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.