Plasma electrolytic oxidation (PEO) is a powerful technique allowing hardening and corrosion protection of valve metals due to formation of an oxide layer on the metal surface. PEO produces much thicker oxide layers as compared to anodizing, which is of critical importance for many technological applications. The present research investigated the influence of the fluoride ion concentration on the composition, structure and morphology of PEO layers on the magnesium alloy AZ91D. The obtained oxide layers were characterized with XRD, SEM, EDS and tested for corrosion resistance by linear sweep voltammetry in 3.5% NaC1 medium. During this investigation it was found that KF addition produces significant changes in the structure and properties of the oxide layers. Fluorine was detected as an amorphous phase in the vicinity of the base metal for both alloys and plausible mechanism was suggested to explain these phenomena. Fluoride ions have pronounced catalytic activity and their presence considerably increases the thickness of the oxide layer. Depending on the process parameters, significant improvement of the corrosion stability of AZ91D alloy is achieved by the use of PEO. C) 2013 Elsevier B.V. All rights reserved.