Transpassive dissolution of ferritic steels in a molten salt electrolyte

The transpassive dissolution of ferritic steels (14% and 18% Cr) in molten KH2PO4 at 270 °C has been studied by voltammetric and impedance measurements. The current vs. potential curves and the impedance spectra in the transpassive region have been found to be relatively similar to those obtained earlier in a phosphoric acid-based electrolyte. Both stationary and transient techniques pointed to two parallel pathways of the overall process. The first reaction path has been proposed to involve oxidative dissolution of Cr as Cr(VI) via a Cr(IV) surface intermediate species. The second path is tentatively identified as the dissolution of Fe as Fe(III) mediated by a surface chemical step involving interaction with an electrolyte-originating species. A simplified kinetic model of the process involving only surface kinetic steps has been found to reproduce successfully the steady-state and the small amplitude ac response of the studied materials. The relevance of the kinetic parameters regarding the influence of the alloy composition on the relative importance of the two parallel pathways is discussed.