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ABSTRACT This project has been concerned with the physical and numerical modeling of the conditions developed under disbonded coatings on steel, with a view to understanding the processes responsible for the conditions that lead to SCC. The physical model (not presented in detail in this paper) has used a polymer film covering a crevice of controlled thickness, with a controlled gas composition (air augmented with additional CO2) on the exterior of the film. pH has been monitored at locations down the crevice using fibre-optic sensors, and potential has been monitored at the same locations using salt-bridge to conventional reference electrodes. A numerical model of the same system has been developed using a commercial finite element package. INTRODUCTION The experimental results will not be presented here, but in order to understand the modeling program, the experimental configuration is summarized. We are concerned with the chemical and electrochemical conditions developed within a crevice under a disbanded coating. The environment assumed is a typical groundwater (see Appendix 1 for compositional details). It is assumed that gas transport will be possible through the coating, but that transport of ionic species (and hence charge) through the coating can be neglected. While it is reasonably well-established that fully-closed crevices reach steady-state conditions at a relatively small depths (typically of the order of 10 cm), it was unclear at the time that this work commenced whether this would also be true in the presence of gas transport through the coating. Therefore a relatively long crevice has been used (0.9 m), with a crevice gap of 0.52 mm. In practical experiments this was covered with a polyethylene film with a gas mixture (air plus CO2) passed over the external surface of the crevice. One end of the crevice was open to the air, and the potential of the steel was potentiostatically controlled at this point. pH and potential values were monitored at locations down the crevice. The crevice geometry was essentially 1-dimensional (treating transport across the width of the crevice as being so rapid relative to the ?along crevice? direction that it can be ignored). Then the potential and solution chemistry at the crevice mouth is taken as being fixed, and gas transport through the coating is treated as a generation term in the crevice solution, with the rate of generation being proportional to the partial pressure difference across the coating. MODEL DETAILS The mathematical model is based on the fundamental equations governing the mass transport of aqueous chemical species in electrolytic solutions. The Nernst-Planck equation was used; this treats the transport and mass balance of every dissolved species through the following equation: (mathematical equation available in full paper) The flux of every species ( Ni ) in the solution is given by: (mathematical equation available in full paper) The total charge of all species in the electrolyte is assumed to be zero, (i.e the electroneutrality condition is assumed), described formally by: (mathematical equation available in full paper) The other equation used in this model relates the flux of species to current density in the solution (along the crevice), given by Faraday?s law: (mathematical equation available in full paper)
ABSTRACT It is known that electrochemical noise can be generated as a result of fluctuations in flow. This work has studied the effect of steady flow in both the laminar and turbulent regimes on the production of electrochemical noise. Experiments have been performed with applied cathodic currents in order to avoid changes in the surface morphology during the experiment. It has been found that flow increases the higher frequency components of the potential noise signal, with the power spectrum tending towards white noise at high Renolds number. INTRODUCTION Flow is an important parameter that can influence corrosion processes. Previous work on the measurement of electrochemical noise (EN) has shown that changes in flow can have a marked influence on the measured EN parameters [1]. The work reported here has used the rotating cylinder electrode (RCE) to study the effect of steady flow on EN production. The RCE was selected on the basis of advantages such as the ability to achieve turbulent flow easily, and the small amount of test solution used [2,3]. The work reported here is of a preliminary nature, and to avoid rapid damage to the specimen as a result of the non-uniform attack to be expected in flow-assisted corrosion, the measurements have been made with an applied cathodic protection current, so that the noise generation process is predominantly hydrogen evolution. EXPERIMENTAL All tests were carried out in 0.1 M NaCl solution with pH adjusted to 7. All tests were performed at room temperature and the solutions were open to air. All solutions were prepared from analytical grade reagent and distilled water. The working electrode was a cylindrical specimen of 1018 carbon steel with a diameter of 1.4 cm and a length of 0.8 cm, giving an exposed area of 3.5 cm. Some specimens (described below as ?static electrodes?) were produced by embedding a carbon steel rod in epoxy resin in epoxy; these were used for preliminary measurements when the working electrode was polarized and used in still solution. Subsequently a rotating cylinder electrode fabricated with PTFE insulation (Figure 1) was used in a commercial rotator. The working electrode surface was polished with SiC paper to 1200 grit, washed with distilled water, degreased with acetone, and dried in air. A saturated calomel electrode (SCE) was used as a reference electrode. The counter electrode was a platinum mesh. Figure 1 Rotating Cylinder Electrode (available in full paper) Electrical connection to the RCE was achieved by a set of four graphite brushes. Two of the brushes were used to supply current to the electrode, and two to measure the specimen potential. There is obviously some concern that slight asymmetry of the contact area might lead to cyclic variations in the measured potential. However, no peaks were detected in the potential noise power spectra at the rotation frequency, and it was therefore concluded that brush noise did not contribute unduly to the measured data. Prior to the measurement of EN, calibration of the mass transport properties of the electrode was performed by measuring the limiting current density for oxygen reduction as a function of rotation speed.
- Materials > Chemicals > Commodity Chemicals > Petrochemicals (0.54)
- Energy > Oil & Gas > Upstream (0.50)