[show abstract][hide abstract] ABSTRACT: The effect of hydrogen on plasma post-oxidation following plasma nitriding of a ferrous alloy was investigated. The increase in the hydrogen concentration in the plasma progressively reduces the hematite/magnetite concentration ratio in the outermost oxide layer down to zero at 25% H(2) concentration. Atomic hydrogen originating from scission of hydrogen molecules in the plasma leads to reduction of Fe(III) in hematite into Fe(II) in magnetite by controlling the gas mixture reduction potential. (C) 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.