Key experimental parameters in steam oxidation testing and their impact on interpretation of experimental results

Abstract

The acceptance of materials for extended duration, safety critical power generation applications usually requires several stages of testing and data generation. Simple, short term exposures under nominally constant atmospheres and temperatures can eliminate materials that are grossly unsuitable, but do not differentiate between materials that have broadly acceptable properties. To better differentiate between candidate materials it is desirable to tailor laboratory tests such that they more closely replicate in service conditions. In terms of components that are exposed to steam oxidation degradation mechanisms, this means replicating the steam conditions with an aim of producing oxide scale morphologies similar to that seen in service. Key experimental parameters have been identified, including water chemistry, pressure, steam delivery and flowrate, and a series of steam exposure tests on ferritic (P92), austenitic (Esshete 1250) and superalloy (IN740) material conducted to evaluate their effect on degradation rate and oxide scale morphology. The oxidation rate of the austenitic, and to a lesser extent the ferritic, material was found to be sensitive to the level of dissolved oxygen in the feed water, low (10 ppb) dissolved oxygen levels producing an increase in oxidation rate. The propensity to spall was also found to be reduced at low dissolved oxygen concentrations. In addition, the steam pressure and steam delivery method were shown to affect the oxidation rate and scale morphology for these materials.

Keywords:

• HIGH TEMPERATURE TESTING
• STEAM OXIDATION TESTING
• OXIDE MORPHOLOGY
• POWER GENERATION COMPONENTS
• STEAM CHEMISTRY
• MATERIALS SELECTION