Long-Term Evolution of Corrosion Potential of Carbon Steel in Alkaline Radioactive Waste Environments

K.J Evans,J Page,B.C Rollins,N Sridhar,S Chawla,J.A Beavers

doi:10.5006/2979

Abstract

The long-term shifts of corrosion potential are important in predicting the likelihood of localized corrosion and stress corrosion cracking (SCC) of carbon steel used for storing radioactive wastes in underground storage tanks. Although considerable work has been done in understanding the passivity and corrosion potential of steel in various electrolytes, an important aspect of the current work is in assessing the effects of multiyear exposures of steel in waste simulants and their effects on corrosion potential. It is shown that SCC susceptibility of steel in nitrate increases at the long-term corrosion potential in solutions without organics (either by applying that potential or letting the corrosion potential increase over time). The long-term increase in corrosion potential results principally from a decrease in the passive current density with time of exposure. The present work shows that such a reduction in passive current density is accompanied by changes in the semi-conductive properties of the passive film, which itself may be a result of changes in stoichiometry of the film over time. Nitrite reduction is the most likely cathodic reaction with a small contribution from oxygen reduction. However, the presence of organic species in the environment can result in additional anodic reactions that may decrease the corrosion potential.

Highlights

A ging structures exist throughout our modern world, including highway infrastructure, water distribution pipelines, nuclear power plants, oil and gas production and transportation systems, and power transmission grids
This paper focuses on the long-term open-circuit potential (OCP) of carbon steel in alkaline solutions containing predominantly NO−3 and NO−2
This paper presents a multiyear study of the evolution of OCP of carbon steel exposed to concentrated NO−3 − NO−2 containing alkaline solutions involved radioactive waste storage. ➣ In most alkaline NO−3 to NO−2 environments, the OCP continues to increase in a logarithmic fashion with time

Summary

Introduction

A ging structures exist throughout our modern world, including highway infrastructure, water distribution pipelines, nuclear power plants, oil and gas production and transportation systems, and power transmission grids. Decades of laboratory studies have yielded much engineering information on how to control localized corrosion and SCC by controlling the waste chemistry.[3,4] While chemistry control is the most practical approach to managing the tank integrity, it has been demonstrated that electrochemical potential has an impact on the failure modes.[4,5] The waste chemistry control specifications are largely derived from relatively short-term, accelerated tests, in which the open-circuit potential (OCP) is relatively low (around −300 mVSCE saturated calomel electrode, SCE).[3] An understanding of the long-term evolution of the OCP of carbon steel under different waste chemistries is important in predicting potential failure modes and mitigating them. Mechanistic studies aid in extrapolating laboratory and field measurements into the future

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Discussion

Conclusion