Passive behaviour of non-equiatomic NiCoCrTa multi-principal element alloys in dilute sulfuric acid

The passive behaviour of non-equiatomic multi-principal element alloys, Ni(58-x)Co20Cr22Tax (x = 0, 0.64, 1.64, 3.39 at.%), was investigated in 0.1 M H2SO4 at 25 °C by electrochemical methods and surface analysis. After 24 h immersion, the alloys were all found to be in a spontaneously passivated state, and the value of the open circuit potential shifted to the positive direction with the increase of Ta content. The grain size of the alloys reduced with increasing the Ta content. The relationship between the passive current density and the grain size was in agreement with a linear model. The capacitance measurements indicated that the passive film formed on the alloys was of n-type semiconductor characteristic and the donor density was decreased with the increase of Ta. The thickness of the passive film improved primarily due to the obstacle to the dissolution of the outer layer as the accumulation of Ta2O5 in the outer layer. The improved corrosion resistance of the alloys with increasing Ta content can be ascribed to the formation of a complex film containing more Ta2O5, which enhanced the stability, thickness, and compactness of the passive film. The developed alloys show good passivity in dilute sulfuric acid.