Brian:
Do you have an opportunity to break open that crack or a similar crack to directly examine the crack surface? The color and tenacity of the oxide layer as observed directly on the surface could be good clues to determine if it is a high-temperature, adherent oxide layer or a lower-temperature oxide from corrosion. Consideration of how thick and dense the oxide layer is as observed in the cross-section is worthwhile. The crack morphology (intergranular, transgranular, mixed) would also give you some clues as to the crack formation conditions, and therefore the most likely oxide formation conditions. In some cases, you may even find clues as to characteristic fracture features by direct examination (SEM/EDS) of the oxide-covered fracture surface.
As you're aware, cast iron will oxidize quite readily, so consideration of the service history will likely be just as important to determining the likely source of the oxidation as will the physical features of the oxide layer itself. I would second Ratnesh's comment on looking at the material microstructure near the crack. X-ray diffraction (particular examination by microXRD so you don't have to collect as much oxide product as you would for a conventional bulk XRD) could be worth considering as well.
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Daniel Grice
Senior Materials Engineer
Materials Evaluation & Engineering,Materials Evaluation And Engi
Plymouth MN
(763) 449-8870
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