Effect of a phosphoric acid treatment on the high temperature oxidation behaviour of γ-TiAl: An overall mechanism

TiAl coupons were dipped in a low-concentration phosphoric acid solution, air dried, then heated up to 700 °C or 800 °C before discontinuous oxidation under laboratory air at these temperatures. At the end of the heating ramp, surfaces of TiAl samples were covered by a layer made of a pyrophosphate compound, resulting from reaction between the deposited acid and oxidized titanium. This layer strongly adhered to the substrate surface and allowed to greatly improve the resistance of TiAl to oxidation. After a certain time, a transition period occurred during which pyrophosphate compound disappeared to be changed into TiO2 rutile. For experiments carried on at 800 °C, it was shown that the phosphorus coming from the pyrophosphate group was present in this titanium oxide and that sample mass gain was still lowered. Performed analyses (XRD, SEM, EDS, μ-Raman spectroscopy) allowed to propose an overall mechanism to account for the “phosphorus effect”.

► 0.5 M phosphoric acid solution used to bring phosphorus specie at the surfaces of TiAl.
► A pyrophosphate compound effective in protecting TiAl against high temperature oxidation.
► Mechanism to account for the role of phophorus, consistent with the TiO2–P2O5 diagram.
► Phosphorus introduced in TiO2 lattice, leading to an increase of the oxidation resistance.