M. Schütze, G.T. Schmidt, A. Naji
Life-time extrapolation has often been performed based on a parabolic oxidation law. The new standards, ISO 21608 and ISO 26146, aim at determining the actual oxidation rate constant and exponent in order to describe the oxidation behavior as closely as possible to the observed situation. The present paper extends this approach by the kinetics of subsurface zone depletion of the protective scale-forming alloying element (for example, chromium). Four different types of characteristic depletion kinetics are combined with considerations on the minimum amount of chromium necessary to maintain the protective situation in order to define the extrapolation basis for oxidation-related life-time assessment. This approach is applied to experimental data from exposure in simulated coal combustion environments at temperatures of 550–750 °C and exposure times up to 15 000 h. The materials ranged from low-alloy steel to high-alloy austenitic steels and nickel-base alloys. As a result, it is proposed to use data from metallographic investigations rather than mass change data for life-time assessment together with the respective kinetics approaches. Furthermore, it is recommended to base life-time extrapolation on experimental data from tests of at least 10 000 h duration. The investigated materials were categorized with regard to their potential of oxidation-related life-time assessment.