PNNL

Abstract

A mechanistic understanding of corrosion and oxidation processes is crucial to ensure long-term resistance to stress corrosion cracking of Ni-based structural alloys in PWR primary and secondary systems. Aqueous corrosion of complex Ni-based alloys under PWR water conditions as well as gaseous oxidation of low-solute Ni binary alloys at high temperatures (>800 °C) have been studied extensively. However, a significant knowledge gap exists between response of Ni-based alloys in aqueous and gaseous environmental conditions. Selective oxidation at different temperatures was investigated in a series of gaseous Rhines Pack experiments for a high-purity Ni-5Cr binary alloy and a commercial Ni-18Cr-9Fe Alloy 600. Focused ion beam milling enabled the extraction of site-specific specimens containing high-energy grain boundaries. Analytical transmission electron microscopy was employed to analyze the microstructure and chemical composition of the resulting localized oxidation. The oxidation mechanism in the gaseous Rhines Pack atmosphere at 420 °C included the formation of a protective Cr-oxide cap above grain boundaries in all cases. At higher temperatures (600 and 800 °C), penetrative intergranular and transgranular internal oxidation was observed. Direct comparisons are made to corrosion response for these same materials in simulated PWR primary water at 320-360 °C. These results reveal a temperature dependence for gaseous oxidation and indicate mechanistic differences between aqueous and gaseous degradation in Ni-Cr alloys. These results imply that selective intergranular corrosion in PWR water is governed by processes beyond mere oxidation. Consideration of complex corrosion/dissolution processes at surfaces and in open cracks is essential for the better understanding of stress corrosion cracking mechanisms in PWR primary water.