Effect of dew point and gas flow rate on the surface oxidation of Advanced High Strength Steels

The chemical and morphological features of oxides, at or near a steel surface, greatly influence the formation of the inhibition layer, critical for controlling the quality of the coating during immersion into the Zn bath. The oxides influence the wettability and the final composition of the Fe-Zn alloy coating. In this study, oxidation behaviors of Advanced High Strength Steels (AHSS) are studied during annealing in a H2-5%N2 atmosphere with controlled dew-points (0 to -40°C) and gas flow rates (5 to 100 l/min.). Compact external oxides, with a high SiO₂, Al₂O₃ ratio, were found to form on the steel surface when the dew point was low and/or the gas-flow rates high. As dew points were increased and gas-flow rates decreased the surface oxides became less continuous and the presence of internal oxides became substantial. The influence of gas-flow rate cannot be attributed solely to the gas phase transport of oxidants to the metal surface and it is suggested that the equilibration state of the gas and consequent amount of molecular oxygen present near the surface needs to be considered.