Atmospheric corrosion behaviour of zinc and zinc alloys: comparison between natural and accelerated exposure

Atmospheric corrosion of galvanized steel is a well-known subject: many experimental results, international research programs, as ISOCORRAG and MICAT, and ISO standards are available. In some applications, as in the case of gabion walls, Zn-Al alloys coatings have been proposed to protect steel wires. These coatings, as a function of thickness and severity of the environment, should guarantee a service life in the range 10 to 120 years. Aim of this paper is to compare the corrosion rate of pure zinc and Zn-Al alloys, in accelerated exposure tests and in natural atmosphere for medium-long time exposure. Obtained data were matched with the prediction suggested by the standards. The results showed that the corrosion rate of zinc alloys is lower than zinc, in both natural and accelerated exposure. The lack of medium-long term data in natural exposure makes difficult, if not impossible, to forecast the real behavior of zinc alloys, especially in very severe exposure conditions (as condition C3 according to ISO 9223).