| A rotating disc electrode technique was used to grow calcareous deposits on low carbon steel specimens in ASTM substitute ocean water at room temperature in the laboratory. Concurrently, a first principle mathematical model was developed to model the formation of calcareous deposits on cathodically protected steel in seawater. The mass transfer of components was described by diffusion, migration, and convection. Three kinds of chemical reactions were taken into consideration in the model: two electrochemical reactions, two precipitation reactions, and one homogeneous reaction. The model equations were cast in finite difference form and solved using Newman's BAND(J) subroutine with an implicit time-stepping technique to obtain the surface coverage of the calcareous films, the concentrations, and potential distributions throughout the diffusion layer. The mechanism of the formation of calcareous deposits may be quantitatively explained from the concentration profiles in the diffusion layer. Also the model is of capability of predicting the changes in current density and deposit composition with time.; A sensitivity analysis was performed to determine the relative sensitivity of parameters to the predicted current density and surface coverage. The results can be used to determine which parameters have the largest influence on the predicted values, and then which parameters are needed to be obtained accurately through experiments or parameter estimation technique. The effects of rotation speed, electrode potential, seawater, salinity, and depth on the formation of calcareous deposits have been examined in the model. The model will be used to predict the conditions necessary for the formation and the maintenance of calcareous films on structural steel in deep ocean water. In addition, the model was used to obtain the time-dependent polarization curves on cathodically protected steel in seawater. |
