Corrosion of heat-treated 304SS in the presence of molybdate ions in hydrochloric acid

The electrochemical behaviors of quenched (non-sensitized) and furnace-cooled (sensitized) 304SS stainless steels have been thoroughly investigated in 0.1M HCl as a function of molybdate ions concentration at 22 °C and 50 °C. Standard corrosion techniques that have been used include potentiostatic polarization, polarization resistance vs. time (R_p vs. t), and cyclic polarization measurements. The non-sensitized steel showed better resistance to pitting corrosion than the sensitized steel when tested in 0.1M HCl at 22 °C and 50 °C. Furthermore, both steels showed better resistance at 22 °C in comparison to 50 °C. While the addition of molybdate ions at sufficient concentrations (0.075M) enhanced significantly both the resistance to general corrosion and to pitting corrosion in both steels, the non-sensitized steel showed better resistance especially when tested at 22 °C. Testing both steels at low concentrations of molybdate ions (0.001M) at 22 °C and 50 °C resulted in increasing the corrosion current densities, i_corr, combined by increasing the repassivation (protection) potential, E_pp. It is suggested that molybdate ions increase resistance to general and pitting corrosion by lowering the current densities in the active and passive regions by means of chemical reactions, enhancing repassivation in the process and making it difficult for stable pits to grow.