Characterization of oxide layer and micro-crack initiation in alloy 316L stainless steel after 20,000 h exposure to supercritical water at 500 °C
Graphical Abstract
Introduction
Materials selection for the supercritical water reactor (SCWR) application poses some serious challenges in terms of chemistry control strategies to increase corrosion resistance of the main parts and corrosion product transport. Oxide scales grown on metal substrates influence properties of engineering materials. Thus, it is vital to identify the mechanisms of processes affecting lifetime of the candidate materials in relevant conditions through extensive experimental tests. General corrosion resistance has been considered as a basic factor as it is a critical property itself and plays a critical role in other detrimental processes such as stress corrosion cracking (SCC) and corrosion fatigue (CF) [1], [2], [3].
Various investigations have been carried out on the corrosion behavior of various stainless steel under SCW conditions [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23]. Stainless steel (SS) alloys are the lowest cost materials with high resistance to corrosion in high temperature SCW due to the presence of Cr and Ni elements. Depending on the amounts of alloy elements in stainless steel alloy and working conditions (temperature and pressure), a complex oxide layer composed of hematite, magnetite, Cr2O3, and FeCr2O4 spinel may form on the metal surface. High-temperature SS alloys are designed to form a uniform, dense, slow-growing Chromium rich oxide scale at high temperatures. However, depending on the operational condition, SS alloys may experience a fast oxidation leading to a change in the composition and microstructure of the scale. This phenomenon is known as “breakdown oxidation” in which an outward-growing iron oxide and a bottom layer consisting of inward-growing spinel oxide and reaction zones containing Cr-rich oxide precipitates and Cr-depleted metal may form [24]. Presence of alloying elements such as Si, Mn, Al, C, and Mo affects the thermodynamic stability of metal oxides in SCW which generally decreases in the order of Al2O3 > SiO2 > MnO > Cr2O3 > FeCr2O4 > Fe3O4 > MoO2 > NiO [4]. Was et al. [25], [26] proposed that the outer oxide layer is composed of non-uniform large grains comprising magnetite, and that the inner oxide layer is made of fine-grained oxides, compact and very adherent to the base metal. The inner layer is generally nonporous, very protective, and chromium rich. Most of the high-temperature oxidation studies of steels have been based on the assumption that oxides on low-alloy steels grow by the outward diffusion of Fe, since the lattice diffusion coefficient of O anions in iron oxides is very small [6], [27].
ASS have been commonly used as in-core materials of nuclear power reactors and susceptible to irradiation-assisted stress corrosion cracking (IASCC), which is a crucial issue for the safe and economical operation of light water reactors [28]. The elevated cracking susceptibility is ascribed to both the aggressive environment of the reactor coolants under irradiation and irradiation damage in ASS. The study of crack susceptibility of ASS is of interest since it is one of the main failure mechanisms, influencing application of austenitic stainless steel in SCWR. Crack susceptibility of alloys has been known as a complicated behavior due to the fact that various in-field parameters may affect the crack initiation and propagation. It has been reported that the microstructural changes (precipitate, dislocation loops, and void formation) and microchemical changes (radiation-induced segregation of solute elements) differences and finally, these micro-scale alterations increase the susceptibility to intergranular stress corrosion cracking (IGSCC) [28], [29], [30], [31], [32], [33]. Generally, the oxidation at the metal surface can lead to the crack initiation, and intergranular cracking can be triggered when the depth of intergranular oxidation exceeds a critical value depending on the applied stress and the strength of the oxidized grain boundary [34]. Consequently, understanding the oxidation mechanism and cracking susceptibility of stainless steels in SCWR environments is of vital importance.
For the first time, we conducted an experiment on the long-term exposure of alloy 316L static capsule exposed to the SCW. In our earlier work [35], we reported the corrosion behavior and weight changes of the alloy 316L after different exposure times to the SCW at 500 °C and 25 MPa. It was found that after long term exposure of the alloy 316L to the SCW at 500 °C, oxide growth rate followed parabolic law, and, 20,000 h exposure to the SCW resulted in formation of scales identified as Fe3O4 (outer layer), Mn-Fe-Ni-Cr spinel (inner layer) on the substrate, and Ni-enrichment (chrome depleted region) in the alloy 316L. In the current work, detailed observations will be conducted to study the oxidation behavior and micro-crack initiation of the alloy 316L using transmission electron microscopy (TEM) equipped with energy-dispersive X-ray spectroscopy (EDS), selected surface electron diffraction (SAD), and electron energy loss spectroscopy (EELS). It will be discussed how elemental segregation may lead to cracking susceptibility of alloy 316L stainless steel after 20,000 h exposure to the SCW. Final remarks will be made with respect to the relevance of the oxidation phenomena on the micro-crack initiation in the applied SCW condition.
Section snippets
Materials, Sample Preparation and Experimental Procedures
Tube sample made of alloy 316L SS with an outer diameter of 9.525 mm and a wall thickness of 1.65 mm was purchased from Swagelok and the chemical composition given in Table 1. In this study, capsule was manufactured according to specifications defined in ASTM-A213 for stainless steels and was supplied in a solution annealed (1 h at 915 °C and then water quench, with the grain size ranging from 15 to 35 μm). Capsule sample was cut into 10 cm long sections to make capsule specimen for SCW exposure.
SEM Study on the top and cross section of the formed oxide layer on the alloy 316L
Fig. 1a illustrates top-view SEM micrographs (using secondary electron detector or SE) of the oxide films on alloy 316L exposed to the supercritical water at 500 °C and 25 MPa for 20,000 h. Coarse polygonal crystals with flat facets (size in the range of 8–15 μm) cover the entire surface of the exposed alloy 316L to the SCW. Fig. 1b–c shows the cross-sectional SEM micrographs (circular backscatter detector or CBS) indication two distinct oxide layer (1) outer oxide layer with thickness of 12–18 μm
Conclusions
The correlation between oxidation and micro-crack susceptibility in the alloy 316L stainless steel exposed to the supercritical water at 500 °C and 25 MPa for 20,000 h was achieved. Thorough advanced electron microscopy analyses were carried out on the samples to characterize the oxide layer as well as corrosion products in the micro-crack tip. Internal oxidation at the grain boundary was observed in the alloy 316L stainless steel exposed to the SCW. It was observed that the crack tip was filled
Acknowledgments
The authors are grateful to many others who participated in sample preparations and data collection, and would like to acknowledge the following individuals: Kai Cui, Douglas Vick [National Institute for Nanotechnology (NINT), National Research Council of Canada, Edmonton, Alberta T6G 2M9, Canada]; Ramin Zahiri, Ermia Aghaie, Zachary Carlyle Tolentino, Dominic Serate [Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta T6G 2V4, Canada]. The authors
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