Stress-corrosion cracking sensitization by hydrogen upon oxidation of nickel-base alloys by water - An experiment-guided first-principles study

This work describes a viable hydrogen-induced sensitisation process toward stress corrosion cracking in chromiaforming nickel base alloys owing to oxidation by water. A mechanistic chemical understanding of the stress corrosion based on repeated cracking and re-healing of the oxide scale emerges from experiment-guided firstprinciples calculations. Piggybacking processes during repeated oxide scale cracking-healing cycles are understood to cause sensitisation towards the formation and growth of macroscopic cracks. Under light -water reactor conditions, the healed oxide scale comprises an outer region composed of chromium depleted nickel ferrite, and non-protective chromia, nickel oxide, and nickel hydroxide. An inner nickel iron chromite layer provides the passivating barrier that controls the oxidation rate at a steady state. Early during the re-healing of a crack in the oxide scale, water is conveyed to the alloy/scale interface by hydration/dehydration of composite nickel oxyhydroxide Ni(OH) 2 center dot NiO inclusions in the scale. At later stages, Ni(OH) 2 center dot NiO serves as oxidant of preferentially chromium, while hydrogen is released as H 2 (g) into the primary water or becomes picked up by the alloy, possibly while assisting in the reduction of NiO to form nickel metal particles. Oxidation by water equivalents at the confining alloy/oxide scale interface favours hydrogen pick-up in the alloy that becomes increasingly detrimental owing to enrichment and inward diffusion of hydrogen along alloy grain boundaries. The pinning of alloy vacancies by hydrogen causes mitigation of outward diffusion of chromium, which in turn hampers crack re -healing while promoting alloy embrittlement.