Work

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

Year: 2025

Type: article

Abstract: • 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.

Source: Electrochimica Acta

DOI: 10.1016/j.electacta.2025.147824

Authors: Carla Fernández-García, María Cruz Alonso

Institutions: Consejo Superior de Investigaciones Científicas, Instituto de Ciencias de La Construcción Eduardo Torroja

Language: English

Topic: Corrosion Behavior and Inhibition

Subfield: Materials Chemistry

Field: Materials Science

Domain: Physical Sciences

Open Access status: hybrid

Estimated APC paid: $3,790

Funders: Fundación General CSIC, HORIZON EUROPE Framework Programme, Comisión Sectorial de Investigación Científica