Page 753 - Corrosion Engineering Principles and Practice

704 C h a p t e r 1 5 H i g h - Te m p e r a t u r e C o r r o s i o n 705

15.4.4 Metal Dusting
Another ultimate effect of carburization can be metal dusting. This
phenomenon may occur in process operations in which oxidizing and
reducing conditions are cycled. When the environment is on the
reducing side (CO predominant), carburization of the metal to a shallow
depth can occur at breaks in the protective oxide film. When the
exposure then changes to oxidizing, the high-carbon area of the metal is
burned out and the metal reacted to the oxide. A depression is left in the
metal surface where the carburized area existed (Fig. 15.28), and the
metal oxide is swept downstream in the process as metal dust.
Metal dusting first appears as fine powders (hence the term “dusting”)
consisting of carbides, oxide, and graphite (soot). The morphology of
attack can be localized pitting or relatively uniform damage. The
underlying alloy may or may not display evidence of carburization in the
microstructure. Metal dusting is manifested at lower temperatures than
carburization, typically between 425 and 815°C. Maximum rates of metal
dusting damage are considered to occur around 650 to 730°C.
Metal dusting is usually associated with gas streams rich in
carbon monoxide and hydrogen. Prediction and modeling of metal
dusting are difficult, and little relevant quantitative data is available
for engineering alloys to assist designers. It appears that most stainless

(a)
FIGURE 15.28 Two alloys tested in the same H -CO-H O metal dusting
2
2
atmosphere: (a) 2.25 Cr 1-Mo at 620°C and 1600 hours showing general
attack; and (b) 9 Cr 1 Mo (P91 steel) at 620°C and 1008 hours, showing metal
dusting pitting attack. (Courtesy of Karl-Winnaker-Institute, DECHEMA, e.v.)