Numerical simulations of turbulent flow have been used to determine the flow structure in a sudden pipe expansion, in order to explain the effects of distributed flow on mass transfer controlled erosion-corrosion. Previously determined rate of mass transfer through the liquid boundary layer have been directly related to the predicted levels of near-wall turbulence. When rust films are present, the turbulent fluctuations affect both the mass transfer through the boundary layer and the removal of the film. The determining resistance to mass transfer, under the conditions examined was that of the rust film