2005 Volume 46 Issue 8 Pages 1890-1896
Preliminary laboratory corrosion tests of two conventional Ni-base alloys were conducted at 650°C for 200 h in various CO–H2–CO2–H2O gas mixtures. For gas mixtures of high ac and CO content, alloy 600 (75 mass %Ni–15%Cr) specimens lost its mass due to metal dusting and deposition of coke was heavy on the surface of the specimen. Meanwhile, any pit formation was not occurred on the alloy 690 (60%Ni–30%Cr) specimens. Microscopic observation indicated that for the corroded Ni-base alloy specimens, inward diffusion of carbon presumably at cracks and flaws in the oxide scale was prominent. Carbon in the diffusion zone reacted with chromium to precipitate carbides in the matrix, followed by the “direct” formation of graphite platelets with a lamellar structure at the revealed metal surface. This direct precipitation of graphite platelets can be considered by a eutectoid reaction, which is similar to the pearlite transformation observed in Fe–C system. In the lamellar structure, the γ matrix thinned with the growth of graphite plates, and small metal particles of Ni and Fe which may catalyze the coke deposition reaction were crumbled and detached from the lamella.
