Crevice corrosion testing methods for measuring repassivation potential of alloy 22

Abstract

Alloy 22 (UNS N06022) belongs to the Ni–Cr–Mo family, and it is highly resistant to general and localised corrosion; however, it may suffer crevice corrosion in aggressive environmental conditions. The susceptibility to crevice corrosion is measured by the value of the repassivation potential. The objective of this work is to compare different electrochemical methods for determining the repassivation potential of alloy 22; these include cyclic potentiodynamic polarisation, Tsujikawa–Hisamatsu electrochemical, potentiodynamic–potentiostatic–potentiodynamic and potentiodynamic–galvanostatic–potentiodynamic methods. The potentiodynamic–galvanostatic–potentiodynamic technique was found to be the most conservative laboratory technique, which gave the lowest repassivation potential in a relatively short testing time. The polytetrafluoroethylene (PTFE)-wrapped ceramic crevice formers formed more severe crevices, leading to higher repassivation potentials than the solid PTFE crevice formers. Torque values higher than 2 N m were needed for obtaining reliable repassivation potentials. Surface roughness did not affect significantly the repassivation potential.