高强铝合金裂纹尖端在3.5%NaCl溶液中的微区电化学特性

采用毛细管微电极测试方法、扫描Kelvin探针技术和数值分析方法,对2024 T351高强铝合金裂纹尖端在3.5%NaCl溶液中的微区电化学特性和腐蚀行为进行了研究.结果表明,裂纹尖端的腐蚀电位较远离裂纹尖端的基体位置更负,裂纹尖端处的电化学活性明显增加.在外加应力的作用下,裂纹尖端处表面氧化膜的厚度减薄,其稳定性和保护性变弱.裂纹尖端处优先发生阳极溶解,浸泡24 h后在裂纹尖端处出现腐蚀产物的堆积.由于腐蚀电位和电化学活性的差异,在裂纹尖端(阳极)和远离裂纹尖端的基体(阴极)之间可形成电偶对,进一步促进裂纹尖端局部区域内腐蚀过程的进行.

LOCALIZED ELECTROCHEMICAL CHARACTERIZATION OF HIGH STRENGTH ALUMINIUM ALLOY AT THE CRACK TIP IN 3.5%NaCl SOLUTION

The electrochemical microcapillary technique and scanning Kelvin probe(SKP) were applied to study the localized electrochemical characterization and corrosion behavior at the crack tip of 2024-T351 aluminium alloy in 3.5%NaCl.To investigate the effect of stress on the localized corrosion process at the crack tip,numerical simulation of the stress distribution on a pre-cracked wedge-open loading(WOL) specimen was conducted using a commercial software package ABAQUS 6.10.Polarization curves revealed that corrosion potential at the crack tip was more negative than that within the region away from the crack tip.Localized electrochemical impedance spectroscope(EIS) showed that the passive film was thinner at the crack tip than within the region away from the crack tip.The results indicate that the crack tip is more electrochemically active than the region away from the crack tip.Furthermore,passive film at the crack tip was less stable than that on other region of the specimen surface.SKP measurements demonstrated that there was a non-uniform distribution of Volta potential on the pre-cracked WOL specimen surface,with a more positive Volta potential occurring at the crack tip after 24 h immersion into 3.5%NaCl solution.This might be explained by the preferential anodic dissolution and the accumulation of the corrosion product at the crack tip. Numerical simulation results showed that a very high local stress concentration was developed at the crack tip,which could enhance the electrochemical activity around the crack tip and promote the dissolution process therein.Moreover,a galvanic couple could also occur between the crack tip acting as the anode and the distant region as the cathode,which results from the differences of the corrosion potentials and the electrochemical activities between them.As a result,the anodic dissolution of 2024-T351 aluminium alloy at the crack tip is enhanced.