Understanding Metal Dusting Mechanisms

Abstract

Examination of iron dusting shows that cementite forms as a layer on the surface and as particles in the overlying coke deposit. On this basis, the mechanism of cementite disintegration is deduced to be one of internal graphite nucleation and growth within the scale. Cementite formation on iron is suppressed by alloying with germanium. Nonetheless, dusting occurs via the direct growth of graphite into the metal, producing nanoparticles of ferrite. This process is faster than dusting via cementite formation and disintegration. Austenitic materials cannot form cementite, and dust via formation of graphite at external surfaces and interior grain boundaries. The coke deposit consists of carbon nanotubes with austenite particles at their tips, or graphite particles encapsulating austenite. TEM studies demonstrate the inward growth of graphite within the metal interior. It is therefore concluded that the mechanism of nickel dusting is one of graphite nucleation and growth within the near surface metal. Alloying copper with nickel strongly represses graphite formation, thereby reducing greatly the overall dusting rate.