| In order to improve the corrosion resistance of the AZ91D magnesiumalloy, electroless Ni-P and Ni-W-P coatings on AZ91D magnesium alloydirectly and indirectly were investigated in this paper. It had beensuccessfully deposited duality and ternary nickel coatings directly on AZ91Dmagnesium. At the same time, the pretreatment had been replaced by newtechnique without Cr, and the process had been made easier and moreexercisable.SEM and laser microscope observations and XRD analysis wereperformed to examine the microstructure (grain size) and phase compositionof the coatings. The corrosion resistance of the coatings was estimated byelectrochemical polarization measurement and 10%HCl solution immergencetest.In this paper, A new protective coating Ni-P/Ni-W-P deposit on AZ91Dmagnesium alloys was obtained by electroless plating ,the Ni-P coating wasdirect electrolessed on the AZ91D magnesium alloy by using a plating bathcontaining sulfate nickel, and the substrate was abraded on 1000# SiC sandpaper before electroless. The Ni-W-P coatings have different W content, andhave higher corrosion potential with W content increase. The Ni-P coating ofabout 12μm and the Ni-W-P coating of about was 14μm has zero porosity.The sample.1 with the Ni-W0.65-P8.18 coating has the best corrosionresistance that can stand in 10% HCl solution for about 170 minutes with thethickness of Ni-W0.65-P8.18 coating about 18μm .It was found that theability of corrosion resistance in 10%HCl was determined by the porosity ofthe coatings. With the pH value increase from 5,7 to9, the W content increase.The coatings with Ni-W-P coatings from different pH value which had thesame thickness of about 30μm can stand in10%HCl solution for170min,49min and 97min respectively.Ni-P coatings were successfully obtained on AZ91D magnesium alloywith the plating bath containing sulfate nickel. The corrosion resistance of thecoating is not high, but it was a good protective coating before the otherprocess. Then it was found that Ni-W-P was better than that of Ni-P.The second experiment is to improve the pretreatment. There had nodifferent except the pretreatment. CrO3 and HF, which were used in thepretreatment, had been replaced and a new conversion firm emergence, withwhich it could electroless duality and ternary nickel coatings.XRD, SEM and EDX analyzed the morphology and the microstructure.It was considered that conversion firm wasn't continuum. The α-phase on thesurface of the AZ91D magnesium alloy was corrupted, and then the contentof β-phase increase. When the potential difference of the surface wasn't tohigh, then it could electroless Ni-P.After the pretreatment was replaced, the corrosion resistance of the Ni-Pcoating was investigated. The substrate was abraded by NO.1500 SiC sandpaper before electroless. It was analyzed that the P content was about5.57wt%, the corrosion potential was –0.455,and the porosity decreasenotability, and the corrosion resistance in 10%HCl solution became muchbetter. That was a coating with thickness of 30μm could stand in 10%HClsolution for about 122min, while the coating obtained with the old techniquecould only stand for 60min at most.It had been known that Ni-W-P had better corrosion resistance than thatof Ni-P, so it was suggested that deposit Ni-W-P directly from a plating bathcontaining sulfate nickel. A traditional technique of electroless Ni-W-Pcoatings was improved, and the new pretreatment without Cr was used. Itwas 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, andthe 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 wasthicker than 8μm, and a coating with the thickness of about 24μm could standin 10%HCl for about 178min.
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