CREVICE CORROSION REPASSIVATION OF ALLOY 22 IN CHLORIDE SOLUTIONS

Alloy 22 (UNS N06022) belongs to the Ni-Cr-Mo family and it is highly resistant to general and localized corrosion, but it may suffer crevice corrosion in aggressive environmental conditions, such as high chloride concentration, high applied potential and high temperature. Alloy 22 is one of the candidates to be considered for the outer corrosion-resistant shell of high-level nuclear waste containers. Repassivation potential (E_co) was investigated as a parameter in the long-term prediction of pitting and crevice corrosion resistance. This parameter is usually extracted from cyclic potentiodynamic polarization curves (CPP) using artificially creviced specimens.  Alternative techniques are also used such as the Tsujikawa-Hisamatsu electrochemical method (THE) or the potential step method.
The present work studied the crevice corrosion repassivation potential (E_co) in chloride solutions at 90ºC as a function of both the chloride concentration and the employed electrochemical technique. Repassivation potentials were obtained from CPP curves, constant potential tests and modified THE experiments consisting of potentiodynamic-galvanostatic-potentiodynamic (PD-GS-PD) tests. The obtained E_co values were highly reproducible in the studied conditions. The crevice corrosion repassivation potential was independent of the value of the current density applied in the galvanostatic step of PD-GS-PD tests.