COMPOSITION DEPTH PROFILES OF POTENTIAL-DEPENDENT ORTHOPHOSPHATE FILMFORMATION ON IRON USING AUGER-ELECTRON SPECTROSCOPY

An elemental analysis using Auger electron spectroscopy (AES) has been conducted in which corrosion-inhibiting films formed on iron in phosp hate-containing solutions were depth profiled by argon ion bombardment . Exposure to the ambient atmosphere was avoided by the use of an elec trochemical cell interfaced to an ultrahigh vacuum surface spectromete r such that specimen transfer is conducted through an inert gas enviro nment. Elemental depth profiles of films formed on polycrystalline iro n at various potentials indicate that solution ions are incorporated i nto the films. These depth profiles have provided insight into film co mpositions responsible for corrosion protection in the corresponding m edia. In calcium-free solutions, orthophosphate is not incorporated in to the film in significant amounts under any conditions of applied pot ential. The composition of these films is essentially iron oxide with inclusions of solution anions. When divalent calcium ion is added to t he solution, the resulting film contains significant amounts of phosph ate, and the film composition depends on the applied potential. Cathod ic polarization leads to essentially calcium phosphate phases whereas anodic polarization produces films consisting of a mixture of iron oxi de and iron phosphate. Under open circuit conditions the film is much thinner and is composed mostly of iron oxide with small amounts of cal cium phosphate.