Role of Methanol on Pitting of Type 316 Stainless Steel

This work is aimed at improving the understanding of the localized corrosion of stainless steel in methanolic solutions. Potentiodynamic polarization curves on Type 316 (UNS S31600) were measured in methanol (CH₃OH)-water (H₂O)-1 M lithium chloride (LiCl) at water concentrations ranging from 1 wt% to 100 wt%. The pitting potential increased from 350 mV vs. saturated calomel electrode (SCE) to –100 mV_SCE as the concentration of water increased from 0 wt% to 99 wt%. Dissolution kinetics were determined as a function of water concentration, from 100 wt% to 10 wt%, in a saturated pit environment. The overvoltage for active dissolution increased with decreasing water (increasing methanol) content. This, however, does not explain the decrease in pitting potential with increasing methanol content. Repassivation transients were determined by pulsing the potential to –150 mV_SCE at different concentrations of water. The time to repassivate increased with decreasing water content, suggesting that the critical concentration of metal ions for repassivation increases with water content. This may indicate that pit growth is stabilized at a lower concentration of metal ions with the decreasing water (increasing methanol) content. The decrease in critical concentration for repassivation explains the increase in pitting potential with increasing water (decreasing methanol) content.