镁合金微弧氧化涂层表面原位制备MgCr-LDH层及其耐蚀防护机制研究

In situ preparation MgCr-LDH layer on MAO coating of Mg alloy and study its anti-corrosion protection mechanism

摘要:

为了进一步提升镁合金耐蚀性,将水滑石(layered double hydroxides LDH)与微弧氧化结合(MAO)制备一种主动防护的高耐蚀复合涂层。本文采用原位法成功在AZ31镁合金MAO涂层表面制备了层间含NO3-的MgCr-LDH层,构成LDH/MAO复合涂层,研究原有MAO涂层表面LDH的微观组织结构及其与原始MAO涂层的相互作用,并利用长时间浸泡法和电化学法测试MgCr-LDH/MAO复合涂层试样在3.5wt%NaCl溶液中的耐蚀性能,揭示LDH层耐蚀保护机理,研究结果显示：LDH易于在镁合金表面MAO层的孔洞中形成,最终生成均匀致密片状结构的层。LDH的原位生长过程对原始MAO涂层没有破坏,MgCr-LDH与MAO涂层之间属于化学结合,具有强的粘附性与机械稳定性。MgCr-LDH/MAO复合涂层明显提高了AZ31镁合金的耐腐蚀性能,LDH层的其防腐保护机制主要表现为两部分,一是LDH均匀形成于原始MAO层上,有效的覆盖了MAO层的孔洞与裂纹,阻挡氯离子通过MAO涂层的固有缺陷进入合金基体产生破坏。二是LDH的硝酸根层间阴离子能够与腐蚀环境中的氯离子进行离子交换,降低溶液中的氯离子浓度,从而延长涂层的耐蚀保护性。

Abstract:

A high corrosion resistant and active protection composite coating designed combining Layered Double Hydroxides (LDH) layer and Micro-arc Oxidation (MAO) to further improve the anti-corrosion of Mg alloy. In this paper, MgCr-LDHs/MAO composite coating prepared through in-situ growing LDH layer including NO_^3- on the surface of MAO coating. The morphology、composition and microstructure of MgCr-LDHs/MAO composite coating were characterized by XRD, XPS, FT-IR, SEM, besides the interaction of LDH and original MAO coating was investigated. The corrosion properties of MgCr-LDHs/MAO coating was tested through immersion in 3.5wt%NaCl solution and electrochemical experiments. The results show that: (1) the LDHs forms a dense sheet like layer on the original MAO coating surface, which can effectively cover the inherent defects of MAO coating, and LDH layer is preferentially formed in the pores of original MAO coating. (2) LDHs don"t damage the original MAO coating during the growth process, besides the adhesion of LDH layer and MAO layer are strong and stable. (3) The composite coating showed the most superior corrosion protection, since MgCr-LDH layer growing on the original MAO coating can hinder the penetration of chloride ions and effectively adsorb a large number of NO_^3? to exchange Cl_^- in corrosion solution.