Quantification of electrochemical and chemical reactions during the anodic passivation of EN AW-6082 in alkaline silicate solutions

To refine the simulation of anodic passivation of Al alloys in alkaline silicate solutions, it is necessary to understand the chemical and electrochemical reactions and to quantify the charge contributions of electrochemical reactions as a function of silicate content and pH. Therefore, polarization experiments were conducted in three alkaline silicate solutions with logging the current trend and video recording of the anodic oxygen evolution. Electrochemical impedance spectroscopy (EIS) was integrated into the measurement cycle to characterize the surface before and after polarization. In addition, the formed gas was collected above the measurement area during the measurement cycle. Electrolyte samples were taken to determine the Al-ion concentration using mass spectroscopy (ICP-MS). After polarization, the surfaces were analyzed using stereomicroscopy, ellipsometry, infrared spectroscopy (ATR-IR), and scanning electron microscopy (SEM). The current efficiency of barrier-layer formation was not larger than 6.5 %. Oxygen evolution was the dominant electrochemical process for all states. This is facilitated by the formation of microstructural defects within the barrier layer due to the anodic oxidation of Fe-rich dispersoids. As a result of oxygen formation and the decrease in pH, SiO2 nanoparticles are precip-itated on the oxide surface. At high pH and low silicate content, pore channels form in the barrier layer due to chemical dissolution of the oxide at gaps in the precipitated layer. In particular when simulating the oxide formation under these conditions, the high electrochemical activity at the onset of polariza-tion and the continuous regeneration of the barrier layer due to dissolution must be considered.