EFFECT OF CATHODIC CURRENT DENSITY ON THE CORROSION PROTECTION OF THE OXIDE COATINGS FORMED ON AZ91D. MAGNESIUM ALLOY BY PLASMA ELECTROLYTIC OXIDATION.

摘要

A promising approach to produce high-quality oxide coatings with good corrosion and wear resistance on magnesium alloys is to use the plasma electrolytic oxidation (PEO) process. The corrosion resistance of PEO coatings, in any particular environment, is dependent on material parameters such as substrate metallurgy and composition of the electrolyte and the PEO process conditions such as current density, current mode and processing time. In this work PEO coatings have been produced on an AZ91D magnesium alloy using a bipolar current mode and varying current density. The morphology and microstructure of the coatings were investigated using Scanning Electron Microscopy (SEM) and the corrosion resistance was evaluated using both potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) tests in an aqueous 3.5% NaCl solution as a function of immersion time. It is concluded that the corrosion protection performance of the PEO coatings is indirectly dependent on the current density and is controlled by the amount of porosity and other coating defects which could slow down the penetration rate of the electrolyte through the PEO coating.