Phosphate coating process

A continuous process of applying a phosphate coating to a ferrous or zinciferous metal substrate by treating the said substrate with an acidic phosphating solution of zinc phosphate in the presence of hydrogen peroxide or of a hydrogen peroxide-liberating substance wherein the phosphating solution comprises:

• (a) 0.005-0.5 g.atoms of zinc (Zn) per liter of solution.
• (b) 0.0002-0.02 g.moles of hydrogen peroxide per liter of solution and
• (c) orthophosphate (PO.sub.4) such that the molar ratio PO.sub.4 /Zn in solution is in the range 0.5-3.7, and wherein, as phosphating proceeds, the solution is replenished with hydrogen peroxide or with a hydrogen peroxide-liberating substance and with at least two other replenishment feeds, replenishment feed (i) and replenishment feed (ii), to maintain the solution at a desired composition and defined in (a), (b) and (c) above and wherein:
• replenishment feed (i) comprises sufficient zinc (Zn) to maintain the desired concentration (a) in the solution, and sufficient orthophosphate (PO.sub.4) together with another anion N.sup.n- to maintain the molar ratio PO.sub.4 /Zn in the solution within the range defined in (c), the replenishment feed (i) having a free acidity of F gram equivalents/Kg of the said feed, and replenishment feed (ii) comprises an alkaline material selected from the group consisting of the hydroxides, carbonates and bicarbonates of alkali and alkaline earth metals, said alkaline material being soluble in water at the concentration of said replenishment feed (ii) and has a total alkalinity of A gram equivalents/Kg of replenishment feed (ii);
• the ratio of the quantities of replenishment feeds (ii) and (i) respectively with are added to the phosphating solution within a significant period of time being (XF/A) where the value of X is in the range 0.5-1.5, and the anion N.sup.n- being selected such that the acid H.sub.n N has a pK.sub.a value in the nth dissociation step not greater than 3.