Enhanced sulphidation/oxidation resistance of Ti–45Al–8Nb alloy by multilayered coatings at 850°C for up to 675 h

The high temperature sulphidation/oxidation behaviour of three multilayered coatings CrAlYN/CrN etched by Y⁺, CrAlYN/CrN etched by Cr⁺ and CrAlYN/CrN etched CrAl⁺ and the uncoated γ-TiAl (Ti–45Al–8Nb (at-%) used as reference sample was studied at 850°C for 675 h. Sulphidation/oxidation test was performed in the environment of H₂/H₂S/H₂O, yielding pS₂ = 10⁻¹ Pa and pO₂ = 10⁻¹⁸ Pa. Kinetic data obtained by discontinuous gravimetric method showed that the multilayered coatings effectively enhanced the sulphidation/oxidation resistance of γ-TiAl alloy. The corrosion resistance decreasing in order: CrAlYN/CrN etched by Y⁺>CrAlYN/CrN etched by CrAl⁺>CrAlYN/CrN etched by Cr⁺>γ-TiAl. Scale development studies using SEM, EDX and X-ray diffraction confirmed two regions of the coated materials: ‘affected,’ where coating cracked and developed non-protective TiO₂ scale, and ‘unaffected,’ where protective (Al,Cr)₂O₃ scale formed. The uncoated γ-TiAl material, after exposure, showed a typical multilayered structure consisting of layers of TiO₂ and Al₂O₃.