Measurements of Localized Ion Distribution and Nanoscopic AFM Observation of Crack Initiation and Propagation of Stress Corrosion Cracking

为了对包括点蚀和应力腐蚀在内的腐蚀损伤的萌生阶段进行现场观察，采用计时电流法对局部腐蚀损伤进行了研究，其中直径为20μm的微米级铂微电极采用方波周期极化：利用电流，分别在阳极和阴极极化条件下测量局部氯离子和氢离子的浓度或pH值。用这种计时电流法测量了点蚀附近的氯离子和氢离子的分布，在点蚀的底部有时会产生应力腐蚀裂纹：成功地测量了腐蚀坑内外离子的分布，并清楚地显示了点蚀周围离子的局部化。利用原子力显微镜(AFM)原位观察了高强度钢应力腐蚀裂纹在持续载荷作用下的扩展过程，探讨了应力腐蚀开裂的纳米机理。为了阐明环境退化行为，还研究了高强度粉末冶金Al-Zn-Mg合金和Ti-6Al-4V合金在动态载荷下的裂纹萌生和扩展行为。重点研究了复合载荷对碳化硅晶须增强铝合金裂纹萌生和断裂行为的影响。

In order to perform in situ observation of an initiation stage of corrosion damage including pitting corrosion and stress corrosion cracking, a so called chronoamperometric method was applied to localized corrosion damage, where a micron size Pt microelectrode (Diameter : 20 μm) is cyclically polarized by using a square waveform : local concentration of chloride ion and hydrogen ion, or pH values, can be respectively measured by using a current by an anodically and a cathodically polarized condition. By using this chronoamperometric method, distributions of chloride and hydrogen ion near a pitting corrosion was measured, at the bottom of which a stress corrosion crack sometimes initiates : the distributions of ions in and out of a corrosion pit was successfully measured, and the localization of ions around a pitting corrosion could be clearly visualized. In situ nanoscopic atomic force microscope (AFM) observation was also conducted to closely examine the growing process of stress corrosion cracking of a high-strength steel under a sustained loading : nanoscopic mechanism of stress corrosion cracking was discussed. Crack initiation and growth behavior was also investigated in a high-strength P/M Al-Zn-Mg alloy and a Ti-6Al-4V alloy under dynamic loading in order to clarify the environmental degradation behavior. Special attention was paid to the influence of combined loading on crack initiation and fracture behavior in a SiC whisker reinforced Al alloy.