Alumina coatings on Ni-based superalloys: The impact of annealing on heavy oil fouling

As the sweet crude oil reserves decline, refiners must treat sulfur-rich heavy oil, requiring harsher operating conditions, which are detrimental to process equipment. Application of coatings on critical components protects surfaces against sulfidation, corrosion, and fouling, extends the equipment's lifetime, and reduces the frequency of costly turnarounds. In the present work, we coated Inconel 625 and Inconel 718 substrates with amorphous alumina thin films at room temperature using reactive RF magnetron sputtering. Annealing of the deposited coatings at 800, 900, and 1000 °C increased hardness, improved adhesion, and generated crystalline polymorphs, predominantly γ-Al 2 O 3 at lower temperatures, while α-Al 2 O 3 was present at 1000 °C. The annealed substrates formed thermally grown oxides (TGOs), which interacted with the alumina coatings. The TGOs followed grain boundaries in the case of IN718 and a crater-like pattern on IN625. Annealed substrate precipitates generated columnar-like protrusions responsible for inducing crack propagation, which exhibited TGO formation. After 2 h exposure to heavy oil (containing 0.06 g g −1 sulfur) at 450 °C and 11.3 MPa the as-deposited amorphous alumina presented no clear sign of adherent fouling, while the 1000 °C annealed crystalline alumina surfaces presented evidence of fouling. • Deposition of amorphous alumina on IN625 and IN718 by reactive magnetron sputtering. • Enhanced coating adhesion is correlated to refractive index homogeneity through the coating thickness. • Annealed alumina presents polymorphism, with γ-Al 2 O 3 prevailing at 800 °C and 900 °C, while α-Al 2 O 3 is present at 1000 °C. • The surface microstructure of annealed alumina at 1000 °C varies when deposited on IN625 or IN718. • Amorphous alumina resists fouling, while 1000 °C annealed alumina shows substrate-dependent inhomogeneous surface fouling.