5,5−二甲基乙内酰脲体系无氰镀镉层的结构及耐蚀性

Microstructure and corrosion resistance of Cd coating electroplated from a cyanide-free 5,5-dimethyl-hydantoin bath

[目的]研究5,5−二甲基乙内酰脲(DMH)体系无氰镀镉层的结构及性能,为拓宽其应用提供实验数据。[方法]选用碱性DMH体系电镀镉,采用扫描电镜和X射线衍射仪分析Cd镀层的微观结构,通过电化学测试、中性盐雾试验及长期浸泡试验研究Cd镀层的耐蚀性及其腐蚀行为演变规律。[结果]DMH体系Cd镀层外观呈亮白色,晶粒呈六棱柱形,尺寸虽略大于氰化物Cd镀层,但更致密,并且在(002)晶面呈高择优取向,其织构系数达到了99.834%。DMH体系Cd镀层在3.5%NaCl溶液中的腐蚀电流密度约为氰化物Cd镀层的40%,可以经受876 h中性盐雾腐蚀。经六价铬钝化的Cd镀层在2384 h的中性盐雾试验后仍未出现红锈。长期浸泡腐蚀过程中,镉镀试样的腐蚀控制步骤及腐蚀速率会随腐蚀产物的生成和破坏而变化。[结论]碱性DMH体系Cd镀层的耐蚀性可与氰化物Cd镀层相媲美,有望用于航空结构钢部件的防护。

[Introduction]The microstructure and properties of cadmium coating electroplated from a cyanide-free bath using 5,5-dimethylhydantoin(DMH)as complexing agent needs to be studied,providing experimental data to broaden its application.[Method]Cadmium electroplating was conducted in an alkaline DMH bath.The microstructure of Cd coating was analyzed by scanning electron microscopy and X-ray diffraction.The corrosion resistance and corrosion behavior of Cd coating were studied by electrochemical measurement,neutral salt spray test,and long-term immersion test.[Result]The Cd coating electroplated in DMH bath was brightly white,featured hexagonal prismatic grains slightly larger in size but more compact in microstructure as compared with the Cd coating electroplated in a cyanide bath,and exhibited a highly preferred orientation in(002)crystal plane with a texture coefficient up to 99.834%.The corrosion current density of the Cd coating electroplated in DMH bath was about 40%of the one electroplated in cyanide bath in 3.5%NaCl solution,and it withstood the corrosion of neutral salt spray for 876 hours.After being passivated with hexavalent chromium,the Cd coating electroplated in DMH bath could endure neutral salt spray test for 2384 hours without any red rust.During the long-term immersion corrosion process,the corrosion control step and corrosion rate of DMH cadmium-electroplated specimen would change with the generation and destruction of corrosion products.[Conclusion]The corrosion resistance of Cd coating electroplated in alkaline DMH bath is comparable to that electroplated in cyanide bath,and it is expected to be applied to the protection of structural steel components used in aviation industry.