Mechanistic role of phosphate and borate species in aluminium passivation in simulated MKPC pore solutions

• Phosphate ≥3000 ppm (early MKPC) promotes rapid passivation by multilayers. • Phosphate-induced layers degrade with exposure, leading to unstable passivity. • Borate ≤300 ppm (advanced MKPC) adsorbs and sustains compact films over time. • Phosphate–borate synergy reflects MKPC chemistry and suppresses H 2 release. • Dual inhibition mechanism underpins MKPC superiority over OPC across exposure. Aluminium corrosion is a key constraint in the formulation of magnesium potassium phosphate cements (MKPC) for the effective immobilisation of this reactive metal in low- and intermediate-level radioactive waste repositories. This study examines the role of phosphate and borate species in controlling passivation through electrochemical analyses in solutions representative of MKPC pore chemistries, benchmarked against OPC-like media. Electrochemical impedance spectroscopy, linear polarisation resistance, and estimated hydrogen release were used to characterise interfacial processes across ion concentrations and exposure periods. Results reveal distinct roles of phosphate and borate in aluminium passivation. Phosphate (H₂PO₄⁻, pH 4–5) at high concentrations (≥3000 ppm) drives rapid passivation via multilayer film formation, while borate (B(OH)₄⁻/H₂BO₃⁻, pH ≈8.5) becomes increasingly influential at lower levels (200–300 ppm), stabilising compact oxide films by adsorption. Mixed phosphate–borate systems reproduced the chemistry of hydrated MKPC solutions, highlighting their complementary action in establishing and maintaining passivity. Compared to OPC conditions (Ca(OH)₂, pH 12.7), which sustain active corrosion, MKPC chemistries favour stable aluminium passivation and suppress hydrogen evolution. The findings provide mechanistic insight into the inhibition processes underlying MKPC corrosion resistance and its suitability for immobilising radioactive aluminium.