Abstract: An improved protective coating system for metal substrates is described. Nickel-aluminide duplex coatings are improved by adding an effective amount of boron in the nickel coating step and by retaining the boron during the aluminization step. An unexpected result of the retained boron is its non-deleterious effect on oxidation protection. During aluminization, the boron surprisingly migrates to the coating/substrate interface, forming a diffusion barrier in situ. The barrier inhibits both inward diffusion of the nickel-aluminide coating and outward diffusion of substrate impurities. The process enables thinner, less expensive coatings.