Internal oxidation, carburization, and nitridation are three forms of internal corrosion commonly experienced by high-temperature alloy components in industrial plants. They are characterized by ingress of an oxidizing species (oxygen, carbon or nitrogen) into the sub-surface region of the affected material, often accompanied by formation of precipitate phases. These microstructural changes can have a significant impact on mechanical performance. Management of such threats to equipment integrity therefore requires a comprehensive understanding of microstructural evolution, including rate information. In this work, we use optical, scanning electron, transmission electron, and atomic force microscopy to study precipitate phases and quantify internal corrosion rates in the austenitic stainless steel Alloy 800H subjected to nitriding in a 95% N2 + 5% H2 atmosphere at service-relevant temperatures (800–1000 °C). We identify and characterize carbide and carbonitride precipitates formed alongside the expected nitrides, and use the thermodynamic software Thermo-Calc to show that formation of carbides during nitridation is thermodynamically possible in this alloy. This work ultimately demonstrates that a wide-ranging analytical approach may be crucial to gaining a full picture of the effects of internal corrosion in complex industrial alloys.