Abstract Al interlayer is precoated on pure iron or steel substrates for diamond film deposition. Microstructures and compositions around the interfacial region of diamond film/Al interlayer/substrates have been comprehensively analyzed by transmission electron microscopy (TEM). Using only a simple Al thin layer is not sufficient, as the integrity of the Al interlayer is easily destroyed during scratching pretreatment. Consequently, the continuity of diamond film is damaged and local carburization corrosion occurs on the substrate. The carburization products primarily consist of voluminous graphite and a large number of fine particles of iron carbide are dispersed at the interfacial region, inducing deteriorated interfacial adhesion. To solve it, an interdiffusion pretreatment of Al interlayer under annealing vacuum is required. The results show that a Fe–Al alloy facilitated the formation of a protective Al oxide layer and improved the diamond film deposition. However, the surface aluminizing process needs to be further optimized, as indicated by a comparison with the diamond deposition directly on Fe–Cr–Al bulk alloy substrate.