Nuclear Engineering Division

Corrosion and Mechanics of Materials

Fossil Energy

 

Conceptual designs of advanced coal-fired combustion systems require furnaces and heat transfer surfaces that operate at much higher temperatures than those in current coal-fired power plants. The combination of elevated temperatures and hostile combustion environments necessitates the development and application of ceramic materials in these designs. However, downstream of the combustion zone, a transition from ceramic to metallic materials will be required, and the metallic components will experience much more elevated temperatures than those in current combustion systems. Furthermore, they will be subjected to combustion environments in which the deposit and gas chemistries could be different from those in current boiler systems. Studies of the high-temperature corrosion of metallic and ceramic materials are being conducted in simulated coal-gasification atmospheres and coal-combustion environments typical of conventional pulverized coal-fired boilers, fluidized-bed systems, and low-NOx boilers and Advanced Fossil Power Systems. This effort is directed at establishing mechanisms of corrosion, predicting the onset of breakaway corrosion, and evaluating the role of stress and deformation in corrosion processes.

Comparison of the temperature dependence of the oxidation rate constant (in post-breakaway region) for air oxidation of steam-preoxidized Zircaloy-4 and Zirlo derived from this project with those for Zircaloy-4, based on previous correlationsComparison of the temperature dependence of the oxidation rate constant (in post-breakaway region) for air oxidation of steam-preoxidized Zircaloy-4 and Zirlo derived from this project with those for Zircaloy-4, based on previous correlations. Click on image to view larger image.

Sulfidation and chloridation resistance of structural alloys (namely, model Fe-Cr, Fe-Cr-Ni, and Fe-Cr-Al alloys; Fe-Al intermetallics with controlled additions of several refractory metals and rare earth elements; and commercial high-Cr alloys) is being evaluated by thermogravimetric tests, and postexposure microstructure of corrosion scales is being characterized by several electron/optical microscopy techniques. Alloy additions are made by bulk alloying and as overlay or diffusion coatings. Another major thrust of the program is to evaluate the effects of corrosive environments on the tensile and creep properties of metallic alloys and fracture toughness properties of monolithic and ceramic/ceramic composite materials. Further collaborative programs, supported by DOE’s Fossil Energy and its Materials Division of Basic Energy Sciences, are in progress to examine fundamental aspects of scaling in structural materials exposed to oxidizing environments and to develop ultrahigh-strength Mo-Si intermetallic alloys and MoSi2-Si3N4 composite materials.

Our recent work has focused on the corrosion behavior of alloys in advanced coal-fired combustion systems. In advanced boiler systems, the presence of slag constituents, sulfur, alkali, and chlorine determine the thermodynamic activity of various deposit constituents. An important difference between the conventional boiler system and the advanced system is in the chemical and physical characteristics of the ash layers that can be deposited on the heat transfer surfaces. Such deposits can lead to corrosion of waterwall boiler tubes, steam superheaters, and air tubes during service in coal-fired systems. Fire-side metal wastage in conventional coal-fired boilers can occur via gas-phase oxidation or deposit-induced liquid-phase corrosion. The former can be minimized by using materials that are oxidation resistant at service temperatures of interest. On the other hand, deposit-induced corrosion of materials is an accelerated type of attack influenced by the vaporization and condensation of small amounts of impurities such as sodium, potassium, sulfur, chlorine, and vanadium, or their compounds, that are present in the coal feedstock. In advanced combustion systems, because the metals in the superheater regions will be at a much higher steam temperature, the alkali sulfate and coal ash will be the predominant deposit. Several factors (including sulfur, alkali, chlorine in coal feedstock; excess air level during the combustion process; and metal temperature) determine the extent of corrosion of superheater materials in coal-fired boilers.

The objective of the present work is to evaluate the corrosion performance of state-of-the-art candidate materials in coal ash, alkali sulfate, and alkali chloride environments at temperatures in the range of 575-800°C. The experimental program is aimed at developing an understanding of corrosion mechanisms as a function of alloy composition and deposit chemistry, and at quantitatively determining the scaling and internal penetration of the alloys.

The figure below (a) shows the weight change data for nine alloys tested in mixtures of coal ash and alkali at temperatures between 575 and 800°C for 1000 h. The results indicate that the weight loss rates increase with temperature up to 725°C and decrease to low values at 800°C. In fact, several of the alloys showed either negligible weight loss and/or gained weight after exposure at 800°C. The corrosion rates followed a bell-shaped curve with peaks near 725°C for all the alloys used in this study. The data in this figure should only be used qualitatively, because the weight loss rates could have been affected by the removal of scales during removal of the coal ash deposit from the specimen surface, even though every effort was made to remove only the ash deposits by brushing. Generally, the scales adhered more to some alloys than others, and caution should be exercised in using this information for corrosion allowance for components.

The following figure (b) shows the weight loss rates for eight alloys at 650 and 800°C, obtained when exposed in a deposit mixture that contained 5 wt.% NaCl. Based on the weight loss data, the presence of NaCl in the deposit has little effect on the corrosion rate at 650°C. Most of the alloys used in this study exhibited similar weight loss rates after exposure in deposits with and without NaCl. On the other hand, at 800°C, several alloys exhibited accelerated corrosion in the presence of deposit that contained NaCl, when compared with the rates observed for the same alloys in the absence of NaCl. Currently, the study is continuing to establish the performance envelopes for advanced alloys for use in DOE’s Vision 21 coal-fired system.

Weight loss data for alloys after exposure in mixtures of synthetic coal ash and alkali sulfates at 575 to 800°CFigure (a): Weight loss data for alloys after exposure in mixtures of synthetic coal ash and alkali sulfates at 575 to 800°C. Click on image to view larger image.

Weight loss data for alloys after exposure in mixtures of synthetic coal ash, alkali sulfates, and NaCl at 650 to 800°CFigure (b): Weight loss data for alloys after exposure in mixtures of synthetic coal ash, alkali sulfates, and NaCl at 650 to 800°C. Click on image to view larger image.

Last Modified: Thu, April 21, 2016 4:56 AM

 

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Corrosion and Mechanics of Materials
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