Analytical electron microscopy of a crack tip extracted from a stressed Alloy 800 sample exposed to an acid sulfate environment.

Abstract Alloy 800 (Fe–21Cr–33Ni) has been found susceptible to cracking in acid sulfate environments, but the mechanism is not well understood. Alloy 800 C-ring samples were exposed to an acid sulfate environment at 315 °C and cracks were found with depths in excess of 300 μm after 60 h. Preparation of a TEM sample containing crack tips is challenging, but the ability to perform high-resolution microscopy at the crack tip would lend insight to the mechanism of acid sulfate stress corrosion cracking (AcSCC). The lift-out technique combined with a focused ion beam sample preparation was used to extract a crack tip along the cross-section of an acid sulfate crack in an Alloy 800 C-ring. TEM elemental analysis was done using EDS and EELS which identified a duplex oxide within the crack; an inner oxide consisting of a thin 3–4 nm Cr-rich oxide and an outer oxide enriched in Fe and Cr. Preliminary conclusions and hypotheses resulted with respect to the mechanism of AcSCC in Alloy 800.