目的 在恶劣环境中使用的铝合金部件通常需要涂层防护,但涂层失效导致铝合金发生腐蚀的初期阶段无法通过外观进行检测确认,因此利用阵列差分涡流检测对铝合金涂层下的腐蚀进行无损成像检测。方法 以涂覆有丙烯酸涂层的2A12-T4铝合金作为研究对象,通过在盐酸溶液中进行不同时间的加速腐蚀实验,获得不同腐蚀程度的损伤样品,随后进行阵列涡流检测。检测后使用脱漆剂除漆,对腐蚀缺陷微观形貌、化学成分和三维尺寸进行表征。获得了腐蚀缺陷尺寸与差分涡流信号间的关联规律。使用有限元分析手段,研究了腐蚀缺陷尺寸对差分涡流信号的影响机制。结果 研究结果发现,涂层下的铝合金腐蚀损伤呈现浅碟型几何特征,在腐蚀初期主要在深度方向生长,而在腐蚀后期向径向和深度2个方向生长。涂层下腐蚀缺陷尺寸越大,检测信号越强。发生腐蚀损伤区域的阵列涡流差分信号会呈现出“凹-凸”的信号特征,并且缺陷实际位置处于检测信号的“峰”和“谷”之间。结论 差分阵列涡流检测可以直观地确定涂层下腐蚀缺陷的存在及腐蚀缺陷的尺寸,实现腐蚀缺陷的涡流成像检测。
Abstract
Aluminium alloys are widely used in aerospace, automotive, machinery, electronic information and other fields because of their low density, high strength, corrosion resistance, easy processing and other performance characteristics. Although the aluminium alloy has a certain corrosion resistance, but in some harsh environments (such as the sea), it is necessary to coat the aluminium alloy to protect its surface, ensuring the safe use of aluminium alloy components. However, the aging of the coating will result in a gradual decrease in the protective ability, and before the coating completely fails, corrosion has already occurred under the intact appearance of the coating, which cannot be detected by naked eyes, posing a serious potential risk. Traditionally, the maintenance method of periodic coating removal is generally used to detect corrosion damage under the coating, but there are disadvantages of high cost and low efficiency, and there is the potential danger of untimely repair. Therefore, there is an increasing demand for non-destructive testing of corrosion damage under the coating, especially for non-destructive visual inspection that can be performed quickly and efficiently.
The work aims to investigate the potential use of an array differential eddy current detection for non-destructive imaging detection of corrosion under aluminium alloy coatings. 2A12-T4 aluminium alloy widely used in aerospace industry, coated with acrylic coating was used as the research object, and damage samples with different levels of corrosion were obtained by accelerated corrosion experiments in hydrochloric acid solution for different time, followed by the array eddy current detection. After the detection, the coatings were removed with a paint stripper, and the corrosion defects under the coatings were characterized in terms of microscopic morphology, chemical composition and three-dimensional dimensions by scanning electron microscope, energy dispersive X-ray detector and white-light interferometer, respectively. The correlation law between corrosion defect dimensions and differential eddy current signals was obtained. The effect mechanism of the corrosion defect size on the differential eddy current signal was also investigated through finite element analysis.
It was found that the corrosion damage of the aluminium alloy under the coating presented shallow disc-type geometrical characteristics, which grew mainly in the depth direction at the early stage of corrosion, and grew in both radial and depth directions at the late stage of corrosion. The larger the size of the corrosion defect under the coating, the stronger the detection signal. This is because there is no eddy current distribution at the corrosion defect due to the corrosion of the aluminium alloy that is conductive, so the coil located above the defect has a small eddy current density, leading to smaller impedance value, while if the defect is larger, the larger the difference in eddy current density between the coils, the stronger the differential signal. In the scanning direction, the defect is almost in the same straight line of the coil compared to other coils, which can reach the highest degree of coincidence with the defect. At this time, due to its differential signal coil and the defect of the lowest degree of coincidence, the detection signal obtained is the strongest. The coils on both sides due to the gradual decrease in the degree of coincidence with the corrosion defects result in a gradual decline in the intensity of the signal. Corrosion damage occurs in the region of the array eddy current differential signal, presenting a 'concave-convex' signal characteristics, and the actual location of the defect is in the detection signal between the 'peak' and 'valley'. The existence of corrosion defects under the coating can be determined intuitively, and the eddy current imaging detection of corrosion defects can be realized.
关键词
铝合金 /
腐蚀 /
涂层 /
涡流 /
无损检测 /
有限元分析
Key words
aluminum alloy /
corrosion /
coating /
eddy current /
non-destructive testing /
finite element analysis
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基金
成都市区域科技创新合作项目（2023-YF11-00021-HZ）
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