Studies of Initial Atmospheric Corrosion of Magnesium Alloys AZ91 and AZ31 with Infrared Spectroscopy Techniques

The initial formation of corrosion products in pure humid air on magnesium alloys AZ91 and AZ31 was studied using infrared reflection absorption spectroscopy (IRRAS), infrared spectroscopic imaging, and SEM-EDS. The kinetics of corrosion product formation were monitored in situ with IRRAS during exposure to humid air (95% relative humidity) un-der two different CO₂ concentrations: low ( 1 ppm) and ambient (400 ppm). For low CO₂ concentrations, the primary corrosion product detected on both alloys was magnesium hydroxide (Mg(OH)₂). In contrast, under ambient CO₂ conditions (400 ppm), magnesium hydroxy carbonate was the dominant product. After 16 hours of exposure, the amount of magnesium converted into corrosion products was approximately 8-10 times higher un-der low CO₂ conditions compared to ambient levels. The smaller formation of corrosion products but increased magnesium carbonate formation on AZ91D is attributed to its higher aluminum content compared to AZ31. Corrosion attack and product formation were largely localized to the centre of the α-phase in AZ91D, with the β-phase likely serv-ing as sites for cathodic reactions.