Chemical conversion coatings are the most widely used prepaint treatment processes for metal substrates. Processes specifically designed for aluminum are of recent origin. These include accelerated chromate phosphates, chromate oxides, anodizing and, very recently, nonchromate formulations. Historically, however, phosphoric acid cleaners, wash primers, and iron and zinc phosphates have all been utilized as paint pretreatments with satisfactory results. Among these, the chromate types provide the most reliable underfilm corrosion protection and paint adhesion. Anodized coatings, especially unsealed sulfuric and chromic acid types, are also comparable in performance, but fall far short of the productivity and cost effectiveness of the chromate processes. A careful evaluation of critical product requirements is essential in selecting the right conversion coating for anticipated field performance. Paint pretreatments must assure these corrosion protection functions: passivation of base aluminum, action as a barrier against moisture, oxygen, and other corrosive agents, electrochemical insulation, and protection against mechanical erosion. In addition, conversion coatings must provide other essential interfacial properties complementary to the paint top coat, including an effective and continuous bonding site; chemical stability during the service life of painted products; remaining insoluble, impervious, and flexible; providing a wettable subsurface for paint application; and maintaining adhesive integrity between the base metal and painted film. See Table I for typical performance data.
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