Enhanced corrosion protection of magnesium oxide coatings on magnesium deposited by ion beam-assisted evaporation

In contrast to other light materials such as aluminium or titanium, the potential of magnesium to form stable, corrosion-resistant films is limited to alkaline solutions. In neutral, acidic or saline solutions the protective hydroxide or carbonate film breaks through immediately and heavy corrosion takes place. The aim of this work was to deposit protective magnesium oxide layers, by ion beam-assisted deposition (IBAD), which do not grow under natural circumstances. These coatings are different from coatings obtained by anodic oxidation, because of their partial crystallinity, good adhesion and very smooth, non-porous character. The MgO coatings were prepared with IBAD on Mg (hp), AZ91 magnesium alloy and AlMgSi0.5 substrates by the evaporation of high-purity MgO under bombardment with Ar+ ions with energies up to 15 keV. The Ar+ to MgO arrival ratio was chosen in order to obtain dense coatings. The coating thickness was 1 mu m. The electrochemical and corrosion behaviour were determined by potentiodynamic controlled current-potential measurements under pitting corrosion conditions and a standard salt-spray test. The crystallinity was determined by X-ray diffraction measurements and the surface quality by atomic force microscopy. The results prove that by IBAD, magnesium oxide coatings with or without the addition of other elements (Sn, Nd) of high hardness and low porosity and with good corrosion protection properties can be formed. The degree of crystallinity and the texture have a strong influence on the quality of the coatings acid depend on the ion energy. (C) 1998 Elsevier Science S.A.