The effect of Sn microalloying on the selective oxidation of Fe-(6-10 at.%)Mn alloys

To evaluate the suitability of Sn microalloying as strategy to decrease the selective oxidation kinetics of Mn-alloyed advanced steels, the effects of 0.01 and 0.03 at.% Sn micro-additions on oxide morphology and oxidation kinetics for Fe-6Mn and Fe-10Mn (at.%) alloys were determined under annealing conditions characteristic of the continuous galvanizing process. Selective oxidation followed a parabolic rate law, where activation energy analysis revealed that internal oxidation is rate-determined by the grain boundary diffusion of O in Fe. The 0.01 and 0.03 at.% Sn additions significantly reduced the internal oxidation of alloys with Mn contents of 6 and 10 at.%, which was attributed to interfacial Sn segregation reducing the solubility of O in Fe, the occupation of adsorption sites for water vapour molecules, and the reduction of short-circuit diffusion of O along internal interfaces. Sn microalloying decreased the external oxidation of Fe-6Mn-(0.01,0.03)Sn, but did not significantly alter external oxide thickness of the Fe-10Mn-(0.01,0.03)Sn alloys. In this case, Sn reduced the outward diffusion of Mn along grain boundaries, but also affected the balance between the inward O flux and outward Mn flux. Incorporating the effect of Sn occupying adsorption sites into a selective oxidation model revealed that Sn microalloying is predicted to cause a transition from internal to external oxidation for Fe-10Mn. For Fe-6Mn alloys, however, a Sn micro-addition of 0.01 at.% (∼0.022 wt.%) was sufficient to significantly reduce selective oxidation kinetics and produce beneficial alterations to the external oxide morphology in order to ease reactive wetting in the continuous galvanizing process.