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Arcing Contact Materials
Published in Paul G. Slade, Electrical Contacts, 2017
Internal oxidation consists of heating a silver alloy to a temperature below the melting point and allowing oxygen to diffuse into the alloy and react with solute atoms to form metal oxide particles. Following is an example of an equation for the oxidation of silver–cadmium alloy to become silver–cadmium oxide. This shows a parabolic relationship between oxidation depth and oxidation time and the oxidation of silver–tin oxide also has a similar parabolic relationship. Wagner [7] and Freudiger et al. [8] developed Equation 16.2 for oxidation of silver–cadmium alloys in the range 700–900°C. Equation 16.2 below is corrected for a typo in the original paper and also has the constant changed to yield mm versus cm: () X2=(2Ke−A/RT(P)1/2t)/NCd
A Study on the Prevention of Hot Corrosion of Boiler Steel with a High-Velocity Oxy-Fuel Spray-Coating Process
Published in Cherry Bhargava, Amit Sachdeva, Pardeep Kumar Sharma, Smart Nanotechnology with Applications, 2020
Jaswinder Singh, Hitesh Vasudev, Sharanjit Singh
The authors investigated the coating behavior of Cr3C225NiCr coating on a T-91 boiler tube under actual boiler conditions at a temperature of 900 °C [7]. A high-velocity oxy fuel process was used for coating the T-91 sample. Coated and uncoated samples were placed inside the steel boiler for cycles of 100 hours of heating and 1 hour of air cool. The results were examined by X-Ray Mapping, SEM/EDS, and cross-sectional analysis technique. It was observed that internal oxidation attack inside the substrate and corrosion scale formation results in metal loss in uncoated T-91 steel samples. In case of a coated metal surface, there are no internal cracks and formation of corrosion takes place. It indicates that coating helps prevent hot corrosion.
Properties and Applications of Molybdenum
Published in C. K. Gupta, Extractive Metallurgy of Molybdenum, 2017
A valuable property of molybdenum is its resistance to corrosion. This property has not been exploited much in the past, partly because of size and fabrication limitations. The current availability of seamless tubing, clad metal, large sheets, and forgings, combined with the accumulated experience in the fabrication of molybdenum parts, has stimulated considerable interest in the corrosion behavior of this metal. Several small-size molybdenum components have been providing good service in petrochemical plants in a high temperature sulfuric acid environment where nickel-molybdenum alloys, tantalum, and glass-lined equipment did not perform satisfactorily. It appears likely that molybdenum may meet the very stringent materials requirements of some chemical processes now in the development stage, particularly those involving liquid metals and mineral acids at high temperatures. It must be mentioned, however, that molybdenum undergoes rapid oxidation at temperatures above about 550°C. This prohibits its continued use at such elevated temperatures. The oxidation rate of solid molybdenum is not so extreme, however, as to cause the metal to be combustible. The effect of temperature on the oxdiation of unalloyed molybdenum is illustrated in Figure 11. No grain boundary weakening or internal oxidation occurs in this metal. It oxidizes evenly, although there may be some preferential attack at comers or protruding sections. Uncoated molybdenum, therefore, performs satisfactorily in situations where short lives are involved (as in some missile parts) or where the surrounding atmosphere is nonoxidizing. Since no molybdenum base alloys that combine high oxidation resistance and good high-temperature properties have been discovered, protective coatings seem to be the answer where oxidation is a problem. Various coatings, differing in maximum time temperature capabilities and in physical and mechanical characteristics, have been developed. The properties of such coatings and the methods of their production will be dealt with in a later chapter.
High temperature oxidation of 9–12% Cr ferritic/martensitic steels under dual-environment conditions
Published in Corrosion Engineering, Science and Technology, 2018
The specimen after exposure of 25 h (Figure 5(a)) does not form a continuous oxide scale; rather Fe-rich oxide nodules separated by localised regions of chromia (Cr2O3) layer. Formation of chromia at this temperature (650 °C) is not continuous allowing the formation of rapidly growing Fe-rich oxide scale. With increasing oxidation time, the chromia layer disappears and is replaced with continuous scale of Fe-rich oxides after 250 h (Figure 5(b)) and 1000 h (Figure 5(c)). T92 steel oxidised for 25 h also shows internal oxidation in few regions of the oxide/alloy interface. Internal oxidation is a typical feature of oxide scales developed in Fe-Cr alloys exposed to water vapour containing environments and is caused by preferential oxidation of Cr to Cr2O3 or Cr-rich (Fe,Cr)3O4 [20]. However, no water vapour is present in the flue gas environment to which the T92 specimens are exposed to. Therefore, the presence of internal oxidation on the flue gas side suggests that hydrogen generated on the steam side diffuses through the base alloy towards the flue gas side. Substantial void formation is also observed in both the outer and inner scales of all the oxidised specimens. The size of the voids is relatively large in the outer scales (Figure 5). Several voids and pores in the oxide scale are supposed to be due to the condensation of cation vacancies, since the growth of the outer oxide is primarily due to the outward diffusion of Fe cations [6,21].