Interference of solution constituents on corrosion inhibition of phosphate species on Grade 202 stainless steel

Strong corrosion resistance of Grade 202 stainless steel (202 SS) in mild acidic conditions is attributed to the presence of chromium which forms 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 202 SS leads to high corrosion inhibition efficiency. Nevertheless, the corrosion stability of 202 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 202 SS, however, has not received sufficient attention despite the widespread use of 202 SS-based machinery in industrial applications. Although the corrosion-inhibitory action of phosphate species on 202 SS has been documented, detailed investigation, especially in the presence of interferents, has not been given due consideration. As such, corrosion inhibition efficiency of HNO3 and H3PO4 of different concentrations on 202 SS, and the impact of chloride ions and various phosphate species, namely PO43−, HPO42− and H2PO4−, on the extent of corrosion of 202 SS would be the industrially important innovative aspect of this study. Weight loss measurements of rectangular stainless-steel specimens immersed separately in HNO3 and H3PO4 acid solutions under ambient conditions conclusively demonstrate the superior corrosion inhibitory behavior of H3PO4 over HNO3 on 202 SS. This inhibitory behavior is evident even in chloride-rich environments under low acidic conditions despite pitting corrosion of chloride species. Polarization resistance determined by Nyquist plots further supports the corrosion inhibitory action of H3PO4 on 202 SS, while open circuit measurements indicate the strong correlation between H3O+ and surface characteristics. The corrosion rate in 0.25 M HCl (7.20 mmpy) is almost 99.92% higher than that in 0.25 M Na3PO4 (5.45 × 10–4 mmpy), indicating that hydrochloric acid has a more severe corrosive effect on steel than sodium phosphate. The order of corrosion inhibition ability of phosphate species on 202 SS, as determined by weight loss measurements, electrochemical impedance spectroscopy, and Tafel slope analysis follows the order, Na3PO4 > Na2HPO4 $$\sim$$ NaH2PO4 > H3PO4. Highlights