Facing the challenge of needs of abundant, cheap and environmental-friendly supply of electricity, the parameters of utility boiler like the ultra-supercritical boilers should be improved to increase the efficiency of power generation, which may seriously influence the safety of thermal power station reduce the useful life of the boiler materials. With the increasing parameters of steam temperature, high temperature corrosion affected by many factors like temperature, corrosive gas and molten salt of water wall or super-heater near the combustion gas might be more and more serious. In additional with the increasing of the parameters of steam, the stress was induced by the nonuniform temperature distribution in the steel. Thus, the external stress in the steel, which plays an important role in the corrosion process, has been outstanding as the temperature increasing. Therefore, the external stress in the steel has been taken in account in the process of hot corrosion happening in the boilers. The hot corrosion behavior of steel T91 was experimentally studied under different stress in complex environment including high temperature and SO2 corrosive atmosphere. Through observing the corrosion products, morphology and compositional changes in corrosion scales formed in the 168h corrosion process through the XRD, SEM and EDS test, it could be informed that the corrosion behavior was strongly associated with the content of chromium oxides in the corrosion scales. Thus under the experimental environment, the oxides scales were mainly made of two layers in which the outer were formed of Fe oxides and the inner one contained Cr oxides and less Fe oxides. Generally, the Cr2O3 or Ni oxides has the most strong corrosion resistance among Cr, Fe, Mn, Ni, so the portion of Cr2O3 in the corrosion scales strongly affect the hot corrosion. Since the corrosion behavior affected by many factors, the effects of external stress might be different in different temperature for example the 60MPa caused most serious corrosion at 700 °C but at 750 °C that is 40MPa. The external stress could accelerate the diffusion of negative ion like S and Cr elements to cause more serious corrosion.

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