The corrosion of carbon steel in oil-in-water emulsions under controlled hydrodynamic conditions
By Becerra, Haydée Quiroga; Retamoso, C & Macdonald, Digby D
Published in Corrosion Science
2000
Abstract
The effect of the oil content on the corrosion of AISI-SAE 1010 carbon steel in oil-in-water emulsions under controlled hydrodynamic conditions was studied by means of potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). The systems that were studied included brine (0.2 wt% NaCl), surfactant solution (Dioctyl sodium sulfosuccinate, 1 wt%, +NaCl, 0.2 wt%), and oil-in-water emulsions, in which the aqueous phase was the surfactant solution and the oil phase was a mineral oil. Corrosion studies employed various controlled hydrodynamic systems, which included a rotating disk electrode (RDE) and a jet impingement electrochemical cell (JIEC), with the working electrode located in the stagnant region on the jet axis or on the wall jet region (displaced from the axis). This study found that the effect of the oil content on the electrochemical activity of carbon steel (as indicated by the current density in the active state) varies with the 'internal phase relationship’, IPR. For emulsion with low IPR (oil contents up to 20 wt%), the electrochemical activity was slightly higher than that of the base surfactant solution. The electrochemical activity of emulsions with medium IPR (oil contents between 20 and 45 wt%) showed no major variation with oil content, while for emulsions with high IPR (oil contents between 45 and 70 wt%) the activity was diminished. The data are explained in terms of a model that postulates the formation of an 'oily phase’ on the steel surface, the stability of which depends on the magnitude of the hydrodynamically induced shear stress at the interface. However, the coverage of the oil phase on the surface is postulated to depend on the normalized IPR, such that as the IPR increases the coverage also increases. Furthermore, the oil phase is envisioned to facilitate the cathodic partial process (reduction of oxygen) due to the enhanced solubility of oxygen compared with water, while inhibiting the dissolution of the metal (anodic partial process). Because the two partial processes are strongly coupled, the corrosion rate is predicted to pass through a maximum with increasing oil content of the emulsion, as observed.