Strong corrosion resistance of Grade 202 Stainless Steel (SS) in mild acidic conditions is attributed to the presence of chromium which is responsible for the formation of a passive surface film of chromium oxide which inhibits corrosion. Additionally, the deposition of polymer coatings, polymer composites with nanomaterials as well as organic compounds containing hetero atoms such as N, O, S, and P on SS, or other metallic objects leads to high corrosion inhibition efficiency. Nevertheless, the corrosion stability of SS in certain environments, especially under aggressive conditions, is questionable. Investigation of the impact of different chemical constituents under moderate and aggressive acidic conditions on the corrosion of SS, however, has not received sufficient attention despite the widespread use of SS-based machinery in industrial applications. Although the corrosion-inhibitory action of phosphate species on SS has been documented, detailed investigation, especially in the presence of interferents, has not been given due consideration. As such, variation of corrosion inhibition efficiency of HNO3 and H3PO4 on SS at different concentrations, and the impact of HNO3 and H3PO4 in the presence of chloride ions along with the effect of various phosphate species were investigated in this study. Despite the pitting corrosion promotion action of chloride species, mass loss measurements of rectangular stainless-steel specimens immersed separately in HNO3 and H3PO4 acid solutions at different concentrations in the presence of HCl, under ambient conditions, conclusively demonstrate the superior corrosion inhibitory behavior of H3PO4 over HNO3 on SS, even in chloride-rich environments under low acidic conditions. Polarization resistance determined by electrochemical impedance spectroscopy further supports the corrosion inhibitory action of H3PO4 on SS, while open circuit measurements indicate the strong correlation between H3O+ and surface characteristics. The order of corrosion inhibition ability of phosphate species on SS, as determined by mass loss measurements, electrochemical impedance spectroscopy, and Tafel slope analysis follows the order, Na3PO4 > Na2HPO4 ~ NaH2PO4 > H3PO4.
