Inderscience Publishers

Effect of substrate dilution on corrosion of colmonoy-6 weld overlays in nitric acid

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In the moderator system of some nuclear power plants, subsequent to the activation of the safety shutdown system, Gd
3+
is removed by ion exchange prior to reactor start-up. During this process, the pH of the system water can get lowered to 3 due to generation of HNO
3
. Colmonoy-6, which is weld-deposited over valve internals, was found to corrode considerably at this pH. This paper presents a detailed study of the corrosion of colmonoy-6 vis-a-vis its extent of dilution by the stainless steel substrate. The corrosion behaviour of these weld overlays was studied in 1 mM HNO
3
at 65?C. The results showed that the extent of dilution affected both the microstructure and hardness of the deposit, its effect being maximum with single layer deposit and the corrosion resistance increased with area fraction of borides and carbides which in turn increased with additional layers.

Keywords: colmonoy-6, 540 MWe PHWR, GTAW, gas tungsten arc welding, corrosion, dilution effect, nitric acid, electrochemical impedance spectroscopy, nuclear power plants, NPP, nuclear energy, stainless steel substrate, weld overlays, pressurised heavy water reactors, microstructure, hardness

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