镁合金导电-耐蚀化学转化膜的研究

A new challenge for the coating technologies rises along with the extensive use of magnesium alloys in 3C electrical products, i.e. the coated surface is mandatory to satisfy the requirements of a low electrical contact resistance (ECR) to ensure good electrical conductivity between integrated circuit broads and internal surface of casing while maintaining high corrosion resistance. This electrical conductivity is essential to guarantee continuous electrical contact, grounding and electromagnetic shielding. As an insulative coating, most of studies focus on the improvement of corrosion resistance of conversion coating, and pay a little attention to electrical conductivity. The desired performances are attributed to the unique microstructure of conversion coating, i.e., β phase protruded from the surface and covered by a thinner passive film, contributing to low ECR, meanwhile, a thicker coating without cracks was deposited on α-phase, providing a good corrosion resistance. Moreover, the conversion bath has some features as following: acidic solution, strong oxidizing agent, the proper pH value and the cation concentration based on thermodynamic calculation. However, in the hygrothermal environment, the conversion coating is easy to lost electrical conductivity due to its poor corrosion resistance. Therefore, in the view point of the electrochemical/chemical reactions on the Metal/Solution interface, the aim of this project is to found the design theory about conversion bath based on chemical bonding theory of crystal growth, clarify the pre-treatment and corrosion resistance of conversion coating, and try to answer the following scientific question: How to manufibricate the high performance conversion coating with high corrosion resistance and low electrical contact resistance based on the controlling of the reactions on the Metal/Solution interface.

为了提高电磁兼容与电磁屏蔽性能，通讯及3C电子工业迫切需求镁合金转化膜同时具有较高的耐蚀性和良好的导电性。传统的转化膜属于绝缘膜层，相关研究几乎全部集中于耐蚀性能方面，对于导电性问题很少涉及。导电-耐蚀转化膜在显微结构、溶液设计思路方面与传统的转化膜有着明显的差异：导电-耐蚀转化膜需要让β相凸出并发生钝化形成导电斑点，以提高导电性，α相成膜较厚以增强耐蚀性能；转化膜溶液需要呈酸性、含强氧化性物质，同时合理调控pH值和成膜阳离子浓度。但是，当前的导电-耐蚀转化膜的耐蚀性能仍旧相对较差，在湿热环境下会丧失导电能力。针对于此，本项目从“金属/溶液”界面反应入手，通过引入“晶体生长化学键合理论”，建立转化膜溶液设计理论；构建“前处理工艺—表面拓扑形状与电化学特性—转化膜性能”的相关性，完善前处理理论。最终回答这样一个科学问题：如何通过调控“金属/溶液”界面的反应过程，获得高性能的导电-耐蚀转化膜。

本项目围绕镁合金“耐蚀-功能性”防护涂层关键科学问题，在“酸比”理论指导下，把握界面反应“溶解-电离-沉积”过程的主要特征，建立热力学与动力学之间的关联，提出了“耐蚀-功能性”防护涂层的溶液设计准则，为开发新型防护涂层提供了理论基础。借鉴金属凝固若干概念开展交叉学科研究，控制处理液的成分过冷度与临界形核尺寸，解决了转化膜开裂与增厚难以兼顾的传统难题，最终形成了系列性的绿色、高耐蚀镁合金表面处理技术。新技术初步应用于国防建设与国民经济领域。“耐蚀-功能性”涂层在华为公司5G通讯设备的镁合金外壳上交付使用，耐蚀-耐磨-导电-低磁性涂层在航天一院试用。研究成果丰富和完善腐蚀电化学的相关理论，为解决镁合金的腐蚀与防护问题提供科学依据和理论支撑。