Preparation And Corrosion Resistance Of The Conversion Coating-electroless Ni-P Plating On Magnesium Alloy

Chemical conversion treatment has attracted more attention in researchers owing to its simple steps, especially for low cost among the anti-corrosion technologies of magnesium alloy. However, the thin thickness of the chemical conversion coating cannot provide a long-term protection to matrix. In this paper, chemical conversion treatment and electroless plating technology were combined. Chemical conversion coating- electroless Ni-P platings with excellent corrosion resistance were deposited on AZ91 D magnesium alloy substrate.In this paper, preparation methods of three chemical conversion coatings on the AZ91 D magnesium alloy substrate were optimized. It can be observed that the stannate conversion coating was composed of nearly spherical particles, the phosphate- permanganate conversion coating and rare earth cerium conversion coating were composed of crack. All these three conversion coatings were completely covered the substrate. Spot tests and electrochemical tests were taken to assess the corrosion resistance of conversion coatings. In the spot tests, the discoloration time of magnesium alloy substrate was 6 s, the discoloration times of three conversion coatings were increased to 209 s, 25 s and 33 s. In the electrochemical tests, the corrosion potential of magnesium alloy substrate was-1.635 V, the corrosion potential of the chemical conversion coatings were moved positively to-1.369 V,-1.502 V and-1.485 V. The results showed a better corrosion resistance of chemical conversion coating than the magnesium alloy substrate.The chemical conversion coating- electroless Ni-P platings were prepared, which showed a typical spherical cell shape of electroless nickel coating. The combination of electroless plating and chemical conversion coating was compact. In the spot test, discoloration times of the specimens after electroless plating have achieved 6 min 13 s, 4 min 47 s and 5 min 48 s.The corrosion potential of the specimens after electroless plating was shifted positively whenobserved from polarization curves, especially improve from-1.635 V to-0.548 V,-0.576 V and-0.582 V. In full immersion corrosion test, the corrosion rate of chemical conversion coating- electroless Ni-P platings was the lowest, which indicated that the composite technology could improve the corrosion resistance of magnesium alloy.The reason of the improvement of composite technology was researched. To be as the interlayer, conversion coating provided roughness surface for electroless Ni-P plating to deposite. The porosity conversion coating was covered by the uniformly continuous electroless plating. In the presence of interlayer, the potential difference between electroless plating and magnesium alloy substrate was decreased, which impeded micro cell corrosion formed between electroless plating and magnesium alloy substrate. The electroless plating occurred active dissolution first when immersed in the corrosive medium. A layer of nickel phosphide passivation coating was deposited on the surface of electroless plating. And then the corrosion was slowed down and stopped. In general, the composite process can play a sealing effect to conversion coating, which can protect the magnesium alloy substrate and improve the corrosion resistance.