X-ray photoelectron spectroscopy (XPS) was used in order to investigate the correlation between the surface chemistry and the atmospheric corrosion of AZ31, AZ80 and AZ91D magnesium alloys exposed to 98% relative humidity at 50ºC. Commercially pure magnesium, used as the reference material, revealed MgO, Mg(OH) 2 and tracers of magnesium carbonate in the air-formed film. For the AZ80 and AZ91D alloys, the amount of magnesium carbonate formed on the surface reached similar values to those of MgO and Mg(OH) 2 . A linear relation between the amount of magnesium carbonate formed on the surface and the subsequent corrosion behaviour in the humid environment was found. The AZ80 alloy revealed the highest amount of magnesium carbonate in the air-formed film and the highest atmospheric corrosion resistance, even higher than the AZ91D alloy, indicating that aluminium distribution in the alloy microstructure influenced the amount of magnesium carbonate formed.
KEYWORDS:Magnesium/aluminium alloys, XPS, corrosion, humidity the breakdown of the passive film [13]. On the other hand, corrosion mechanism of magnesium alloys in many other environmental conditions remains ill-defined. For instance, published investigations regarding corrosion of magnesium alloys in non-polluted atmospheres are rather scarce [14,15].Magnesium reveals a good resistance to oxidation at room temperature in dry environments [16][17][18]. However, corrosion susceptibility increases with the relative humidity and the temperature.According to Splinter's theory the detrimental effect of relative humidity is associated with the dissociation of water vapour, which participates in the surface oxidation process, thus, the surface film thickens quicker at higher exposure to water vapour [19]. With regard to the influence of temperature, it has been suggested that, at low temperatures, the oxide film grows according to a Cabrera-Mott mechanism, where the thickness of the film is proportional to the temperature [20].The presence of NaCl and/or CO 2 in the atmosphere affects the mechanism of corrosion of magnesium alloys in humid environments. High corrosion susceptibility was found in the presence of NaCl and absence of CO 2 , with localized corrosion and formation of magnesium hydroxide as the main corrosion product [21]. On the other hand, in the presence of 350 ppm of CO 2 and NaCl concentrations in the range of 0-70 g/cm 2 , several studies reported corrosion rates between 3 and 4 times lower compared with CO 2 -free atmospheres [22,23]. In the presence of CO 2 , corrosion mechanism appears in form of general corrosion with development of gray oxide film over the surface [24]. The increased corrosion resistance has been attributed to the formation of magnesium carbonates on the surface which may block the corrosion process [22].
