Mixed-oxidant attack of high-temperature alloys in carbon- and oxygen-containing environments

A Model for High-Temperature Pitting Corrosion in Nickel-Based Alloys Involving Internal Precipitation of Carbides, Oxides, and Graphite

“…In the past, several authors have suggested that the internal precipitation of carbides and oxides can be a source of surface disintegration during the high-temperature exposure of heat-resistant alloys [20,43,51] and, thus, can be related to the observed pitting corrosion behavior at lower temperatures. In particular, the in-situ oxidation of internal carbides is associated with large volume changes, because phases such as Cr 2 O 3 , Al 2 O 3 , and graphite have a low density compared to nickel.…”

“…[35] This suggests that a complex transport mechanism is operative during the steady-state part of the degradation process, which also involves the counter-diffusion of molecular species across the boundary layer or across the reaction zone established at the surface. [35] However, since the alloy is often protected by a stable surface oxide layer, [42,43] the accompanying carbon penetration into the bulk of the material with consequent carbide formation must occur by diffusion through already existing pores or cracks in the oxide. [44] Therefore, in order to prevent subsequent carbide formation, a certain amount of oxidation is needed to maintain or restore the protective oxide layer, which, in turn, requires the diffusion of oxygen and oxide-forming species toward the metal surface.…”

High Temperature Corrosion of Cast Alloys in Exhaust Environments. II—Cast Stainless Steels

“…Here, the supply process is quite complex since it involves Cr and Mn depletions from the matrix as well as Cr carbide depletion. The later, due to the growth of Cr 2 O 3 layers, have already been described [1,5,47]. As Cr 2 O 3 forms, the subjacent bulk gets depleted in Cr to such a level that Cr carbides are no longer stable within the matrix and dissolve.…”

“…Carbon diffuses away towards the interior of the sample to suppress this concentration gradient. It has been observed [5,47] that in highly oxidative atmospheres, C would redistribute to the interior of the sample and form carbides. This may be the case if the oxide scale is dense enough to forbid the transport of C to the surface through pores or channels.…”

“…Thus, the high temperature behavior of cast stainless steels has been the subject of many investigations, e.g. in [1][2][3][4][5][6]. It has been recognized [1][2][3], that the very pronounced microstructural features of these types of alloys can affect the oxidation behavior of the alloy.…”