Type I Hot Corrosion Of Platinum-Containing Model γ and γ′ Alloys

Understanding of the hot corrosion behavior of each constitutive phase of the new Pt-enriched γ/γ′ nickel-based single-crystal TROPEA superalloy is of significant interest for further optimization. Therefore, single-phase γ and γ′ materials containing a low content of Pt (0.13% at.) were manufactured by inductive melting and their behavior was investigated at 900°C with 1 mg/cm2 of Na2SO4 deposit using thermogravimetry in air and air + 400 ppm of SO2. After 24 h in air, the scale grown on the γ′ model alloy consisted of an outer heterogeneous NiO layer and of an internal porous α-Al2O3 layer. The addition of 400 ppm of SO2 to air did not really change the nature and morphology of the oxides formed for γ′, but the measured linear rate was tenfold higher than that recorded in air. The γ alloy underwent an incubation period during which the corrosion rate was limited, followed by a propagation stage. In air, the alloy developed an external continuous NiCr2O4 spinel layer and a thin internal Cr2O3 subscale. The γ alloy was able to efficiently re-passivate after the propagation period. With 400 ppm SO2, only a thin protective layer Cr2O3 formed on the surface of γ, with large Cr2O3 crystals growing within the residual Na2SO4. The occurrence of Cr-rich sulfides was also greater in the γ′ model alloy compared to the γ counterpart both in air and in air + 400 ppm of SO2. Therefore, the results clearly evidenced the effect of SO2 on the type I hot corrosion.