Computer models of corrosion in passivating systems
Analysis of corrosion in marine and acid environments is a complicated task, involving the interaction of thermodynamic, kinetic and geometrical factors. Two mathematical models which predict corrosion behaviour have been implemented for personal computers. The first program uses an assumption of unidirectional current flow to simplify the prediction of potential distributions for systems of essentially cylindrical geometry containing natural seawater-based electrolytes of differing strength. Using experimentally determined electrochemical and flow rig data, experimental and theoretical results were compared. The correlation between the two was shown to be poor, and this is attributed to the unrepresentative nature of the electrochemical data input to the model. The second model involves the synthesis of polarization curves. Several algorithms to model passivating behaviour have been studied, and one was selected and incorporated into the calculation routine. A number of kinetic and thermodynamic parameters are used in algorithms describing such behaviour, along with activation, concentration and solution polarization effects, for a number of redox reactions, which are then combined to produce an overall potential-log current density curve. Experimentally determined data for pure iron and different stainless steels in marine and acid environments of differing dissolved oxygen content and temperature were obtained. Theoretical models were constructed for each system, and compared to experimental data. Excellent correlation between experimental and theoretical data was obtained for potential ranges in excess of 2 V. Trends in parameter values were discussed, and compared to published data. The transition between stable and unstable passivity of stainless steels was shown to be dependent on the oxygen reduction diffusion limited current density and the iron dissolution reaction free corrosion current density, which in turn was linked to the dissolved oxygen content and temperature of the electrolyte. A new model for the behaviour of stainless steels in the transpassive region was proposed.