High temperature alloys are often exposed to controlled atmospheres with very low partial pressure of oxygen and other elements as major corrosive species such as carbon or even nitrogen. Nitridation is a corrosion form occurring in such environments where low oxygen activity is combined with a high availability of nitrogen-containing species at high temperatures.  If an alloy is not able to grow a dense and protective oxide scale, in this case alumina,  uncontrolled nitrogen ingress occurs. Earlier studies on high temperature nitridation of iron chromium aluminum alloys (FeCrAl) at 900°C in N2–H2 repeatedly showed the formation of locally confined corrosion pockets reaching several microns into the alloy. These nitrided pockets form underneath chromia islands laterally surrounded by the otherwise protective alumina scale. Chromia renders a nitrogen‐permeable defect growing on material inherent chromium carbide particles under the given conditions.
Considering these findings on FeCrAl, a focused ion beam–scanning electron microscope tomography study has been undertaken on a FeNiCrAl sample nitrided under equivalent conditions. Corrosion features, i.e. oxide scales, defects, and internal nitride morphologically and evolution has been characterized and associate to the nitridation of chromium carbide particles  Besides the confirmation of the complete encapsulation of the corrosion pocket from the alloy by a closed and dense aluminum nitride rim, very large voids have been found in the said pockets. Furthermore, metallic particles comprising nickel and iron are deposited on top of the outer oxide scale above such void regions. To obtain information on the spatial distribution of the pocket, FIB-SEM 3D Tomography on a Zeiss Auriga System in combination with 3D reconstruction was used. The results were used to formulate a void formation mechanism basing on the impact of gaseous species in the alloy similar to the assumptions made in previous studies on cracking mechanisms in pressurized ferritic materials upon methane evolution.