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The purpose of this study was to determine the mechanism by which austenitic iron-and nickel-based alloys oxidize in pure supercritical water. Austenitic stainless steel alloys Type 304 (UNS S30400) and Type 316L (UNS S31603) and nickel-based Alloy 625 (UNS N06625) and Alloy 690 (UNS N06690) were exposed in deaerated supercritical water over the temperature range from 400°C to 550°C. Exposure experiments resulted in the formation of a two-layer oxide on Type 304 and Type 316L in which the outer layer is porous magnetite (Fe 3 O 4 ) and the inner layer is a denser iron-chromium spinel. In Alloy 690 the outer layer is iron-rich and the inner layer is composed of chromia (Cr 2 O 3 ), nickel oxide (NiO), and Ni(Cr,Fe) 2 O 4 . The oxide on Alloy 625 was too thin to measure. Inner and outer oxides have the same grain orientation as the matrix on the stainless alloys, and on Alloy 690, the inner oxide has a common orientation with the matrix but the outer layer consists of randomly oriented oxides. The inner/outer oxide interface is coincident with the original metal surface. The activation energies for oxide growth on each of the alloys, determined from weightgain measurements, are ~210 kJ/mol for the stainless alloys and ~140 kJ/mol for the nickel-based alloys. In both alloy systems, the outer layer grows by outward iron cation diffusion (iron in stainless steel and nickel in nickel-based alloys) and the inner layer grows by inward diffusion of oxygen to the oxide-metal interface. The likely rate-limiting step of the corrosion mechanism is inward oxygen diffusion by a short-circuit process.

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The effects of thermomechanical processing (TMP) with iterative cycles of 10% cold work and strain annealing, on corrosion and stress corrosion cracking (SCC) behavior of alloy 600 was studied. The associated microstructural and cracking mechanisms were elucidated using transmission (TEM) and scanning electron microscopy (SEM), coupled with precession electron diffraction (PED) and electron back scatter diffraction (EBSD) mapping. TMP resulted in increased fraction of low coincident site lattice (CSL) grain boundaries whilst decreasing the connectivity of random high angle grain boundaries. This disrupted random grain boundary network and large fraction of low CSL boundaries reduced the propensity to sensitization, i.e. carbide precipitation and Cr depletion. After TMP, alloy 600 (GBE) also showed higher intergranular corrosion resistance. Slow strain rate tests in 0.01 M Na 2 S 4 O 6 solution at room temperature show TMP lowered susceptibility to intergranular SCC. To better understand the improvements in corrosion and SCC resistance, orientation maps of regions around cracks were used to analyze the interactions between cracks and various types of grain boundaries and triple junctions (TJs). Detailed analysis showed that cracks were arrested at J1 (1-CSL) and J2 (2-CSL) type of TJs. The probability for crack arrest at special boundaries and TJs, calculated using percolative models, was found to have increased after TMP, which also explains the increase in resistance to corrosion and SCC in GBE alloy 600. A clear correlation and mechanistic understanding relating grain boundary character, sensitization, carbide precipitation and susceptibility to corrosion and stress corrosion cracking was established.

In this study, the effects of laser shock peening (LSP) on stress corrosion cracking (SCC) behavior of nickel based Alloy 600 in tetrathionate solution were investigated. The LSP induced compressive residual stresses and changes in the near surface microstructure, hardness were characterized. The effects of LSP on SCC susceptibility of Alloy 600 in tetrathionate solution were evaluated by slow strain rate tests and constant load tests. The results indicate a significantly longer time to failure and decreased susceptibility to SCC. These improvements were attributed to LSP induced compressive residual stresses, increased yield strength and hardening caused by near-surface microstructural changes.

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In this paper, we demonstrate a novel method for grain boundary engineering in Alloy 600 using iterative cycles of ultrasonic nanocrystal surface modification (UNSM) and strain annealing to modify the near surface microstructure (~ 250 µm) for improved stress corrosion cracking (SCC) resistance. These iterative cycles resulted in increased fraction of special grain boundaries whilst decreasing the connectivity of random grain boundaries in the altered near surface region. A disrupted random grain boundary network and a large fraction of low CSL boundaries (Σ3-Σ27) reduced the propensity to sensitization. Slow strain rate tests in tetrathionate solutions at room temperature show that surface GBE lowered susceptibility to intergranular SCC. Detailed analysis of cracks using Electron Back-scattered Diffraction showed cracks arrested at J1(1-CSL) and J2 (2-CSL) type of triple junctions. The probability

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