*3.9. Corrosion Tests*

The results of corrosion tests are presented in Figure 22. The corrosion rate is plotted as a function of immersion time. It can be seen that the renewal of SBF after 7 and 14 days which removes corrosion products from the liquid and adjust the pH value back to 7.35 resulted in the boosts of hydrogen gas evolution and increases of corrosion rates.

**Figure 22.** Corrosion rates of (**a**) Mg0.3Ca (**b**) Mg5Zn and (**c**) Mg5Zn0.3Ca in different conditions determined by immersion testing in simulated body fluid at body temperature. The SBF was renewed all 7 days.

It can be concluded from Figure 22 when comparing the curves of the as-cast with those of the IS samples that solid solution treatment decreases the corrosion rate in all the investigated alloys. The difference is least pronounced in the binary Mg-Ca alloy.

HPT-processing increased the corrosion rate of Mg5Zn slightly, while it had no detectable influence on corrosion rate in the case of Mg0.3Ca. In the case of Mg5Zn0.3Ca, HPT resulted in an increased corrosion rate in the first week and a decreased one in the third week.

The heat treatments carried out after solid solution treatment or after HPT-processing have no significant influence on the corrosion rates of Mg0.3Ca and Mg5Zn. In case of the ternary alloy Mg5Zn0.3Ca, the heat treatment after HPT leads to a decrease in the corrosion rate while the same heat treatment in the initial state leads to an increase in the corrosion rate.

The difference between the three alloys is largest in the IS as most of the primary precipitates become dissolved in this condition. It shows that the corrosion rate of Mg5Zn0.3Ca is highest especially in the first week of immersion. Mg5Zn alloy shows the lowest total hydrogen evolution and is less sensitive to the change of SBF. However, microscope images in [47] show that in the case of both Zn containing samples some parts are already missing after three weeks, while the Mg0.3Ca alloy shows a rather homogenous corrosion behavior.

The general appearance of the corrosion behavior is rather independent of the material treatment, meaning that Mg0.3Ca always shows a rather homogenous corrosion behavior and a thick surface layer of degradation products while Mg5Zn shows localized corrosion, and parts of the samples are missing after three weeks. Mg5Zn0.3Ca exhibits a strong surface roughness and some parts of the samples also completely dissolve. The amount of completely dissolved parts in both Zn-containing agrees well with the overall corrosion rates determined by the hydrogen evolution method (Figure 22).
