A better understanding of PEO on Mg alloys by using a simple galvanostatic electrical regime in a KOH-KF-Na3PO4 electrolyte

AZ91 Mg alloys were anodized by PEO under galvanostatic regime in order to evaluate the role of the different constituents of a KOH + KF + Na3PO4 containing electrolytic bath. The morphology of the anodized coatings was observed by scanning electron microscopy and the composition was evaluated by energy dispersive X-ray spectroscopy. The corrosion resistance of the grown films was studied by electrochemical methods such as impedance spectroscopy, voltammetry and chronoamperometry. It was shown that 10 min anodizing is sufficient to grow a protective layer by PEO under galvanostatic conditions (10 mA cm(-2)), with a relatively low voltage (50 to 70 V). Fluoride anions act as "sparking initiators" in KOH electrolyte. So a threshold content of fluoride anions (0.2 M) is necessary to form an initial insulating layer and therefore reach the sparking phenomenon. Then, the addition of phosphate anions to a KOH/fluorides electrolyte allows the "sealing" or "the densification" of the coating, and promotes the growth of thicker films, without increasing the corrosion resistance of the coating. (C) 2011 Elsevier B.V. All rights reserved.