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

During hot-dip galvanizing dross particles form in the zinc bath and develop over the surface of submerged pot hardware. This accumulation of dross on the hardware is known as dross build-up. Dross build-up is a critical factor in the pot hardware lifetime. The surface quality of the hardware directly influences the quality of products produced on the galvanizing line. More dross build-up could lead to more defects on the coated product, which is undesirable for customers and creates higher scrap rates and lost production tonnage. The mechanisms of dross build-up in a typical GI zinc bath have been extensively researched in recent years. In addition to typical GI zinc coatings, Tata Steel also produces Zn-Al-Mg coated steels with 1,60 wt% Al and 1,60 wt% Mg called MagiZinc. The pot hardware is used in production for both GI zinc pots and subsequently in MagiZinc coating baths. There is some evidence that the dross build-up layer on the bath hardware created in the typical GI bath changes when the hardware is submerged in the MagiZinc bath. This thesis project aimed to identify the characteristics of this modifying behaviour in MagiZinc of the dross build-up that is formed on bath hardware when submerged in typical GI zinc bath. In general, it was observed that the intermetallic dross particle/liquid Zn interface changes the structural morphology of the dross build-up creating a more open structure with reduced cohesion of the grain boundaries of the intermetallic dross particles. This reduction in cohesion may lead to particle detachment at the grain boundaries when enriched with Zn. Furthermore, heating and cooling of the pot hardware during pot changes also contributed to breakdown of the dross build-up layer. Based on the results obtained from experiments in this study it can be concluded that the separation process of the intermetallic dross particles in a MagiZinc bath is a combination of intergranular diffusion of Zn and crack formation as a result of thermal shock.