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|Title:||Degradation of materials under hot corrosion conditions|
|Citation:||TRANSACTIONS OF THE INDIAN INSTITUTE OF METALS, 51(5), 279-290|
|Abstract:||The performance of materials at high temperatures is determined by their resistance to mechanical deformation and attack by the environment. High temperature corrosion may involve attack in the gas phase or the combined effects of hot gases and solid/molten deposits. Hot corrosion can be thought of as deposit modified, gas induced degradation of materials and therefore requires that a molten salt deposit is present on the materials surface. Due to an appraisal of the operating conditions within utility turbines which form part of a combined cycle coal fired power plant the deposition of alkali sulphates onto hot parts is an inherent product of the process. Thus hot corrosion can occur at high temperatures where the deposit is in the liquid state right from the beginning; or the solid deposit turns into liquid during the exposure as a result of reaction with the environment. These two types of hot corrosion processes are termed as High Temperature Hot Corrosion (HTHC) or Type I and Low Temperature Hot Corrosion (LTHC) or Type II, respectively. Both these types of hot corrosion are encountered in many industrial applications such as boiler and gas turbine components in coal based power plants, aircraft industries and certain chemical plants. etc. The role of the liquid salt is to dissolve the protective oxide scale by the acid or basic fluxing mechanism of the salt. The dissolution process is further complicated if the impurities such as chlorine and sulphur is present. Since the molten sulphate inducing the corrosion is an electrolyte, hot corrosion should really be considered as an electrochemical phenomenon and useful information regarding hot corrosion resistance of alloys and coatings can be obtained using electrochemical polarization tests. Control of aggressive species in coal and fuel oils and use of corrosion resistant materials are the two approaches to counter hot corrosion. The alloys or protective coatings are selected on the basis of their resistance to hot corrosion by forming protective oxides which have little tendency to dissolve in the liquid melt. This review paper discusses the basic mechanism of materials degradation due to hot corrosion, recent advances in electrochemical methods for the measurement of hot corrosion, and possible remedial methods for its control.|
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