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

Dual phase (DP) steels are known for having issues with hot dip galvanizing due to the formation of surface alloy-oxides [1]. These oxides appear during the annealing step, preventing a good zinc adhesion to the base metal [2]. This work aims to give a fundamental understanding of the nucleation and growth mechanisms of the oxides. DP800 steel samples were oxidised in a flowing argon atmosphere, with approximately 10ppm O2. Due to the minimal moisture content within the atmosphere, oxidation occurs at a reduced rate enabling a thorough and in-depth, stepwise investigation of the oxide formation as function of increasing temperature. In an infrared (IR) furnace under vacuum, samples were heated sequentially from 450°C to 850°C in increments of 50°C. The samples were held at their respective temperatures for a few minutes to allow the oxidation process to take place. These temperatures were chosen as they imitate the heating step of the annealing cycle during Hot Dip Galvanizing. The surfaces and oxides were then analysed by Scanning Electron Microscopy (SEM) for visualisation; Energy-Dispersive X-ray Spectroscopy (EDS) for elemental analysis; Transmission Electron Microscopy (TEM) for visualisation and elemental analysis and Raman Spectroscopy (RS) for oxide characterisation. In the temperature range 550°C - 600°C well-defined particles started to appear on the surface, which are attributed to the oxides. Islands of oxides form in a manner which is consistent with differing oxidation behaviour of the multiphase/high strength steel phases. As will be shown, as the temperature reached 800°C – 850°C, large thick regions of oxides develop. Silicon was found to be mostly in defined globular regions, which were the oxides themselves, as opposed to aluminium which was speckled across the whole surface. An explanation for this variance in elemental behaviour is discussed and a suggested mechanism for the oxidation of the alloying elements is proposed. This initial research was performed to fully characterise the steel surface as precursor to a coating step, either via hot dip galvanizing (HDG) or physical vapour deposition (PVD) and to understand the specific surface oxidation mechanisms influencing the surface before coating deposition.