Corrosion of galvanised steel in volcanic ash leachates from the 2021 Tajogaite eruption: implications for metallic infrastructure

Volcanic eruptions can generate corrosive environments that compromise the durability of metallic infrastructure. This study investigates the corrosion behaviour of hot-dip galvanised steel exposed to volcanic ash from the 2021 Tajogaite eruption (La Palma, Spain). Fine and coarse ash fractions were characterised by XRF, XRD, particle size analysis, and SEM/EDS, revealing silicates and oxide-rich compositions with water-soluble chloride, sulphate, and fluoride ions. Galvanised steel samples were partially buried in volcanic ash for two years under periodically wetted conditions, while complementary electrochemical tests were performed in ash-derived leachate solutions. The results showed heterogeneous corrosion, with localised zinc coating losses of up to 35 µm after two years of exposure. Electrochemical measurements in ash leachates indicated mildly corrosive conditions, with corrosion current densities in the µA·cm⁻² range, and revealed a strong dependence on the solid-to-liquid ratio used for leachate preparation. Long-term burial tests showed progressive and highly heterogeneous degradation of the protective zinc coating. These findings highlight the potential corrosion risks associated with volcanic ash deposition and underscore the need for careful assessment of metallic infrastructure in affected environments.