Formation of Calcareous Deposition Layers on AM50 Magnesium Alloy in Presence of Ca2+ in De-Icing Salt Solutions: Immersion Vs. Salt Spray Test

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.