Microstructure and Corrosion Behaviour of Carbon Steel and Ferritic and Austenitic Stainless Steels in NaCl Solutions and the Effect of p-Nitrophenyl Phosphate Disodium Salt

The microstructure and corrosion behavior of carbon steel (CSA516) and ferritic (SS410) and austenitic (SS304L) stainless steels were studied and compared. Corrosion tests were carried out in 0.5 M NaCl solutions. Rates of corrosion were monitored based on weight loss, Tafel extrapolation and linear polarization resistance (LPR) methods. Rates of corrosion were ranked following the order: CSA516 >> SS410 > 55304L. The impact of p-Nitrophenyl phosphate disodium salt (NPP) on the corrosion rate of CSA516 was also studied using Tafel polarization and LPR measurements. Optical microscopy (OM), scanning electron microscopy (SEM/EDX), and X-ray photoelectron spectroscopy (XPS) were employed to assess the chemical compositions and morphologies of the corroded and inhibited surfaces. FT-IR analyses were also performed to assess the functional groups of the inhibited sample in a comparison with NPP itself. XPS and FT-IR studies revealed the presence of phosphate groups originating from tested inhibitor, thus proving formation of the protective layer on the steel surface. The microstructural and defect investigations of as-polished, corroded, and inhibited CSA516 samples were also carried out using positron annihilation lifetime (PAL) and positron annihilation Doppler broadening (PADB) techniques. Experimental findings revealed that NPP acted as an efficient mixed-type inhibitor with anodic predominance. It reached about 97% inhibition efficiency at a low concentration of 0.02M.