Morphology of rust phases formed on weathering steels in various laboratory corrosion tests

Abstract

The morphology and growth characteristics of rust phases formed on ASTM A-588 weathering steel in three different types of laboratory tests—accelerated atmospheric exposure simulation tests (AAEST), salt fog test, and continuous immersion test in plain as well as salt water—are analyzed using microstructural information obtained from representative exposed specimens studied in a scanning electron microscope (SEM). The ultimate and most dominant phase in the AAEST was α-FeOOH whereas an amorphous phase designated as amorphous bulk (AB) appeared as “cotton bolls” in the adherent, sedimentary layer formed on the steel surface during continuous immersion. Crystalline phases α-, δ-, and γ-FeOOH as well as γ-Fe₂O₃.H₂O were found developed on top of the first-formed sedimentary amorphous layer, containing another amorphous phase designated as amorphous mix (AM). Magnetite was the dominant phase obtained in the salt fog test. It forms in layers and seems to transform to α-FeOOH through formation of whiskers and rods on its surface. Sandy grains of γ-Fe₂O₃.H₂O were also seen in the rusts obtained in this test.