Moisture Induced Damage in Oxide Scales

Water vapor is present in most environments used in high temperature applications. The water vapor content can vary from a few Vol.% for example in coal combustion environments, to as much as 100 Vol.% in steam turbines. The effect of water vapor on oxide scale growth has been the focus of high temperature materials research for over a decade, revealing for example increased growth rates or more pronounced inward growing oxides in the presence of water vapor. However, the effects of water vapor and liquid water on oxide scale integrity and mechanical properties have, up to now, not been investigated in detail, even though a few reports exist that show increased alumina scale spallation on contact with liquid water [1, 2]. Also, moisture induced failure of thermal barrier coatings has been reported [3, 4]. Since high-technology materials used in high temperature applications rely on protective oxide scales, a loss of the protective oxide limits the materials life time and has to be avoided.

The aim of this project is therefore to investigate the influence of water vapor as well as liquid water on the development of stress states and damage evolution in oxide scales (Moisture Induced Damage in Oxide Scales, MIDOS). Several metals that are commonly used in high temperature applications and the respective oxides were chosen for this investigation. The growth of oxide scales as a result of high temperature oxidation will be studied in humid and dry environments, respectively, and the intrinsic growth stresses will be monitored in-situ in combination with acoustic emission measurement. The mechanical integrity of the formed oxide scales will be studied after cooling to room temperature by means of 4 point bending experiments and acoustic emission investigations.