The superior corrosion resistance of hot dip galvanized steel (HDG) alloyed with addition of Al and Mg compared to conventional zinc coating has been shown in numerous atmospheric conditions including accelerated tests and natural exposures. Despite significant work in understanding the mechanisms of corrosion of Zn-Al-Mg coatings, there are still gaps of knowledge such as, on the exact interactions between the complex microstructure of theses coatings and their corrosion properties. A large European (RFCS) project namely MicroCorr has been initiated in 2015 with the aims of better understanding these aspects. On task was to examine the effect of surface pH and contaminations as well as relative humidity and temperature on the corrosion performance of well-defined microstructures of alloyed zinc coatings with Al and Mg.

The present study focuses on the effect of carbon dioxide depletion on the atmospheric corrosion stability of Zn alloyed coatings compared to conventional HDG (Zn-0.2Al) and Galfan (Zn-5Al). Zn-Al-Mg and Zn-55Al coatings produced with various microstructures differing in the grain size were investigated in low and ambient concentration of CO₂ with two levels of chloride concentration. In addition to mass loss data, the surface pH was measured using wide range pH indicator in agar-agar gel. The corrosion products were analyzed by FTIR spectroscopy and X-Ray diffraction.

The corrosion of HDG and zinc alloyed coatings with low content of Al and Mg was significantly enhanced in low CO₂ conditions particularly at high chloride concentration and whatever the microstructure. This was connected to a high surface pH in the absence of CO₂. Surprisingly, similar weight losses were measured for Zn-55Al coatings whatever the level of CO₂ and no increase of surface pH was observed even in depleted CO₂ conditions. However, important localised corrosion was observed for this coating