Metal Finishing Guide Book

2013

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Potassium fluoride (KF.2H2O), 2–3 oz/gal Hydrofluoric acid, 2–3 fl oz/gal Room temperature Anodizing can be accomplished by making the part anodic at 10 VAC for 20 minutes in a 70 to 80 oz/gal solution of potassium hydroxide. Steel tanks can be used as cathodes and to hold the solution. Titanium will heat and the solution will produce a caustic spray, so it should be covered and handled carefully. Both of the treatments will provide satisfactory paint adhesion. ZINC A phosphate coating applied after electroplating will render zinc-plated parts paintable. A solution containing 10% phosphoric acid (85% ortho) and zinc phosphate or dissolved zinc (0.5–2.0 oz/gal) will provide a coating satisfactory for painting; however, the proprietary processes intended for heavy phosphate coating are also satisfactory. New galvanize should be thoroughly washed and given a phosphate conversion coating prior to painting. An alkaline resisting primer will give best results for outdoor exposures. Assembled and weathered galvanize can be washed with a solution of 25% phosphoric acid (85% ortho), 25% isopropyl alcohol, 0.25% wetting agent, and balance water. Allow the solution to stay wet on the part for a few minutes, then wash off by spraying with water (hose or spray bottle, depending on size), and then dry and paint. Wash primer can also be effective on weathered galvanize, but it may not work well on new work. ALLOYS The zinc-nickel and tin-zinc alloys that are emerging as cadmium replacements will respond to both chromate and phosphate conversion coats. Owing to environmental pressures the chromates are not recommended; hence, the phosphate coatings are excellent. Wash primer without conversion coating also works well for a pretreatment. 133

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