Using FIB Secondary Ion Images to Observe the Presence of Corrosion

Comparative FIB images of a crack in pipeline steel are shown, acquired using secondary electron and secondary ion signals. The ion image (inset) highlights the corrosion product in the crack itself, while the electron image (lower right) shows the microstructural features within the plastic zone around the crack tip. This sample was metallographically polished prior to FIB imaging.

FIB secondary ion images are particularly sensitive to the presence of oxides and carbides in metallic systems due to the effect these elements have on the secondary ion yield of the metal. The oxygen or carbon "enhanced yield" can cause order-of-magnitude or more increases in the brightness of a region containing metal oxides or carbides, creating a chemical contrast effect that makes FIB SII an ideal technique for identifying corrosion or grain boundary segregation without resorting to chemical etching. Eliminating the need to use chemical etching is significant as many etchants contain elements that are also expected to be found in the native corrosion products. Etching frequently contaminates the sample to a depth of several micrometers into precipitates or porous grain boundaries. FIB secondary ion and secondary electron imaging can be combined with careful metallographic polishing techniques that minimize mechanical deformation introduced during preparation prior to FIB work. This permits the use of FIB micromachining to produce TEM specimens with a near one-to-one correspondence between the TEM specimen and the FIB imaged region. There is therefore little concern concern that the chemical analysis to be performed in the TEM will be compromised, as would be the case by chemicals in a conventional etchant.