X-ray-Excited Optical Luminescence Imaging for On-Site Analysis of Alumina Scale

On-site analysis of the composition, morphology, and thickness of surface scale on heat-resistant alloys helps to efficiently prevent serious problems such as failure and corrosion during their operation and to predict their remaining life. Currently, there are no analytical methods available that satisfy the requirements for the on-site analysis of oxide scale, which include short measurement time, nondestructive measurement, and portability of the analyzer. This study proposes a nondestructive analytical method to simultaneously identify alumina scale, which is one the most important protective oxide scales for base alloys, and to evaluate its morphology and thickness within 10 s by obtaining X-ray-excited optical luminescence (XEOL) images of the alloy surfaces. This was verified on Fe–25%Al, Fe–15%Al–10%Cr, and NiAl alloys heated at 900 or 1000 °C for different holding times. The XEOL images allow identifying alumina scale and observing its morphology from the infrared luminescence at 695 nm. The alumina scale thickness can be determined from the R value of the XEOL images in the range of 0.20–1.50 μm. The XEOL measurement can be performed in the air, and the setup primarily requires a digital camera and an X-ray tube used in portable analyzers such as X-ray fluorescence analyzer. The results suggest that the XEOL imaging method is suitable for the on-site evaluation of oxide scales on practical heat-resistant alloys.