<i>2014 W.R. Whitney Award Lecture:</i> Change in Bonding Strength at Grain Boundaries Before Long-Term SCC Initiation

The purpose of this work is to understand the quantitative processes which are the underlying causes of the initiation of stress corrosion after long-term exposures in high-temperature water. Crack initiation tests were performed using blunt-notched compact-tension type specimens of cold-worked (CW) carbon steel (ASTM A106 [UNS K03006]), CW mill annealed Alloy 600 (UNS N06600, MA600), and CW thermally treated alloy 690 (UNS N06690, TT 690), which were exposed in air, hydrogenated pure water, and in the primary coolant environment in pressurized water reactors (PWR) under static load condition in the range of temperatures between 360 and 465°C. Four important patterns were observed: first, intergranular cracking was observed in all tested materials, even in those exposed in air, under static load conditions when materials had been cold worked. Secondly, 1/T type temperature dependencies of initiation times were observed for CW carbon steel; the crack initiation time in an operating plant (Point Lepreau Nuclear Generating Station, New Brunswick, Canada) seemed to lie in the extrapolated line of the experimental results. Third, cavities in the metal were identified in all of the test materials at the grain boundaries at the bottom of a notch before cracks initiated in both water and air. The cavities seem to result from the condensation of vacancies and affected the bond strength of grain boundaries. Consequently, the bond strength is assumed to be weakened during the incubation period. Judging from the similar apparent activation energy of crack initiation time (145.2KJ/mol) and lattice diffusivity (148.6 KJ/mol) in CW carbon steel, the rate-limiting process of crack initiation caused by cavity formation seems to be lattice diffusion of CW-induced vacancies to grain boundaries. Fourth, the rate of formation of cavities depends on the material, cold work, temperature, grain size, and stress gradient. The rates of cavity formation were examined using 20% CW materials of TT690, MA600, Alloy 800 (UNS N08800), 316 stainless steel (USN S31600, 316), and carbon steel in air. Four important results were obtained: First, 10 to 10,000 times different rates of cavity formation were observed among in the materials. These results suggest that the times of crack initiation that were caused by cavity formation depend strongly on the properties of the materials. Cavities form ten times more rapidly in carbon steel than in TT690 and MA600. The causes of these different rates are more likely because of rapid lattice diffusivities in carbon steel rather than TT690, and the smaller grain size of carbon steel (~20 μm) when compared to TT690 (~100 μm). The formation rates of cavities in Alloy 800 and Type 316 are about 100 times slower than TT690. The cause of this difference is assumed to be caused by the effects of grain boundary carbides, judging from the observations that cavities are always nucleated near carbides. This result suggests that crack initiation caused by the formation of cavities will be controlled by lattice diffusivity, grain size, and conditions of nucleation sites, such as grain boundary carbides. As a model for the initiation of stress corrosion cracking (SCC) after long-term operations of CW materials in high-temperature water, the combination of local corrosion and the formation of cavities from the collapse of vacancies seem to dominate the initiation of SCC after long periods of time in high-temperature water.

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