Unsteady state mass transfer coefficients controlling steel pipe corrosion under isothermal flow conditions

Abstract

Corrosion tests of commercial steel pipe in aerated natural water under variable flow isothermal conditions were made by weight loss determinations over periods up to 210 days. The porosity, density, and the average thickness of the corrosion product layer were determined. Friction coefficients were evaluated through precise pressure drop measurements as a function of exposure time for the various flow rates (Reynolds number range 4000–25,000). The corrosion mechanism was analysed as a mass transfer operation by considering all the plausible unsteady state process characteristics, involving growth of corrosion product on the metal surface and change of surface roughness. The general mathematical model postulated from that analysis fitted the experimental data with acceptable precision. The model permitted, using corrosion data only, the prediction of values for the porosity and friction coefficient which were in good agreement with the experimental data. The average thickness of the damped turbulence layer for the smooth pipe was also calculated from the corrosion data, using the same model, and showed an excellent agreement with values estimated from usual hydrodynamic relationships. The effect of the surface roughness on the mass transfer coefficients in the damped turbulence layer, K L, and in the corrosion product layer, K s, were examined in detail. It was found that K L controls the overall mass transfer process for the time range and flow rate range examined.