Corrosion Evolution of T91 Steel in Static Lead-Bismuth Eutectic Under an Oxidising Environment

TL;DR

This study investigates how T91 steel corrodes in liquid lead-bismuth eutectic under oxidizing conditions, revealing grain boundary attack, the role of chromium and oxygen, and a surprising iron-enriched surface layer.

Contribution

It provides new insights into corrosion mechanisms of T91 steel in LBE under oxidizing environments, including the formation of an iron-enriched surface layer.

Findings

Corrosion progresses from intergranular to broader areas.

Chromium and oxygen diffusion are key factors in corrosion.

An iron-enriched BCC phase forms on the surface, contrary to previous studies.

Abstract

Understanding corrosion in liquid metal-cooled nuclear systems is essential in order be able to control it. While much literature exists detailing corrosion rates and mechanisms of structural materials in liquid metals, much still remains to be discovered in new regimes of temperature, chemistry, and impurity content. We focus on a less-studied set of conditions, specifically to investigate how liquid lead-bismuth eutectic (LBE) corrodes ferritic/martensitic steels under high-temperature oxidizing conditions. We find that corrosion follows grain boundaries, transitioning from intergranular attack to broader area corrosion as it progresses. Both chromium and oxygen diffusion play vital roles in this process. Mechanistically speaking, the ingress of LBE induces regions of martensite decomposition to ferrite via localized chromium depletion, somewhat slowing corrosion. A stable, coherent oxide scale appears to be the deciding factor that controls whether intergranular LBE attack occurs or not. Most surprisingly, a layer of iron enriched body-centred cubic phase forms on the surface of LBE-corroded T91 at these conditions, contradicting previous studies, which reported only oxide-based surface layers.