Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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pretreatment PAINT PRETREATMENTS FOR ALUMINUM BY ANTHONY O. ITA HOUGHTON METAL FINISHING CO., ALPHARETTA, GA. 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 ox- ides, anodizing and, very recently,nonchromate formulations. Historically, how- ever, 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 under film 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 pro- ductivity 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, elec- trochemical insulation, and protection against mechanical erosion. In addi- tion,conversion coatings must provide other essential interfacial properties com- plementary to the paint top coat, including an effective and continuous bond- ing 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. CRYSTALLINE PHOSPHATES These are primarily accelerated iron and zinc phosphate processes adapted from iron and steel pretreatment. Phosphating solutions typically contain metal hy- drogen phosphate salts with limited free acidity. The metal phosphates are sol- uble in strong acids but crystallize out when the acidity is reduced. This mech- anism occurs as the acid ions react with the aluminum surface, become neu- tralized, and produce an integral crystal growth on the metal surface. The alu- minum surface is therefore converted to a finely crystalline phosphate film with acceptable texture for paint bonding. Crystalline phosphate films may be iri- 46

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