Nanoporous films have been formed on 316L stainless steel in 5 mol dm(-3) sulphuric acid at 60 C by square wave pulse polarization between active or transpassive and passive potentials, and characterized using glow discharge optical emission spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, Rutherford backscattering spectroscopy and nuclear reaction analysis. The films are shown to be chromium- and molybdenum-rich relative to the substrate, and to consist mainly of sulphates, oxides and hydroxides. The morphology and composition of the films are discussed in relation to the polarization conditions and mechanism of film formation. (C) 2011 Elsevier Ltd. All rights reserved.