Abstract Amorphous silica coatings have been deposited by plasma assisted chemical vapor deposition (PACVD) process on M2 steel and 304 stainless steel substrates. The chemical inertia of the coating has been established by resonant quartz crystal microbalance (RQCM), and no porosity has been observed at the AFM scale. The barrier effect has been evaluated on M2 steel by applying the concept of dipolar relaxation. The protection is very efficient, and no deleterious galvanic coupling with the uncoated metallic surface can occur. A coated stainless steel was also investigated in NaCl solution in order to understand how the passive properties are influenced by such a barrier coating. When deposited on stainless steel, coated part keeps the beneficial passive behaviour inherent to the substrate. They evidence almost no corrosion. The silica based layer behaves as a quasi perfect dielectric. The corrosion rate is then greatly reduced, and the pitting resistance is improved. The excellent localized corrosion protection has been explained by a tiny porosity rate, and a limited access of chloride ions through open pores.