Effect of Superficially Applied Y 2 O 3 Coating on High-Temperature Corrosion Behavior of Ni-Base Superalloys
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INTRODUCTION
NICKEL-BASE superalloys are the commercial alloys commonly used for the manufacture of components used in aggressive environments such as those in gas turbines and steam boilers, where these alloys are vulnerable to degradation by high-temperature corrosion. High-temperature corrosion (commonly known as hot corrosion) is the degradation of metals and alloys as a result of the oxidation process at increased temperatures, which is aggravated by a liquid salt deposit. During this mode of corrosion, the materials are subjected to degradation at much higher rates than in gaseous oxidation, with a porous, nonprotective oxide scale formed at their surface, and sulfides in the substrate. Hot corrosion is a serious problem in power generation equipment, in gas turbines for ships and aircraft, and in other energy conversion and chemical process systems such as in boilers, internal combustion engines, fluidized bed combustion, and industrial waste incinerators.[1] A prime example is the attack of Ni-, Co-, or Fe-base alloys by Na2SO4, which has served as a model for degradation process in gas turbines and other fossil-fueled devices. This type of corrosion causes heavy GITANJALY GOYAL, Associate Professor, is with the National Institute of Technology, Srinagar, Jammu and Kashmir 190001, India. HARPREET SINGH, Assistant Professor, is with the Indian Institute of Technology Ropar, Roopnagar, Punjab 140001, India. Contact e-mail: [email protected] SURINDRA SINGH and SATYA PRAKASH, Professors, are with the Indian Institute of Technology Roorkee, Roorkee, Uttrakhand 247667, India. Manuscript submitted December 20, 2009. Article published online September 25, 2010 METALLURGICAL AND MATERIALS TRANSACTIONS A
economic losses to the concerned industries in terms of more downtimes, repair, and maintenance costs. However, it has been learnt from the literature that these corrosion costs can be reduced at least by 30 pct by devising effective and industrially viable corrosion control strategies. The current study is an attempt toward this direction and outcome of the study would provide useful data for designing corrosion-control methodologies for high-temperature applications such as power plant boilers and gas turbines. Furthermore, the superalloys are the potential candidate materials for these high-temperature applications, such as super- and ultra-super-critical boilers of new generations. Several countermeasures have been investigated to control the hot corrosion of materials such as use of protective coatings, controlling the corrosion environment/process parameters, application of oxide additives, or inhibitors. Numerous inhibitors are commercially available that have been investigated to reduce the severity of hot corrosion, such as Mg- and Mn-based additives, CaO, MnO2, Al2O3, ZnO, BaO, PbO, SiO2, BaO, Ba(OH)2, Ca(OH)2, CaCO3, as well as oil-soluble Ni, Al, Fe, and other compounds.[2] Yedong and Stott[3] studied the effects of surface-applied Y2O3, Al2O3 and Cr2O3 coatings or films on the selective oxidation o
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