Alloy 22 (N06022) belongs to the Ni-Cr-Mo family and it is highly resistant to localized corrosion. The anodic and cathodic behavior of mill annealed (MA) and thermally aged (10 hours at 760°C) Alloy 22 was studied in chloride solutions with different pH values at 90°C. Thermal aging leads to a microstructure of full grain boundary precipitation of topologically closed packed (TCP) phases. Electrochemical tests included monitoring of open circuit potential, potentiodynamic polarization and electrochemical impedance spectroscopy. Assessment of general and localized (crevice) corrosion was performed as well as determination of Tafel slopes in the studied environments. Repassivation potentials were obtained from cyclic poten-tiodynamic polarization tests. Results indicate that MA and TCP material show similar general corrosion rates and crevice corrosion resistance in the tested environments. MA and TCP specimens suffered general corrosion in an active state when tested in low pH chloride solutions. The grain structure of the alloy was revealed for MA material, while TCP material suffered a preferential attack at grain boundaries. The passive film behaved as an n-type semiconductor in the passive potential range, showing the same 1/C2 vs. E response for MA and TCP specimens.

Subject
Scanning electron microscopy, Corrosion resistant alloys, Corrosion rate, Passive films, Repassivation potentials, Sodium chloride, Chloride solutions, Crevices, Corrosion attacks, Crevice corrosion, Immersion, Alloys, Corrosion potential
Keywords:
N06022, chloride, crevice corrosion, repassivation potential, thermal aging
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2008
Association for Materials Protection and Performance (AMPP)
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