无氰镀银层高温防变色性能初步探究

Preliminary Study on Discoloration Resistance of the Cyanide-free Silver Coating at High Temperature

目的探寻无氰镀银层高温变色的原因,以增强无氰镀银层的防高温变色性能。方法采用硫代硫酸盐体系在紫铜片上镀银,并进行无惰性气体保护的300℃×1 h烘烤。借助扫描电镜(SEM)观察镀层高温烘烤后的微观形貌,采用能谱仪(EDS)检测镀层中各元素的含量与分布,探讨银层厚度、供电方式、后处理工艺对镀层防高温变色性能的影响。结果当直流电镀层厚度达到9μm,脉冲电镀层厚度达到6μm时,镀层经高温烘烤后不变色。而经水溶性银保护剂、重铬酸钾、PMTA处理后的镀层(厚6μm)均在高温烘烤后发生了变色情况。扫描电镜观察显示,高温变色镀层表面有凸起状裂纹,能谱仪检测到有铜原子外渗现象。结论高温下铜原子的外渗导致了镀层变色。采用脉冲方式进行电镀,可以使镀银层孔隙率降低。通过增加镀银层厚度和采用脉冲电镀,能够提高镀银层的防高温变色性能。

Objective To search for the cause for the high-temperature color change of cyanide-free silver coating,in order to enhance the discoloration resistance of the cyanide-free silver coating at high temperature. Methods Thiosulfate system was used to plate silver on the copper sheet,followed by baking at 300 ℃（ without inert gas protection） for 1 h. Scanning electron microscopy（ SEM） was used to observe the microstructure of the coating after high-temperature baking,and energy spectrometer（ EDS） was used to detect the content and distribution of elements in the coating. The effects of silver layer thickness,power supply mode,and the post-processing technology on the discoloration resistance of the coating at high temperature were discussed. Results When the thickness of direct current（ dc） plating layer was 9 μm and the thickness of pulse electroplating layer was 6 μm,the coating did not change color after high-temperature baking. But the coatings（ with a thickness of 6 μm） after treatment with water soluble silver protectant,potassium dichromate,and PMTA all changed color after high-temperature baking. Scanning electron microscopy showed convex-shape cracks on the surface of the high-temperature discolored coating,and spectrometer detected copper atoms leakage phenomenon. Conclusion Leakage of copper atoms at high temperature led to discoloration of the coating. Pulse plating could decrease the porosity of the coating. Increasing the silver coating thickness and using the pulse plating could improve the high-temperature discoloration resistance of silver coating.