The reaction of SiH4/H2 mixtures with iron and steels was studied at a total pressure of 1 atm and temperatures above 500 °C. When the amount of water vapor in the gas mixture is carefully controlled, a metal silicide diffusion coating forms at low temperatures (below 900°C). Composition and structure of the Si diffusion coatings were determined with Auger depth profiling and x-ray diffraction. Kinetics of the surface reaction between SiH4, and the metal substrate as well as the behavior of these films in severe environments at high temperatures were studied by a microgravimetric technique. Characterization of these Si coatings on iron, low carbon steel (1010), 9% Cr/1% Mo steel (alloy A182F9), and stainless steels (310) and their applications to reduce oxidation, nitriding, or coking at high temperatures or corrosion in mineral acids are described.