Raman identification of corrosion products on automotive galvanized steel sheets

Iron oxides (haematite, maghemite, magnetite), (oxy)hydroxides (lepidocrocite), carbonates, as well as zinc carbonate and oxide, have been identified on corroded galvanized steel samples after corrosion accelerating tests in the laboratory and compared with those observed on samples taken from vehicles that have been in circulation for five years in severe weather conditions. Spectra recorded on the corroded parts are compared with synthesized compounds. (Hydroxykarbonates are clearly evidenced on galvanized and phosphated steel sheets. Corrosion layers beneath the paint could be detected. White regions always correspond to a ZnO-rich phase but maghemite (gamma-Fe2O3) and sometimes akaganite (fi-FeOOHCI) are observed at the centre (maroon) of very corroded spots. Maghemite is observed in strongly corroded regions. Goethite (a-) and lepidocrocite ()-FeOOH) (and akaganite) are observed at the surface of less corroded regions of phosphate-free galvanized steel and are absent for phosphate-coated steel. Green rust is observed only on galvanized samples corroded in the laboratory. Copyright (c) 2008 John Wiley & Sons, Ltd.