模拟海洋环境中Ni-Co合金镀层对AZ91D镁合金的腐蚀防护研究

Corrosion Resistance of Ni-Co Alloy Coating Prepared on AZ91D Magnesium Alloy in Simulated Marine Environment

目的提高AZ91D镁合金的腐蚀防护性能。方法采用化学镀前处理在AZ91D镁合金表面制备一种保护性的Ni-Co合金镀层。分别采用环境扫描电镜(ESEM)、X射线衍射(XRD)和能量散射谱(EDS)分析合金镀层的表面形貌、微结构特点和化学成分。采用动电位极化(PC)和电化学阻抗谱(EIS),分析测试在模拟海洋环境(中性3.5%Na Cl溶液)中Ni-Co合金镀层对AZ91D镁合金的腐蚀防护性能。结果镁合金表面化学镀Ni-P镀层均匀覆盖,晶粒生长较致密,表面呈菜花状形貌,Ni-P镀层中P质量分数约为5.6%。Ni-Co合金镀层表面均匀且呈金字塔状形貌,形成了面心固溶体(FCC),镀层中Co质量分数约为31%。Ni-P镀层和Ni-Co合金镀层的厚度分别约为11μm和19μm。在模拟海洋(中性3.5%Na Cl溶液)环境中,镁合金裸基体、化学镀前处理Ni-P镀层、Ni-Co合金镀层的腐蚀电位分别为-1485、-372、-284 m V,其腐蚀电流密度分别是3.4×10-5、1.8×10-6、2.9×10^(-7) A/cm2,所拟合的电荷转移电阻分别为4.72×103、1.70×104、2.06×106?/cm2。结论化学镀前处理Ni-P镀层可为镁合金提供较好的腐蚀防护,Ni-Co合金镀层能够为镁合金提供更显著的腐蚀防护。

The work aims to improve corrosion resistance of AZ91D magnesium alloy. Protective Ni-Co alloy coating was prepared on AZ91D magnesium alloy by performing electroless plating pretreatment. Morphology, microstructure feature and chemical composition of the coating were analyzed by using environmental scanning electron microscope （ESEM）, X-ray dif- fractometer （XRD） and energy dispersive spectrometer （EDS）, respectively. Corrosion resistance of Ni-Co alloy coating for AZ91D magnesium alloy in simulated marine environment （neutral 3.5wt.% NaCI solutions） was analyzed and tested by using potentiodynamie polarization curve and electrochemical impedance spectroscopy （EIS）. The electmlessly plated Ni-P coating on magnesium alloy was uniformly covered, cauliflower-like grains grew densely, and mass fraction of P element in the Ni-P coat- ing was approximately 5,6wt.%. The Ni-Co alloy coating had uniform coating surface and was in pyramid-shape, formed face-centered cubic （fee） solid solution, mass fraction of Co in the coating was approximately 3 lwt.%. Thickness of Ni-P and Ni-Co alloy coating was approximately 11 μm and 19 μm, respectively. In simulated marine environment （neutral 3.5wt.% NaCI solution）, corrosion potential （Ecor~） of bare magnesium alloy, Ni-P and Ni-Co alloy coatings pretreated by electroless plating was -1485 mV, -372 mV and -284 mV, respectively; corrosion current density （Jcorr） was 3.4×10^-5Ω/cm^2, 1.8×10^-6 Ω/cm^2, 2.9×10^-7Ω/cm^2, respectively; and fitted charge transfer resistance （Rot） was 4.72x 10^3 Ω/cm2, 1.70×10^4 Ω/cm2 and 2.06×10^6Ω/cm2, respectively. The Ni-P coating pretreated by electroless plating can provide better corrosion protection for magnesium alloy while Ni-Co alloy coating provide more significant corrosion resistance for magnesium alloy.