The Application of X-Ray Photoelectron Spectroscopy in Understanding Corrosion Mechanisms of Magnesium and Mg-Al Alloys

ABSTRACT

The authors present a review of their research work on the application of X-ray photoelectron spectroscopy (XPS) to characterise thin (~ 3 nm) natural oxide films that spontaneously form on the surface of magnesium and its alloys in contact with the oxygen in air, and thick (> 1 im) corrosion films rich in carbonates and other species that form during exposure to a humid atmosphere. The results obtained can provide much useful information about the protection mechanisms of these materials. Emphasis is placed on the relationship between the alloy's Al content and the characteristics of the natural oxide film and the relationship between segregation of calcium impurities from the bulk to the outer surface of the metal and repercussions on electrochemical behaviour. The natural oxide film may alone be sufficiently protective in less aggressive environments, making it of interest to know its characteristics. A detailed study will also be made of the changes in chemical composition that may occur in this passivating film with increasing exposure time to a humid atmosphere, deteriorating or reinforcing its anticorrosive properties. In particular, an attempt will be made to research the effect on corrosion behaviour of: (a) the time the different specimens are exposed to the aggressive agent (humid atmosphere); (b) the possible formation during corrosive attack of layers that are rich in carbonates and other species; and (c) the presence of aluminium (alloying element) on the protective properties of the corrosion layers.