Corrosion mechanism of laser-melted AA 2014 and AA 2024 alloys

The corrosion performance of laser-melted AA 2014-T6 and AA 2024-T351 alloys, using a 2 kW CW CO2 laser, has been examined to gain insight into the factors influencing pitting corrosion resistance. Examination of laser-melted surfaces in terms of microstructure and phase analysis was performed using scanning electron microscopy (SEM), with associated elemental analysis by energy dispersive X-ray (EDX) spectroscopy, electron probe microanalysis (EPMA) and X-ray diffraction (XRD). Pitting corrosion resistance was evaluated using potentiodynamic anodic polarisation in 1 M NaCl solution. The work revealed that there was an improvement of pitting corrosion resistance for the laser-melted AA 2014-T6 alloy, but no improvement for AA 2024-T351 alloy. It indicated that the refinement of the microstructure, per se, with finer intermetallic particles, did not play an important role in corrosion performance. More importantly, the extension of copper solubility in the a-Al matrix, leading to an increased corrosion potential, was considered to be the key factor responsible for the corrosion behaviours of the laser-melted aluminium alloys. For the AA 2014-T6 alloy, due to the cathodic nature of the Al2Cu phase relative to the a-Al solution, the rise of corrosion potential of the a-Al solution reduced the galvanic coupling between the Al2Cu and a-Al matrix. As a result, the driving force for pitting corrosion in the a-Al solution was reduced. For the AA 2024-T351 alloy, due to the anodic nature of the Al2CuMg phase relative to the a-Al solution, the driving force for pit initiation at the Al2CuMg phase was enhanced. Therefore, the laser melting promoted the pitting corrosion of the AA 2024-T351 alloy.