Effect of solution pH on the electrochemical behaviour of AISI 304 austenitic and AISI 430 ferritic stainless steels in concentrated acidic media

In this study, the effect of solution pH on the electrochemical behaviour of AISI 304 austenitic and AISI 430 ferritic stainless steels at open circuit potential (OCP) in concentrated acidic solutions was studied. Polarization curves showed that the corrosion current density of both stainless steels increased with decreasing pH. Mott–Schottky analysis revealed that passive films formed on AISI 304 and AISI 430 stainless steels behave as n-type and p-type semiconductors and the donor and acceptor densities increased with decreasing pH. Electrochemical impedance spectroscopy (EIS) results showed that the reciprocal capacitance of the passive film is directly proportional to its thickness which decreases with decreasing pH. Thus for both austenitic and ferritic stainless steels in acidic solutions, increasing the solution pH offers better conditions for forming passive films with higher protection behaviour, due to the growth of much thicker and less defective films.