Experience with the use of the acoustic emission technique for monitoring scale failure in wet and dry environments

Abstract

A brief survey of the acoustic emission technique for monitoring scale cracking and failure on 2.25–24% Cr steels in wet and dry environments is given. A number of acoustic emission test rigs are described. Some of the more simple test rigs are used for testing small oxidation coupons during isothermal oxidation. More sophisticated rigs have been used for testing full size heat exchanger tubes during thermal cycling.Most acoustic emission measurements in a wet environment come from testing at temperatures below 650°C. There are examples from Alloy 800 and thermal barrier coatings that were tested at higher temperatures, 900°C and 1100°C, respectively. Through the years acoustic emission tests have been performed in dry air, dry air+10%H2O, dry air+0.5%SO2, and Ar+5%H2+50%H2O. Consequently, a wide variety of exposure temperatures and atmospheres can be investigated using acoustic emission techniques.Qualitative acoustic emission results can detect when scale cracking occurs at exposure temperature, where such cracks are produced by growth stress. Acoustic emission signals have been measured during sample cooling, where the signal arises from scale cracking that is caused by the thermal expansion mismatch stress. Measured results have clearly shown that scale cracking caused by both growth stress and thermal expansion mismatch stress are affected by water vapor in the exposure environment. Post-test metallographic investigations show that crack orientation and the oxide scale phases are also affected by the gas composition in the test rig. Additionally the sample mass gain and scale thickness is affected by water vapor content.Finally, acoustic emission techniques are helpful for understanding the phenomena of breakaway oxidation and spallation/exfoliation.