Mechanism of transpassive dissolution of nickel-based alloys studied by impedance spectroscopy and rotating ring-disc voltammetry

A generalised kinetic model of transpassive dissolution of industrial nickel-based alloys is presented in this paper, It is based on electrochemical impedance spectroscopic (EIS) and rotating ring-disk (RRDE) measurements on several nickel-based alloys in 0.5 M sulphate and chloride solutions with pH ranging from 2 to 7. The model assumes that the dissolution of Cr from the transpassive film occurs via a Cr(VI) surface intermediate species. The dissolution of Ni as Ni(II) is proposed to occur via two parallel paths featuring two reaction intermediates. The role of Fe as a secondary passivating agent and Mo as an accelerator of transpassive dissolution is also taken into account. The kinetic model has been found to reproduce quantitatively the current vs. potential curves and the impedance spectra for the studied nickel-based alloys in the transpassive region. The proposed mechanism is sensitive to the impact of alloy and electrolyte composition on the dissolution process. Thus it can serve as a starting point to describe the transpassive behaviour of engineering alloys. (C) 2002 Elsevier Science Ltd. All rights reserved.