Elsevier

Journal of Nuclear Materials

Volume 493, September 2017, Pages 40-52
Journal of Nuclear Materials

Radiolysis driven changes to oxide stability during irradiation-corrosion of 316L stainless steel in high temperature water

https://doi.org/10.1016/j.jnucmat.2017.05.042Get rights and content

Highlights

  • Stainless steel is irradiated with a proton beam in high temperature water.

  • Radiolysis is found to cause an increase in corrosion potential.

  • Irradiated oxides were found to be deficient in inner oxide chromium.

  • The stability of species in the oxide film is altered due to radiolysis.

Abstract

316L stainless steel samples were irradiated with a proton beam while simultaneously exposed to high temperature water with hydrogen (320 °C, 3 wppm H2, neutral pH) to study the effect of radiation on corrosion. The inner oxides on irradiated samples were found to be depleted in chromium when compared to the inner oxides on unirradiated samples exposed to the same conditions. Additionally, hematite was found on the oxide surfaces of irradiated samples, but not on unirradiated samples. Sample areas which were not directly irradiated but were exposed to the flow of irradiated water also exhibited chromium-deficient inner oxides and had hematite on their surfaces, so it is concluded that water radiolysis is the primary driver of both effects. Thermodynamic calculations and radiolysis modeling were used to show that radiolytic production of hydrogen peroxide was sufficient to raise corrosion potential high enough to cause the dissolution of chromium-rich spinel oxides which make up the inner oxide layer on stainless steel in high temperature water.

Introduction

The degradation of stainless steel components in light water reactor cores is of continuing concern to the nuclear power industry. Accurate knowledge of core component lifetimes is required for safe operation and life-extension of the light water reactor (LWR) fleet. Among the life-limiting degradation modes, irradiation assisted stress corrosion cracking (IASCC) is of particular concern for components exposed to radiation. To inform modeling efforts for current reactors, and to aid in the design of new materials, a fundamental understanding of the factors affecting IASCC is needed. Among those factors is the influence of irradiation on the corrosion process.

The effect of radiation on corrosion of LWR core structural components has been a subject of some interest [1], [2], [3], [4], [5], [6], [7]. In water, radiation is known to create several strong oxidizing species due to water radiolysis. Further, neutron irradiation may accelerate diffusion of species to the surface and to grain boundaries by creating point defects in the metal and oxide [8]. Prior works have reported on the results of electro-chemical measurements on stainless steel irradiated in LWR environments [9], [10], [11]. They did not, however, include a characterization of the oxide film. Other studies have used injected H2O2 [12], [13], [14], [15] and/or dissolved O2 [14], [15], [16], [17] as a substitute for radiation to increase the corrosion potential of stainless steel in LWR conditions, and have observed changes to the oxide morphology and composition. Kumai and Devine [18] found an increased presence of hematite on the oxide surface as dissolved oxygen was added to the corrosive environment. Some works have examined the effect of gamma irradiation on corrosion potential [19], [20] and on oxide stability [3], but gamma irradiation does not capture the possible effects of displacement damage. To further the understanding of radiation and corrosion, there is a need to examine the simultaneous effects of radiolysis and displacement damage on the corrosion process.

The objective of this work is to determine how the thermodynamic driving force for corrosion is altered by irradiation. The work focuses on experiments in which stainless steel is irradiated with a proton beam while simultaneously exposed to 320 °C water with 3 wppm H2. Radiolysis modeling is used to show how irradiation altered conditions at the oxide-solution interface. Oxide films were characterized to determine how the irradiated environment altered the stability of species in the oxide film. Thermodynamic calculations are presented to support a mechanistic explanation of irradiation accelerated corrosion (IAC) in which the spinel oxides on stainless steel in high temperature water dissolve under proton beam irradiation. An accompanying work in this issue examines how irradiation altered the morphology of the oxides on the stainless steel samples, presents kinetic effects of radiation on corrosion, and discusses the possible role of displacement damage as a factor contributing to oxide dissolution during irradiation [21].

Section snippets

Irradiation

Experiments were conducted using the Irradiation Accelerated Corrosion facility at the Michigan Ion Beam Laboratory at the University of Michigan, which was described in detail in a previous work [22]. The facility consists of a 10 mL corrosion cell mounted to the end of a dedicated proton beamline. A schematic drawing of the corrosion cell assembly is shown in Fig. 1. The sample mount was fitted over a hole in the beamline flange and sealed with a Conflat flange on the beamline side, and with

Results

An optical image of sample Hi12 after exposure is shown in Fig. 4. The region through which the proton beam was transmitted, marked as “irr”, is visible as a circle with a diameter of approximately 1 mm at the center of the sample. A discolored region extending up and slightly to the left of the irradiated region, marked “Flow”, is also visible and is attributed to a higher concentration of radiolysis products in the discolored area, due to the direction of water flow across the sample face.

Discussion

The objective of this work is to determine how irradiation alters the stability of oxide species during stainless steel corrosion. In the results section it was shown that regions exposed to either direct irradiation or to radiolysis product flow exhibited hematite on their surfaces, a shift in their spinel A1g peak, and a loss of inner oxide chromium. The discussion will begin with a consideration of radiolysis to determine whether the irradiation conditions could lead to an increase in

Conclusions

Radiation has been shown to raise the corrosion potential at the oxide-solution interface of 316L stainless steel, and affect the thermodynamic stability of the species that make up the oxide film. Radiolysis modeling showed that H2O2 is the dominant radiolysis product responsible for elevating corrosion potential, and that the concentration of H2O2 produced by the experiments is consistent with observed changes to spinel oxide stability. Hematite was found on irradiated samples, and is

Acknowledgements

This work was supported by the DOE-NEUP, grant number DE-AC07-05ID14517 and EDF, Contract No. 8610-BVW-4300243004.

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