In depth analysis of corrosion mechanism of U-tube under conditions of differential aeration

Abstract

• A two dimensional model for the study of concentration corrosion in U-pipe is proposed creatively. • Based on the knowledge of electricity, the discrete equation of corrosion potential after polarization is derived and solved. Through the analysis of the results, the corrosion rate of the inner wall is predicted and the mechanism of concentration difference corrosion is revealed. • It is a meaningful supplement to the traditional FAC(flow accelerated corrosion) theory. The corrosion mechanism of fluid in pipeline is very complex, and many mechanisms often work together. Among these mechanisms, differential concentration corrosion(DCC) mechanism is often ignored by researchers. Due to the difficulty of experimental method, a numerical simulation method is used to carefully study it. Firstly, the flow field inside U-tube was calculated through FLUENT, and the oxygen concentration distribution of the ionic conductive layer near the wall is got; secondly, the ionic conductive layer is extracted and discretized, and the natural corrosion potential and current density of the surface unit are calculated according to the oxygen concentration; finally, according to Kirchhoff's Second Law, the discrete equations of the corrosion potential of the surface ionic conductive layer are derived. By solving these equations, the corrosion potential of the unit after polarization is obtained, and the final corrosion current density distribution on the surface is got too. The results show that when DCC is not considered, the natural corrosion potential and corrosion current density of the pipeline wall with high oxygen concentration are higher and larger, while those with lower oxygen concentration have lower natural corrosion potential and smaller natural corrosion current, when DCC is considered, the cathodic polarization will occur at the position with higher nature corrosion potential, and the corrosion rate will be reduced; while the anodic polarization will occur at the position with lower nature corrosion potential, and the corrosion current will increase. These greatly change the distribution of the corrosion current and the corrosion potential tends to homogenize.