ABSTRACT

An important factor in determining the breakdown of the barrier layer of the passive film on carbon steel in halide-containing solutions is the anion size. In this study, the influence of the size of aggressive anions on the passivity breakdown of UNS K02700 grade carbon steel exposed to saturated Ca(OH)2 solutions with the addition of different halides was investigated by using the potentiodynamic polarization (PDP) experiments. The PDP results were interpreted by using a mechanistic description based on the Point Defect Model (PDM). The experimental results revealed a linear dependence of the critical breakdown potential (Ec) on the logarithm of the activity of the breakdown-inducing halide (F-, Cl-, Br-, and I-), as predicted by the PDM. Furthermore, the PDM successfully accounted for the order with which the halides induce passivity breakdown, F- < Cl- > Br- > I-, in terms of competitive Gibbs free energy of anion dehydration and expansion of surface oxygen vacancies, into which the halide must absorb as the initial event in the breakdown process.

INTRODUCTION

UNS K027001 grade carbon steel is currently the reference material for the fabrication of the overpack in the supercontainer design that is developed for the geological disposal of the High-Level Nuclear Waste (HLNW) in Belgium.1 Damage to the supercontainer is envisioned to possibly occur by localized corrosion resulting from the presence of aggressive species in the annulus between the carbon steel overpack and the stainless steel envelope, which is filled with a cementitious material similar to Portland cement. The aggressive species may lead to passivity breakdown of carbon steel and to the development of pits. Therefore, the reliability and the corrosion performance of these metallic containers are of great importance in assuring the public safety. However, only using the empirical approach for studying the corrosion damage and passivity breakdown of the metallic parts of the barrier layer, cannot satisfy the need for predicting the corrosion performance over many millennia.

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