Comparative Assessment of the Pitting Corrosion Resistance and Passivation Behaviour of 439L Ferritic and 904L Austenitic Stainless Steels for Application in Extreme Process Environments

Abstract

Potentiodynamic polarization studies of the passivation behaviour and pitting corrosion resistance of 439L ferritic (ST439L) and 904 austenitic (ST904L) stainless steels in 3.5 M H2SO4 and neutral chloride solution at specific chloride concentrations were performed. Results showed ST439L to be relatively more resistant to general corrosion in H2SO4 solution at 1–5% NaCl (wt%) concentration due to the higher tendency of ST904L to oxidize. Their corrosion rate values were comparable at 0% NaCl (0.130 mm/year and 0.189 mm/year). At 6% NaCl, the corrosion rate of ST439L increases to 2.355 mm/year which is significantly higher than 0.778 mm/year for ST904L. In the neutral solutions, both steels generally have the same corrosion rates. However, Cl− ions caused unstable passivation of both steels resulting in additional anodic–cathodic peaks and formation of 904L exhibited slightly higher resistance to pitting corrosion in the presence of chlorides with peak passivation range value of 1.213 VAg/AgCl compared to 1.117 VAg/AgCl for ST439L at 5% NaCl concentration, signifying higher pitting resistance in sulphate/chloride environment. The passivation current value for ST904L increases with increase in Cl− ion concentration due to its higher tendency to reform after metastable pitting activity. Both steel exhibited thermodynamic stability in the acid chloride and neutral chloride media though ST439L exhibited some potential transients at 6% NaCl in the neutral chloride soluthion. ST904L was more electronegative in the acid chloride and neutral chloride solution at 6% NaCl. The metallurgical configuration of both steels was clearly visible from optical microscopy analysis due to acidification of the steel surfaces. Excessive corrosion pits formed on the morphology of ST439L compared to ST904L which had very few macro-pits.