The microstructures of 800 H samples affected by natural corrosion within industrial-like environmental conditions have been studied, and their potential effects on the creep response have been investigated based on literature information. Smooth cylindrical specimens of the alloy extracted from rolled sheet material have been placed into an industrial furnace, where they were exposed to high temperature thermal cycles within an air atmosphere. Samples spending 0 to 5 years inside the furnace underwent a microstructural characterisation campaign. Optical microscopy images reveal no grain coarsening. Macroscopic Vickers hardness shows a relatively wide surface hardness range (115 less than or similar to HV10 less than or similar to 140). Compared to as-received material, micro-hardness profiles from specimens after years of high-temperature exposure exhibit a surface hardening trend within the vicinity of the edge exposed to the environment. Scanning electron microscopy analyses revealed two coexisting corrosion mechanisms: oxidation and nitridation. The latter is identified as the cause of the micro-indentation hardening trend.