5,5-二甲基乙内酰脲配位体系酸性镀镉工艺优化

Optimization of acidic cadmium plating process using 5,5-dimethylhydantoin as complexing agent

针对无氰镀镉体系镀液稳定性差、镀层性能不佳等问题,选用5,5-二甲基乙内酰脲(DMH)为主配位剂、CdCl2为主盐研究了新型镀镉工艺。通过赫尔槽试验、阴极极化等技术研究了镉盐和DMH含量、辅助络合剂类型及浓度、pH值以及电流密度等因素的影响,优化了酸性DMH镀镉工艺方案。结果表明:酸性DMH镀镉体系中可以选择柠檬酸铵作为辅助络合剂。适当增加镉盐与DMH含量,可拓宽光亮电流密度范围,提高镉镀层致密性;增加柠檬酸铵浓度有利于提高镀速,但过多的镉盐和DMH会降低镀液稳定性。提高镀液pH值,镉沉积极化电位不断负移,有助于获得细致结晶,但pH>4后镀液易出现沉淀;此外,该体系许可的电流密度不宜超过1.5 A/dm~2。综合确定较佳的酸性DMH镀镉工艺方案为:CdCl230 g/L,DMH 60 g/L,柠檬酸铵40 g/L,KCl 30 g/L,pH=3,电流密度1.0~1.5 A/dm~2。在此体系下所得镉镀层表面结晶细致、均匀致密,晶粒尺寸为650 nm左右,镀层中Cd元素含量为95.6 wt.%。

In order to solve the problems of bad bath stability and poor coating performance for the cya‐nide-free cadmium plating system,a new cadmium plating process with 5,5-dimethylhydantoin(DMH)as the main complexing agent and CdCl2as the main salt was studied.The influence of cadmi‐um salt and DMH content,the type and concentration of auxiliary complexing agent,pH value,curren density was researched by Hull cell test and cathodic polarization,and the process scheme for acidic DMH cadmium plating was optimized.The results show that ammonium citrate can be selected as auxil‐iary complexing agent in acidic DMH cadmium plating system.Appropriately increasing the content of cadmium salt and DMH can widen the range of bright current density and improve the compactness of cadmium coating.Increasing the concentration of ammonium citrate is beneficial to increase the plating speed.But too much cadmium salt and DMH will reduce the stability of plating bath.The polarization potential of cadmium deposition continues to move negatively as the pH value of the bath increased,which is helpful for refining the grains.However,the plating bath is prone to precipitation if pH value is greater than 4.In addition,the current density allowed by this system should not exceed 1.5 A·dm-2.The optimum process parameters of acid DMH cadmium plating are as follows:30 g·L-1CdCl2,60 g·L-1DMH,40 g·L-1ammonium citrate,30 g·L-1KCl,pH=3 and the current density of 1.0~1.5 A·dm-2.Under this condition,the prepared cadmium coating is fine,uniform and dense,the grain size is about650 nm,and the content of Cd element in the coating is 95.6 wt.%.