This paper presents a study of mass transfer and wall shear stress in slug flow in a large diameter horizontal pipeline using the electrochemical limiting current technique. Since the mass transfer in slug flow is very different from that in full pipe flow, the extrapolation of the mass transfer correlation developed in single-phase flow to multiphase flow will cause a large error in the corrosion mechanistic modeling. Hence the new mass transfer correlation was studied in slug flow. The high wall shear stress derived from the measured mass transfer coefficient may explain the mechanical effect of multiphase flow on corrosion product layer and its breakage.