Standard surface defects produced by water vapour oxidation in steels used in fossil fuel fired power plants

The present work describes the population of surface defects produced by steam in steels typically used in superheater, reheater and water wall tubes of fossil fuel fired power stations at working temperatures. An accurate statistical description of material defects (in the as-manufactured condition and of those defects induced in service), and the service conditions (temperature, stress, time, ambient…) is necessary to assess correctly the life of a component on a sound, scientific bases and not only in an empirical way. Oxidation-induced defects were generated in the laboratory using Ar, saturated with water vapour for different times and at various temperatures, representative of those expected in service. The population of surface defects has been measured by conventional metallographic procedures. An evolution is observed, with time and temperature, of the mean size of surface defects in carbon steels but not in SA-213-T22 ferritic steel. The surface density of defects for carbon steel SA-210 decreases with time by growth and coalescence of neighbouring pits. For carbon steel SA-178-C, the oxidation takes place by lateral growth of the pits that meet, eventually spalling the metal. The defect densities in samples that operated under real conditions (true service 1, TS.1 and TS.2) are smaller than in the as-received material and laboratory-oxidised samples. A coalescence mechanism among neighbouring pits is also observed for these samples.