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

Suitable sealants for cracked aluminized austenitic steels and their oxidation behaviour

“…7. Therefore, both the concentration and time increase shall thus result in thicker phosphate layers that shrink upon drying to result in the dry-mud morphology as also demonstrated by Brou et al on TiAl surfaces [24]. In spite of the cracked appearance of the deposits, the evolution of the wettability with time was the same (see Fig.…”

“…Since their decomposition temperatures are well above 650°C, these compounds shall remain stable at our oxidation temperature of 650°C. Moreover, Brou et al [23][24][25] showed that the formation of metal pyrophosphates [AlxPyOz] could also occur. Depending on the temperature and time, the pyrophosphates evolve into Al2O3 and P2O5 [24].…”

“…Moreover, Brou et al [23][24][25] showed that the formation of metal pyrophosphates [AlxPyOz] could also occur. Depending on the temperature and time, the pyrophosphates evolve into Al2O3 and P2O5 [24]. Diffusion of P in the aluminide coating could occur but we have no experimental evidence for this.…”

“…on TiAl but at higher temperature [23][24][25], the phosphate layer seems to block diffusion, hence limiting oxidation while the growth of alumina would be delayed by the transformation of the aluminium phosphates. As a result, the oxide scales are much thinner than in the untreated specimens ( After more than 1000 cycles of exposure to air at 650°C, the microstructure and the composition of the sealed aluminide coatings remained similar to the untreated ones.…”

“…Sealing is commonly achieved by impregnating the coating with organic or inorganic solutions [20]. Orthophosphoric acid (H3PO4) and commercial phosphate solutions have been successfully used as sealants for porous oxide coatings [20][21][22] while mixed H3PO4 and acetone were employed to seal γ-TiAl [23][24][25]. With the latter, a gel-like deposit was left over the surface and the high temperature oxidation resistance in air was greatly improved.…”