THE PREPARATION AND CHARACTERIZATION OF A CERIUM-BASED RARE EARTH CONVERSION COATING ON MAGNESIUM ALLOY AZ91D AND ITS DEGRADATION IN 3.5% NaCl SOLUTION

For rare earth conversion coatings (RECCs) on the surface of magnesium alloy (MA), their electrochemical impedance spectroscopy (EIS) evolution and characteristics during the process of coating degradation in environmental media were involved relatively few. In this work, a cerium-based RECC (Ce-RECC) was prepared on the surface of MA AZ91D with chemical conversion technique; for Ce-RECC, its morphology was characterized with optical microscope (OM) and scanning electron microscope (SEM) observations, its composition was characterized with energy dispersion spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) detections, and its protection was characterized with potentiodynamic polarization and EIS tests. Ce-RECC exhibited a mud-crack morphology and was composed of Ce and O mainly, and the corrosion current density (ic) and the coating resistance (Rc) for Ce-RECC-coated MA AZ91D were smaller and larger than those for bare MA AZ91D, respectively. During the process of Ce-RECC degradation in 3.5% NaCl solution, the EIS of Ce-RECC-coated MA AZ91D always comprised a capacitive loop at high frequency region and an inductive loop at low frequency region, which was independent of time extension; however, the value of Rc for Ce-RECC decreased gradually with the extension of time, and the total degradation of Ce-RECC consumed 77h.