Crevice Corrosion Repassivation of Alloy 22 in Aggressive Environments

The resistance of Alloy 22 (UNS N06022) to crevice corrosion in aggressive environments was studied using various modifications of the Tsujikawa-Hisamatsu Electrochemical (THE) technique. THE technique on a creviced specimen involves potentiodynamic polarization to initiate crevice corrosion, holding the specimen at constant current for a period to grow the crevice and then stepping the potential downward until the crevice repassivates. In this work, a new criterion for determining the crevice repassivation potential (E_R,CREV) is proposed, i.e., the potential at which the current reaches the passive current measured on the forward scan. This value of E_R,CREV is unambiguous, conservative, and more reproducible for Alloy 22 in the solutions studied. Furthermore, this definition allows for a modification of the THE technique that simplifies the measurement and decreases the time of experiment. With this definition, it is possible to use a potentiodynamic-galvanostatic-potentiodynamic approach, which results in similar values of E_R,CREV for non-welded and as-welded Alloy 22 specimens in chloride solution. Moreover, E_R,CREV was independent of hold current density in the crevice region. THE technique was further modified to allow determination of the crevice re-passivation temperature (T_R,CREV). T_R,CREV values for non-welded and as-welded Alloy 22 specimens were within 5°C. Effects of potentially inhibitive oxyanions in chloride solutions were also evaluated for Alloy 22.