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.