EIS Investigations of Transpassive Dissolution of Ferritic Steels in Aqueous and Molten Electrolytes

Abstract

The present paper compares the kinetics of transpassive dissolution of industrial ferritic steels in a phosphoric acid-based electropolishing solution to that for the same materials in molten KH 2 PO 4 . Both steady state and transient techniques point to two parallel pathways of the overall process, the first corresponding to oxidative dissolution of Cr as Cr(VI). The first and the second path is identified as the isovalent dissolution of Fe as Fe(III) mediated by a surface chemical step involving interaction with an electrolyte-originating species. Kinetic models of the process involving only surface reaction steps have been found to reproduce successfully the steady-state and the small amplitude ac response of the studied materials in both electrolytes. The kinetic parameters are estimated by a calculation procedure that involves simultaneous fitting of the impedance spectra at several polarisation potentials and the steady state current vs. potential curve for a particular material-electrolyte combination to the model equations. The ability of the kinetic models proposed for aqueous solutions to account for the behavior of materials in molten salt medium is discussed.

Influence of Additives on the Transpassive Dissolution of Ferritic Steels in Phosphoric Acid-Acetic Acid Electrolytes

“…The curves are qualitatively similar to these in Ref. (1)(2)(3) and comprise a Tafel-like region and a current plateau. It is noteworthy that the plateau current densities are higher for Fe-14%Cr in electrolyte 1 and for Fe-18%Cr in electrolyte 4, the values for both alloys in electrolyte 3 being almost identical.…”

“…Industrial ferritic steels containing 14% and 18%Cr were used as working electrode materials, in analogy to the results reported in Ref. 3. The working electrode pretreatment consisted of mechanical polishing on emery paper up to 2400 grade, degreasing and rinsing with bidistilled water.…”

“…In previous papers (1)(2)(3) we presented and discussed experimental results for the effect of electrolyte composition, temperature and stirring on transpassive dissolution of industrial ferritic steels in 14.8 M H 3 PO 4 -15 M CH 3 COOH mixtures. Calculations based on a quantitative kinetic model (1)(2)(3) have shown that the transpassive dissolution of such steels in electrolytes used for electropolishing and anodic levelling is basically a surface process.…”

“…In previous papers (1)(2)(3) we presented and discussed experimental results for the effect of electrolyte composition, temperature and stirring on transpassive dissolution of industrial ferritic steels in 14.8 M H 3 PO 4 -15 M CH 3 COOH mixtures. Calculations based on a quantitative kinetic model (1)(2)(3) have shown that the transpassive dissolution of such steels in electrolytes used for electropolishing and anodic levelling is basically a surface process. The proposed model (1-3) comprised two parallel dissolution pathways, namely, oxidative dissolution of Cr as chromate and isovalent dissolution of Fe(III), mediated by an adsorption/surface complexation step involving a solution-originating species.…”

“…An overall conclusion was that the relative importance of surface kinetic and transport steps in the overall mechanism of transpassive dissolution of stainless steels in concentrated acid mixtures has to be reconsidered. In the present communication, experiments to evaluate the extent of influence of sulphuric acid and glycerol additives to the electrolytes proposed in [3] are described and discussed in order to estimate more systematically the effect of individual components in quasi-non-aqueous electrolytic mixtures on the kinetics of transpassive dissolution.…”