Changmin Chun

Carbon‐Induced Corrosion of Nickel Anode

In high‐temperature solid oxide fuel cells where natural gas is used as a fuel, high‐carbon‐activity environments can be encountered in the anode compartment. Under these conditions, nickel could corrode by a process known as metal dusting. In the present study, metal dusting corrosion of pure nickel is simulated in high‐carbon‐activity environments at temperatures between 350 and 1050°C. The focus of this research is to understand reaction mechanisms by characterizing interfacial processes at the nanometer level. Nickel corrodes by a combination of carbon diffusion and precipitation in the bulk metal and atom migration through surface carbon deposits. The nature of the carbon deposit is important in the overall corrosion process. At lower temperatures closer to about 350°C, nickel forms a carbide. , which is rather stable and does not decompose.

Mechanisms of Metal Dusting Corrosion of Iron

Metal dusting is a severe form of corrosive degradation that Fe, Co, and Ni base high-temperature alloys undergo when subjected to environments supersaturated with carbon (a c > l). This corrosion process leads to the break-up of bulk metal into metal powder. The present study focuses on the fundamental understanding of the corrosion of Fe in carbon-supersaturated environments over the temperature range 350-1050°C. Building on earlier research, the role of deposited carbon in triggering corrosion is further clarified. The corrosion rate peaks at ∼575°C with a sharp decrease in rate on either side of the maximum. High-resolution electron microscopy reveals, in addition to metal particles, a mixture of graphitic carbon, amorphous carbon, and filamentous carbon in the corrosion product. While the presence of a surface layer of Fe 3 C is characteristic of corrosion up to 850°C, such a layer is absent at the higher temperatures. The focus of this research is to understand reaction mechanisms by characterizing interfacial processes at the nano level.

Metal-Dusting Corrosion of Low-Chromium Steels

Metal dusting is an aggressive form of corrosive degradation that Fe−, Ni− and Co-base, high-temperature alloys undergo when subjected to environments supersaturated with carbon (ac > 1). This corrosion process leads to the actual conversion of bulk metal to powder or dust. The present study focuses on the fundamental understanding of the corrosion of low chromium steels containing about 1.25–13 wt.% Cr in carbon-supersaturated environments (CO–H2) over the temperature range, 650–1100 °F (343–593 °C). With increasing Cr content the overall corrosion rate decreases and the corrosion becomes more localized. All low-chromium steels in an overall sense disintegrate by metastable surface M3C growth and its subsequent decomposition upon carbon deposition in good agreement with earlier research on the metal-dusting mechanism of pure iron. While the presence of a continuous surface layer of M3C is characteristic of general corrosion, such a layer is absent in low chromium steels having more than 5% chromium (e.g. 9Cr and 13Cr). High-resolution electron microscopy of such steels reveals, in addition to metal particles and a mixture of graphitic and amorphous carbon, stable carbide (M7C3) particles in the corrosion product. The mechanistic aspects of metal dusting are discussed with particular attention to stages of microstructure degradation process of low chromium steels.

Metal Dusting Corrosion of Cobalt

Metal dusting is a severe form of corrosive degradation of metals and alloys at high temperatures (350-950°C) in carbon-supersaturated gaseous environments. Fe, Ni, and Co, as well as alloys based on these metals are all susceptible. The corrosion manifests itself as a break-up of bulk metal to metal powder, hence, the term metal dusting. In the present study, metal dusting corrosion of pure cobalt is simulated in high carbon activity environments at temperatures between 350 and 950°C. The focus of this research is to understand reaction mechanisms by characterizing interfacial processes at the nanometer level. Cobalt corrodes by a combination of carbon diffusion and precipitation in the bulk metal and atom migration through surface carbon deposits. The nature of the carbon deposit is important in the overall corrosion process.

