COMSOL 6.4 - Tertiary Current Distribution, Nernst-Planck (TCD)

Tertiary Current Distribution, Nernst-Planck (TCD)

Now set up the physics interface for the tertiary current distribution. Start with the domain transport properties.

Electrolyte 1

In the Model Builder window, under click .

In the window for , locate the section. In the Dc text field, type D_O2(PS).

Next, in the same window, locate the section.

From the σl list, choose . In the associated text field, type sigma(PS).

The argument PS to the functions D_O2 and sigma is the pore saturation parameter defined in the Parameter list in Global Definitions.

Initial Values 1

In the window, under click .

In the window for , locate the section.

In the c text field, type C_O2_ref.

Electrode Surface1

In the window, right-click and choose Electrode Surface.

Select Boundary 1 only (the left boundary of the geometry).

Electrolyte Reaction1 (Zinc Oxidation)

To implement a fast reaction kinetics assumption, a constant potential is set at the anode surface. To achieve a constant potential, set a constant value for the equilibrium potential and use the Primary Condition (Thermodynamic Equilibrium) type of electrode kinetics at the Electrode Reaction child node.

In the Model Builder window, expand the Electrode Surface 1 node.

Right-click Electrode Reaction 1 and choose Rename.

In the Rename Electrode Reaction dialog, type Zinc oxidation in the New label text field.

Click OK.

In the Settings window for Electrode Reaction, locate the Equilibrium Potential section.

In the Eeq text field, type Eeq_Zn.

Locate the Electrode Kinetics section. From the Kinetics expression type list, choose Thermodynamic equilibrium.

Electrode Surface 2

In the Physics toolbar click Boundaries

and choose Electrode Surface

In the Model Builder under Secondary Current Distribution, click Electrode Surface 2

Select Boundaries 6 and 7 only (the left and right edges of the rebar).

In the Settings window for Electrode Surface, locate the Electrode Phase Potential Condition section.

In the ϕs,ext text field, enter E_app.

Electrode Reaction 1 (Oxygen Reduction)

Three different reactions occur at this electrode surface: oxygen reduction, iron oxidation and hydrogen evolution.

In the Model Builder expand the Electrode Surface 2 node and click Electrode Reaction 1.

Press F2 on the keyboard to rename the node (or right-click the node and choose Rename). Enter Oxygen reduction as the new name.

In the Settings window for Electrode Reaction (for the newly named Oxygen reduction node), locate the Stoichiometric Coefficients section.

Enter 4 in the Number of participating electrons field.

Enter -1 in the Stoichiometric coefficients field.

In the Settings window for Electrode Reaction, locate the Equilibrium Potential section. In the Equilibrium potential Eeq text field, enter Eeq_O2.

Under Electrode Kinetics from the Kinetics expression type list, choose Cathodic Tafel equation.

In the Exchange current density i0 text field, enter c/C_O2_ref*i0_O2.

The exchange current density thus depends on the oxygen concentration.

In the Cathodic Tafel slope (<0) Ac text field, enter A_O2.

Next add, rename, and define two more Electrode Reaction nodes.

Electrode Reaction 2 (Iron Oxidation)

In the Physics toolbar, click Attributes and choose Electrode Reaction. Electrode Reaction

Press F2 to rename Electrode Reaction 2 to Iron oxidation. Click OK.

In the Settings window for Electrode Reaction under Equilibrium Potential, enter Eeq_Fe in the Equilibrium potential Eeq text field.

Under Electrode Kinetics from the Kinetics expression type list, choose Anodic Tafel equation.

In the Exchange current density i0 text field, enter i0_Fe.

In the Anodic Tafel slope (>0) Aa text field, enter A_Fe.

Electrode Reaction 3 (Hydrogen Evolution)

In the window, under right-click and choose .

Press F2 to rename Electrode Reaction 3 to Hydrogen evolution. Click OK.

In the Settings window for Electrode Reaction under Equilibrium Potential, enter Eeq_H2 in the Equilibrium potential Eeq text field.

Under Electrode Kinetics from the Kinetics expression type list, choose Cathodic Tafel equation.

In the Exchange current density i0 text field, enter i0_H2.

In the Cathodic Tafel slope (<0) Ac text field, enter A_H2.

The concrete is in contact with air at the left, and the concentration is therefore constant at this boundary.

Concentration 1

In the window, right-click and choose .

Select Boundary 1 only.

In the window for , locate the section.

Select the checkbox.

In the c0,c text field, type C_O2_ref.