Formation of the Fe(II)–Fe(III) hydroxysulphate green rust during marine corrosion of steel

Abstract

Rust layers formed on steel sheet piles immersed 1 m above the mud line for 25 years were analysed by Raman spectroscopy, scanning electron microscopy and elemental X-ray mappings (Fe, S, O). They consist of three main strata, the inner one mainly composed of magnetite, the intermediate one of iron(III) oxyhydroxides and the outer one of hydroxysulphate green rust GR(SO 4 2−). Simulations of GRs formation in solutions having large [Cl −]/[SO 4 2−] ratios revealed that the hydroxysulphate GR(SO 4 2−) was obtained instead of the hydroxychloride GR(Cl −), as demonstrated by X-ray diffraction and transmission Mössbauer spectroscopy analyses. Measurements of the [S], [Fe] and [Cl] concentrations allowed us to establish that GR(SO 4 2−) formed along with a drastic impoverishment of the solution in sulphate ions; the [Cl −]/[SO 4 2−] ratio increased from 12 to 240. The GR, acting like a “sulphate pump”, may favour the colonisation of the rust layers by sulphate reducing bacteria.