CORROSION SCIENCE 546 CORROSION–JULY 1994 ✫ Submitted for publication October 1993; in revised form, January 1994. Presented as paper no. 005 at the 12th International Corrosion Congress, September 1993, Houston, TX. * Bethlehem Steel Corp., Homer Research Laboratories, Bethlehem, PA, 18016. ** Air Products and Chemicals Inc., 7201 Hamilton Blvd., Allentown, PA, 18195. Structure of Rust on Weathering Steel in Rural and Industrial Environments ✫ H.E. Townsend, T.C. Simpson,* and G.L. Johnson** ABSTRACT Rust on weathering steels exposed for up to 11 y in industrial and rural environments exhibited a spotty, nonuniform appearance as a result of contrast between light and dark areas on the surface. The area of the lighter regions increased with exposure time, as well as with increasing alloy content. Microscopic examination of cross sections revealed the dark regions had a porous appearance. The lighter regions were similar except for the presence of a relatively thin, smooth outer-surface layer. Raman spectroscopy indicated the dark, porous rust was comprised of goethite and lepidocrocite, while the smooth outer layer on the light areas was composed of hematite (-Fe 2 O 3 ) and magnetite (Fe 3 O 4 ). Electrochemical potential measurements indicated the lighter areas were more passive. Results suggested formation of the thin outer layer of -Fe 2 O 3 and Fe 3 O 4 is an important part of the mechanism of protective rust formation on weathering steels in the environments studied. KEY WORDS: A588 steel, A242 steel, goethite, hematite, industrial environment, lepidocrocite, magnetite, potential, Raman spectroscopy, rural environment, rust, weathering steels INTRODUCTION The term “weathering steels” describes a class of low- alloy steels with enhanced resistance to atmospheric corrosion. Although the underlying mechanisms are not understood completely, it is well known that alloying elements such as copper (Cu), chromium (Cr), nickel (Ni), phosphorus (P), and silicon (Si) somehow promote formation of protective rust layers during outdoor exposure. Weathering steels generally contain a combination of these elements totaling < ~ 3 wt%. Because of barrier protection imparted by the protective rust layers, weathering steels are used outdoors without paint or other protective coatings. Corrosion testing of weathering steel for up to 16 y in a variety of environments has been reported previously. 1 However, certain environmental conditions may be unfavorable for development of protective rust on weathering steel. According to United States government guidelines, the following conditions should be avoided: high levels of salt (chloride > 0.5 mg/100 cm 2 /day), high wetness times (average time of wetness > 60%), and direct exposure to high levels of industrial pollutants (i.e., sulfur trioxide > 2.1 mg/100 cm 2 /day). 2 Unpainted weathering steels have been used successfully in applications such as bridges, utility towers, and highway guardrails when these guidelines have been followed. During long-term corrosion tests, the rust layers on weathering steel exposed at rural and industrial test 0010-9312/94/000125/$5.00+$0.50/0 © 1994, NACE International