High Temperature Oxidation Behavior of Hypoeutectic Mg-Sr Binary Alloys: The Role of the Two-Phase Microstructure and the Surface Activity of Sr

The high temperature oxidation behavior of Mg-Sr hypoeutectic alloys was investigated. The oxide layer was characterized after isothermal heating experiments that were conducted at 500 degrees C in dry airflow for 12h. It was seen that a non-uniform oxide scale formed on hypoeutectic Mg-Sr alloys with a two-phase structure whereby the oxide over the interdendritic intermetallic phase had a finer grain size than the oxide over the -Mg dendrites. The interrupted tests on selected alloys showed that the two phases exhibit different oxidation sequences of Sr and Mg. It was observed that the intermetallic phase oxidizes first with the formation of both MgO and SrO on the surface. The oxidation proceeds with SrO and MgO formation on the -Mg. This sequence is different from what is predicted as possible via thermodynamic calculations based on the activities of the elements on the two-phases. SEM/EDS analyses revealed that the Sr surface-enrichment has occurred, which explains the differences between the observation and the assessment. Thermodynamic re-assessment suggested that the Sr enrichment on the surface due to the surface activity of Sr in Mg could largely determine the initial oxidation sequence.