Formation and analysis of corrosion products deposited on steel under HCl atmosphere by means of conversion electron Mössbauer spectrometry at room temperature and 80 K

A conversion electron Mössbauer spectrometer, which allows measurement to be performed at low temperature (80 K), was fabricated using a ceramic electron multiplier. It was used to discriminate the CEM spectrum of β-FeOOH (TN = 273 K) from γ-FeOOH (TN = 50 K), which were produced on steel surfaces as corrosion products following exposure of steel in an air-HCl-H2O atmosphere. Thin films of FeCl2·2H2O and FeCl2·4H2O were formed on the steel during exposure to air, containing HCl vapor evaporated from HCl solution, while stored in a closed vessel. These hydrated films of iron(II) chloride remained unchanged when kept in completely dry air, but were transformed to β-FeOOH when they were exposed to slightly humid air. Raising the air humidity up to 100% favored an increase in the ratio of γ-FeOOH to β-FeOOH. Only γ-FeOOH was formed by the exposure of FeCl2·xH2O coated steel to the water saturated air.