PROCESS OF FORMING PHOSPHATE COATINGS ON METAL SURFACES

摘要

A process for forming phosphate coatings on metal surfaces is provided which comprises the steps of contacting said metal surface with an Fe(II)-containing phosphating solution comprising 0.4 to 30 g/l Zn 4 to 30 g/l P2O5 5 to 50 g/l NO3 up to 10 g/l Fe(II) ; and up to 0.3 g/l (Fe(III) wherein the weight ratio of free P2O5 to total P2O5 is (0.04 to 0.50):1 and, replenishing said phosphating solution with Zn, NO3 and P2O5 in a weight ratio of Zn:NO 3 P2O5 =(0.60 to 0.30):(0.2 to 0.4):1. wherein the Fe(II) content is adjusted by oxidation with nitrate or nitrite derived from nitrate optionally employed with an oxygen-containing gas, H2O2 and/or nitrous gases; and rinsing said metal surface with a cascade of at least two aqueous rinsing baths in the opposite direction of travel of said metal surface wherein water having a low salt content or no salt content derived from the phosphating bath is fed to the last rinsing bath and the overflowing water from said rinsing bath is fed to the next preceding rinsing bath and ultimately to the phosphating bath, and wherein the low salt content rinse water derived from the phosphating bath is withdrawn therefrom as a rate effective to permit the addition of phosphate-enriched rinsing water from the cascade of rinsing water to the phosphating bath while maintaining the desired species concentration in said phosphating bath.