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Abstract

Undissolved particles in alloy 690 were identified as titanium nitrides with a hexagonal crystal structure. On subsequent thermal treatment of the alloy these inclusions were found to act as preferred sites for the precipitation of chromium carbide. The alloy in three heat treated conditions (solution annealed and solution annealed followed by thermal treatment at 700°C for 4 or 16 h) displayed active–passive–transpassive behaviour in H₂ SO₄ solution. Passivation of the alloy in H₂ SO₄ solution was found to be adversely affected by the presence of discrete chromium carbide precipitates at the grain boundaries. In HCl solution, the alloy displayed unstable passivation in all three heat treated conditions. Microscopic studies revealed that the TiN inclusion/matrix interface may provide a preferential site for pit initiation in alloy 690.

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Microstructural and corrosion aspects of alloy 690

Abstract

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References