Study of an alternative phosphate sealer for replacement of hexavalent chromium

Abstract

This study evaluates the possibility of replacing the hexavalent chromium passivation treatment used as a sealer after phosphating of carbon steel (SAE 1010) by a treatment with niobium ammonium oxalate (Ox). Samples of carbon steel (SAE 1010) after being phosphated in a zinc phosphate bath (PZn+Ni) were immersed in solution of niobium ammonium oxalate (250 mg L−1 of Nb) either at pH 3.0 or pH 8.0. A passivation treatment with a solution with CrO3 (200 mg L−1 of Cr6+) was also used for reference. The corrosion resistance of the phosphated samples after passivation treatments was analyzed in a NaCl 0.5 mol L−1 solution using electrochemical impedance spectroscopy (EIS) and anodic polarization curves. Salt spray tests were also performed to evaluate their corrosion resistance. The results showed that the highest corrosion resistance was obtained by passivation in a solution with (250 mg L−1 of Nb) at pH 8.0.