激光熔覆铁基合金涂层在HCl溶液中的腐蚀行为

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

采用激光熔覆方法在30CrMo合金钢表面制备了一种铁基合金耐腐蚀涂层,利用EIS、极化曲线、浸泡腐蚀实验等测试方法,结合XPS研究了该涂层在0.5 mol/L HCl溶液中的腐蚀行为,并与304不锈钢和30CrMo钢进行对比。电化学结果表明,在0.5 mol/L HCl溶液中,与30CrMo钢相比,铁基合金涂层极化曲线出现了明显的钝化区,且具有较低的腐蚀电流密度和较高的自腐蚀电位,熔覆层的耐腐蚀性能显著提高。与304不锈钢相比,铁基合金涂层维钝电流略微变小,钝化区变宽,其耐蚀性与304不锈钢相当。涂层钝化膜主要由Cr_2O_3、FeCr_2O_4和MoO_3组成,涂层具有优良耐蚀性能的本质原因是复合氧化膜的钝化作用对腐蚀介质产生机械阻隔作用。

30 CrMo 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 30 CrMo 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 30 CrMo 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 g-Fe phase and the eutectics g-Fe/M_（23） C_6. 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 30 CrMo alloy steel. This indicates that the corrosion resistance of the Fe-based coating is superior to 30 CrMo 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 HCl solution are slowed down due to the passivation of Cr and Mo. The passive film is mainly composed of Cr_2 O_3, FeCr_2 O_4 and MoO_3. 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.