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Abstract
Metal dusting corrosion is a form of high temperature corrosion that is caused by carbon supersaturated gas, leading to a rapid degradation of the metals. Usually, metal dusting corrosion is associated with temperatures between 500 and 800°C, and very low oxygen partial pressures, due to the nature of the main chemical reactions in the gas phase. In nickel, metal dusting corrosion is caused by the catalytical deposition of graphite on the metal that eventually grows inward and causes a rapid degradation of the metal. One way to mitigate metal dusting corrosion is by physically separating the metal from the reaction gas with an oxide layer that blocks gas and carbon diffusion. This layer can be achieved by alloying with metals that form stable, thermally grown oxides under the low oxygen partial pressures typically present. In this study, the effect of aluminium on the dusting of Ni-Al model alloys and commercial Fe-Ni-Cr alloys has been studied. Reactions were carried out in a CO-H2-H2O gas mixture, using temperature cycling with a reaction temperature of 650°C, leading to a carbon activity of 36.7 and an oxygen partial pressure of 2.8E-26 atm. It was found that the protective effect of aluminium was strongly dependant on the amount of aluminium alloyed, as concentration and diffusion determine the protective properties of the alumina. Low Al alloys were severely attacked at a rate similar to that of pure nickel, while the intermetallic β-NiAl phase proved to be very resistant. The γ'-Ni3Al phase with an intermediate amount of aluminium, showed pitting-type attack, indicating partial protection of the alloy. The commercial Fe-Ni-Cr alloys featuring only low Cr and Al concentrations were all heavily attacked. It was also found that alloying of low levels of hafnium into the Ni-Al alloys generally did not improve metal dusting resistance without a high-temperature preoxidation treatment that produce dense, slow growing α-Al2O3.