DEGRADATION OF PASSIVE LAYERS OF IRON STUDIED BY CONVERSION ELECTRON MOSSBAUER-SPECTROSCOPY

Integral electron Mossbauer spectroscopy (ICEMS) and additionally some electrochemical methods were used to characterize the passivation pro cess of iron (low carbon steel) in sulfate, sulfate + sulfite (a possi ble model solution of acid rain) solutions and in phospate buffer. The phase compositions and thicknesses of the passive layers formed due t o the electrochemical polarizations were analyzed in dependence on the duration of the anodic passivations and on the pH of the used electro lytes. The passive layer, as determined from the Mossbauer spectra, co nsists mainly of gamma-FeOOH, however in sulfite containing sulfate aq ueous solution at pH 3.5 Fe3C and despite ex-situ circumstances FeSO4 . H2O was detected after the shortest polarization time. The film thic kness, which was found to grow nearly linearly with polarization time in pure sulfate solution and in phospate buffer, reached a maximum of 60-160 nm (depending on pH) in sulfate + sulfite solution after a pass ivation time of about 4 hours. It has been proved, that HSO3--ion, whi ch is contained by acid rain, initiate pit formation under acid condit ions and so enforces the corrosion of iron. The experimental results f urthermore suggest, that not the whole oxidic layer is responsible for the passivity but only a very thin intermediate layer formed between an inner oxide layer of a cubic structure and the rhombic oxide (gamma -FeOOH) cover.