No abstract

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 450° and 850°C. 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 650°C. 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.

The corrosion phenomenon, metal dusting, has been observed in many high-temperature industrial plants, such as ammonia, syngas and steam-reforming industries. An experimental research programme has been carried out into the degradation resistance of wrought and cast commercial high-temperature nickel-base alloys in H 2 /CO gas mixtures at 650 C.The results in this paper are focused on the effect of the experimental set-up on the response of the alloy during exposure to the gas mixture; in particular, the specimen holders had an important role on the onset of carbon transfer from the gas to the alloy surfaces. Specimens of most alloys exposed to the gas mixture while suspended from quartz hangers were shown to have good resistance to the environment for periods up to 5000 h. On the other hand, specimens of these alloys, exposed while located in porous refractory ceramic holders, gained large amounts of coke and underwent severe damage. A series of short tests were carried out in order to ascertain the main factors influencing the alloy reactivities in the experimental rig. The presence of specimens of a less-resistant alloy and slight variations in gas composition (with the addition of CO 2 ) were observed to have a marked impact on the alloy reactivities. Factors such as catalysts, contaminants and gas composition were considered and are discussed.