Transpassive dissolution mechanism of Ni-based alloys in a simulated bleaching solution: Effect of alloying elements

The transpassive dissolution of several nickel-based alloys has been studied in a simulated bleaching solution (0.014 M SO2−4, 0.017 M Cl−, and 0.0025 M diethylenetriaminopentaacetic acid, DTPA, pH 2.8) by electrochemical techniques.
The alloys have been prepared by welding superaustenitic stainless steels (UNS S31254 and S32654) with nickel-based filler metals (UNS N06625 and N06059). According to the rotating ring-disk voltammetric studies, the release of soluble high-valency products from the alloys plays a significant role in the transpassive potential region.
Electrochemical impedance spectroscopic data point to a competition between transpassive dissolution and secondary passivation reactions. The experimental data have been compared to a previously proposed model for the transpassive dissolution of nickel-based alloys.
The model treats the transpassive dissolution of Cr as a two-step reaction featuring a Cr(VI) intermediate. It assumes that the dissolution of Ni follows two parallel reaction paths, one of which involves a self-catalytic step.
The kinetic parameters of the process for the studied alloys have been determined, and a discussion of the role of alloying elements such as Fe and Mo on the transpassive dissolution mechanism is given