characterization of the early surface films dynamically formed on molten az91d and am60b alloys under different atmospheres: a comparative study

In the present study, the early stages of the surface oxidation and fluoridation of liquid az91d and am60balloys under ultra-high purity (uhp) argon, dry air, and air mixed with two different protective fluorine-bearinggases were studied. the chemical composition, morphology and thickness of the surface films formed inside thetrapped bubbles were characterized by sem equipped with eds analysis. it is found that the molten am60b alloyis more sensitive to impurities under uhp argon gas than az91d alloy. under dry air atmosphere, the entire surfaceof molten az91d alloy is covered with an oxide layer and thinner thickness than the surface film formed on am60balloy which has rough surface exhibiting granular growth in later stages of oxidation. the eds analysis shows thatthe film chemistry is mainly composed of mg, al and o elements. under fluorine-bearing gas/air mixtures witheither sf6 or hfc-r134a at 3.5%vol., a fresh surface film formed with a flat and dense morphology with a uniformthickness consisting of mg, f, al and o elements. it is observed that there is a lower o:f intensity ratio in the surfacefilm formed on the molten az91d alloy under 1,1,1,2-tetra-fluoroethane (hfc-r134a) mixed with dry air comparedto the am60b alloy under both air/ r134a and air/sf6 mixtures which shows a higher fluorine concentration in thesurface film a leading to a better oxidation resistance.