NATURE OF THE CORROSION PROBLEM
Unprotected waterwall tubes (carbon steel) in steam generating boilers suffer degradation due to chemical corrosion and erosion.The sulfur content of thefuel and high operating pressures of the boiler (and,therefore,tube temperature) are the main factors causing wastage of the carbon steel tubes.The sulfur content and metal temperature result in the formation of free sulfur which then reacts with the iron of the tubes,coverting it to iron sulfide(Fe+S--FeS)(Plumley and Roczniak,1974).Moreover,the rate of corrosion is ofen related to the operating conditions of the boiler.For example,operating the boiler above rated performance capacity can accelerate the corrosion process.Too,the introduction of excess air into the boiler may create abnormally high levels of SO2 and lead to accelerated corrosion.A number of other factors,such as operating PH,preheat temperatures and solid contents of the black liquor fuel,can also lead to accelerated corrosion of the tubes.
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