Influence of pH and chloride concentration on the pitting and crevice corrosion behavior of high-alloy stainless steels

Localized corrosion resistance (pitting and crevice corrosion) of two high-alloy stainless steels (superduplex and superaustenitic) was studied in solutions with chloride concentrations of 200, 400, 600, and 6,000 ppm, at pH values ranging from 2 to 6.5. Critical temperatures for pitting and crevice corrosion were calculatedfor these test media using electrochemical techniques (continuous current). From results obtained for cyclic polarization, the critical pitting temperature (CPT) and critical crevice temperature (CCT) of these materials in the different test media were determined. Under the tested conditions, the resistance of these materials to localized corrosion was very high. Only in test conditions of higher aggressivity (6,000 ppm Cl- and pH 6.5), pitting or crevice corrosion was observed, In those cases. values of pitting potential (E-pit) and crevice potential (E-cre) showed little tendency to decrease with an increase in Cl- concentration, temperature, and pH. Moreover, the CPT of these steels was determined in a ferric chloride (FeCl3) medium, which corresponds to the standard ASTM G48 practice (Method A).