Deterministic modeling of the corrosion of a low-carbon steel by carbon dioxide and the effect of acetic acid.

摘要

The current work is carried out with the aim of developing a deterministic model of the corrosion of low-carbon steel by carbon dioxide including the effect of acetic acid. The interaction of acetic acid with the corrosion-products layer is studied and the system is modelled by considering reactions and the transport processes within the boundary layer along with protective film formation and its dissolution. The Point Defect Model (PDM) is used to describe passive film processes using fundamental chemical reactions and physicochemical properties of the metal-film-solution interface. The underlying mechanisms for describing currents, thicknesses, impedances, and structures are based on the defect chemistry of the layers formed on the metallic surface.;Ellipsometry Spectroscopy, Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), Focus Ion Beam (FIB) and Electrochemical Impedance Spectroscopy (EIS) are used to determine the nature of the layers formed on the metallic surface. Transient analysis is carried out to obtain the parameters for the PDM from the impedance analysis under steady state conditions. Ellipsometry, SEM and FIB are applied for a further understanding on the composition and morphology of the film.;According to Electrochemical Impedance Spectroscopy (EIS), acetic acid effect is markedly dependent upon the pH of the environment, showing a decrease in the charge transfer resistance at pH=4, and an increase of this parameter at pH=6. Acetic acid decreases the corrosion caused by dissolved carbon dioxide but its presence does not change the mechanisms of corrosion. The PDM accounts for most of the experimental observations on the structure and behavior of the films formed on the metallic surface predicting the dissolution of the oxide layer adjacent to the metal surface when the outer layer resistance is similar in magnitude to the applied potential; this is supported by experimental findings from FIB and SEM. The porosity of the outer layer, the electric field strength across the barrier layer, the polarizability of the barrier layer/outer layer interface and the standard rate constants for the point defect generation and annihilation reactions at the barrier layer interfaces, and other parameters, are found to be independent of the applied potential.