Environmental influence on the formation of α-FeOOH on the surface of weathering steel

Abstract

A CEMS and XMS study of the surface oxidation of weathering steel, ASTM A242 Type 1, has shown that after exposure times of 5 1/2 years in a marine or industrial environment, the steel has not yet reached a stage at which the surface is fully protecting the substrate from further corrosion. The formation of α-FeOOH, an intermediate stage of protection, occurs mainly in the form of small particles, the properties of which exhibit an appreciable amount of superparamagnetic relaxation. For the steel exposed to a more moist marine environment, α-FeOOH exists only in the form of small particles. However for the steel exposed to a much dryer and corrosive industrial environment, more α-FeOOH is present. The formation of bulk α-FeOOH is more complete. The temperature dependence of the superparamagnetic hyperfine field and also the field distributions indicate that, in a marine environment, the clusters of α-FeOOH are both smaller in size and have a larger distribution of particle size. The XMS results indicate that α-FeOOH is present below a layer of γ-FeOOH and ferrihydrite which are generally accepted to constitute the early oxidation phases observed on steels.