An adhesion promoting layer is formed on a metallic substrate by generating a non-thermal plasma in air at atmospheric pressure, and exposing a surface of the metallic substrate to the plasma. The plasma oxidizes the metallic substrate to form metal oxide from metal atoms of the metallic substrate. The metal oxide is formed as a metal oxide layer disposed directly on an underlying bulk metallic layer of the metallic substrate. Alternatively, the plasma nitridizes the metallic substrate to form metal nitride from metal atoms of the metallic substrate. The metal nitride is formed as a metal nitride layer disposed directly on an underlying bulk metallic layer of the metallic substrate.