Solution and procedure for depositing a protective coating on galvanized

Abstract

Solution and procedure for depositing a coating, comprising hydrated zinc pyrophosphate, on the surfaces of galvanized steel parts, to protect against corrosion in the presence of water. The aqueous solution contains, per liter of final solution, hexametaphosphate equivalent to 10 to 70 grams of sodium hexametaphosphate, metasilicate equivalent to 1 to 40 grams of sodium metasilicate, an amount of orthophosphoric acid equivalent to 15 to 40 milliliters of orthophosphoric acid having a density of 1.71 g/ml, zinc chloride in an amount equivalent to 10 to 50 grams of anhydrous zinc chloride, and calcium carbonate sufficient to bring the pH of the solution to between 2.0 and 3.0. Nickel, equivalent to 0.5 to 20 grams per liter of final solution of hexahydrated nickel chloride, and chlorate, equivalent to up to 20 grams of sodium chlorate per liter, may be added. The solution is used by contacting it with the parts to be treated for a time determined on the one hand by the solution temperature (between 10.degree. and 70.degree. C.) and on the other hand by the desired deposit thickness. The deposit is hydrated zinc pyrophosphate; it has good abrasion resistance, good mechanical behavior and is insoluble in water. A regenerating solution, containing sufficient metaphosphoric acid to replace the phosphate consumed in forming the coating, and sufficient zinc chloride and zinc oxide to replace the consumed zinc and to maintain the pH of this regenerating solution at between 3.0 and 3.3, may be added to the used coating solution.