铝合金表面铈基转化膜研究进展

Progress of Cerium-based Conversion Coating on Aluminum Alloy Surface

铝合金具有成本低、强度质量比高、可加工性能良好等优点,被广泛应用于航空、交通、建筑等领域。但铝合金表面易发生小孔腐蚀和晶间腐蚀,需要进行防腐处理才能满足应用要求。稀土元素铈因具有特有的电子层结构和物理化学特性,是制备铝合金稀土转化膜最具优越性的元素,以此为防腐基材开发的稀土转化膜技术被认为是最有可能替代铬酸盐钝化的技术。目前所报道的铈基转化膜工艺有化学浸泡法、溶胶-凝胶法、电化学法、磁控溅射法等。其中,化学浸泡法制备过程简单,但铈离子的转化和沉积速率较难控制一致,膜层微米级裂纹较多;溶胶-凝胶法制备的膜层与基体结合强度高,耐蚀性好,但需要大量铈盐,且产生较多废酸、碱液,成本高,环境污染大;电化学法在低温下实现性能可控的铈基转化膜,成本低,但成膜有大量晶间裂纹,结构疏松,成膜质量差;磁控溅射法制备的涂层均匀,成分可控,但靶材的利用率低,难以实现强磁性材料的低温高速溅射。总体来看,目前所报道的铈基转化膜工艺存在溶液体系稳定性差,所制备的转化膜层不稳定、容易开裂,以及制备成本较高的问题,难以满足工业化应用的要求。铈基转化膜成膜机理被普遍认为是阴极成膜理论,即铝合金表面形成了氧化还原微电池。溶液中的溶解氧以及加入的H_(2)O_(2)可作为羟基的供体,通过改变溶液局部pH值、物质、电子和电荷浓度来影响稀土元素的氧化反应和在基体表面的沉积。铈基转化膜的耐蚀机理是阴极抑制机理,即在铝合金表面形成的铈基转化膜阻止了氧的传输和电子的转移和传递,从而阻止了阴极微区上的还原反应,防止了铝合金表面的腐蚀溶解。但该机制只强调了阴极抑制,而忽略了也可能发生的阳极抑制。同时,由于涉及到电化学动力学抑制,因此铈基转化膜有“自愈能力”。本文综述了国内外铝合金铈基转化膜制备工艺的研究进展,指出了各工艺的优点和缺点,阐述了铈基转化膜主流成膜和耐蚀理论的研究,并对制备工艺的改进方向及未来理论的研究方向进行了展望。

Aluminum alloy has been widely applied in aviation,transportation,construction and other fields with a wealth of merits such as low cost,high strength-to-mass ratio,good processability,etc.The corrosion protection,however,is necessary for applications as the pinhole corrosion and intergranular corrosion are prone to emerge on its surface.Cerium is one of the most advantageous rare-earth elements in terms of the preparation of conversion films on the substrate surface of aluminum alloys due to its unique electronic layer structure and physical and chemical properties,and the cerium-based conversion film technology developed is considered to be the most promising alternative to the chromate passivation.Until now the technologies reported of cerium-based conversion films include chemical immersion,sol-gel method,electrochemical process,magnetron sputtering et al.Among them,chemical immersion is the simplest for the preparation process,but both conversion rate and deposition rate of cerium ions are difficult to control,and therefore there are many micron cracks present in the film layer.The sol-gel method endows the film with high bonding strength with hypokeimenon and excellent corrosion resistance,but requires substantial contents of cerium salts while producing a vast amount of waste acid or alkali solution.While the electrochemical process can achieve controllable cerium-based conversion films even at low temperature at a low price,the film causes a lot of intergranular cracks,loose structures as well as poor quality.The coating prepared by magnetron sputtering is uniform with controllable composition,but due to low utilization rate of the target this method fails to achieve low temperature and high speed sputtering for the strong magnetic materials.Overall,these methods still face various issues,such as poor stability of the solution system,the prepared conversion film layer being unstable and easy cracking,and high cost,which make it difficult to meet requirements of industrial application.The formation mechanism of cerium-based conversion film is principally believed to be the cathode formation film theory,that is,the redox microcell is formed on the surface of aluminum alloy.The dissolved oxygen in the solution and added H_(2)O_(2) can be used as the donor of hydroxyl group,and the local pH value,components,electron and charge concentration of the solution can be changed,benefitting the oxidation reaction of cerium elements and the deposition on the substrate surface.The corrosion resistance mechanism of the cerium-based conversion film is the cathodic inhibition,that is,the film formed on the surface of aluminum alloys prevents the oxygen transmission and electron transfer,consequently,preventing the reduction reaction on the cathode micro-zone and the corrosion and dissolution of the aluminum alloy.However,this mechanism only emphasizes the cathode inhibition,while ignoring the anode inhibition.Furthermore,these cerium-based conversion films demonstrate a self-healing ability since electrochemical kinetic inhibition takes place.In this paper,the technological progress on the cerium-based conversion film for aluminum alloys at home and abroad was reviewed,and the advantages and disadvantages of technologies were also described.Moreover,the mainstream formation and anticorrosion theories were expounded.Finally,we predict the direction of process improvement and theory research in future.