Long-term atmospheric corrosion of Zn–5%Al-coated steel and HDG during outdoor worldwide exposures
Dominique Thierry, Dan Persson, Anne‐Laure Le Gac, N. LeBozec, Ari Peltola, Pasi Väisänen
Abstract
Hot-dipped galvanised (HDG) steel and Zn5Al-coated steel were exposed for 6 years at different atmospheric weathering sites worldwide. The mass loss of the coatings has been measured after 1, 2, 4 and 6 years of exposure. In addition, corrosion products have been determined using scanning electron microscopy, X-ray diffraction and FTIR spectroscopy. It is shown that Zn5Al is better than HDG-steel independently on the exposure duration and location. The mass loss of Zn5Al is also less dependent of the environmental parameters (such as chloride deposition, relative humidity and temperature) than HDG-steel. The ratio of performance after 6 years of exposure varied from 1.3 to 5.1 with the mean value of 3.1. The analyses of the corrosion products indicate the formation of protective Zn/Al -CO32− layered double hydroxide (LDH) and sulphate-containing corrosion products of Zn and Al after 6 years of exposure. A mechanism explaining the superior properties of Zn5Al is proposed.