化学镀镍磷合金镀层碱性无铬钝化膜的耐蚀性研究

Study on corrosion resistance of alkaline chromium-free passivation film on surface of electroless nickel–phosphorus alloy deposit

为了解决六价铬钝化工艺的毒性问题,采用碱性无铬钝化工艺对A3钢上化学镀Ni–P合金镀层进行钝化处理,在镀层表面获得了抗氧化性与耐蚀性良好的钼酸盐钝化膜。研究了钝化液组分(包括碳酸钠和钼酸钠)的质量浓度以及钝化温度对镀层耐硝酸点滴腐蚀时间的影响,获得了最佳的钝化工艺:NaOH 10 g/L,Na2CO3 4.0 g/L,钼酸钠40.0 g/L,复合添加剂(四硼酸钠与亚硝酸钠组成的复合缓冲体系)5 g/L,温度60°C,时间15 min。通过扫描电镜(SEM)和电化学测试,对比研究了钝化前、后Ni–P合金镀层的表面形貌及耐蚀性能,采用X射线光电子能谱仪(XPS)分析了最佳工艺得到的钝化膜的组成。经最佳钝化工艺处理得到的钝化膜无色透明,由Ni、P、O、Mo等元素组成。钝化后,Ni–P合金镀层在3.5%Na Cl溶液(p H=6.8)中的电荷转移电阻由钝化前的11.6 k?·cm2增大至448.2 k?·cm2,耐硝酸点滴腐蚀时间提高了60倍以上,耐蚀性明显提高。

In order to solve the toxicity problem ofhexavalent chromium passivation process, the electroless plated Ni-P alloy deposit on A3 steel surface was passivated by an alkaline chromium-free passivation process, forming a molybdate passivation film on its surface with good resistance to oxidation and corrosion. The effects of the mass concentrations of the components （including sodium carbonate and sodium molybdate） of the passivation solution and passivation temperature on the resistance of the deposit to nitric acid dropping corrosion were studied. The optimal passivation conditions were obtained as follows： NaOH 10 g/L, Na2CO3 4 g/L, sodium molybdate 40.0 g/L, composite additive （a buffer comprising sodium tetraborate and sodium nitrite） 5 g/L, temperature 60 ℃, and time 15 rain. Surface morphologies and corrosion resistance of the Ni-P alloy deposit before and after passivation were comparatively studied by scanning electron microscopy （SEM） and electrochemical measurement. The composition of the passivation film obtained by the optimal passivation process was analyzed by X-ray photoelectron spectroscopy （XPS）. The passivation film is colorless and transparent, and mainly composed of Ni, P, O, and Mo elements. The molybdate passivation increased the charge transfer resistance of the NiP alloy deposit in 3.5% NaCl solution （pH = 6.8） from 11.6 kΩ·cm^2as-deposited to 448.2kΩ·cm^2, and its resistance to nitric acid dropping corrosion by more than 60 times. The corrosion resistance of electroless Ni-P alloy deposit is remarkably enhanced.