Influence of black annealing oxide scale on the anodic behavior of Alloy 22

The resistance of Alloy 22 (UNS N06022) to localized corrosion, mainly crevice corrosion, has been investigated extensively in the last few years. The effect of influencing variables such as temperature, applied potential, chloride concentration, and nitrate inhibitor concentration have been addressed previously. At this time, it was important to address the effect an oxide film or scale has that forms during the high-temperature annealing process or solution heat treatment (SHT) and its subsequent water quenching. Electrochemical tests such as cyclic potentiodynamic polarization (CPP) have been carried out to determine the repassivation potential for localized corrosion and to assess the mode of attack on the specimens. Tests have been carried out in parallel using mill-annealed (MA) specimens free from oxide on the surface. The comparative testing was carried out in six different electrolyte solutions at temperatures ranging from 60°C to 100°C. Results show that the repassivation potential of the specimens containing the black anneal oxide film on the surface was practically the same or higher than the repassivation potential for oxide-free specimens.