A pre-passive state observed for the passive film formed on Alloy 625 in a hydrochloric acid solution

Investigation on oxide films of Inconel alloy 625 formed in 0.5 M HCl solution was performed, mainly by means of Electrochemical Impedance Spectroscopy (EIS) measurements, potentiostatic polarization and X-ray photoelectron spectra (XPS) analysis. The steady state current density at elevated temperatures in a low anodic range was found to be lower than that at 25 °C. Through the EIS measurements in the entire anodic range, two time constants were observed, while a semi-infinite spectrum associated with mass transport was found at only room temperature in the low anodic potential range. A new electrical equivalent circuit (EEC) was proposed for the spectrum in low potential range. The EEC indicated that there was a pre-passive process at room temperature before the passivity, and the film formed was incomplete. The increase of temperature promoted the pre-passive state into the passive state. XPS results confirmed that the film thickness was susceptible to temperature.