The effect of immersion of type 304 stainless steel in pyrrolidium-2-one trifluoroacetate ionic liquid on surface composition and structure was studied. A passive film with increased protective properties was formed on the steel surface due to enrichment of chromium in the inner region of the oxide film and enrichment of nickel beneath the oxide film. The presence of fluorine bonded to metal and carbon was revealed on the surface of steel that was attributed to interaction of the steel and anions of the ionic liquid. Newly developed plasma profiling time-of-flight mass spectrometry was successfully employed for semi-quantitative analysis of the elemental depth distributions in the thin passive film with high resolution. (C) 2015 Elsevier B.V. All rights reserved.