Effect of pH on corrosion behaviour of CoCrNi MEA imposed by alternating current in Na2CO3/NaHCO3 solution

Abstract

ABSTRACT The corrosion behaviour and mechanism of CoCrNi medium-entropy alloy (MEA) imposed by alternating current (AC) in HCO3 −/CO3 2− solution with various pH were systematically investigated via a series of corrosion tests. The results showed that with the augment of pH value, the polarisation resistance of the MEA decreased significantly, Ep shifted negatively, and ipI value increased, as well as corrosion rate was enhanced, revealing that the corrosion resistance was weakened. As solution pH increased, the formation of secondary passivation zone became easier, which may be attributed to the reaction conversion process of Cr(III) to Cr(VI) species. Moreover, the increase in pH led to an increase in defect density and a thinning of passivation film, facilitating the corrosion of the MEA. AC interference accelerated the ion exchange process and promoted the formation possibility of stable pitting. Higher iAC and pH inhibited the repassivation of metastable pitting, and increased the pitting susceptibility. Highlights The effect of pH on the corrosion behaviour of CoCrNi MEA with imposed AC was clarified. High pH and iAC accelerated the nucleation and growth of metastable pits. With increased pH and iAC, the stability and protective ability of passive film were weakened. The secondary passivation of the MEA was mainly due to the conversion of Cr(III) to Cr(VI) species.