Kinetics and mechanisms of nickel metal dusting I. Kinetics and morphology

The kinetics of nickel metal dusting were investigated at different gas compositions at 680 C. The carbon uptake rate on nickel exposed to supersaturated H-2/CO/H2O gas mixtures (nominal a(C) = 19) increased as p(CO) increased from 0.31 to 0.68 atm, but decreased with further increase in peso. This behaviour was ascribed to the existence of parallel, independent reaction paths, and the rate was well described by Rate = k(1)p(CO)p(H2) + k(2)p(CO)(2) - k(3)p(H2)(2) where the k(i) are constants. Examination of reacted cross-sections showed that general coke layer formation at the nickel surface was accompanied by internal precipitation of graphite along grain boundaries deep inside the nickel. The internal precipitation reflected the large grain of the nickel. Prior cold working of the surface caused its recrystallisation, which affected carbon deposition and coke morphology. The carbon morphologies formed on the sample surface could generally be classified as bulk graphite, carbon filaments and graphite particle clusters. Carbon filaments were nanoscale in size, and uniform. The particles in the graphite particle clusters were relatively large (0.1-2.5 mu m) and non-uniform. The morphological development of the carbon deposit differed from one sub-grain to another. (c) 2006 Elsevier Ltd. All rights reserved.