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

Abstract

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.