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

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 LiNO₃ 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 LiNO₃. 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. (authors)