Effect of holding time on the microstructure evolution of titanizing coating on 09CrCuSb steel

ND steel (09CrCuSb steel), as a material resistant to sulfuric acid dew point corrosion, still faces severe corrosion during long-term service in low-temperature sulfur-containing flue gas environments. Titanium possesses excellent passivation characteristics which is widely used in surface coatings for iron-based materials. The phases and microstructure of titanizing coatings are crucially important for its corrosion resistance. However currently, there are very few studies on the growth evolution of phases and morphologies, which is explored by this study using the powder pack cementation method, with a pack mixture of Ti powder, NH₄Cl, and Al₂O₃ at 1020 °C for 1–4 h. Its phase and microstructure were characterized by SEM, XRD, and EPMA to reveal the evolution mechanisms of titanizing coating. Results show that “ant-hole-like” pores and irregular net-like protrusions formed on the coating surface, which gradually grows and extends with holding time. The titanizing coating consists of Ti-poor layer, “needle-like” transition layer, Ti-rich layer, and subsurface particles. The formation of transition layer is closely associated with the coating micropores and the cyclic reactions induced by elemental diffusion, which thickness increases significantly with holding time. The main phases of coating include Ti₄Fe₂O, TiAl₃, TiN, AlN, TiFe, and FeAl compounds, and there are point-like particles of TiN and AlN in the substrate subsurface.