Quantification of the role of parameter variation under cyclic oxidation testing - Development of a european standard for the investigation of high temperature oxidation resistance (COTEST)

The effect of cycling parameter variation (i.e. oxidation temperature, upper and lower dwell time, humidity in test gas) of the oxidation/ spallation kinetics on four alloys was investigated within the framework of an EC funded research project (COTEST). For this purpose, specimens of AISI 441, Alloy 800H, CM 247 and P91 were subjected to thermocyclic testing in dry or humidified air. It was found that a minimum of 300 h accumulated hot dwell time is required for meaningful test results. Detailed characterisation of the corrosion products was performed using OM, SEM/EDX and XRD. The net weight change curves were evaluated with regards to the characteristic quantitative parameters describing oxide growth rate, time to onset of spallation/breakaway and weight of spalled oxide. Analysis of these values was made by the ANOVA method, which allows assessment of the significance of the roles of different test parameters and parameter variations on the oxidation/ spallation kinetics from a limited number of experiments. A code of practice have been developed and is currently discussed in the working group ISO TC 156 WG 13.