Investigation of the effect of the alloy 600 substrate for the stability of a Ni3Sn2 coating for metal dusting protection at 620°C

C. Geers, M. Galetz, M. Schütze

Under specific conditions carbon containing gas species can attack and degrade steels and nickel base alloys via a corrosion mechanism called metal dusting. These conditions occur at temperatures in the range of 400 to 800 °C, low oxygen partial pressures and carbon activities higher than one. Carbon activity higher than one means that the gas phase is out of equilibrium but due to high gas velocities carbon segregation is kinetically hindered by simply reducing the holding time of the gas in the critical temperature range. The kinetic hindering is superimposed by the catalytic activation of the deposition process by the metal surfaces of reactors. High amounts of atomic carbon deposit on the metal surface, diffuse inwards into the material and precipitate as graphite in the metal matrix causing the total disintegration and the formation of ‘metal dust’.

Metals triggering this corrosion attack include iron, nickel and cobalt. To inhibit the catalytic activation of the carbon deposition a catalytic poisoning concept has been invented. Tin has been shown to be very useful to occupy the catalytical active centers of nickel on the metal surface via the formation of the intermetallic Ni3Sn2-phase for at least 3000 h of exposure at 620 °C.

The performance of the coating does not only depend on its stability towards the aggressive atmosphere but is also strongly determined by the diffusion processes occurring at the coating/substrate interface. On nickel base alloys the Ni3Sn2-phase should transform into the Ni3Sn phase due to the vicinity of a high nickel reservoir but this was not observed for the substrate alloy 600 after 3000 h of exposure. This behavior was investigated in this work and shall be focused on in this article to gain a better understanding of the coating/substrate interaction for a better lifetime prediction.

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