Electrochemical Methods for Repassivation Potential Measurements

The susceptibility of Alloy 22 (N06022) to crevice corrosion may depend on environmental and metallurgical variables and it was mostly tested to obtain crevice repassivation potentials.  Regardless of all the combinations or variations of the electrochemical test methods (Cyclic Potentiodynamic Polarization CPP, Tsujikawa – Hitsamatsu Electrochemical, Potentisotatic POT, and Mixed CPP - POT), whenever crevice corrosion readily occurs, the values of repassivation potential for Alloy 22 seem comparable if all environmental and metallurgical variables remain the same.  This finding suggests that the crevice repassivation potential is a property of the alloy in each testing condition and does not depend greatly in the manner it is obtained provided that highly aggressive conditions be used.  Under less aggressive conditions, when crevice corrosion is more difficult to initiate, the value of repassivation potential may differ from method to method.  In this work three different electrochemical methods (CPP, THE, and mixed CPP-Potentiostatic methods) were used to evaluate the susceptibility Alloy 22 to crevice corrosion in two different solutions: one highly aggressive (1M NaCl at 90ºC), and the other one weakly aggressive (NaCl + NaNO₃).  The effect of applied torque was also investigated in these systems.