As Cr-Ni stainless steel has high strength and excellent corrosion resistance, it is widely applied in industrial machinery parts. However, the surface of Cr-Ni stainless steel is easy to be damaged in a severe corrosion industrial environment and long-term wear conditions, resulting in failure of industrial parts. Advanced surface strengthening technologies can greatly improve the service period and repair economic cost of parts. Laser cladding of composite coatings is an ideal technology to strengthen the surface of Cr-Ni stainless steel. To solve the problem of insufficient corrosion resistance of Cr-Ni stainless steel applied in a severe corrosive industrial environment, a Ni-Al2O3 composite coating was fabricated by laser cladding. It was expected to strengthen the surface of Cr-Ni stainless steel by combining the high chemical stability of metal Ni with the high hardness strengthening effect of ceramic particle Al2O3. At present, there are many studies on corrosion resistance of the composite coating, but there are relatively few studies on the corrosion resistance of the Ni-Al2O3 composite coating prepared by laser cladding with different Al2O3 content. The microstructure, phase composition and elemental distribution of the Ni-Al2O3 composite coating were studied with an X-ray diffraction (XRD), a scanning electron microscopy (SEM) and an energy dispersive spectroscopy (EDS). The influences of Al2O3 content on the morphology, microhardness and corrosion resistance of the composite coating were investigated by a SEM, a microhardness tester and an electrochemical workstation. Besides, the strengthening mechanism of the Ni-Al2O3 composite coating was also studied. The results showed that a homogeneous and defect-free composite coating was successfully prepared. An obvious metallurgical bonding zone (MBZ) was observed at the interface of the composite coating and the substrate. The microstructure along the direction of the composite coating depth successively exhibited cellular crystal, oriented columnar crystal and fine equiaxed crystal. The phases of the composite coating were composed of Al2O3 ceramic particles evenly distributed on top of the composite coating and Fe-Ni, Fe-Ni-Cr solid solution. With the increase of Al2O3 content, the microhardness of the composite coating increased and then decreased. The corrosion potential of the Ni-Al2O3 composite coating increased first and then decreased, while the weight loss corrosion rate and corrosion current density decreased and then increased, resulting in the enhanced corrosion resistance of the coating and then weakened. Among these Ni-x%Al2O3 (x=0, 15, 25, 35, mass fraction) composite coatings, the Ni-25%Al2O3 composite coating possessed both the highest microhardness and the strongest corrosion resistance. The microhardness of the Ni-25%Al2O3 composite coating reached 1 026.3HV, and its weight loss rate for corrosion was 0.15 mg/(cm2·h). The corrosion potential and corrosion current density of the Ni-25%Al2O3 composite coating were –326.6 mV and 38.6 µA/cm2, respectively. Once the Al2O3 content exceeded the 25% limit, the ceramic layer thickened with the increasing channels for corrosion, defects such as pores and cracks increased, resulting in a decrease of both microhardness and corrosion resistance of the composite coating. Based on mechanism studies, both microhardness and corrosion resistance of the Ni-x%Al2O3(x≤25) composite coating varied with the laser cladding and the addition of Al2O3 particles, which induced a synergistic effect from grain refinement strengthening, solid solution strengthening and particle strengthening. The Ni-25%Al2O3 composite coating prepared by laser cladding possesses both the highest microhardness, strongest corrosion resistance and can effectively provide protection for Cr-Ni stainless steel, which is conducive to the high corrosion resistance and long-term service life of industrial machinery parts in a severe corrosive industrial environment. © 2024 Chongqing Wujiu Periodicals Press. All rights reserved.
