Electron work functions (EWF) and open potentials of electrodeposited nickel, TiC microparticle-reinforced and TiC nanoparticle-reinforced Ni matrix coatings were studied with the objective of investigating the effects of TiC particle size on the surface activity and corrosion tendency of the coatings. First-principles calculation was conducted to elucidate the underlying mechanism. Results of the study demonstrate that the overall EWF and open potential of Ni coating decreased when TiC microparticles (diameter = 2 μm) were added. However, TiC nanoparticles (diameter = 200 nm) showed opposite effects. Such changes are ascribed to the effect of particle size on the TiC/Ni interfacial state. It was shown that TiC microparticle/Ni interface had a low interfacial EWF. While the TiC nanoparticle resulted in a stronger TiC/Ni interface with elevated EWF. The first-principles calculation revealed that valence electrons were more localized in the region of the nano TiC/Ni interface, resulting in higher EWF, stronger interfacial bonding, and higher surface stability that limited the electrons available to participate in corrosion reactions.