The coupled effects of dissolved oxygen, permeated hydrogen, and crystallographic orientation on the intragranular oxidation of Alloy 600TT in high temperature water

The coupled effects of dissolved oxygen (DO) and permeated hydrogen (H) and crystallographic orientations on the general oxidation of Alloy 600TT in high-temperature water were investigated using dual-exposure test and density functional theory (DFT) calculations. The experimental results indicate that DO dictates the primary oxidation regime, where low DO conditions promote a thin, compact Cr-rich oxide layer, whereas high DO induces thick, porous duplex layers. Permeated H acts as a kinetic accelerator, exacerbating internal dissolution and void formation, particularly under high DO conditions. Crucially, the role of the metal substrate crystallographic orientation becomes significant under high DO conditions, with discrepancies reaching hundreds of nanometers. In contrast, under low DO conditions, the oxide layers remained uniformly thin with negligible orientation-dependent variations. The oxide thickness exhibits a strong correlation with the surface energy and atomic packing density (APD). Surfaces with high energy and low APD undergo more severe oxidation, whereas those with low surface energy and high APD exhibit reduced oxidation. Density functional theory (DFT) calculations revealed that this behavior is driven by a significant reduction in the vacancy formation energy of chromium on high-energy crystallographic planes. Consequently, while DO determines the thermodynamic baseline, the oxidation rate is ultimately governed by the intrinsic atomic stability of the substrate surface.