| The open circuit potential oscillation of nickel-containing UNS S30403 stainless steel in 93.5 wt% sulphuric acid has been studied. The study is aimed to explore the mechanism of the oscillation therefore to pursue a deep understanding of both the activation and the passivation of this steel in concentrated sulphuric acid. Literature review was conducted to acquire fundamental knowledge for the understanding of the phenomenon. Electrochemical measurements and surface analysis techniques including Auger Electron Spectroscopy, Laser Raman Spectroscopy and X-ray Photoelectron Spectroscopy were employed in experiments.; Open circuit potential measurements, potentiostatic and potentiodynamic polarizations were conducted. Results showed that the oscillation was between the active and passive states of the steel and was both activation and mass transport controlled. The re-passivation process during the oscillation is found to be solely activation controlled, which is consistent with the proposed mechanism that nickel salt is nucleated and grown in this process.; It was further revealed that the nickel salt is not stable at above 0.263 VSHE ("SHE" is standard hydrogen electrode potential) but stable at 0.163 VSHE and below. XPS analysis results showed that the nickel salt was nickel sulphide/polysulphides. During the oscillation, the particular stability limit of the sulphide permits it to nucleate and grow in the active region where it is stable. It catalyzes the reduction reaction to a higher exchange current density on a large sulphide surface area and thereby passivates the steel. In the passive region, the sulphide is unstable. It dissolves to an extent at which the re-activation is permitted while the stable region of the sulphide is not reached, therefore the steel is activated. Sulphide is produced again in the active region to start the cycle. However, if a critical coverage of the nickel sulphide is exceeded, the excessive sulphide will block the surface therefore the passive film cannot form. Instead, the steel will be subject to active corrosion.; According to our model, the stable passivity of the alloy may be achieved by changing the composition of the steel. Future research work with respect to improved alloy design has also been proposed. |
