The Ni3Al intermetallic compound has been in situ reaction synthesized from elemental powders to form a surface coating material and then diffusion bonded with three representative steels, i.e. a carbon steel, a stainless steel and a tool steel, in order to improve the high-temperature corrosion and wear resistance of these conventional materials. The as-reaction-formed intermetallic has been found to have an unstable crystalline structure. Diffusion-induced recrystallization takes place in the region close to the interface during subsequent diffusion bonding. A conformable contact between the as-reaction-formed intermetallic and the steel substrate is essential for subsequent interfacial bonding, which can be achieved by heating the as-reaction-formed intermetallic up to a high temperature to allow local melting to wet the interface prior to diffusion bonding. During diffusion bonding via an annealing step, an interdiffusion zone is formed and its thickness depends mainly on annealing temperature and duration. As a result of the microstructural development at the interface during annealing, different interfacial properties, i.e. a hardened interface or a softened one, can be obtained. The current success in coating the steels with the intermetallic opens up a new way to broaden the applications and prolong the service life of a wide range of conventional materials.