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ANODIZING AS A PRETREATMENT FOR ALUMINUM by Earl Groshart Consultant, Sumner, Wash. Anodizing is an excellent pretreatment for nonclad aluminum prior to painting. Anodizing meeting the requirements of Mil-A-8625 is, in fact, a required pretreatment for aluminum used on the exterior of military aircraft. This section discusses nonchromic acid anodizing processes for aluminum to be used as a paint base. Although chromic acid anodizing is a viable base for paint, it is not discussed here because of the environmental and health problems posed by hexavalent chromium ion. The environmentally acceptable anodizing solutions are based on sulfuric and phospho- ric acids as follows: Standard Sulfuric Acid • Sulfuric acid (66'B6), 20-25 oz/gal • Aluminum, 2.5 oz/gal max • Room temperature • Current density (see note I), 10-15 A/ft 2 • Mild air or mechanical agitation Note I: Start with a low voltage and increase as the coating starts to form. Continue to increase voltage until the current holds steady. Sulfuric/Glycolic Acid • Sulfuric acid (66'86), 25-30 oz/gal • Additive (10 parts by volume of glycolic acid 70% mixed with 7 parts by volume of glycerin 95%), 2-4 vol% • Aluminum. 0.5 oz/gal • Temperature, 50-7CfF • Current density (see note I) 10-15 A/ftC Sulfuric/Boric Acid (This solution is covered by U.S. Patent No. 4,894,127-license required) • Sulfuric acid (66'86), 4-7 oz/gal • Boric acid, 0.7-1.4 oz/gal • Aluminum, 0.35 oz/gal • Temperature, 80:!:4T • Current density (see note 2) Note 2: Immerse parts and apply voltage starting at O.Increase within 3-5 minutes to 15 :!: I Y, anodize at this voltage for 18-22 minutes. This solution, designated Mil-A-8625 Type IC, provides a coating that is equivalent to that produced by chromic acid solution Mil-A-8615 Type I, which is not recommended because of environmental considerations. 92 Phosphoric Acid • Phosphoric acid (H,P04 ) (see note 3), 13-16 ozJgal • Temperature, 72-82'F • Current density (see note 4) Note 3: It may be necessary to filter this bath periodically to remove fungi that tend to grow in phosphoric acid at room temperature. Note 4: Immerse parts at zero voltage. Raise the voltage to 15 ± I V within 5 minutes and maintain for 20-25 minutes. PROCESSING For successful anodizing as a pretreatment to painting, the following general sequencing is satisfactory. All mechanical processing should have been acceptable prior to anodizing. The parts should now be ready for cleaning. If they are excessively dirty, it may be necessary to preclean them prior to racking; if not, they can be racked. This step is very important. The electrical connections must be firm, as they will be required to handle large currents, and arcing at a loose connections may destroy the part. The connections should be made in nonfunctional areas of the part whenever possible because the anodic coating will not form under the connection. Cleaning is standard, remove grease and oily soils with solvents or in emulsion cleaners. If this doesn't result in a water-break-free surface. a standard alkaline cleaner should be used. Deoxidize This step removes the natural oxides so that the anodize (oxide) coating is formed uniformly over the part. There are a number of nonchromic deoxidizer solutions on the commercial market. The following solution, based on nitric/hydrofluoric acid or ammonium bifluoride, will work. • Nitric acid, 120-250 gL • Hydrofluoric acid/ammonium bifluoride, to maintain etch rate at 0.15-0.5 mil/surface/hour This is followed by anodizing in one of the solutions above. POSTANODIZE TREATMENT Following rinsing and drying, the parts should be primed as soon as possible but before the freshly formed aluminum oxide adsorbs water and partially seals. This will give excellent adhesion of the first paint (primer) coating. Otherwise, the anodized coating can be sealed in hot water. It is recommended that for better adhesion only a partial seal be accomplished (i.e., 5% weight gain of adsorbed water) or for maximizing unpainted corrosion resistance but poor paint adhesion, the sealing can be complete (15% weight gain). 93
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