金属杂质对碱性DMH体系无氰镀银的影响

Effects of metal impurities on silver electroplating in a cyanide-free alkaline 5,5-dimethylhydantoin-based bath

通过霍尔槽试验、阴极极化曲线测试、扫描电镜(SEM)和X射线衍射(XRD)研究了不同金属杂质(包括Cu^(2+)、Ni^(2+)和Pb^(2+))对碱性5,5−二甲基乙内酰脲(DMH)体系中银电沉积行为及镀层结构的影响。结果表明,无金属杂质时,碱性DMH镀银体系的光亮电流密度区为0.5~1.0 A/dm^(2),Ag镀层结晶细致,晶粒平均尺寸为30.7 nm。3种金属杂质均会减弱碱性DMH镀银体系中银电沉积的阴极极化作用,加速银配离子的放电还原过程,导致镀层晶粒粗大,外观发灰、发黄。Cu^(2+)和Ni^(2+)会显著缩小银电沉积的工作电流密度范围,二者的质量浓度应分别控制在0.50 g/L和0.20 g/L以内。Pb^(2+)会改变银的结晶方式,使所得Ag镀层的Ag(111)晶面衍射峰减弱,Ag(200)晶面衍射峰增强,其质量浓度不宜超过0.10 g/L。

The effects of different metal impurities including Cu^(2+),Ni^(2+),and Pb^(2+)on the elctrodeposition behavior of silver in a cyanide-free alkaline 5,5-dimethylhydantoin(DMH)-based bath and the microstructure of the silver coating obtained therefrom were studied by Hull cell test,cathodic polarization measurement,scanning electron microscopy(SEM),and X-ray diffraction(XRD).The results showed that the current density range for elecrodepostion of bright silver coatings in the alkaline DMH-based bath without metal impurity was 0.5-1.0 A/dm^(2),and the Ag coatings obtained thereat was fine and compact with an average grain size of 30.7 nm.All of the three metal impurities could accelerate the discharge reduction of silver complex ions for they could weaken the cathodic polarization of silver electrodeposition,resulting in the coarse grain,gray and yellow appearance of silver coatings.Both Cu^(2+)and Ni^(2+)could significantly reduce the allowable current density for silver electrodeposition,and their contents should be controlled lower than 0.50 g/L and 0.20 g/L,respectively.Pb^(2+)could change the crystallization of silver coating as be shown by a decrease for diffraction peak of Ag(111)plane but an increase for Ag(200)plane.The mass concentration of Pb^(2+)in the bath should be controlled lower than 0.10 g/L.