Alloys are often found to suffer much greater metal-dusting attack under high-pressure conditions compared to ambient-pressure conditions. Ultimately, the resistance of a given alloy to metal dusting depends on the formation of an oxide scale that is impermeable to carbon which, in turn, depends on scale composition and structure. This paper reports the metal-dusting behavior of several Ni-based alloys having relatively high Cr contents (about 30 wt.%) and different controlled minor levels of Fe, Al, and/or Si. Testing was conducted under 20 bar total pressure of a high-carbon-activity gas at 600 °C (1112 °F). The exposed alloys were analyzed by SEM and TEM techniques to evaluate the oxide scales and evidence of carbon ingress. It was found that aluminum is beneficial to improve metal dusting resistance by reacting to form a continuous inner layer of alumina; whereas iron is detrimental to resistance. Mechanistic aspects of the role(s) played by minor elements in affecting metal-dusting resistance are considered.
High-Temperature Hydrogen Attack (HTHA) is a phenomenon that involves the formation and accumulation of methane (CH4) in steels operating under conditions where there is hydrogen ingress. To account for the phenomenon, it is necessary to know how the supply of solute carbon atoms occurs. What is discussed here concerns only low-carbon steel within the range 0.08-0.30 wt % carbon that has no intended additions of alloying element such as chromium (Cr) or molybdenum (Mo), and that it is typically delivered in the as-hot worked or normalized condition, resulting in microstructure consisting of pearlite colonies within a matrix of ferrite grains. Carbon steels do not normally contain carbon atoms in solid solution, but most are tied to cementite (Fe3C), except when retained in supersaturated solid solution by rapidly cooling from just below the subcritical temperature Ac1, 727 °C (1340 °F), in which case, the solute carbon atoms do not remain in supersaturated solution for long, they precipitate, but the resulting precipitates are rather unstable and get quickly thermally activated when heated to temperatures that are considered relatively too low to significantly affect the cementite in existing pearlite colonies. Thus, these precipitates may supply solute carbon atoms for HTHA damage to occur at temperatures that would not otherwise occur if there were only cementite in existing pearlite colonies.