在镁合金防腐技术中，化学转化处理具有操作简单、工序少、成本低等优势，使得化学转化处理吸引了很多研究者的目光。但化学转化膜厚度偏薄，不能给基体提供长久的保护。因此，本文将化学转化处理和化学镀技术相结合，以AZ91D镁合金为基体，成功地制备了具有优异耐蚀性的转化膜-化学镀Ni-P涂层。 本文针对AZ91D镁合金基体优化了锡酸盐体系、磷酸-高锰酸盐体系和稀土铈盐体系三类转化膜的制备方法。其中，锡酸盐转化膜表面由近似球形的颗粒组成，磷酸-高锰酸盐转化膜和稀土铈盐转化膜表面均呈现裂纹状，三种转化膜均完全覆盖基体；通过点滴实验和电化学测试评定了转化膜的耐腐蚀性，点滴实验中，镁合金基体的变色时间为6 s，三种转化膜的变色时间分别提高到209 s、25 s和33 s；电化学测试表明，镁合金基体的自腐蚀电位为-1.635 V，经过化学转化处理后自腐蚀电位分别正移到-1.369 V、-1.502 V和-1.485 V，结果表明化学转化能够提高镁合金基体的耐腐蚀性。 通过不同转化膜表面化学沉积镍磷获得具有双层结构的涂层，三种涂层表面均呈现化学镀镍磷典型的球形晶胞形状，镍磷镀层与转化膜成紧密的“锯齿”状结合。点滴实验中，三种转化膜表面化学镀层变色时间分别达到了6 min 13 s、4 min 47 s和5 min 48 s；从极化曲线得知化学镀镍磷后试样的自腐蚀电位分别正移到-0.548 V、-0.576 V和-0.582 V；全腐蚀浸泡实验中，转化膜及化学镀层的腐蚀速率相比镁合金基体均有所降低，与转化膜和镁合金基体相比，转化膜表面化学镀层的腐蚀速率进一步降低，复合工艺使基体的耐蚀性获得显著提高。 研究了复合工艺对基体耐蚀性提高的原因：转化膜作为中间层，具有一定的粗糙性和裂纹，化学镀层可以很好的沉积在这些不平整的部位，封闭了转化膜的孔隙，形成均匀连续的镀层。同时由于中间层的存在，降低了化学镀层与镁合金基体之间的电极电位差，阻碍了镀层与基体直接形成腐蚀微电池。镍磷化学镀层在遇到腐蚀介质后，化学镀层先发生活性溶解，然后马上形成磷化镍钝化膜，进而减缓和阻止了腐蚀的继续发生。总体来讲，该复合工艺能够对转化膜起到封孔作用，很好的保护基体、提高镁合金的耐腐蚀性能。

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 AZ91D magnesium alloy substrate. In this paper, preparation methods of three chemical conversion coatings on the AZ91D 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 -1.635V, 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 when observed 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.