An electrochemical and surface analytical investigation of iron oxidation, passivation and corrosion inhibition

A combined electrochemical and surface analytical study of iron oxidation, passive film formation and corrosion inhibitor interaction is presented. Ultrahigh vacuum surface analysis by Auger electron spectroscopy (AES), low energy electron diffraction (LEED), thermal desorption mass spectrometry (TDMS) and ion bombardment (depth profiling) was used to investigate iron oxide films formed on iron by gas phase oxygen exposure as well as electrochemical polarization in borate buffer and corrosion inhibitor solutions.; Oxygen exposure to the Fe(100) single crystal surface has unambiguously shown the formation of a c(2 x 2) LEED pattern during the oxide evolution. This step in the oxidation of the (100) surface is often not observed, most likely as a result of prior surface contamination. A calibration technique is also outlined in which the oxide film formed by oxygen exposure can be used to estimate thicknesses of films formed in solution.; Passive film investigations by composition depth profiling and TDMS after exposure to borate buffer at various potentials have shown the film is hydrated. Composition depth profiles also indicate that a two-phase oxide is present. An inner barrier layer likely forms by direct substrate oxidation while an outer deposit layer is formed by deposition of solution complexes, especially those containing boron. Surface crystallographic differences are evident in the cyclic voltammetry, the potential-dependent AES surface spectra, and the eventual film thicknesses.; Composition depth profiles of films formed on iron after exposure to inhibitor solutions under varying conditions of dissolved ions, pH, and potential have provided clues to reaction mechanisms. Inorganic orthophosphate is not incorporated into the films under deaerated conditions at any potential. In the presence of divalent calcium ion in solution, the film composition is potential dependent. Cathodic polarization produces calcium phosphate films while anodic polarization leads to iron phosphate phases. Again, two-phase barrier and deposit layer films are evident.; An organic phosphonic acid, hydroxyethanediphosphonic acid (HEDPA), behaves similar to orthophosphate in deaerated solutions. Aerated solutions produce thicker films as do lower pHs. The mechanism of inhibition in both inorganic and organic inhibitor solutions appears to be through anodic or cathodic polarization.