Oxidation of Alloy 600 and Alloy 690: Experimentally Accelerated Study in Hydrogenated Supercritical Water
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INTRODUCTION
LABORATORY stress corrosion crack (SCC) initiation and crack growth testing of nickel-based alloys has been conducted using accelerating methods to characterize the susceptibility of cracking in pressurized water reactor (PWR) primary water conditions.[1–5] Increasing the testing temperature beyond PWR conditions, 558 K to 598 K (285 C to 325 C), has been used to accelerate cracking in more resistant alloys. The extent to which this can be done is limited by the transition to the supercritical water (SCW) phase at 647 K (374 C). The idea of testing in SCW is appealing, especially for the characterization of alloys that are resistant to cracking in subcritical water. However, it is not clear if the mechanisms of corrosion and stress corrosion cracking are the same in subcritical and supercritical water. Similar corrosion behavior between subcritical and supercritical would add support for a consistent SCC mechanism between the two regimes. Oxide films on nickel-based alloys in PWR conditions consist of a duplex oxide structure with an iron-rich spinel outer oxide and a chromium-rich inner oxide.[6–14] The composition and thickness of the oxide layers are highly dependent upon exposure conditions.[7,11,15,16] By TYLER MOSS and GARY S. WAS are with the University of Michigan, 2355 Bonisteel Blvd, Ann Arbor, MI 48109. Contact e-mail: [email protected] GUOPING CAO is with the University of Wisconsin-Madison, 1500 Engineering Drive, Madison, WI 53706. Manuscript submitted June 21, 2015. Article published online February 1, 2017 1596—VOLUME 48A, APRIL 2017
varying the hydrogen concentration around the Ni/NiO transition in experiments on Alloy 600, Peng et al.[11] were able to create a thick inner oxide layer deficient in Cr at low hydrogen concentrations, and a thin chromium-rich oxide layer at high hydrogen concentrations. The surface condition of exposed coupons can affect the oxide formed, and Carrette[16] observed an increase in the thickness of the oxide layer and a Cr-depleted surface layer in the alloy with higher cold work. No Cr-depleted layer was observed on electropolished samples, and the oxide layer was discontinuous, highlighting the importance of short-circuit diffusion in low-temperature exposures. It is also observed that surface cold work increases the cation release kinetics.[16,17] The metal ion concentration of the test solution has also been observed to affect the size, density, and formation of crystallite oxidation on the surface. Crystallite size and density decreased[16] when corrosion tests were carried out in continuously cleaned and purified solutions. Experiments conducted in titanium autoclaves have shown an absence of surface oxide particles usually observed on coupons exposed in stainless steel autoclaves, demonstrating the effect of metal ion contribution from the vessel material.[7] In experiments conducted in PWR primary water in stainless steel autoclaves, surface films observed on Alloy 600 have exhibited variations in the microstructure. However, intergranular oxidatio
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