Abstract
An experimental study of the corrosiveness of citric acid, oxalic acid, and EDTA mixtures to sensitized AISI 304 stainless steel (SS) was conducted. In initial tests, the corrosivity of various solutions at 125 C was assessed by measuring anodic polarization curves and estimating corrosion current densities (CDs), critical CDs for passivity, and passivation CDs. Based on the electrochemical results, solutions were ranked from highest to lowest according to their corrosiveness when applied to sensitized AISI 304 SS as follows: citric acid/oxalic acid, citric acid/oxalic acid/EDTA, and citric acid/ EDTA. Scanning electron microscopy (SEM) and metallographic examinations of specimens exposed at rest potential to solutions of varying corrosiveness show that intergranular attack (IGA), IGA with pitting at grain boundaries, and general pitting occurred only in samples immersed in solutions containing oxalic acid. No surface attack was detected in specimens immersed in citric acid/EDTA mixtures. These results and the anodic polarization results show that of the three reagents commonly used to prepare decontamination solutions, oxalic acid is the most likely to cause IGA in sensitized AISI 304 SS. In subsequent tests, the effects of ferric ion addition were investigated using electrochemical techniques and SEM and metallographic examinations of specimens exposed to solutions of varying ferric ion concentrations. The addition of ferric ion to citric acid/oxalic acid/EDTA mixtures results in up to a threefold reduction in corrosion CDs and reduces IGA to insignificant levels in cases where it had previously been widespread and had penetrated to depths of 85 µm. For citric acid/oxalic acid mixtures, an addition of ferric ion results in decreases or increases in corrosion CDs and surface attack, depending on the initial concentration of oxalic acid present. Addition of ferric ion to citric acid/EDTA mixtures has no effect on corrosion CDs or surface damage. Hence, corrosion damage for sensitized AISI 304 SS exposed to citric acid/EDTA mixtures is negligible with or without added ferric ion.
