This communication reports the effects of multiphase flow on the corrosion products formed on the surface of 1018 C-steel coupons in oil and gas lines. The main objective of this paper is to understand the effects of multiphase flow (full pipe and slug flow Fr. No. 6 and 9 at various flow and oil-water mixtures) on the morphology and the porosity of the C-steel coupon subjected to a horizontal test loop. The corrosion product layer formed on the surface of the steel coupons were analyzed using Scanning Electron Microscopy (SEM), Electron Dispersive Spectroscopy (EDS), X-ray mapping, Atomic Force Microscopy (AFM) and X-ray Photoelectron Spectroscopy (XPS). Detailed porosity calculation has been carried out using metallographic software. The porosity increases as the flow velocity is increased. The corrosion product layer is primarily made of iron carbonate crystals along with Na and CI as indicated by EDS measurements. Detailed XPS analysis show the deterioration of inhibitor film as the flow is increased from full pipe (Fr = 0) to slug flow (Fr = 6). XPS shakeup satellite lines in the N (1s) spectra have been useful in the analysis. © 2000 by NACE International.