Transport through chromia films

Abstract Chromium oxide surface films that form in situ on alloy surfaces are the basis for providing high-temperature corrosion resistance when such alloys are used in high-temperature service. While the slow growth kinetics of chromium oxide is integral to its acting as a corrosion barrier, its periodic growth and spallation finally render alloys unprotective when the chromium concentration in the alloy gradually decreases from about ∼25–30% to about 10–15%. At these latter concentrations the ability of the alloy surface to form a continuous chromium oxide film becomes severely compromised. The ability to decrease the growth kinetics of chromium oxide films can thus prolong the service life of such alloys. Certain rare earth elements such as Ce and Y, whether they are introduced into the alloy as a dispersion of oxides or ion-implanted on the surface, have the ability to significantly reduce the growth rate of chromium oxide. Concomitantly, the major migrating species in the oxide film changes from chromium to oxygen. There is controversy in the literature on the mechanisms leading to these effects. The present study provides further advances in our understanding of this important effect.

Carbon-Induced Corrosion of Metals and Alloys

Many high temperature alloys are based on Fe, Ni and Co with significant levels of chromium added for corrosion resistance. During service in carbon-rich environments, such alloys can degrade by two distinct corrosion processes. One is carburization, which generally occurs at temperatures in the range, 800 - 1100° C, while the other is metal dusting which typically manifests itself in the range, 400 - 800° C. In the present paper the sequential stages of alloy degradation when exposed to mixed carburizing-oxidizing environments having a carbon activity of 1 are discussed. Four distinct stages of microstructure evolution are described. In the final stages, carbon diffusion into the alloy interior followed by the precipitation of stable, brittle carbide phases affect the mechanical integrity of the material. By contrast metal dusting is a process that occurs in carbon- supersaturated environments (carbon activity > 1) and results in the actual conversion of bulk metal to powder or dust. The mechanistic aspects of metal dusting are discussed with particular attention to the behavior of Fe.

Metal Dusting Resistant Copper-Based Materials

Cu and Cu-based alloy coatings suppress carbon transfer to the metal surface from carbon-supersaturated gaseous environments. The metal dusting resistance of such coatings has been investigated in CO-H2 gas mixtures at temperatures ranging from 450°C to 700°C. Conventional plating and cladding methods have been used to deposit Cu-based materials on the surfaces of commercial materials such as 2.25Cr-0.5Mo steel and carbon steel. In addition to the performance of Cu-based materials under laboratory conditions, the effect of alloying elements such as Sn, Zn, Ni, Si, and Al in Cu on the corrosion process and the coating/substrate interface stability are discussed with particular attention to commercial scale applications.

Electron-Ion Transport in ZrO2-Y2O3-CeO2 Ceramics

Simultaneous conduction of oxide ions and electrons in solid ceramic systems provides the capability for oxygen transport under a concentration gradient without the need for an externally applied electric field. In the present study, ionic transference numbers have been measured in the ZrO2-5.8%Y2O3-10%CeO2 system by open circuit Emf measurements involving different metal/metal oxide electrodes. In order to correlate the ionic transference number with grain size, high-density ceramic discs of different grain sizes (50 nm–5 μm) were prepared by sintering pressed powders at various temperatures and times. Hydrothermal synthesis was used to prepare nanocrystalline powders of the above material with uniform crystallite size (10 nm) and chemistry. Emf measurements on the samples suggested both ionic and electronic transport, the ionic transference number decreasing with increase in the grain size. This observation was attributed to an increase in the amount of continuous crystalline grain boundary phase in the ceramics as the grain size increased. The presence of crystalline silicate and zirconate phases in the grain boundary region was confirmed by electron microscopic imaging combined with microanalysis. In the large grain (5 μm) ceramics, the ionic transference number decreased linearly with temperature. As the grain size decreased, a maximum occurred in the ionic transference number vs. temperature curve. This maximum became more pronounced at smaller grain sizes. Better grain-grain contact and the doping effect of trivalent Ce in the grain boundary core are proposed to explain this observation.