Nital etch is used to prepare nitrided steel cross section samples for microscopic examination. But I'm assuming you mean nital used for NDI such as surface temper etch inspection of finished steel part surfaces to check for abusive grinding/machining or issues with a carburized case. Surface temper etch inspection uses an etchant solution that reacts with areas that have experienced localized overheating after heat treatment, creating various shades of gray or gray/white discoloration based on the severity of the condition. The inspection itself is performed visually. So I think the answer to your question is that the most common etchant solution used (3-5% nitric acid) does not react properly with the nitrided case compounds for use with the visual examination procedure of surface temper etch inspections.
