Corrosion of cast Stellite-3 analogue in simulated PWR conditions

• Published in: Corrosion science Vol. 140; pp. 402 - 411
• Main Authors: Karimi, G.N., Shipway, P.H., Stewart, D.A., Hussain, T.
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier Ltd 01-08-2018; Elsevier BV
• Subjects: Alloys; Chromium; Cobalt; Cobalt alloy; Cobalt base alloys; Corrosion resistance; Corrosive wear; Electrochemistry; Hydrothermal corrosion; Inhomogeneity; Interfacial oxide; Pressurised water reactor; Pressurized water reactors; Primary circuits; Superalloys; Wear resistance; Electrochemistry; Hydrothermal corrosion; Pressurised water reactor; Interfacial oxide; Cobalt alloy
• ISSN: 0010-938X; 1879-0496
• DOI: 10.1016/j.corsci.2018.05.023

Figs (a & b) BSE-SEM images of before and after exposure respectively showing the changes in the microstructure and Fig (c) shows the STEM foil of the lift out of the preferential oxide formed between the Co-rich matrix and Cr-rich carbide. Fig (d) AFM contact-mode current map indicating that the Co-rich matrix conducts the highest amount of current, followed by the W-rich carbide with the Cr-rich carbide conducting the least current and Fig(e) BSE-SEM image of the region where the AFM was conducted.
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•General corrosion of the surface caused recession of the Co-rich matrix, Cr-rich carbide and the W-rich carbide.•The surface of the Co-rich matrix receded more than the surface of the Cr-rich carbide.•Preferential corrosion was seen at the interface between the Cr-rich carbide and the Co-rich matrix, but not elsewhere.•The interfacial corrosion product was CoCr2O4.•The preferential corrosion was shown not to be related to chemical heterogeneity, but was instead an electrochemical effect.
Wear resistant alloys are required for deployment in pressurised water reactor primary circuits and in this context, the corrosion behaviour of a cast cobalt-based alloy following exposure for 30 days in lithiated water at 300 °C was investigated. Corrosion of the surface was observed, resulting in the formation of a ∼100 nm thick chromium- and silicon-rich oxide. Preferential corrosion of the matrix at its interface with just one of the carbide types was observed to a depth of ∼1 μm; for the first time this was shown to be not due to any inhomogeneity in the matrix but instead to be an electrochemical effect.