环保型 AZ91D 镁合金直接化学镀镍工艺研究

目的开发一种无铬、低氟、稳定的镁合金直接化学镀镍工艺。方法通过SEM,EDX及浸泡实验、动电位极化曲线、划格试验等方法 ,以稳定系数和镀速等参数为对象,对比新工艺和传统工艺对镀层或镀液性能的影响。结果在以硫酸镍为主盐的基础液中,添加质量浓度为0.5 mg/L的硫脲,镀液的稳定性可得到明显的提高。在使用硝酸+磷酸酸洗后的镁合金试样表面,获得了良好的腐蚀形貌结构,这种结构有利于增强镀层与基底间的机械咬合作用。新工艺获得的镀层属高P镀层(P的质量分数约为11%),在Na Cl溶液中的自腐蚀电位由-1.5 V正移至-0.5 V,腐蚀电流密度降低了约3个数量级。结论以硫酸镍主盐镀液获得的镀层耐蚀性优于碱式碳酸镍主盐镀液获得的镀层,镀液的最佳p H=5,化学镀镍温度为82℃。

An Environmentally-friendly Process of Direct Electroless Nickel Plating on AZ91D Magnesium Alloy

Objective To develop a chromium-free and stable direct electroless nickel plating ( ENP) technology for magnesium alloy. Methods SEM, EDX, immersion test, polarization curves, scribe and grid test as well as the stability factor and deposition rate were used to compare the effects of the new and traditional processes on the characteristics of the performance of Ni-P coating and plating bath. Results The stability of the base solution with nickel sulfate as the main salt could be obviously improved when 0. 5 mg/L of thiourea was added. A good corrosion morphology structure of the AZ91D magnesium alloy was achieved after pickling in a mixture of nitric acid and phosphoric acid. This specific surface structure could improve the mechanical adhesion between the substrate and the coating. The coating prepared using the new technology belonged to high-P coating, with a P content of about 11%. Compared to the bare magnesium alloy, the potentiodynamic polarization test in NaCl solution showed that the corrosion po-tential and the corrosion current density of the new coating increased positively from -1. 5 V to -0. 5 V and decreased by three or-ders of magnitude, respectively. Conclusion The corrosion resistance of coating obtained from a plating bath using nickel sulfate as the main salt was better than that from a plating bath using basic nickel carbonate as the main salt. The optimal pH and temperature of the ENP processes were 5 and 82 ℃, respectively.