Passivity of Nickel-Containing Stainless Steels in Concentrated Sulphuric Acid

The spontaneous periodical oscillation of corrosion potential between the active and passive states, typical of nickel-containing stainless steels in concentrated (93.5%) sulphuric acid is used to determine the nature of passivity in such systems. Detailed analysis of the electrochemical kinetics of the oscillating potential in combination with (ex-situ) surface analysis has indicated the presence of nickel sulphides with a limited potential range of stability. A minimum critical coverage of sulphide on the surface is required for the mixed potential to be shifted into the passive range. The sulphide catalyses the cathodic reaction thereby raising the potential. However, when the coverage exceeds a critical value, the sulphides effectively block the formation of a passive film. Consequently, there is a critical range of nickel content required in these alloys for their effective use in sulphuric acid production plants.