Trikur A. Ramanarayanan

The influence of yttrium on oxide scale growth and adherence

Alloys and coatings for high-temperature service are designed to form selectively chromia scales, alumina scales, or, to a limited extent, silica scales upon exposure to the environment. For such oxide scales to be protective, they should be both slow growing and adherent. It turns out that the addition of yttrium to such alloys can often impart both characteristics to the oxide scale. However, the actual operating mechanisms continue to be a matter of controversy among researchers in the area of oxidation. In the present study, the growth and adherence of alumina and chromia scales on alloys containing yttrium, either as an oxide dispersion or as an intermetallic phase, have been investigated in conjunction with detailed oxide scale characterization using the techniques of scanning electron microscopy (SEM), transmission electron microscopy (TEM), and secondary ion mass spectrometry (SIMS). The results of the study are used for critical assessment of the proposed mechanisms, especially the more recent ones, and to suggest some new mechanisms for adherence.

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.

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

The corrosive degradation of high-temperature alloys in environments containing more than one oxidant cannot, in general, be predicted from a knowledge of the response of the materials to the individual oxidants. In the present study, the phenomenological changes associated with the degradation of iron-nickel-chromium base alloys in carbon-oxygen environments have been investigated by examining the microstructural changes in samples exposed to such environments for extended periods of time. The results of these studies have led to the formulation of a model which proposes that the material exposed to the reaction environment experiences five stages of microstructural changes close to the surface before severe degradation sets in. The end of Stage V is the start of severe degradation, which contributes to a complete modification of the microstructure. This, in turn, leads to a rapid deterioration of the mechanical properties of the material.

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.

Computational modeling of mixed oxidation-carburization processes: Part 1

Heat-resistant alloys used in mixed-oxidant environments rely on the formation of a chromia, alumina, or silica surface film for corrosion resistance and the presence of second-phase precipitates in the matrix often for their strength properties. The growth of the oxide film on such alloys is often accompanied by the dissolution of precipitates in the alloy subsurface region. Continued oxidation combined with oxide-scale spallation tends to decrease the content of the oxide-forming constituent to such a level that protective scaling can no longer occur and severe degradation can develop. In the present work, the initial corrosion processes involving the complex coupling between oxide scale growth and precipitate dissolution is simulated computationally. As an example, a Ni-Cr alloy containing Cr23C6precipitates was exposed to an oxidizing-carburizing environment. An approach combining finite difference and Newton-Raphson methodologies is developed to model this diffusion/ dissolution process, incorporating the point-defect-chemistry aspects of the oxide scale. The model is able to predict the chemical and microstructural evolution of high-chromium austenitic alloys during the initial stages of oxidation-carburization.

Carburization of high chromium alloys

Iron-nickel-chromium based heat resistant alloys are designed to operate at high temperatures in corrosive gaseous environments. Under mixed oxidizing-carburizing conditions, the microstructure of such materials changes progressively during service and the physical and mechanical properties are altered. One instance where such microstructural changes are encountered is in furnace tubes for pyrolysis applications. In the present study, kinetic experiments in the laboratory are combined with microstructural observations on alloys which have undergone long service times to develop an understanding of the fundamental processes that induce property changes in the material. Based on this study, four distinct stages are identified. These include: Initial oxidation, Oxidation in a carburizing environment, Direct carburization and Internal oxidation. Each of these stages is described. Questions are posed with respect to the sudden alteration in process stream chemistry or temperature. For instance, how does a drop in the oxygen partial pressure to levels where a chromium oxide film is unstable affect a preformed film? What beneficial effects are provided by inhibitors such as H2S especially under conditions where an oxide film cannot form?

The Role of Morphology and Structure in the Kinetic Evolution of Iron-Sulfide Films on Fe-Base Alloys

Sulfidation corrosion of 4130 steel in CH3SH was studied in the temperature range 250–550°C. The rate of sulfidation attack was found to be a function of temperature and sulfur activity. Investigations of the corrosion process led to the proposal of two mechanisms of sulfidation, dependent on temperature. Cation diffusion through the iron sulfide corrosion product is the rate-determining step at higher temperatures (>370°C), while a surface reaction was identified as the rate-limiting step at lower temperatures. The corrosion scale has preferred orientation as determined by X-ray diffraction and morphological observations. The lower-temperature corrosion product is made up of columnar grains of pyrrhotite crystals with the c-axis aligned nearly perpendicular to the steel substrate. At high temperatures, a whisker morphology developed with the whiskers having variable texture with respect to the steel substrate. A preformed-surface-oxide layer on 4130 steel does not appear to significantly reduce sulfidation corrosion.