High temperature corrosion of valve steels in atmosphere containing water vapor

The kinetics and mechanism of high temperature corrosion of two valve materials (X33CrNiMn23-8 and X50CrMnNiNbN21-9 steels) in water-vapor-containing atmosphere have been studied as a function of temperature (973-1,273 K) and gas composition, being the mixture of nitrogen, oxygen, and water vapor. In all experiments this atmosphere contained 50 vol.% of water vapor and the concentration of oxygen was changed from 0.001 to 50 vol.%. The oxidation kinetics have been studied thermogravimetrically in microthermogravimetric equipment, enabling the registration of weight changes of the oxidizing samples as a function of time with the accuracy of the order of 10(-6) g. It has been found that the corrosion process of both steels is rather complex and can not be described by one kinetic rate law. X33CrNiMn23-8 steel, containing higher chromium concentration, shows better corrosion resistance than the X50CrMnNiNbN21-9 steel, but both these materials undergo more rapid corrosion in water vapor than in oxygen atmospheres. This phenomenon results from non-protective, heterogeneous scales, formed in water vapor, as compared to protective Cr2O3 scales, developing in oxygen atmosphere. It may be then concluded that the presence of water vapor in corrosive atmosphere plays a definitely negative role in corrosion processes.