Corrosion of stainless steels in acidic, neutral and alkaline saline media: Electrochemical and microscopic analysis

This study presents how SS304 and SS316 corrode in saline media, including in the presence of acid and alkali, through an electrochemical and microscopic analysis. SS304 and SS316 samples were studied for their corrosion resistance in 1M NaCl, and in the presence of either 1M HCl or 1M NaOH. Potentiodynamic polarization studies were carried out to determine the corrosion potentials and currents. From the anodic polarization, regions corresponding to corrosion, passivation and transpassivation were identified. Samples were corroded at different anodic potentials corresponding to the different regimes and were analysed using scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX). In 1M NaCl medium, SS304 exhibits pitting and passivation whereas SS316 does not. Transpassivation occurs through intergranular corrosion that connects the different pits. Making the medium acidic results in much higher corrosion rates for both SS304 and SS316, but the corroded surfaces bear vastly different structures. SS304 surface forms deep pits with large intergranular cracks resembling chimney stacks. However, SS316 surfaces show dendritic morphologies of deposited corrosion products, further evidenced by a higher O/Fe ratio in EDX measurements. In alkaline media, SS304 shows evidence of pits on whose edges corrosion products are deposited while no significant pitting is seen with SS316.