Nitrate → Zinc Phosphate

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An aqueous acidic zinc phosphate coating solution was prepared in a five-gallon laboratory spray tank to contain 1.2 g/l zinc ions, 1.0 g/l nickel ions, 1.0 g/l fluoride added as silicofluoride, 5.0 g/l phosphate ions, 2.0 g/l nitrate ions, and 0.1 g/l nitrite ions. This solution, when heated to 150° F., and when titrated in the conventional manner, was found to have a total acid of between 13.0 and 14.6 points, and a free acid of between 1.0 and 1.3 points. In a second spray tank, a conventional alkaline cleaner was prepared at a concentration of one ounce per gallon and was heated to 145°-150° F. A third spray tank was used to contain the melamine-formaldehyde resin/tannin post-treatment solutions detailed in Table XVIII. A number of 4 × 12 inch SAE 1010, cold rolled steel panels (CRS) and 4 × 12 inch minimum spangle commercial hot-dipped galvanized panels (GALV), temper rolled one percent, were sprayed in the sequence one minute cleaner, one-half minute fresh warm water spray rinse, 1 minute zinc phosphate solution, one-half minute cold water rinse, and one-half minute melamine-formaldehyde resin/tannin solution detailed in the table, after which they were dried in a circulating air oven for 5 minutes at 325° F. Control panels were prepared in the same manner except that they were oven dried following a one-half minute tap water rinse and a one-half minute deionzed water rinse, or alternatively treated in a commercial post-treatment solution containing about 0.04% chromic acid at a pH of about 4 at 80° F. for one-half minute following the cold water rinse and oven dried. Other control panels were prepared in the same manner except that they were treated in a solution containing 0.25 g/l of tannin T9 at a pH of 4.6 (adjusted with H3PO4) at 80° F. for one-half minute following the cold water rinse and oven dried. In this example, all panels were oven dried for 5 minutes at 325° F.

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