Effects of Alloying with Sn and Mg on the Microstructure and Electrochemical Behavior of Cast Aluminum Sacrificial Anodes

The influence of alloying with Sn and Mg instead of In during casting of Al–5 wt% Zn sacrificial anodes on their microstructure and electrochemical properties was investigated. The addition levels were 0.1–1.0 wt% Sn and 0.5–2.0 wt% Mg. The different microstructures were studied, and the effect of particles distribution on the performance of the alloys as sacrificial anodes was discussed. It was observed that increasing amount of Sn distributes its particles locally on the grain boundaries which remarkably affected the electrochemical behavior of the alloys. Adding Sn up to 1.0% gradually shifted the potential of the Al–5 wt% Zn alloy toward more negative values (~ − 0.92 to − 1.1 V) and enhanced the breakdown of the oxide layer. Accordingly, the alloys containing Sn in amounts that range between 0.3 and 1.0 wt% exhibited electrochemical properties close to that of the In-containing alloy. Magnesium addition of 0.5 wt% showed remarkable grain refinement where the grain size range of the base alloy was reduced from (150–200 μm) down to (70–100 μm) due to Mg distribution on the grain boundary area. With increasing Mg wt%, grain coarsening occurred due to the segregation of Mg in the matrix which encouraged the breakdown of the passive film and hence increased the corrosion rate. However, the potential levels of the investigated Mg alloys showed more positive values compared to that of the In-containing alloy, which may hinder using this group of alloys for sacrificial anodes applications.