In the frame of conceptual design for the long-term management of high level radioactive waste defined by Andra (France), it is assumed that carbon steel (C-Steel) overpack would be mainly affected by general corrosion. As it is widely accepted that precipitation of a corrosion product layer (CPL) can limit the corrosion rate, the aim of this work is to simulate the growth of a thick FeCO 3 CPL and its impact on the evolution of the corrosion rate. We use a 1D finite element method numerical model based on the resolution of Nernst-Planck equation in free potential conditions. Two numerical models of the evolution of the corrosion rate of low alloy steel in anoxic conditions are developed. These models take into account the growth of a porous CPL assuming either a constant and homogeneous porosity or a varying and heterogeneous porosity and results are compared to experimental data. ARTICLE HISTORY
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...
Long-distance transmission of oil is usually carried out in large-diameter steel pipelines; water present therein may cause severe internal corrosion. An effective method of mitigating such corrosion is to inject organic corrosion inhibitors (CIs). Their surface adsorption, via heteroatom functionalities, can markedly enhance the corrosion resistance of metals. In this study, three CI model compounds with different head groups but the same alkyl tail length (-C14H29), specifically tetradecyltetrahydropyrimidinium (THP-C14), tetradecylphosphate ester (PE-C14) and tetradecylimidazolinium (IMID-C14) were synthesized, their purities being determined using nuclear magnetic resonance spectroscopy (NMR). The critical micelle concentrations (CMCs) of each compound were measured using surface tensiometry (Du Noüy ring) and fluorescence spectroscopy techniques, differences being found between these indirect and direct methods. In addition, linear polarization resistance was used to determine inhibition efficiencies (IEs) for carbon steel immersed in a 5 wt.% NaCl electrolyte saturated with CO2. CI surface saturation concentrations, with maximum IEs, were compared with the determined CMCs. Excellent IEs were observed at concentrations of THP-C14, PE-C14 and IMID-C14 which do not correspond with their CMCs, the differences involved being significantly greater than what has previously been reported for tetradecylbenzyldimethylammonium (BDA-C14). These results demonstrate that there is no direct link between CMC and metal surface saturation/corrosion inhibition efficiency that can be made on a generalized basis for the different head groups but with the same tail length corrosion inhibitors, indicating that the selection of the appropriate CI concentration for an industrial application should not be based on CMC alone.
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