08579 Anodic and Cathodic Behavior of Mill Annealed and Topologically Closed Packed Alloy 22, in Chloride Solutions

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Product Number: 51300-08579-SG

ISBN: 08579 2008 CP

Author: Ricardo M. Carranza, Martin A. Rodriguez, and Raul B. Rebak

Publication Date: 2008

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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 760ºC) Alloy 22 was studied in chloride solutions with different pH values at 90ºC. 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 potentiodynamic 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.