Automated Mapping of High-Temperature Corrosion Products on Iron Chromium Alloy Using Raman Microscopy

An automated Raman microscope mapping system is described which uses single point analysis combined with a stepper-motor-driven microscope stage. Methods for one-dimensional full spectra line-scans and repetitive frequency selective line-scans providing a two-dimensional species map are reported. The technique is applied to a surface feature appearing on a sample of Fe-Cr steel oxidized at 800°C for 143 h. The results suggest that Fe 3 O 4 , FeCr 2 O 4 and Cr 2 O 3 develop fairly uniformly across much of the surface and that the formation of raised areas of more extensive corrosion is due to the absence of Cr 2 O 3 in these regions. In addition, a sample oxidized at 675°C for two hours was ball-cratered to provide a Raman depth profile. The corrosion scale was complex—the outermost layer comprising Fe 2 O 3 and some Fe 3 O 4 , while the inner layer consisted mainly of FeCr 2 O 4 , with some evidence of Cr 2 O 3 .