Surface corrosion studies by Mossbauer spectroscopy.

Transmission [57]Fe Mossbauer Spectroscopy has been used to investigate two standard iron oxides and two environmental corrosion products in the temperature range 10-300 Kelvin. This was achieved using a specially developed Air Products cryogenic refrigeration system using gaseous helium as the refrigerant - thereby removing the requirement of an expensive liquid helium facility normally required for low temperature work. Characteristic spectra and transitions were observed for both the standard oxides. The environmental corrosion products demonstrated the differences in observed spectra between a sample which exhibits only bulk properties and one in which the particle size distribution within the sample is such that relaxation phenomena become important. The interpretations-made from the variable temperature Mossbauer data were confirmed by the complementary technique of X-ray diffraction.Conversion electron Mossbauer spectroscopy (C.E.M.S.) and conversion X-ray Mossbauer spectroscopy (C.X.M.S.) backscatter techniques have also been developed and are now routinely available in our laboratory.The greater escape depth of the conversion X-ray allows the C.X.M.S. method to be used to record spectra from samples to which a protective layer such as a paint, varnish, grease or oil layer has been applied. To ensure a surface sensitive signal, the substrate surfaces had to be enriched in the Mossbauer isotope 57Fe. This was achieved by vacuum evaporation of 57Fe onto the substrate surface and subsequent diffusion of the 57Fe into the near surface region - this diffusion had to be accomplished without oxidising the surface. Samples thus prepared were subsequently exposed to aggressive atmospheres and their CXM spectra recorded.The limitation of the method is that only room temperature spectra can be recorded at present and difficulties are encountered in the assignment of spectra that contain only quadrupole doublets.