Corrosion of iron by a perfluorpolyalkylether identified by Mössbauer spectroscopy

Mössbauer spectroscopy was used to identify the corrosion products produced when various samples of pure iron were heated in a branched perfluoropolyalkylether (PFPAE) at temperatures between 232°C and 343°C (450°F and 650°F). The samples that were treated are: (1) a 2 mm thick piece of 99.99% pure natural iron, (2) a 567 Å film of iron enriched in 57Fe evaporated onto 1018 steel, and (3) powdered iron. The predominant products produced for treatments of 24 to 50 h at 288°C, 316°C, and 343°C, and with minimal oxygen present, are FeF2, and Fe3O4 modified by the substitution of fluorine for oxygen at the Fe2+ site (site B). A treatment at 232°C for 25 h did not produce any FeF2 but did show some sites in the spectrum that may be fluorides or oxyfluorides in addition to the modified Fe3O4. For the seven day treatment at 232°C FeF2 is observed. When powdered iron was treated at 343°C in a vessel where oxygen was introduced, FeF3, FeF2, and the Fe3+ site (site A) of Fe3O4 with little or no site B were observed. These results suggest that the formation of FeF3, which catalyzes the decomposition of PFPAE fluids, involves the production and fluorination of Fe3O4.