Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 57 of 331

TITANIUM Untreated titanium exists in the atmosphere as a passive material, and as such, it is paintable; however, adhesion is only moderate. A conversion coating or an anodize coating is recommended. A phosphate–fluoride conversion coating is ap- plied by immersion for 2 to 3 minutes in the following solution: Sodium phosphate (Na2PO4), 6–7 oz/gal 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 coat- ing are also satisfactory. New galvanize should be thoroughly washed and given a phosphate conver- sion 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, de- pending 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 envi- ronmental pressures the chromates are not recommended; hence, the phosphate coatings are excellent. Wash primer without conversion coating also works well for a pretreatment. 56

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