Cyclic oxidation - guidelines for test standardisation, aimed at the assessment of service behaviour

This paper reviews current procedures used for cyclic oxidation testing and their ability to simulate service behaviour for life time prediction model ling. Test complexity varies from a simple laboratory, slow cycle, furnace test to the use of high velocity dynamic combustion rigs. It is shown that the response of an alloy under cyclic oxidation testing varies, depending o n the exact test conditions chosen and the length of test exposure, but for many alloy systems this response has common features. It is proposed that ail alloys that are protected by a stable, slow growing oxide scale conform to a common, generic behaviour under cyclic oxidation conditions. This lif e cycle of such alloys involves first the formation of a stable, protective oxide. At a critical thickness this may start to spall, increasing the rat e of scale Forming element consumption. When the activity of such elements falls below a critical level, in the near surface region, internal oxidatio n results together with the formation of less protective scales. Ultimately , it is no longer possible fur the alloy to self repair following a thermal cycle and then breakaway corrosion ensures, marking the end of the alloys life. Thus the choice of cyclic test procedure must be tailored to the particular phase of the alloy life cycle under investigation.