Effects of equal-channel angular pressing (ECAP) on the pitting corrosion resistance of Al-Cu (AA2024) alloy with and without anodization were investigated using surface analysis and polarization curves in solutions containing 300 ppm or 0.6 mol·dm−3 of Cl−. The pitting corrosion potential and corrosion rate of Al-Cu alloy revealed no effect of ECAP on the pitting corrosion resistance of Al-Cu. The Al-Cu alloy contained such precipitates as Al2Cu, Al2CuMn and Al-Cu-Si-Fe-Mn intermetallic compounds, around which pitting corrosion occurred. Actually, ECAP pulverized these precipitates to decrease their size, but it had no effect on the pitting corrosion resistance of Al-Cu alloy because pitting corrosion occurred even around small precipitates of Al2Cu. In contrast, the time to initiate pitting corrosion of Al-Cu alloy while maintaining a constant potential of 1.2 V was longer with ECAP than without, indicating that ECAP improved the pitting corrosion resistance. Although the precipitates of Al2Cu and Al2CuMn disappeared because of oxidation during anodization, Al-Cu-Si-Fe-Mn intermetallic compounds were present and formed defects in anodic oxide films. Pitting corrosion of the anodized Al-Cu alloy occurred around the Al-Cu-Si-Fe-Mn precipitates. The improved pitting corrosion resistance of anodized Al-Cu alloy by ECAP appears to be attributable to the decreased size of the precipitates that initiate pitting corrosion.
Using a rotating disc electrode, electropolishing behavior of titanium (Ti) was studied in mixed ethanol (EtOH) -ethylene glycol (EG) solvents dissolving metal chlorides. Effects of the electrolyte composition on the anodic behavior of the rotating titanium were discussed in terms of the balance in ionic conductivity and solution viscosity. A mirror surface of Ti was obtained by anodic polarization in a mixed alcohol system of EtOH and propan-2-ol (i-PrOH), which dissolves ZnCl2 and AlCl3. However, substitution of i-PrOH by EG with lower vapor pressure did not proceed to anodic oxidation of Ti when ZnCl2 was used as the co-electrolyte of AlCl3 in the mixed solvent system. The use of LiCl with AlCl3 with a proper mole ratio produced a mirror surface of Ti in the mixed solvent system of EtOH and EG. The polarization behavior of the system was almost identical to that in the mixed alcohol system of EtOH and i-PrOH dissolving ZnCl2 with AlCl3. The best electropolishing result was obtained for a solution containing 0.225 M AlCl3 and 1.8 M LiCl (M = mol dm−3). Results of conductivity and viscosity measurements for the electrolyte solution suggest that anodic dissolution of Ti coupled with the formation of a viscous surface layer would be balanced in EtOH+EG mixed solvent systems containing LiCl+AlCl3 with moderate values of Walden's product.