chromium-free products will eventually meet the stringent requirements of the architectural and transportation industries. Some of the shortcomings of current products are being gradually eliminated. New high performance top coat paints such as silicones, fluoropolymers, and powder coats can minimize performance requirements for these prepaint treatments. Advances in application technology of electrocoat systems are also proving beneficial. PROCESS GUIDELINES Conversion coating processes are essentially multistep operations. Preconditioning stages, such as cleaning, rinsing, and postrinse treatments are significant factors in assuring critical final results. Spray and immersion systems usually show the same order of operation. Stages consist of the following: 1. Clean 2. Rinse 125

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