An Evaluation of Some Electrochemical Techniques for the Determination of Pitting Potentials of Stainless Steel^⋆

Three different techniques for the determination of pitting potentials were evaluated: slow potential scan (260 to 520 mV·hr⁻¹), fast potential scan (50 V·hr⁻¹ and the scratch technique. Evaluation was performed with high-purity experimental ferritic stainless steels containing 18% Cr and 0 to 5% Mo, and with commercially produced ferritic stainless steels 18Cr-2Mo-Ti, 21Cr-3Mo-Ti, and E-Brite 26-1. Pitting potentials were determined in M NaCl at 25 C (77 F) and in some cases at 45 C (113 F). The pitting potentials found by the three different methods were of comparable magnitude for 18Cr, 18Cr-1Mo, 18Cr-2Mo, 18Cr-2Mo-Ti, and 18Cr-3.5Mo stainless steels; however, for the 18Cr-5Mo, 21Cr-3Mo-Ti, and E-Brite 26-1, the scratch technique gave consistently less noble pitting potential than either the slow or fast scan techniques. The scratch technique is recommended for the determination of pitting potentials, at least for the ferritic stainless steels.