Steam Oxidation of 9% to 12% Cr Steels: Critical Evaluation and Implications for Practical Application

In new and future generation power plants, creep-resistant steels selected for high-temperature components are subjected to several service environments. In many of these applications, water vapor has been found to substantially alter the technologically relevant properties, mainly growth rate and adherence, of the surface oxides. Various data sets exist in the literature for steam oxidation for this type of steel; however, these data show a substantial variability. These differences can be related to, e. g., batch-to-batch variations, oxidation conditions, surface treatment, and differences between laboratory/power plants data. Therefore, these literature data need careful and critical verification when being used to estimate the long-term materials behavior in real power generation plants. The paper discusses the existing steam oxidation data for the martensitic steels P91 (9% Cr-V-Mo), P92 (9% Cr-W-V-Mo), and VM12 (12% Cr-Co-W-V-Mo), with the main emphasis on discussing the parameters that are responsible for the observed variations in oxidation rates for the various types of materials. Differences in minor alloying additions and surface treatment have as a result that for the 9%Cr steels the scale thickness after a given oxidation time may easily differ by a factor of two. Because of the batch-to-batch variation in oxidation behavior, the common procedure used to determine service temperatures of tubes made of low-Cr steels on the basis of measured oxide layer thicknesses, has to be used with some care in case of the 9% to 12%Cr martensitic steels. This is especially the case for the 12%Cr steels because their oxidation rates between 500 degrees C and 650 degrees C may not steadily increase with increasing temperature.