Weathering steels are low-alloy steels containing up to 3% of alloying elements such as copper, chromium, vanadium and phosphorus. Corrosion tests in the USA had shown that the corrosion rate of weathering steel was much lower than the rate for mild steel in the same environment. In fact the rate was considered low enough for the steel to be used unprotected in structures such as bridges. The expected loss in thickness over the life of a structure was considered negligible. Engineers were understandably attracted by a material which seemed to eliminate the need for both initial corrosion protection and subsequent maintenance.