To lower the freezing point of deicing salt, calcium chloride (CaCl 2 ) is commonly used. The influence of the presence and absence of the deicing salt ingredient Ca 2+ on the corrosion behavior of die-cast AM50 magnesium alloy was investigated using an electrolyte close to the deicing salt composition with and without Ca 2+ addition. The goal of the work was to point out the differences in the corrosion mechanism with and without Ca 2+ addition by electrochemical investigations, H 2 evolution, and weight-loss characterization. Under polarizing conditions the resulting current densities are significantly lower in the presence of Ca 2+ . Electrochemical impedance spectroscopy (EIS) measurements up to 4.5 h showed an increasing polarization resistance, R P , with time, and after 4.5 h, a breakdown of R P values in the presence of Ca 2+ combined with the appearance of an inductive loop. Mass-gain, mass-loss, and hydrogen evolution measurements confirmed the inhibiting behavior in the presence of Ca 2+ up to 1 day. During immersion, the pH does not exceed a value of 10. Therefore, under these experimental conditions it is not possible for the system to reach the passivating pH of 12. The influence of the pH value on the corrosion rate was found to be limited in the pH region of 8 to 11.
The corrosion inhibiting effect of Ca2+—which was presented in a previous publication of the authors under immersion conditions in the first 4.5 h—cannot be attributed to the incorporation of Ca2+ into the surface layer as demonstrated by energy dispersive x-ray spectrometer (EDX) and x-ray photoelectron spectroscopy (XPS) measurements. XPS depth profiling indicates that an increase of the corrosion product layer thickness and a higher amount of more protective magnesium carbonate 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 Fourier transform infrared spectroscopy (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. X-ray diffraction and FTIR measurements proved the presence of calcite, with layer thicknesses of up to 155 μm, shown by scanning electron microscope 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 and partial pressure.
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