Reaction mechanisms and diagnostic mineralogy of intertidal steel corrosion: An X-ray photoelectron spectroscopy study

The products of intertidal and super tidal (splash zone) corrosion on steel piles have been characterised at 3 UK sites with contrasting environmental conditions in order to determine corrosion reaction mechanism and if a common mechanism for accelerated low water corrosion occurs across sites. Intertidal corrosion samples at Shoreham and Newhaven ports show an internal composition of iron mono- and bi-sulphide, with intermediate sulphur oxidation state compounds, and an outer surface dominated by iron oxides and oxyhydroxides with a component of iron sulphates. The FTIR spectra are characteristic of sulphate green rust. In contrast, samples from Southend have all sulphur species below detection levels and are dominated by iron oxides and oxyhydroxides. Carbon binding energy spectra are consistent with the development of biofilms at all sites except for a splash zone sample at Southend. The results demonstrate a common mechanism for ALWC at Newhaven and Shoreham, involving the action of sulphate-reducing bacteria generating iron sulphides on the steel surface. These are subsequently oxidised to produce sulphate green rust, which may in turn oxidise to produce lepidocrocite. At Southend differences in environment are inferred to restrict the activity of sulphate reducing bacteria, resulting in direct oxidation of steel to generate iron oxyhydroxide gels, which subsequently recrystallise, dehydrate and oxidise to goethite, magnetite and ultimately hematite in both splash zone and intertidal samples. The multi-technique approach used here characterises the full range of corrosion products.