Degradation of materials under hot corrosion conditions

The performance of materials at high temperatures is determined by their re sistance to mechanical deformation and attack by the environment. High temp erature corrosion may involve attack in the gas phase or the combined effec ts of hot gases and solid/molten deposits. Hot corrosion can be thought of as deposit modified, gas induced degradation of materials and therefore req uires that a molten salt deposit is present on the materials surface. Due t o an appraisal of the operating conditions within utility turbines which fo rm 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 c orrosion can occur at high temperatures where the deposit is in the liquid state right from the beginning; or the solid deposit turns into liquid duri ng the exposure as a result of reaction with the environment. These two typ es of hot corrosion processes are termed as High Temperature Hot Corrosion (HTHC) or Type I and Low Temperature Hot Corrosion (LTHC) or Type II, respe ctively. Both these types of hot corrosion are encountered in many industri al applications such as boiler and gas turbine components in coal based pow er plants, aircraft industries and certain chemical plants. etc. The role o f 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 com plicated if the impurities such as chlorine and sulphur is present. Since t he molten sulphate inducing the corrosion is an electrolyte, hot corrosion should really be considered as an electrochemical phenomenon and useful inf ormation regarding hot corrosion resistance of alloys and coatings can be o btained using electrochemical polarization tests. Control of aggressive spe cies 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 me asurement of hot corrosion, and possible remedial methods for its control.