Crystal structure of green rust formed by corrosion of cast iron

Abstract

THE principal solid corrosion products of iron in water are magnetite and hydrated forms of ferric oxide, depending on the degree of oxidation. The occurrence of intermediate products, termed green rusts, was reported by Keller¹ in 1948, since when the formation and decomposition of these compounds, precipitated from aqueous solution by controlled oxidation of ferrous hydroxide, have been extensively studied^2–5. It has been proposed^2,3 that specific anions are incorporated in the crystal structure of some green rusts, although no substantial evidence has been found to confirm this. A number of green rusts have been classified crystallographically³ as follows: first, rhombohedral green rusts I (GRI) formed in Cl⁻, SO₄²⁻ and Br⁻ solutions and (second), hexagonal green rust II (GRII) formed only in SO₄²⁻ solutions by decomposition of GRI. The interrelationships between the various products involved in the oxidation of ferrous hydroxide have been summarised by Misawa et al.⁵ In this paper a preliminary account is given of the crystal structure of a rhombohedral green rust formed by corrosion of iron which, although of similar unit cell dimensions to GRI, differs structurally from those previously reported.