Characterization of rust layers on a plain-carbon steel and weathering steels exposed to industrial and coastal atmosphere for years

Abstract

Four types of weathering steels and a plain-carbon steel were exposed to air for 17 years at an industrial coastal region, and at a rural region, in Japan. Two types of steels, a weathering steel and a plain-carbon steel were exposed for 18 years at coastal regions, in Japan. All specimens were placed under bridges with their faces horizontally set and were investigated by EPMA, XRD, SEM, TEM and ED. The following conclusions were drawn: (1) Rust consists essentially of three layers; the inner, outer and outermost layers irrespective of whether a plain-carbon steel or a weathering steel. The rust layer consists of α-FeOOH, β-FeOOH, γ-FeOOH, magnetite and amorphous rust. (2) The outermost layer is about 3 μm thick and enriched with atmospheric deposits. In the outer layer, Cl and S are enriched. Alloying elements, Cu, Cr and Si are concentrated in the inner layer while P and Ni are mostly distributed uniformly over the whole rust layer. (3) Regardless of steel, the major component on the outermost rust layer is α-FeOOH and minors are β-FeOOH and γ-FeOOH. β-FeOOH is especially abundant on the skyward surfaces. (4) In the outer and inner layers, the main constituent is α-FeOOH on all specimens. α-FeOOH appears almost homogeneously through the rust layer. Its concentration is higher on weathering steels than on a plaincarbon steel when the environment is not very severe. (5) Rust layer usually lacks uniformity; it has thick parts and thin parts. β-FeOOH exists at thick parts of the rust layer, but is scarce at the thin parts of the rust layer. β-FeOOH also exists in considerable quantity on severely corroded specimens. (6) Average abundance of α-FeOOH is higher and that of β-FeOOH is much lower in the rust of weathering steels than that on a plain-carbon steel. (7) Amorphous rust is located at the bottom of the rust layer irrespective of steel type and exposure environment. (8) Concentration of magnetite is negatively correlated with concentration of β-FeOOH when steels are exposed in mild environments. From the negative correlation of magnetite and β-FeOOH, a model of degradation mechanism of rust layer was suggested. (9) There are almost no differences between specimens exposed at the industrial coastal region and the rural mountainous region because of effect of calcium chloride sprayed for melting snow on road during winter at the rural mountainous region. (10) The α/γ* ratio, the quantity ratio of α-FeOOH to β-FeOOH + γ-FeOOH + magnetite, obtained by ED, is larger than 1 on a weathering steel, and less than 1 on a plain-carbon steel. However, when we take α = α-FeOOH + amorphous rust, all values become greater than unity. Moreover, the α/γ* ratio is very large on a plain-carbon steel for specimens exposed at very severe environment. It is not convenient to use the ED determined data to evaluate α/γ* value of the rust layer.