Corrosion behavior of ferritic steels on the air sides of boiler tubes in a steam/air dual environment

The corrosion behavior of heat resistant ferritic steels for boiler tubes in a steam/air dual environment simulating the non heated areas at 600°C has been investigated. Corrosion rates of the ferritic steels on the air sides under a condition of steam/air dual environments were increased more significantly than those in simple air conditions because of the permeation hydrogen from the steam side. Amounts of hydrogen permeated through the test tubes were measured electrochemically using a proton conductive solid electrolyte (5mol%Y2O3–SrCeO3) The results of hydrogen permeation current measurements show that the amount of permeated hydrogen from the steam sides decreased parabolically with time and decreased with increasing Cr content in the steels. The corrosion attack of the outer surfaces saturated under a certain permeated hydrogen level and the marginal hydrogen content needed to accelerate the corrosion of the air side depended on the Cr content in the steel. Therefore, a part of the permeated hydrogen react with oxygen to form H2O at the metal/oxide interface on the air sides. That is, the corrosion mechanism may be the same as that of steam oxidation.