Improving the oxidation resistance of 316L stainless steel in simulated pressurized water reactor primary water by electropolishing treatment

doi:10.1016/j.jnucmat.2015.09.029

Journal of Nuclear Materials

Volume 467, Part 1, December 2015, Pages 194-204

https://doi.org/10.1016/j.jnucmat.2015.09.029 Get rights and content

Highlights

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Duplex oxide films on ground and mechanically polished specimens.
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Compact oxide on electropolished specimen after 120 h immersion.
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Large spinel outer layer rich in Fe and fine spinel inner layer rich in Cr.
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Electropolishing improved oxidation resistance especially at the early stages.
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Inhomogeneous Cr-rich inner layer with granular areas affected by surface treatment.

Abstract

The oxidation behavior of 316L stainless steel specimens after emery paper grounding, mechanical polishing, and electropolishing were investigated in simulated pressurized water reactor primary water at 310 °C for 120 and 500 h. Electropolishing afforded improved oxidation resistance especially during the early immersion stages. Duplex oxide films comprising a coarse Fe-rich outer layer and a fine Cr-rich inner layer formed on all specimens after 500 h of immersion. Only a compact layer was observed on the electropolished specimen after 120 h of immersion. The enrichment of chromium in the electropolished layer contributed to the passivity and protectiveness of the specimen.

Introduction

There are many austenitic stainless steels components in contact with the water in the primary loop (primary water) in pressurized water reactors (PWRs) [1], [2], [3], [4], [5], [6]. Several occurrences of stress corrosion cracking (SCC) in austenitic stainless steel components, such as the pressurizer heaters of PWRs [1] and stainless steel safe ends of the water inlet nozzles of steam generators, have been reported [2]. Localized hardening and high residual stress as a result of fabrication and surface finishing are thought to be important factors inducing stress corrosion in these components [1], [2], [3], [4], [5], [6]. It is recognized that surface states, including surface roughness, superficial microstructures, and residual surface stresses/strains, have important effects on the oxidation behavior and SCC of stainless steels in high-temperature water environments [1], [2], [3], [4], [5], [6], [7]. The surface oxidation behavior is also crucial to the cation release leading to corrosion product activation and subsequent re-deposition on out-of-core surfaces. Many studies have been conducted on the influence of different surface treatments on corrosion resistance. Ziemniak et al. [8] studied the corrosion behavior of 304 stainless steel (SS) in hydrogenated water at 260 °C, and suggested that electropolishing treatment could remove the surface micro-strain, thereby benefiting the homogeneous nucleation of oxides. Sarata Cissé et al. [9] compared the corrosion behaviors of different surface-treated 304L SS specimens (via mechanical polishing and finishing grinding) in PWR primary water. The authors found that the ground surface exhibited a thinner oxide layer and a thicker nano-crystallized layer due to friction during grinding below the oxide film. Robertson [10] concluded that the effects of electropolishing (EP) and mechanical polishing (MP) on the oxidation behavior of iron- and nickel-based Ni–Cr–Fe alloys were related to the temperature and water chemistry. However, Zhang et al. [11] reported that in the case of 690 TT alloy examined in simulated hydrogenated primary water at 325 °C, the ground surface could benefit from the growth of a protective oxide film, and thus showed a lower oxidation rate than the polished alloy specimens. Perez et al. [12] suggested that surface finishing could generate a sub-surface deformation layer with high dislocation density, which would impact the corrosion mechanisms. The properties of the oxide film (e.g., microstructure, porosity, protective aspect) also play important roles in the corrosion process [13], [14], [15], [16], [17], [18], [19]. Therefore, it is of great significance to investigate the effect of surface preparation on the oxidation behavior of 316L SS in simulated PWR primary water.

The present work investigated the morphologies, cross-section characteristics, and chemical composition of oxide films grown on 316L SS using different surface conditions. The surface-treated stainless steels specimens were exposed to simulated PWR primary water at 310 °C for 120 and 500 h. Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) were used as the characterization techniques.

Section snippets

Materials and specimens preparation

The material studied is a 316L austenitic SS, provided as a plate. The test specimen was solution treated at 1100 °C for 20 min then quenched in water. The chemical composition of the test specimen is shown in Table 1. Inverse pole figure (IPF) and kernel average misorientation (KAM) obtained by electron backscattering diffraction (EBSD) measurements are shown in Fig. 1. The average grain size was about 23 μm including the twins. The KAM image shown in Fig. 1b indicates that 316L SS exhibited a

Surface layer composition of 316L SS after surface treatment

Fig. 2a shows the XPS depth profiles of the surface on the Ground specimen before immersion. The signals generated after 85 s of sputtering were attributed mainly to the alloy matrix. Analysis from the outer to the inner layer showed that Cr content initially increased from the outermost surface and then gradually decreased to a similar value to that determined in the matrix. In contrast, the Ni content gradually increased to a similar value to that in the matrix with increasing sputtering

Conclusions

The morphology, chemical composition, and microstructure of oxide films formed on 316L stainless steel specimens with various surface treatments, and exposed to simulated PWR primary water at 310 °C were investigated. The conclusions of the study are as follows.

1.
After exposure to PWR primary water for 120 h, duplex oxide films formed on emery paper-polished and mechanically polished specimens. In contrast, a compact oxide film monolayer was formed on electropolished 316L SS.
2.
After exposure to PWR

Acknowledgments

This work has been supported by the International Cooperative Project sponsored by Science and Technology Commission of Shanghai Municipality No. 13520721200, the Ph.D. Programs Foundation of Ministry of Education of China No. 20123108110021 and Shanghai Pujiang Program No. 12PJ1403600.

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Improving the oxidation resistance of 316L stainless steel in simulated pressurized water reactor primary water by electropolishing treatment

Highlights

Abstract

Introduction

Section snippets

Materials and specimens preparation

Surface layer composition of 316L SS after surface treatment

Conclusions

Acknowledgments

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Corros. Sci.

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Corros. Sci.

Mater. Chem. Phys.

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Scr. Mater

Solid State Ionics

J. Nucl. Mater.

J. Nucl. Mater.

Corros. Sci.

Corros. Sci.

PWSCC of austenitic stainless steels of heaters of pressurizers

Cracks on the Inner Surface of the Welds at Primary Water Inlet and Outlet Nozzle to Steam Generators (Title Translated)

Effect of cold work and stress on oxidation and SCC behavior of stainless steels in PWR primary water environments

Corrosion

Stress corrosion cracking of cold-worked austenitic stainless steels in PWR environments

Corrosion

Corrosion