Nickel-Based Alloys in Carbonaceous Gases

High-temperature corrosion in carbonaceous gases can occur by carburization (i.e., internal carbide formation at C activities [a_C] < 1 and at temperatures > 800°C) or by metal dusting, a disintegration of alloys into graphite and metal particles observed at a_C > 1 and in a lower temperature range. In both cases, C entry and diffusion in the metal matrix is a decisive step, and in comparison to high-alloy steels, the C ingress in Ni-based alloys is retarded by the decrease of C solubility and diffusivity with increasing Ni content. Therefore, Ni-based alloys are recommended for applications in carbonaceous gases (e.g., for the steam-cracking of hydrocarbons). Since operating temperatures > 1,100°C are desired in ethylene (C₂H₄) production, where chromium oxide (Cr₂O₃) is no more stable, the alloys must contain Si and/or Al to form a protective scale. At temperatures ≤ 650°C, high Ni-based alloys with ≥ 25% Cr and some Al proved to be very resistant to metal-dusting attack by the carbonaceous syngas (CO-H₂) environment. Such alloys showed no attack in laboratory and pilot plant exposures for up to 10,000 h, whereas all high-alloy steels were susceptible.