The influence of Ca2+ in de-icing salt on the chemistry of corrosion products formed on AM50 magnesium alloy – calcareous deposition

The corrosion inhibiting effect of Ca²⁺ – which was presented in a previous publication of the authors [1] under immersion conditions in the first 4.5 h – cannot be attributed to the incorporation of Ca²⁺ into the surface layer as demonstrated by EDX and XPS measurements. XPS depth profiling indicates that an increase of the corrosion product layer thickness and a higher amount of more protective magnesium carbonates in the outmost surface layer seem to be responsible for the inhibiting effect in presence of Ca²⁺. Furthermore, the corrosion products formed in presence of Ca²⁺ exhibit less incorporation of water and hydroxyl species under short time immersion conditions, as shown by FTIR-measurements.
After several days of immersion in the presence of Ca²⁺ in the solution a white, open-porous deposition covers the sample surface and the element Ca could be detected on the surface by EDX analysis. XRD and FTIR measurements proved the presence of calcite, with layer thicknesses up to 155 µm, shown by SEM investigations. The alkalization of the electrolyte during magnesium alloy corrosion and the presence of Mg²⁺ trigger the deposition of an intermediate deposition product CaMg(CO₃)₂ which transforms to CaCO₃ under conditions of low CO₂ partial pressure.