Pitting corrosion of stainless steels in acidic environment

In halogenated environments, especially those containing chlorides, stainless steels can suffer localised corrosion. Alloying elements such as chromium, molybdenum and nitrogen impart resistance to pitting corrosion. In the present investigation, both de and ac electrochemical methods have been investigated as a means of elucidating the pitting susceptibility of austenitic and duplex stainless steels. Acid chloride environments containing hydrochloric acid mixed with either perchloric acid or sodium chloride to give either a constant hydrogen ion ol a constant chloride ion concentration of 1M were used. The results of the electrochemical tests have been correlated with pitting resistance equivalence (PRE) values based on the molybdenum, chromium, and nitrogen contents of the steels. for austenitic stainless steels based on 18Cr-15Ni-0.2Mo and 25Cr-24.5Ni-2.5Mo (wt-%) with PRE numbers of approximately 20 and 35.6 respectively: the electrochemical tests revealed generally low resistance to pitting for hydrochloric acid concentrations of 0.1, 0.5, and 0.9M. Greatly improved resistance to pitting in these environments was indicated for a 20.5Cr-25Ni-6.4Mo alloy for which the PRE number was 45.5. Two duplex stainless steels based on 26.5Cr-4.8Ni-1.5Mo and 25.5Cr-5.7Ni-3.1Mo with PRE numbers of 34.8 and 38.8 respectively performed well in the electrochemical tests in 0.1M hydrochloric acid, but less so with hydrochloric acid concentrations of 0.5 and 0.9M. Polarisation resistance values from linear polarisation and ac impedance methods were found to be generally comparable and of use for assessing the pitting susceptibility of stainless steels in acidified chloride media.