A surface treatment is disclosed for welding rod or wire of ferrous composition to greatly enhance its resistance to corrosion and considerably extend shelf life. The treatment consists of forming a relatively thick (0.0001 inches) black oxide surface layer on the rod, by an aqueous alkali-nitrate immersion process, and is applied to the wire or rod after the final drawing operation in which the outside diameter of the wire or rod is sized to closely controlled tolerances to insure proper feeding in reel and other automated arc welding operations. The aqueous alkali-nitrate immersion process enables the formation of relatively thick layer of black oxide without appreciably affecting the diameter of the treated welding rod, such that mechanical working may be entirely completed prior to treatment, insuring a smooth, uninterrupted coating.