One-Dimensional Porous Electrode Model for Predicting the Corrosion Rate under a Conductive Corrosion Product Layer

Abstract

General corrosion is the main form of corrosion likely to affect carbon steels in an anoxic and near neutral environment such as encountered in the context of long term storage of steel canisters in a deep geological repository. This paper aims at studying the influence of the electrical and geometrical properties of a siderite corrosion product layers (CPL) formed in such conditions on its stability and on its subsequent protective properties against corrosion. A 1-D numerical model describing general corrosion under a porous conductive CPL and accounting for chemical evolution in the electrolyte is presented. It is demonstrated that a conductive layer with a cathodic activity increases the corrosion rate and the Fe 2+ ions concentration. Otherwise, a conductive layer leads to high saturation levels of siderite and high pH values within the CPL and consequently to a stabilization of the CPL. It is shown also that the stability of the CPL is promoted when it is initially thick and/or when it has a low porosity. In the context of geological disposal of High Level radioactive Waste (HLW) over several thousand years it is necessary to predict long term corrosion of the metals in contact with liquid or solid phases present in the geological environment. In France, these HLW would be stored in a near neutral deep Callovo-Oxfordian (Cox) argillite, and low alloyed steel will be used for most of the metal components (overpack, liner) in contact with this environment. 1In this context, different corrosion steps are expected: a dry corrosion step during operated phase, a step of aqueous corrosion under oxidizing conditions (oxygen reduction) and a step of aqueous corrosion under anoxic conditions. This latter step, much longer than the first ones, is considered in our research. During this phase, in the absence of passivation, the main form of corrosion likely to affect carbon steels is general corrosion leading to the formation of a thick corrosion products layer (CPL) mainly composed of siderite (FeC O 3 ) in an argilleous environment.2 Then, predicting corrosion evolution of steel components in contact with the Cox environment necessitates to account for the role and properties of this siderite CPL. It must be emphasized that the formation of siderite CPL have been considered in some numerical models to predict the evolution of general corrosion occurring on carbon steel in acidic (C O 2 and/or H 2 S) environments in the petroleum context, using either an empirical / semi-empirical approach 3-5 or mechanistic models. 6-10In a near neutral environment in the field of HLW repository, semi-empirical models based on experiments, 11,12 and mechanistic models [13][14][15][16][17] have also been developed to predict the evolution of the corrosion rate of carbon steel.Furthermore, the role of the CPL has been highlighted and implemented in their models by other authors. [18][19][20][21][22] However, the electrical properties of the CPL were not implemented. The CPL was only considered as an additional diffusive barrier. For i...

Review of the Modelling of Corrosion Processes and Lifetime Prediction for HLW/SF Containers—Part 1: Process Models

“…In contrast to the modelling of passivity using the PDM and DPCM, a number of reactive transport models have been developed on the basis of active dissolution of the container surface, along with the formation of porous non-passive surface films. Models have been developed for a range of corrosion systems, including copper in O 2 -containing Cl − environments [39,40], copper in the presence of sulphide [70][71][72], carbon steel in O 2containing bentonite [5], carbon steel in bentonite under anaerobic conditions [73], and carbon steel in contact with bicarbonate-containing argillite clay [74,75].…”

“…In this early model, the steel surface was assumed to dissolve actively, and there was no effect of film formation on the rates of the interfacial processes. Mohamed-Said et al [74,75] describe a model for the corrosion of carbon steel and the effects of FeCO 3 film formation on the corrosion behaviour. The various attempts at modelling the effect of porous film formation will be addressed in more detail below.…”

“…In addition, the rates of transport of reactants to, and of dissolved corrosion products away from, the metal-film interface will be restricted by the pore structure. Lastly, if the film is electrically conducting, it could support electrochemical processes and could galvanically couple to the underlying metal surface [74].…”

“…A more rigorous approach to this problem is to model the chemistry of the pore solution in the CPL since the continued corrosion suggests that open porosity is maintained by acidification of the pores due to the hydrolysis of dissolved metal ions. Mohamed-Said et al [74,75] included the prediction of pore water pH, although the H + produced acted primarily as a cathodic reactant. In modelling the spatial distribution of the porosity of a FeCO 3 CPL, Mohamed-Said et al [75] found that high rates of siderite precipitation resulted in denser and more protective CPL layers.…”

Modelling of Lead Corrosion in Contact with an Anaerobic HCl Solution, Influence of the Corrosion Product Presence

“…This is a first step in allowing the handling of the code, but is entirely necessary for the validation of these first developments. The model presented in this study is based on the PhD theory of Mohamed-Saïd that simulated the corrosion of steels in a deaerated and carbonated environment [33][34][35]; however, this theory was adapted for HCl in the present study. Our study focuses more particularly on the impact of the corrosion products layer of PbCl 2 .…”