AZ91镁合金和MAO涂层的点蚀行为研究

doi:10.11902/1005.4537.2021.320

中国腐蚀与防护学报

2022, Vol. 42

Issue (6): 1034-1042 DOI: 10.11902/1005.4537.2021.320

研究报告

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AZ91镁合金和MAO涂层的点蚀行为研究

刘玉项(

), 徐安阳

陆军装甲兵学院装备再制造技术国防科技重点实验室北京 100072

Characterization of Pitting Corrosion Behavior of AZ91 Mg-alloy without and with MAO Coating

LIU Yuxiang(

), XU Anyang

National Key Laboratory for Remanufacturing of China, Academy of Army Armored Force, Beijing 100072, China

引用本文:

刘玉项, 徐安阳. AZ91镁合金和MAO涂层的点蚀行为研究[J]. 中国腐蚀与防护学报, 2022, 42(6): 1034-1042.
Yuxiang LIU, Anyang XU. Characterization of Pitting Corrosion Behavior of AZ91 Mg-alloy without and with MAO Coating[J]. Journal of Chinese Society for Corrosion and protection, 2022, 42(6): 1034-1042.

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摘要:

采用循环极化曲线研究了AZ91镁合金及其表面微弧氧化 (MAO) 涂层在3.5%NaCl溶液中的点蚀行为。采用光学显微镜和扫描电镜观察循环极化不同阶段的点蚀形貌,探讨了点蚀在AZ91镁合金和MAO涂层上的萌生和扩展机制。结果表明,合金上点蚀倾向于在α-Mg相上萌生,而涂层上点蚀在多孔结构和裂纹处萌生,合金和涂层上点蚀的初始形态均为开口的“火山口”形貌。合金上点蚀坑中沉积一层腐蚀产物层对点蚀产生一定钝化效果,导致了点蚀在合金上横向扩展。而涂层上点蚀造成涂层的剥离和腐蚀产物的溶解,无法对点蚀形成钝化效果,导致点蚀在涂层上向纵深扩展。点蚀在循环极化过程中持续扩展的微观形貌验证了合金和涂层的循环极化曲线上出现正的滞后环,而不同的点蚀扩展现象也验证了涂层上较大的滞后环面积。

关键词 ： AZ91镁合金, MAO涂层, 循环极化曲线, 腐蚀形貌和产物, 点蚀萌生及扩展

Abstract：

The use of micro-arc oxidation (MAO) coating is a promising approach for controlling corrosion for Mg alloys. However, MAO coating and the underneath Mg-alloy substrate are constantly threatened by pitting corrosion. In order to reveal the relevant mechanism, the pitting corrosion behavior of AZ91 Mg alloy without and with MAO coating was studied via cyclic potentiodynamic polarization (CPDP) test. Meanwhile, the pitting corrosion morphology and corrosion products at different CPDP stages were characterized by means of optical microscope (OM), scanning electron microscope (SEM) and energy dispersive X-ray (EDS) spectroscope. Results show that pits initiate on α-Mg phase rather than on β phase for AZ91 Mg alloy, however, they initiate around pores and cracks on MAO-coating. The initial pits show a volcano-like morphology for both the alloy and the coating. Moreover, inside the pits on the alloy there is a corrosion products film, which act as protective barrier for the substrate, as a result, pits on the alloy widened with time. By comparison, corrosion products of poor protectiveness exfoliate and dissolve inside the pits on the MAO coating, thus, where pits grow deeper with time. The morphology evolution of pits validates the positive hysteresis loops that exist on CPDP curves for both the alloy and coating, and can also explain the larger area of the positive hysteresis loop on the MAO coating. Overall, the MAO-coated alloy might fail without proper post-treatments due to the severe pitting corrosion, therefore, it is necessary to conduct pore sealing treatment for MAO coating to provide a promising corrosion protectiveness for Mg alloys.

Key words： AZ91 Mg-alloy MAO coating cyclic potentiodynamic polarization corrosion morphology and products pits initiation and propagation

收稿日期: 2021-11-09

ZTFLH:

TG174.41

基金资助:装备再制造技术国防科技重点实验室基金(614005180101)

作者简介: 刘玉项,男,1988年生,博士,助理研究员

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https://www.jcscp.org/CN/10.11902/1005.4537.2021.320 或 https://www.jcscp.org/CN/Y2022/V42/I6/1034

图1 MAO涂层表面/截面的SEM微观形貌和EDS面扫描元素分布

图2 AZ91镁合金和MAO涂层XRD谱

图3 AZ91镁合金和MAO涂层在3.5%NaCl溶液中的开路电位曲线

图4 AZ91镁合金和MAO涂层在3.5%NaCl溶液中开路电位测试之后的循环极化曲线

表1 AZ91镁合金和MAO涂层循环极化曲线上A,B和C点的电位和电流密度

图5 AZ91镁合金和MAO涂层的循环极化曲线上A,B和C三点对应的腐蚀形貌及实时氢气释放情况

图6 AZ91镁合金和MAO涂层的循环极化曲线上A,B和C三点的表面腐蚀形貌

图7 AZ91镁合金和MAO涂层的循环极化曲线上A,B和C三点的截面腐蚀形貌

图8 AZ91镁合金和MAO涂层的循环极化曲线上B1和B2点的SEM表面腐蚀形貌和EDS面扫描

图9 AZ91镁合金和MAO涂层的循环极化曲线上B1和B2点XRD谱

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