Passivation of aluminium A1050 in MKPC matrices: effect of MgO type and chemical retarder composition

• Effect of MgO and retarder types on Al passivation in MKPC mortars and solutions. • Electrochemical analyses validate the corrosion inhibition role of thiosulfate. • Reactive MgO with thiosulfate enhances Al passivation, limiting H₂ release. • Low reactive hard-burnt MgO reduces the passivation efficiency of boric acid. • Dual retarder systems destabilise the passive layer, increasing H₂ evolution. Magnesium potassium phosphate cement (MKPC) is a promising material for encapsulating low- to intermediate-level radioactive metallic wastes, including aluminium. This study investigates the influence of magnesium oxide (MgO) type and chemical retarders—boric acid and sodium thiosulfate—on the passivation of aluminium A1050 alloy. Corrosion behaviour and hydrogen evolution were monitored over 25 days in aqueous solutions and 230 days in mortars using electrochemical impedance spectroscopy and linear polarisation resistance. In aqueous solutions, boric acid improved passivation by shifting the corrosion potential to more anodic values, while thiosulfate promoted rapid passivation through adsorption, leading to slightly lower anodic potentials. In mortars, the near-neutral pH facilitated passivation, though variations were observed depending on the MgO type and retarder composition. Soft-burnt MgO combined with thiosulfate accelerated the formation of the alumina layer, enhancing passivation and reducing corrosion kinetics due to its higher reactivity and refined pore structure. In contrast, hard-burnt MgO with boric acid resulted in slower passivation and more cathodic potentials, likely due to its lower reactivity and heterogeneous pore network. When both retarders were used together, a competitive effect was observed, reducing passivation efficiency and increasing hydrogen evolution despite anodic potential shifts. These findings underscore the critical role of MgO type and retarder selection in optimising aluminium passivation within MKPC systems.