Under a stratified flow regime and under dewing conditions, internal corrosion can occur at the top of horizontal pipelines. Corrosive gases such as carbon dioxide dissolve in the freshly condensed water on the inner pipewall where continuous injection of inhibitors of corrosion is not possible. In absence of such inhibitors, a thorough understanding of the phenomena of corrosion under dewing conditions is needed to predict the corrosion risk.
The corrosion and condensation rates were studied experimentally in a full-scale flow-loop. The experimental data were compared with theoretical results obtained from a mechanistic model for the prediction of the corrosion rate during Top-of-the-Line Corrosion (TLC). This model solves the hydrodynamics of the condensed liquid and the heat and mass transfer in the gas phase to predict the condensation rate in a horizontal pipeline in the presence of a non-condensable gas. Further, the mass transfer in the condensed liquid is coupled to the thermodynamics of the vapor-liquid equilibrium and to an electrochemical model for the corrosion reaction at the wall. A Computational Fluid Dynamics (CFD) code was implemented to study the influence of mass transfer in the condensed water on the corrosion rate. Based on the chemistry of the condensed liquid the corrosion rate in the absence and in the presence of an iron carbonate scale is computed. The mechanistic model is tuned to a large set of experimental corrosion rates through the use of the superficial porosity accounting for the partial blockage of the corroding surface by the deposition of an iron carbonate scale.
Increasing the gas temperature led to larger condensation and corrosion rates. However, at a higher temperature the corrosion rate was found to decrease due to the formation of a corrosion products scale. The gas velocity was found to influence the condensation rate, which, in turn, influenced the corrosion rate. The total pressure and partial pressure of CO2 played a less significant role, particularly at low condensation rates. Temperature and condensation rate were found to be the two main parameters influencing the corrosion rate during TLC.