12th International Conference on Zinc & Zinc Alloy Coated Steel Sheet - GALVATECH 2021

The end plates and flanges used in retractable snouts are fabricated from 253MA, to overcome failure modes associated with long service periods at temperature above 500oC. Alloy 253MA is creep resistant micro-alloyed stainless steel developed for applications requiring high temperature oxidation resistance. In 253MA the alloying elements Si, Ce, and N promote the formation of a very adherent, impervious and fine-grained chromium oxide scale that is protective in high temperature service environments. Ce in 253MA reduces oxide growth rate by promoting the formation of a thin SiO2 film at the interface between the chromium oxide scale and the 253MA substrate. SEM/EDS analyses of a corroded flange fabricated from 253MA confirms that the protective chromium oxide scale can react with condensed zinc resulting in the rapid deterioration of the end plate. This occurs in the regions of the retractable snout that are not adequately heated to prevent the condensation of zinc vapour. The corrosion of the protective chromium oxide scale leads further to diffusion of Fe, Ni and the microalloying elements Si, and Ce from the substrate into the condensed Zn leading to rapid degradation of the steel. In this paper we propose a mechanism for condensed zinc induced degradation of 253MA. We also propose a mechanism for the formation of periodic and layered structure of Fe-Zn based intermediate phases. The proposed mechanism further considers the long-range diffusion of Fe, Si, Ce, Cr and Ni atoms from the substrate steel and the diffusion of Zn from the surrounding matrix which leads to the formation of alternating and repeating patterns of the intermediate phases observed in the microstructure of the corroded flange.