Formation of the Fe(II–III) hydroxysulphate green rust during marine corrosion of steel associated to molecular detection of dissimilatory sulphite-reductase

Accelerated corrosion phenomena of carbon steel constantly immersed in seawater could be simulated in situ via a galvanic coupling of the samples with steel port structures. Three harbours located on different seas and various conditions of immersion were considered so as to study the eventual correlation between dissimilatory sulphite-reductase genes and sulphate-containing corrosion products. In each case, after 6 or 12 months, the rust layers proved to be made of an inner black layer, close to steel surface, and an orange outer layer. Scanning electron microscopy, chemical analyses by inductively coupled plasma/atomic emission spectroscopy, X-ray diffraction and micro-Raman spectroscopy were used to obtain a detailed characterisation of these layers. The inner one proved to be mainly composed of iron sulphides FeS and Fe(II–III) hydroxysulphate green rust GR(SO42-), the outer one of Fe(III) oxyhydroxides, with lepidocrocite γ-FeOOH as a major component. The molecular detection of dissimilatory sulphite-reductase, the key enzyme in dissimilatory sulphate reduction by micro-organisms, was applied for the first time to rust layers. This detection was positive in most cases, especially for the inner part of the rust layers. This demonstrates that sulphate reducing bacteria are associated to GR(SO42-) inside the rust layers, GR(SO42-) more likely playing the role of a source of sulphate. The systematic presence of iron sulphides also testifies the activity of sulphate reducing bacteria and/or thiosulphate reducing bacteria.