CORROSION BEHAVIOUR OF MRI 230D AND AZ91D MAGNESIUM ALLOYS WITH PLASMA ELECTROLYTIC OXIDATION TREATMENT

The applicability of Plasma Electrolytic Oxidation treatment for the corrosion protection of high-pressure die-cast AZ91D and high-pressure die-cast creep resistant MRI 230D magnesium alloys in 3.0 wt% NaCl aqueous solution and corrosion behaviour of these alloys was studied in this work by several electrochemical and non-electrochemical techniques. Plasma Electrolytic Oxidation (PEO) of an MRI 230D and AZ91D alloys was accomplished in a silicate-base electrolyte with KF addition using an AC power source. The corrosion process of both alloys before and after PEO treatment was evaluated by open circuit potential (OCP) measurements, electrochemical impedance spectroscopy (EIS), linear polarization tests, linear sweep voltammetry (Tafel extrapolation) and chemical methods: mass loss and hydrogen evolution. Corrosion rates for each method were calculated and compared and the morphology of corroded surface studied. According to the tests results, the corrosion of both alloys is localized so that corrosion pits are developed on a specimen surfaces. Corrosion rates measured by different methods are all on the order of magnitude of 0.25 – 3.7 mm / year. The protection of both alloys by the PEO decreases the rate of corrosion and shifts the corrosion potential to a more noble value. However, the effect that PEO process has on corrosion resistance of MRI 230D magnesium alloy is not always unambiguous. An attempt to explain the influence of the PEO process on the corrosion behaviour of the MRI 230D alloy is presented. The localized character of corrosion causes some deviations in the results obtained even by the same method and deviations of corrosion rates are observed for all the methods. SEM and light microscope observation of corroded specimens demonstrated the localized character of corrosion as well, at least at the initial stages.