Corrosion and electrochemical behaviour of 316L stainless steel in acetic acid solutions

The corrosion and electrochemical behaviour of 316S11 stainless steel in acetic acid solutions typifying chemical process environments has been investigated. Acetic acid concentrations tested were in the range 70-90% and included addition of 1500 ppm Br⁻ and 200 ppm Na⁺. Of key interest was the impact of Cl⁻ ions, representing an uncontrolled excursion in system chemistry. Corrosion potential-time and electrochemical polarisation measurements were made for the different environments at 90 °C and the characteristics of the surface film formed at different stages of exposure analysed using X-ray photo-electron spectroscopy (XPS).The most distinctive feature of the results was the step increase in potential with exposure time in the 70% acetic acid solution, in the absence of Cl⁻ ions, indicating a sharp transition from active corrosion to some degree of passivity. No such transition was observed in the 90% acetic acid solution. Addition of chloride to the 70% acetic acid solution after the step in potential resulted in a step decrease in potential once a critical level of chloride had been exceeded. If the chloride were present on initial immersion, the potential stayed relatively low and the steel remained active. XPS analysis suggested that local enrichment of Mo was important in initiating the passivation process but the precise details of the mechanism remain speculative.