Corrosion Behavior of Fe-Based Laser Cladding Coating in Hydrochloric Acid Solutions

30CrMo alloy steel has a wide range of applications in the petrochemical industry such as the valve bodies and valve covers of subsea Christmas tree, and oil drilling pipes that working in strong acid environment. Therefore, the methods to improve the corrosion resistance of 30CrMo steel by surface modification techniques have become a hot topic of research. Laser cladding Fe-based coatings are regarded as promising materials, because of their high bonding strength, good hardness and excellent wear and corrosion resistance, and they might replace more expensive Co-based or Ni-based alloys. Additions of Cr, Mo, Y, Co and Ni are benefit to improve the corrosion resistance of Fe-based coatings. However, Cr, Y, Co and Mo are expensive. With consideration of reducing the materials cost, and at the same time maintaining the excellent corrosion resistance, a novel Fe-based alloy without, Y, Co and minor Mo content is synthesized. Therefore, in this study, to improve the corrosion resistance of 30CrMo alloy, the novel synthesized Fe-based powder was prepared on the surface by laser cladding. The microstructure, chemical and phase compositions of the fabricated coating were measured systemically by using a SEM equipment with EDS spectrometer, and XRD. The corrosion behavior of this Fe-based coating in 0.5 mol/L HCl solution were studied by polarization curve and EIS measurements, combined with immersion tests. The passive film formed on the surface of the alloy after immersion in the 0.5 mol/L HCl solution for 3 d was analyzed by XPS. The microstructure is mainly composed of dendrites and interdendritic phases, which are confirmed as austenite gamma-Fe phase and the eutectics gamma-Fe/M23C6 . Similar to 304 stainless steel, the Fe-based alloy coating with a very broad passive region, shows positive corrosion potential and less corrosion current density than that of 30CrMo alloy steel. This indicates that the corrosion resistance of the Fe-based coating is superior to 30CrMo alloy steel, and almost the same as 304 stainless steel. The immersion tests show that the corrosion mechanisms of the coating are the combination of anodic dissolution and passive film protection. As for the eutectic region rich in Cr and Mo, the destruction and corrosion of this area in HCI solution are slowed down due to the passivation of Cr and Mo. The passive film is mainly composed of Cr2O3, FeCr2O4, and MoO3. The main reason for the excellent corrosion resistance of the coating is the mechanical barrier effect of the passivation effect of the high density composite oxide film.