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目的：研究硫脲对电镀镍层形貌、晶型、晶粒尺寸以及抗腐蚀性能的影响。方法在添加和不添加硫脲两种条件下,对304不锈钢进行电镀镍。通过XRD和SEM对比镀层的晶相结构、晶粒尺寸和微观形貌,通过Tafel曲线和EIS电化学阻抗谱对比分析样品的抗腐蚀性能。结果添加硫脲前后,镍镀层的择优生长方向从<200>晶面转变为<111>晶面,晶粒平均尺寸从60.2 nm减小为20.7 nm。由于硫脲吸附在金属表面,阻滞了溶液中金属离子放电,从而提高了阴极极化作用；同时,硫脲分子吸附在晶体生长的活性点上,有效抑制了晶体生长；因此,镍晶粒得到细化。关于腐蚀性能,镀镍304不锈钢的耐腐蚀性能最好,其次是304不锈钢基体,最差的是含硫镀镍304不锈钢。结论硫脲能够改变镍镀层晶体的择优取向,并且细化晶粒。硫脲影响电镀镍层抗腐蚀性能的机理是：阴极吸附的硫脲通过电化学还原产生H2 S,进一步与Ni2+反应生成NiS,NiS再与富集的Ni反应生成Ni3 S2。随着电镀的进行,Ni3 S2被更多地沉积在镍镀层的表面,降低了镍层表面钝化膜的致密性；而且,Ni3 S2与镍层存在电势差,故易发生原电池腐蚀。此外,晶粒细化导致晶界和晶面的缺陷增加,也使得腐蚀加速。故镀液中加入硫脲,镀层的耐腐蚀性能大大降低。

ABSTRACT:Objective To investigate the effect of thiourea on crystalline structure,grain sizes,morphology and corrosion resis-tance of Ni coatings. Methods Ni coatings were prepared on the surface of 304 stainless steel with or without thiourea. The crystal-line structure,grain sizes and microscopic morphology of the coatings were characterized by XRD and SEM. The corrosion resis-tance of the samples was analyzed by Tafel and EIS. Results After adding thiourea, the preferred orientation of Ni coatings was changed from the<200>crystal plane to the<111>crystal planes and the average grain size of Ni coatings was reduced form 60. 2 nm to 20. 7 nm. Due to the adsorption of thiourea on the metal surface,the discharge of metal ions was blocked and the cathodic po-larization was enhanced. Meanwhile,thiourea molecules were adsorbed on the active sites of the crystal growth,which effectively in-hibited the crystal growth. Thus, the grain size of Ni coatings was refined. For corrosion resistance,304 stainless steel with Ni coa-tings was the best, followed by 304 stainless steel substrate;304 stainless steel with nickel coatings containing Ni3 S2 was the worst. Conclusion The preferred orientation of Ni coatings was changed and the average grain size of Ni coatings was reduced by thiourea. The mechanism of thiourea influencing anti-corrosion resistance of Ni coatings was proposed. Thiourea which was adsorbed on the cathode was restored to H2 S by electrochemical reduction,then NiS was generated by the reaction of H2 S and Ni2+. Ni3 S2 was fur-ther generated by the reaction of the enrichment of Ni and NiS. With the deposition of Ni,Ni3 S2 was deposited on the surface of Ni coatings and reduced compactness of Ni passivation film. Meanwhile,due to the potential difference between Ni and Ni3 S2 , corro-sion of the original cells may occur. In addition, grain refinement leaded to increased defects of grain boundaries and crystal plane, which would accelerate corrosion. Therefore,the corrosion resistance of the coatings with adding thiourea was greatly reduced.