Insights into Interfacial Structure of Slag–Metal Interface During Desulfurization Through XPS and DFT Simulations

Ultra-low sulfur steel has excellent resistance to pitting and hydrogen-induced cracking, and is mainly used for gas pipeline steel, drilling steel and steel for structural parts of offshore platforms. This study reveals the limiting aspects of desulfurization at slag–metal interface from the perspective of experimental and theoretical calculations. Based on XPS results, the quantitative analysis shows that the relative contents of Si and O are decreasing and that of Ca, Mn and S are increasing as the etching depth increases at the slag–metal interface. Mn exists mainly as a divalent (Mn²⁺), S exists as a negative divalent (S²⁻), and Fe exists as a Fe–O and Fe–S at the same time. Under the conditions in this study, the analysis shows that the slag–metal interface is mainly composed of bridged oxygen (BO) and non-bridged oxygen (NBO), and the content of BO gradually decreases and that of NBO gradually increases as the depth deepens. With increasing basicity, the interfacial bridged oxygen gradually decreases and the NBO gradually increases. The increase in basicity leads to faster diffusion of sulfur and therefore to a decrease in Fe–S at the interface. Based on the experiment and simulation results, in this system, two main exchange reactions occur at the slag–metal interface, one is the exchange reaction between Fe and Mn/Si, and the other is the exchange of sulfur with oxygen. Afterwards, the Sulfur enters the slag and forms stable sulfides with Ca/Mn at the interface.