Interaction of combinations of Zn and Sn in different forms with Cr-Al-B MAB phase

Abstract

MAB/MAX phases with high corrosion resistances have gained significant attention because of their promising in energy and catalysis applications. Sn/Zn whisker could grow on these Sn/Zn-containing MAB phase solid solutions as reported in our previous work. Herein, a comprehensive investigation on the effects of the combinations of Zn and Sn in different forms (atom during hot-dip aluminizing and cation during immersion in molten chloride salt) on the microstructure and whisker growth on the hot-dip aluminized coating on Fe-Cr-B cast steel was conducted by hot-dip aluminizing-thermal diffusion treatment, and immersion in molten chloride salts. We demonstrated that all the interactions of the combinations of Zn and Sn in different forms (atom and cation) with the Cr-Al-B MAB phases could partially replace the Al atoms in the Cr-Al-B MAB phases contained in the periodic layered structure, leading to the formation of the corresponding Cr-Al-B MAB phase solid solutions and the spontaneous growth of whiskers. The replacement ability during immersion in molten chloride salt was stronger than that during hot-dip aluminizing under the driving force of formation of the volatile chlorides (like AlCl₃). The replacement ability of Zn and Sn in different forms as they substituted the Al atoms in the Cr-Al-B MAB phase was in the order: Sn²⁺＞Zn²⁺＞Sn＞Zn. Sn facilitates the formation of Zn vacancy, which promoted that whisker with complex compositions (head being Zn and body being Sn and head being Sn and body being Zn) grew on the Cr-(Al, Sn, Zn)-B MAB phase solid solution. The matter source to feed the growth of whisker played a key role in the spontaneous growth of whisker under the both mechanisms of diffusion and stress. Our work would expand the MAB phase family and tailor the growth of whisker, also enrich the theory on the growth of whisker on the MAB/MAX phase.