Metal dusting performance of nickel-base alloys

The corrosion phenomenon of metal dusting is observed in many high-temperature industrial plants. It usually occurs in environments of high carbon activity and in a temperature range between 450degrees and 850degreesC. A sufficiently high carbon activity, combined with a low oxygen partial pressure, yield an atmosphere that is able to deposit carbon on metal surfaces where an oxide scale is not able to develop or be maintained. The carbon may transfer into the metal phase, leading to an oversaturated, unstable solid solutions, following by metal dusting damage Filamentous growth is associated with damage to the alloy, leading to small metallic particles (corrosion products) becoming embedded in the deposits. In the present research, nickel-base alloys (Alloys 600, 602CA, 603XL, 693, H46M) were exposed to 20 vol.% H-2 - 80%CO at 650degreesC. Specimens of each alloy were suspended from a quartz hanger and exposed for periods up to 5000h. Alloy 600 was sensitive to damage in this environment, but the other alloys were resistant to attack. A chromium oxide scale developed on all the alloys, while underlying aluminium and/or silicon oxide layers were observed for those containing a sufficient amount of the relevant alloying element. A light carbon deposit, in the form of filaments, was observed on the surfaces of all the alloys, even after short exposure times. However, such deposits did not give rise to the dusting reaction in most cases. The exposed specimens have been analysed by EDX, XRD and EPMA. The results are discussed in terms of the formation of oxide scales and their resistance to the environment.