Corrosion behaviour of an experimental nickel aluminum bronze within an artificial crevice

The corrosion behaviour within an artificial crevice of an experimental nickel aluminum bronze (NAB) containing chromium (0.6 %) was studied with a short-term exposure and microscopic analysis. The various metallurgical phases within the as-cast material corroded at different rates in the following order of decreasing susceptibility: 1) grain boundary material and the α component in the 'eutectoid α plus κIII', 2) α and 3) the κIII component in the 'eutectoid α plus κIII' and κI. The electrochemical current pattern for this specimen during the crevice corrosion test revealed that it was more susceptible to localized corrosion than a standard as-cast NAB. Cross-sections of individual α and κI particles were obtained by focused ion beam (FIB) milling. The cross-section of the α grain revealed that corrosion attack occurred by pitting. The cross-section of the κI grain revealed that the interdendritic zone between the arms of the precipitate was marbled with 'eutectoid α plus κIII' and that the α component in this phase was preferentially attacked with a subsequent redeposition of copper. A thermodynamic analysis revealed that copper deposition is favoured as a cathodic reaction in stagnant rather than flowing conditions.