镁合金化学镀镍溶液稳定性

Bath stability of electroless plating nickel on magnesium alloys

为了开发出长寿命高稳定性镁合金化学镀镍溶液,通过添加硫脲、碘酸钾等稳定剂,研究了pH值、温度对镀液的稳定性能、镀速、镀层质量等因素的影响。采用称重法测定镀层的沉积速率,沉积到镀件上的镍量占溶液中消耗镍量的百分比来表示溶液的稳定性,NaCl溶液浸泡实验评定镀层的覆盖度,热震实验评定镀层的结合力,极化曲线表征镀层的耐蚀性。结果表明,沉积速率随硫脲、碘酸钾浓度的增大先升高后降低,碘酸钾对镀速的影响不如硫脲显著。添加硫脲0.5 mg·dm-3时稳定常数最大值达89.25%,添加碘酸钾5 mg·dm-3时,稳定常数达82.45%。采用pH值为5.0的含硫脲的镀液,(82±1)℃施镀,获得的Ni-P镀层和镁合金基体之间没有缝隙,结合紧密,而且Ni-P镀层均匀致密。硫脲不仅能提高沉积速率,而且也催化镁合金表面,提高沉积效率。

The main purpose of the research was to develop an electroless nickel bath with long cycles and high stability. Thiourea and potassium iodate were added in the eleetroless bath as the stabilizing agents. The effects of temperature and pH values on the bath stability, plating rate and coating quality were studied. Weighing was used to measure the deposition rate of coating. The stability of the bath was characterized by percentage of the nickel on the surface of work piece to total nickel consumption in the bath. The coverage of coating was evaluated by immersion test in NaC1 solution. The adhesion of coating was evaluated by thermal shock test. Polarization curves were used to characterize the corrosion resistance of coating. The results showed that the deposition rate increased at first, then went down with increasing concentration of thiourea or potassium iodate. The deposition rate was more significantly affected by thiourea than that by potassium iodate. The maximum value of stability constant reached 89.25% in the bath of 0.5 mg mg·dm^-3 thiourea and the stability constant was 82.45% in the bath of 5 mg·dm^-3 potassium iodate. When plating was carried out at （82 ± 1）℃ in the pHS. 0 bath of 0.5 mg · dm^-3 thiourea, uniform and dense coating was obtained, and there was no gap between Ni-P coating and Mg substrate. Thiourea not only improved the deposition rate, but also catalyzed the surface of magnesium alloy and raised the deposition efficiency.