The transpassive corrosion of highly alloyed austenitic stainless steels UNS N08904, UNS S31254 and UNS S32654 was investigated at 20 and 70 °C in a range of simulated bleaching solutions with conventional and rotating ring-disc electrode voltammetry, as well as electrochemical impedance spectroscopy. The overall transpassive oxidation rate of UNS S32654 was found to be much higher than that of the other two alloys. The general features of the impedance spectra demonstrate that transpassive dissolution is favoured for UNS S32654 and secondary passivation predominates for the two other steels. The addition of oxalic acid resulted in a significant increase of the transpassive oxidation rate at both temperatures. At room temperature, the addition of diethylenetriaminopentaacetic acid (DTPA) led to a decrease of the transpassive oxidation rate, especially at pH 3. Conversely, the addition of DTPA to the pH 3 solution at 70 °C has been found to increase the transpassive oxidation rate. A kinetic model of the process is proposed, featuring a two-step transpassive dissolution of Cr via a Cr(VI) intermediate species and taking into account the dissolution of Fe(III) through the anodic film. The model has been found to be in quantitative agreement with the steady-state current versus potential curves and the impedance spectra. The kinetic parameters of transpassive dissolution have been determined and the relevance of their values is discussed.
- stainless steel
- simulated bleaching solution
- electrochemical impedance spectroscopy
- rotating ring-disc voltammetry
- kinetic model