Flow-dependent corrosion in turbulent pipe flow

Abstract

This thesis describes the application of the low Reynolds number (LRN) k-$\varepsilon$ eddy viscosity model and an electrochemical (or corrosion) model to simulate the corrosion rate, the metal/solution interface concentration of both oxidants and corrosion products, and the film formation conditions in turbulent pipe flow. The overall objectives of this study were the numerical simulation and experimental investigation of the effects of flow on the corrosion rate and film formation and disruption under both attached and disturbed flow conditions. The turbulence model used in this study is based on the standard k-$\varepsilon$ model proposed by Launder and Spalding (1974). The model was modified with a recently developed LRN model (Abe et al, 1994) in the near wall region which enables the calculation of species concentration all the way down to the wall. The mass transfer was modelled by simultaneously solving the transport equation for mass, momentum, kinetic energy of turbulence and its dissipation as well as species concentration. The corrosion model was developed to construct E/logi diagrams which take into account the effects of mass transfer resistance on the anodic and cathodic reactions. The pipe-wall mass transfer in the mass transfer entrance region has been simulated with LRN k-$\varepsilon$ model and tested against the experimental results of Berger and Hau (1977), in the range Re = $10\sp4$ to $10\sp5,$ for Sc = 2244, and of Son and Hanratty (1967), in the range Re = $10\sp4$ to $5 10\sp4,$ for Sc = 2400, in both the mass transfer entrance region and the fully developed region and good agreement was reported. The application of small cathodes embedded in a larger active cathode to measure local mass transfer rates was also simulated. The size of the electrode and the thickness of the electrical insulation around the small electrode give rise to errors that increase as the insulation thickness increases and the electrode size decreases. Effects of mass transfer on the corrosion of metals were simulated throughout the mass transfer entrance region in turbulent pipe flow. Iron was chosen to represent metals with a low exchange current density and copper those whose anodic dissolution is more reversible. Profiles of corrosion rate, surface metal ion concentration and surface pH for corrosion under charge transfer control; oxygen-mass transfer control; and anodic partial mass transfer control are presented. The turbulence models have also been applied to the numerical simulation of the effect of electrode misalignment on corrosion rate measurements in turbulent pipe flow. The effect of the mass transfer entry length was also taken into consideration. The formation and disruption of the corrosion films on a flat copper surface in flowing 3% NaCl solutions were also studied. (Abstract shortened by UMI.)