Morphological Studies of Coking on Heat-Resistant Alloys

The deposition of coke from a propylene--hydrogen mixture on to a range of austenitic Fe--Cr--Ni base alloys (Incoloy 800, Incoloy 800H, PG2535 Nb) has been studied. Morphological investigations reveal the formation of a Cr-rich carbide layer on the surface of the alloys. This layer is initially protective, but eventually develops defects from which coke filaments grow. The formation of these filaments is catalysed by the Cr-depleted metal which becomes accessible to the gas following failure of the carbide layer. Once catalytic coke formation commences, it is maintained by the presence within the coke of small metal particles rich in Fe and Ni. 7 ref.--AA