Steel in concrete: Part II Electron microscopy analysis

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Abstract
The microstructure, crystallinity and the chemical and mineralogical constitutions of corrosion products formed under exposure of a steel-reinforced concrete fence post to normal atmospheric conditions have been investigated by analytical electron microscopy techniques. A bi-layered structure is observed comprised of a dense inner spinel phase (Fe3O4,/γ-Fe2O3,) in epitaxial orientation to the steel substrate and a relatively porous outer layer of mainly χ-FeOOH. A weak but apparent band showing a relatively elevated chloride concentration, rims the inner corrosion layer. The permeation through the corrosion layer of the products of cement hydration is indicated by the detection of nodules of Ca(OH)2, crystals at the steel reaction boundary. The mechanism of reinforced steel corrosion is thus evaluated on the basis 0fthese.finding.s and with special reference to the role of chloro-complexes and the modes of Eh, and pH buffering actions in the composite steel-corrosion product-cement system.

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