Formation and evolution of phosphorus-containing species during high-temperature oxidation of TiAl dipped in a low-concentrated phosphoric acid solution

TiAl samples were dipped in a low-concentrated phosphoric acid solution, air dried, then heated to 800 °C under argon gas before oxidation under reconstituted air at this temperature. At the end of the heating ramp, sample surfaces were covered by a titanium pyrophosphate (TiP₂O₇) layer, well adhered to the substrate. This pyrophosphate was shown to be detectable from 400 °C. Chemical reactions have been proposed for the formation of this compound. TiP₂O₇ coating strongly increased the oxidation resistance of TiAl at 800 °C for 100 h. After 100 h, a transition period occurred leading to the change of TiP₂O₇ into TiO₂, which was achieved after 230 h, total mass gain always remaining below that of raw substrate. This evolution was demonstrated not resulting from a thermal decomposition of pyrophosphate compound.