Metal dusting on an alumina forming Ni-base alloy

A high-performance commercial alumina-forming Ni-base alloy was studied after a 2 years field exposure at 540 °C in a methanol plant with a gas composition of 10–20%CO and 20–40%H2O, with some CO2 and the remainder H2. The same material was also used in laboratory studies performed at 650 °C using a gas mixture with higher CO and lower H2O content; 50%CO + 3%H2O + 47H2 (carbon activity ac = 39). Post-exposure metallographic examinations together with thermodynamic calculations were used to identify and describe the metal dusting processes.A growth mechanism for metal dusting in nickel base alloys, which is independent of metal bulk diffusion, is identified. The process involves a separation of the carbon-saturated metal into a network of discontinuous precipitated carbides and a depleted Ni–austenite matrix followed by selective oxidation of the carbide network. The corrosion product consists of Cr-depleted Ni-particles, Cr-rich oxides and free carbon. The estimated metal dusting corrosion rate in the field exposure was 20–25 μm/year, based on metallography and it was correlated to a theoretical model based on boundary diffusion processes.