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.
The influence of the presence and absence of de-icing salt ingredient Ca2+ (CaCl2) on the corrosion behavior and morphology of calcareous deposition products on die cast AM50 magnesium alloy was investigated by using an electrolyte close to the de-icing salt composition (NaCl, MgCl2) with and without Ca2+ addition. The aim of the work is to point out the difference of corrosion mechanisms between the presence or absence of Ca2+ and between immersion and salt spray conditions. Electrochemical measurements (OCP, EIS, potentiostatic and potentiodynamic polarization) and several surface analysis techniques (FTIR, XRD, XPS, SEM / EDX) were used. During immersion experiments in the presence of Ca2+, a grey-white, voluminous and open-porous deposition layer has been formed on the surface of magnesium samples, which covered the whole surface after 12 days with layer thicknesses above a tenth of a millimeter. XRD was used to identify Calcite (CaCO3) as a major component in the deposition layer. It has been shown previously by the authors [1, 2] that Ca2+ has an inhibiting effect on Mg alloy corrosion during short time immersion experiments (up to 4.5 h). The inhibiting effect of the Ca2+ in the first 4.5 h cannot be attributed to the incorporation of Ca2+ into the surface layer, which was shown by XPS. Further, an increase of the thickness of the corrosion product layer in presence of Ca2+ seems to be responsible for the inhibiting effect, shown by XPS depth profiling. After several days of immersion in the presence of Ca2+, the element Ca could be detected on the surface by EDX. XRD and FTIR proved the presence of calcite with layer thicknesses up to 155 µm, shown by SEM cross section investigations. The inhibiting effect of Ca2+is not present anymore as Calcite occurs in the surface layers at larger immersion times (> 4,5 h). Furthermore, the influence of the used testing method was investigated. In addition to immersion experiments, salt spray testing was performed up to 13 weeks which resulted in a different morphology of corrosion and deposition products (see Figure 1). Mass gain and mass loss measurements were carried out and showed a 5-times higher mass loss under immersion conditions in comparison to salt spray testing. The presence of a higher amount of carbonates after salt spray testing could be proven. Calcite is present in the surface layers in significant amounts after both testing methods. The increasing OH- concentration during Mg corrosion triggers the deposition of CaCO3 under conditions of low CO2 partial pressure, whilst the carbonate anion derives from the CO2 in the atmosphere. References [1] Grabowski, M., Bluecher, D., Korte, M., & Virtanen, S. (2014). Influence of Ca2+ in Deicing Salt on the Corrosion Behavior of AM50 Magnesium Alloy. Corrosion, 70(10), 1008-1023. [2] Grabowski, M., Bluecher, D., Korte, M., & Virtanen, S. (2015). The influence of Ca 2+ in de-icing salt on the chemistry of corrosion products formed on AM50 magnesium alloy–calcareous deposition. Corrosion (in press, 2015). Figure 1: Comparison of surface fraction and appearance of corrosion and deposition products after 1 d, 3 d, 7 d and 12 d of salt spray testing and immersion of AM50 Mg-alloy in presence of a Ca2+-containing de-icing salt solution. Figure 1
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