Anodizing of MgZn2 Phase in Sulfuric Acid: Mechanism of O2- Diffusion in Mixed (Mg,Zn)O at High Voltage

Abstract

The η-MgZn2 phase is an important intermetallic phase present in some galvanized coatings on steel, but also is present as strengthening phase in many aluminum alloys belonging to the 7XXX series. The objective of this work is to characterize the electrochemical behavior of MgZn2 phase in the classical electrolyte of aluminum anodizing, H2SO4 2 M. Firstly, bulk phase was synthesized by melting pure zinc and magnesium at 700 °C during 1 hour in a furnace under argon flux and then the sample was annealed at 400 °C during 12 hours. As expected by the E-pH diagram of zinc and magnesium, the MgZn2 phase undergoes a rapid dissolution in sulfuric acid medium at open-circuit potential. The surface analyses show a classical morphology of metallic dissolution.At high voltage (between 3V to 30 V), the dissolution of MgZn2 phase is greatly reduced and the electrochemical measurements reveal the presence of a passivation plateau. Even though the passive current density remains relatively high (around 20 mA cm-2 at 10 V), the formation of a thin layer of oxide on surface was observed by microscopic analysis on surface and metallographic cross-sections (FEG-SEM). X-ray diffraction at grazing incidence and X-ray photoelectron spectroscopy allowed us to determine the overall composition of the oxide, MgxZn1-xO. The growth kinetic of oxide was characterized versus the voltage and the anodizing time by in-situ electrochemical impedance spectroscopy measurements at high voltage. At low voltage (< 5 V), the growth of the anodized layer is governed the diffusion of O2- in the solid state under the electric field. Nevertheless, the growth rate is low because the apparent diffusion coefficient, determined by EIS, is low in this voltage range and the oxide dissolution is important in the electrolyte.At high voltage (> 10 V), the in-situ EIS measurements show a blocking behavior of the electrode. The growth rate is managed by the metal/oxide interface and the growth rate of oxide layer remains low in acid sulfuric medium. In-situ electrochemical impedance constitutes an important tool to determine the conditions of anodizing and the useful anodizing range of most of metallic materials.