| Magnesium alloy has low density, high strength-to-weight ratio and other good physical and mechanical characteristics. However, the application of magnesium alloys has been limited due to their undesirable properties including poor corrosion and low wear resistance. Thus, the formation of anticorrosion and high wear-resistance coatings on the surface of Mg or Mg alloys is necessary in practical applications.In order to improve the corrosion resistance of the AZ91D magnesium alloy, electroless Ni-Sn-P and Ni-W-P coatings on AZ91D magnesium alloy directly and indirectly were investigated in this paper. It had been successfully deposited ternary nickel coatings directly on AZ91D magnesium. The preparing technologies, characteristics and anticorrosion mechanisms have been investigated in the paper. At the same time, the pretreatment had been replaced by new technique without Cr, and the process had been made easier and more exercisable.SEM and laser microscope observations and XRD analysis were performed to examine the microstructure (grain size) and phase composition of the coatings. The corrosion resistance of the coatings was estimated by electrochemical polarization measurement and 10%HCl solution immergence test.The first experiment is to improve the pretreatment. There had no different except the pretreatment CrO3 and HF, which were used in the pretreatment, had been replaced and a new conversion firm emergence, with which it could electroless duality and ternary nickel coatings. XRD, SEM and EDX are used to analyze the morphology and the microstructure. It was considered that conversion firm wasn't continuum. As theα-phase on the surface of the AZ91D magnesium alloy was corrupted, and then the content ofβ-phase increase. When the potential difference of the surface wasn't to high, then it could electroless deposite ternary nickel coatings.After the pretreatment was replaced, the corrosion resistance of the Ni-Sn-P coating was investigated. A new protective coating Ni-Sn-P deposit on AZ91D magnesium alloys was obtained by electroless plating. The substrate was abraded by NO.2000 SiC sand paper before electroless. The ternary nickel coating directly on AZ91D magnesium was obtained. It was found that the Ni-Sn-P had the compound of Sn: 2.48wt%, P: 8.51wt%. The corrosion potential was–0.645, and the porosity decrease notability, and the corrosion resistance in 10%HCl solution became much better. That was a coating with thickness of 35μm could stand in 10%HCl solution for about 608mins, while the Ni-P coating could only stand for 76mins at most.It had been known that Ni-W-P had better corrosion resistance than that of Ni-P, so it was suggested that deposit Ni-W-P directly from a plating bath containing sulfate nickel. A traditional technique of electroless Ni-W-P coatings was improved, and the new pretreatment without Cr was used. It was found that the Ni-W-P had the compound of W: 4.50wt%, P: 4.84wt%. The ternary nickel coatings directly on AZ91D magnesium was obtained, and the corrosion resistance was actually much better than that of Ni-P coating. The porosity of the Ni-W-P coating decreased to zero when its thickness was thicker than 8μm, and when the thickness is up to 15μm.the resistance of the coating is increasing rapidly.
|
PDF Full Text Request