Stability of Rust Layers Formed on Corrosion Resistant Low Alloy Steels

Abstract

This investigation has been carried out in order to make clear the prot ective nature of rust layers. Measurements have been made of the amount of water-soluble compounds dissolved into rain-water from rust layer for 1 month, exfoliated rust, retained rust and absorbed SO₂ by the rust. The results are summarized as follows:
1) Leaching rate of ferrous and sulfate ions into rain-water and exfoliation rate of rust from specimens decreased with increasing the exposure time.
2) The amount of ferrous ions in water-soluble compounds was found to be equivalent to that of sulfate ions: that is, the water-soluble compound in the corrosion products can be estimated to be FeSO₄ or FeSO₃.
3) Insoluble sulfur compounds gradually accumulated in the rust layers during exposure and such rust layers became inactive for SO₂ in the atmosphere. Further, our laboratory experiments on absorption of SO₂ by rusts indicated that the rate of SO₂ absorption or its re-dissolution in water decreased with increasing the exposure period of the specimen.
4) The corrosion losses of steels were more closely related with the amount of exfoliated rust rather than that of the retained rust or water-soluble compounds.
These results indicate the importance of both chemical and structural study in considering the stability of rust layers on corrosion resistant low alloy steels.