Applied Surface Science, Vol.490, 535-545, 2019
Electrochemical, TOF-SIMS and XPS studies on the corrosion behavior of Q-phase in NaCl solutions as a function of pH
The corrosion behavior of a bulk-synthesized Q-phase (Al4Cu2Mg8Si7) in NaCl solutions at different pH has been investigated using anodic and cathodic potentiodynamic polarization, time-of-flight secondary ion mass spectroscopy (ToF-SIMS) and X-ray photoelectron spectroscopy (XPS). Potentiodynamic polarization reveals that the corrosion of Q-phase is pH dependent and Q-phase supports rapid anodic and cathodic reaction kinetics in an acidic and alkaline environment. It is seen that the electrochemical processes on the Q-phase changes from activation controlled in the acidic region to diffusion controlled in the neutral region, then to a mixed activation/ diffusion controlled process in the alkaline region. The trend of the corrosion rate is pH 1 > pH 13 > pH 12 > pH 2 > pH 10 > pH 4 > pH 6. On the other hand, the corrosion potential is seen to shift to more negative values with increase in the pH of the solution. ToF-SIMS and XPS analyses of the surface corrosion film revealed that selective dissolution of more active elements such as Mg, Al and Si occur during the corrosion of the Q-phase leading to Cu rich remnants discretely dispersed in the neutral region and more uniformly dispersed in the alkaline region. In the acidic region, the corrosion film formed on Q-phase is composed of mainly of oxides while in the neutral/alkaline region the corrosion film is composed mainly of oxides and hydroxides although the corrosion film formed in the alkaline region is thicker and richer in hydroxides compared to that formed in the neutral region.