Kinetic Model of Aluminum Behavior in Cement-Based Matrices Analyzed by Impedance Spectroscopy

Abstract

Aluminum reactivity in cement-based matrices, generally used for conditioning low-and intermediate-level radioactive wastes, is a problem due to dihydrogen produced by the corrosion reaction which depends on the pH of the pore solution. Electrochemical impedance spectroscopy has been used to propose a mechanism for aluminum corrosion in cementitious matrices based on ordinary Portland cement (OPC, pH ≈ 13) or magnesium phosphate cement (MKP, pH ranging between 4 and 9) containing or not LiNO 3 as a corrosion inhibitor. The fit of impedance diagrams recorded on aluminum electrode as a function of time has been realized using electrical parameters to model the cement and kinetic constants for the faradaic impedance. The so determined kinetic constants are used to calculate the corrosion current and the dihydrogen production as a function of time. Comparison of these results with experimental measurements of dihydrogen release obtained by gas chromatography shows a very good agreement except in the MKP matrix containing LiNO 3 . In the OPC matrix, dihydrogen production is between 500 and 1000 times higher than in the MKP matrix, which put in evidence the significant benefit in using MKP cements for aluminum encapsulation. It has long been common practice to stabilize and solidify lowand intermediate-level radioactive wastes using cement before sending them to disposal.1-3 The wastes are confined with a cement or a mortar in a steel container which is closed. Ordinary Portland Cement (OPC) is one of the most widely used materials for conditioning radioactive wastes. Indeed, it is inexpensive, it can be easily supplied and, after hydration, it can exhibit high stability over time. 4 However, wastes produced by nuclear activities are very diverse and, under certain circumstances, can chemically react with cementbased phases or aqueous interstitial solution, thus reducing the quality of the conditioning material. For instance, the dismantling of old nuclear reactors generates a large volume of low and intermediate-level wastes, some of them containing metallic aluminum. Aluminum is a reactive amphoteric metal, forming a protective oxide layer on contact with air or water. This layer is generally stable in the pH range 4-9. [5][6][7] However, in a strongly alkaline medium, such as that encountered in conventional cementitious materials based on OPC, this layer is soluble, resulting in continued corrosion with associated liberation of dihydrogen. Then, the use of Portland cement to encapsulate large amounts of aluminum is prohibited. The formation of dihydrogen contributes to increase the pressure in the container which is able to crack. It is the reason why the main criterion for metallic waste management is based on the volume of dihydrogen production.In this context, our work concerns the aluminum reactivity in cement-based matrices for conditioning radioactive wastes. To overcome the Al corrosion, the strategy proposed consists in (i) replacing OPC by an alternative mortar leading to a pore solution with an appropria...

Corrosion of aluminium in ordinary Portland cement paste: Influence of matrix porosity and the presence of LiNO3 corrosion inhibitor

“…Ordinary Portland cement (OPC) is one of the most widely used materials for conditioning radioactive wastes because it is inexpensive, it can easily be supplied and it exhibits high stability over time after hydration. [ 3 ] However, OPC possesses a high pH pore water solution. The natural oxide layer formed on the aluminium surface is amphoteric and, therefore, not resistant to corrosion when brought in contact with the highly alkaline cement pore water because the protected oxide film dissolves to form, for example, aluminium oxides and hydroxides, with the liberation of hydrogen gas.…”

Evaluation of several conditioning matrices for the management of radioactive metal beryllium wastes

“…The reactivity of Be has been studied using the same electrochemical approach that the one adopted previously to study the reactivity of Al and U encapsulated in a conditioning matrix: open circuit potential and electrochemical impedance spectroscopy measurements [44][45][46].…”

“…We have previously shown that cement brings an electrical contribution to the impedance measurement [45]. In order to discriminate the impedance related to the cement from the one related to the beryllium reactivity, measurements on a platinum electrode, as an inert electrode, have been performed in parallel.…”

“…The first part corresponds to the impedance contribution to the cement matrix (Zmatrix). The details of this impedance are already described in the reference [45]. Each matrix has its own impedance contribution.…”

Corrosion Mechanisms of High-Mn Twinning-Induced Plasticity (TWIP) Steels: A Critical Review

“…Aluminum acts as a reactive, amphoteric alloying element, which generates a protective oxide layer in the pH range 4-9 [63]. However, the oxide layer is unstable in acidic or alkaline solutions, therefore corrosion occurs with hydrogen gas evolution [64].…”