The Oxidation of Iron as Studied by Conversion-Electron Mossbauer Spectroscopy

The potential of Mössbauer spectroscopy in investigating the oxidation and corrosion behavior of iron has been adequately demonstrated by a number of investigators [1-8]. These studies involved transmission Mössbauer spectroscopy of relatively thick oxide layers and of corrosion products that had been removed from the iron surfaces. Back-reflection Mössbauer spectroscopy has also been used in a preliminary way in studying advanced stages of corrosion [9-10]. There are many corrosion conditions, on the other hand, that result in the formation of thin (10-500 A) films such as found in low-temperature oxidation or during anodic treatment. Mössbauer spectroscopy has the capability to provide qualitative and quantitative information about the chemical and physical properties of thin corrosion layers. Since chemical treatments to remove films from the iron substrate may alter the corrosion products, it is desirable to study thin films that are still in contact with the iron, i. e. in situ. Although reference has been made to the possibility of obtaining Mössbauer spectra from oxide thicknesses down to 30-40 A in situ, few results have yet been published [9-11]. The primary purposes of the present paper are to describe the technique of reflection Mössbauer spectroscopy by detection of conversion electrons and to present results from the application of this technique to studies of thin oxide layers formed during the thermal oxidation of iron.