Metal Finishing Guide Book

2013

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cleaning, pretreatment & surface preparation 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 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 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 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. CRYSTALLINE PHOSPHATES These are primarily accelerated iron and zinc phosphate processes adapted from iron and steel pretreatment. Phosphating solutions typically contain metal hydrogen phosphate salts with limited free acidity. The metal phosphates are soluble in strong acids but crystallize out when the acidity is reduced. This mechanism occurs as the acid ions react with the aluminum surface, become Table I. Comparative Performance Data Conversion Coating Film Weight (mg/ft2) Iron Phosphate with chrome seal Zinc Phosphate with chrome seal Anodize Primer Chromate Chrome Phosphate Alkaline chromate oxide 20–30 100–300 1000–3000 3000+ 15–70 30–100 10–30 aChromic Paint Salt Spray Adhesion Resistance Painted Fair Good Good Very Good Very gooda Very Good Excellent Excellent Excellent- Excellent Excellent Excellent Very Good Excellent Humidity Resistance Painted Good Very good Good Good Excellent Excellent Good acid anodized, unsealed sulfuric films, and chromate sealed films. 123

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