Effects of Cu on the passive film stability of Fe-20Cr-xCu (x = 0, 2, 4 wt.%) alloys in H₂SO₄ solution

Effects of Cu on the stability of passive film formed on Fe-20Cr-xCu (x = 0, 2, 4 wt.%) alloys in deaerated 0.1 M H₂SO₄ solution were examined by potential decay test, X-ray photoelectron spectroscopy, and the Mott-Schottky analysis. The potential decay curves of Fe-20Cr-xCu (x = 0, 2, 4 wt.%) alloys passivated for 1 h at 400 mV_SCE exhibited a 1st stage-potential decay to a potential of approximately -300 mV_SCE. Especially, the stability of passive film decreased with an increase in Cu content, suggesting that the film formed on the Cu-containing alloys was less stable than that of Fe-20Cr alloy. The rapid potential decrease in the Cu-containing alloys was primarily due to the Cu-dissolution from the film, which increased the defect density of the passive film and hence decreased its stability. After the 1st stage-potential decay, Fe-20Cr alloy exhibited the 2nd stage-potential decay to -654 mV_SCE or to an active state in 9000 s. However, the 2nd stage-potential decay did not occur in the Cu-containing alloys because of the Cu enrichment on the surface of the alloy. The Cu enrichment in the Cu containing alloys decreased the defect density in the film, and hence increased the stability of the passive film.