STRUCTURE OF RUST ON WEATHERING STEEL IN RURAL AND INDUSTRIAL ENVIRONMENTS

Rust on weathering steels exposed for up to 11 y in industrial and rur al environments exhibited a spotty, nonuniform appearance as a result of contrast between light and dark areas on the surface. The area of t he lighter regions Increased with exposure time, as well as with incre asing alloy content Microscopic examination of cross sections revealed the dark regions had a porous appearance. The lighter regions were si milar except for the presence of a relatively thin, smooth outer-surfa ce layer. Raman spectroscopy indicated the dark, porous rust was compr ised of goethite and lepidocrocite, while the smooth outer layer on th e light areas was composed of hematite (alpha-Fe2O3) and magnetite (Fe 3O4). Electrochemical potential measurements indicated the lighter are as were more passive. Results suggested formation of the thin outer la yer of alpha-Fe2O3 and Fe3O4 is an important part of the mechanism of protective rust formation on weathering steels in the environments stu died.