Electrochemical Behavior of Al/Mg Alloys Immobilized in a Magnesium Potassium Phosphate Cement-Based Mortar

Abstract

Portland cement is extensively used for the conditioning of radioactive waste. However, its high alkalinity is a serious obstacle to the stabilization of waste containing aluminum metal since aluminum is oxidized by the pore solution with the production of dihydrogen. This work investigates the potential of an alternative binder, magnesium potassium phosphate (MKP) cement, for the stabilization of Al–Mg alloys comprising 2 to 4.5 wt% of Mg and other metallic impurities. The objective is to assess the influence of the alloy composition on its reactivity in the cementitious matrix at earlier ages, as well as at later ages, when the cement has reached a significant reaction degree. Two complementary techniques are used. Gas chromatography shows that the dihydrogen release, resulting from the corrosion process, is not influenced by the magnesium content in the alloy. Electrochemical impedance spectroscopy provides qualitative information about the corrosion but also makes it possible to assess the corrosion current using an equivalent electrical circuit linked to the kinetic parameters of the postulated corrosion mechanism. Over a one-year period, the corrosion current of the alloys, regardless of their Mg content, is reduced by almost three orders of magnitude in MKP mortar as compared to Portland-cement-based mortar.

MgO/KH2PO4 and Curing Moisture Content in MKPC Matrices to Optimize the Immobilization of Pure Al and Al-Mg Alloys

“…The inflection point close to the real impedance (Z’) axis has been used to determine the electrical resistance evolution with matrix maturity. Under isolated conditions (see Figure 5 c), the matrix electrical resistance increases over time with a shift towards lower frequencies in the inflection point of the high-frequency domain, as also detected in Poras et al [ 50 ]. This is attributed to the acid–base reaction progression with pore water consumption.…”

“…For both moisture contents, AlMg (AA5754) has a higher probability to locate in a corrosion stage. This effect is not appreciated in the Al Pourbaix diagram for both moisture content, where AlMg alloy (AA5754) has the same trend as pure Al (A1050), as also observed by [50]. However, as the pH of 1M MKPC at isolated curing is lower than at 100%RH, pure Al (A1050) and AlMg alloys (AA5754) presented fewer corroding risks under isolated curing.…”

“…The corrosion response is corroborated by the Pourbaix diagram, drawn in Figure 11 (right subfigure) for aluminium and magnesium metals. A cross-merger of both E-pH Pourbaix diagrams was made according to [ 50 ]. Both 1M MKPC mortars (100%RH and in isolated curing) are in the Al passivity range but below the water-reduction potential with H 2 release associated.…”

“…As described above, MKPC matrices have been considered a promising alternative to OPC cementitious matrices for the immobilization of low- to intermediate-level radioactive aluminium alloys [ 6 , 7 , 9 , 11 , 12 , 17 , 18 , 50 ]. OPC alkaline matrices have a pH outside of the aluminium passive domain, which is primarily responsible for the corrosion of Al alloys, limiting the formation of the passive layer, as Pourbaix suggests [ 9 ], with a high risk of hydrogen release.…”

The Influence of the Magnesium-to-Phosphate Molar Ratio on Magnesium Potassium Phosphate Cement Properties Using Either Wollastonite or Volcanic Ash as Fillers

“…At pH values above 10.5, brucite occurs. For aluminium metal encapsulation, a pH value in the range of 8-8.5, ensuring the stabilization of K-struvite that provides chemical and mechanical stability and maintaining electrochemical conditions for Al 2 O 3 •3H 2 O passivation [45], would be ideal. Only the 2 M formulations maintain those pH conditions in the long term (>30 days), but since the electrochemical behaviour includes complex processes that do not only depend on pH, an evaluation of the interaction between Al metal and MKPC prepared at higher Mg/P molar ratios than two should be performed to either consider or discard those formulations.…”