Galvanic Corrosion of a Magnesium Alloy in Contact with Steel

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Galvanic Corrosion of a Magnesium Alloy in Contact with Steel

Magnesium alloys are attractive alternatives as lightweight materials in various fields of engineering. Magnesium is however relatively non-noble and may suffer considerable galvanic corrosion when being used in conjunction with other metals, for instance when mounting a Mg alloy component using steel fasteners.

This model example simulates a galvanic corrosion couple consisting of a magnesium alloy (AZ91D) and steel (4150), with salt water (5% NaCl) as electrolyte. The example is based on a paper by J.X. Jia and others (Ref. 1).

Model Definition

The model is made in two dimensions using axial symmetry, see Figure 1, with a single electrolyte domain of radius 10 cm and height of 7.5 cm. The electrolyte conductivity is set to 7.95 S/m.

Figure 1: Model geometry. Electrolyte domain with axial symmetry.

Boundary conditions

The cathode is a disc made of steel 4150, placed at the center of the geometry at z=0, extending in the r direction. Three different disc radii are investigated: 5, 10, and 30 mm.

The Mg alloy AZ91D electrode is placed outside the steel disc on the z = 0 line.

The electrode kinetics at both the steel and Mg alloy surfaces is described using the experimental polarization data available in corrosion material library.

The electric potential of both electrode surfaces is set to ground.

The outer (r = 10 cm) and top boundaries (z = 7.5 cm) are insulated.

Results and Discussion

Figure 2 shows a revolved surface plot of the electrolyte potential for a disc radius of 30 mm.

Figure 2: Electrolyte potential for a 30 mm disc radius.

Figure 3 shows the electrode current densities for the three different disc radii. The local current density of the anode reaction in the vicinity of the steel disc increases significantly with an increase in steel disc radius which is attributed to the higher cathode to anode area ratio.

Figure 3: Electrode current densities for the three different disc radii.

Notes About the COMSOL Implementation

The Secondary Current Distribution interface is used to model the problem, using Electrode Surface nodes for the two electrode surfaces.

Due to the faster kinetics and larger area of the anode, the initial value for the electrolyte is set to correspond to a zero anode polarization.

A stationary study step is used to solve the problem, with a parametric sweep to vary the disc radius.

A free triangular mesh is used for meshing, with an additional smaller size setting for increasing the resolution at the contact point between the anode and cathode.

1. J.X. Jia, A. Atrens, G. Song, and T.H. Muster, “Simulation of galvanic corrosion of magnesium coupled to a steel fastener in NaCl solution,” Materials and Corrosion, vol. 56, no. 7, pp. 468–474, 2005.

Corrosion_Module/Galvanic_Corrosion/galvanic_corrosion_mg_alloy

Modeling Instructions

From the menu, choose .

In the window, click

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In the window, click

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In the tree, select .

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In the tree, select .

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Global Definitions

Load the model parameters from a file.

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In the window, under click .

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In the window for , locate the section.

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Browse to the model’s Application Libraries folder and double-click the file galvanic_corrosion_mg_alloy_parameters.txt.

Draw the geometry as a rectangle. Use a point to divide the bottom boundary into two sections. (The two sections will be the anode and cathode, respectively, when setting up the physics).

Rectangle 1 (r1)

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In the toolbar, click

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In the window for , locate the section.

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In the text field, type R_outer.

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In the text field, type H.

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In the toolbar, click

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In the window for , locate the section.

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In the text field, type R_inner.

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button in the toolbar.

Use the Corrosion Material Library to set up the material properties for the electrode kinetics at the magnesium and steel electrode surfaces.

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In the toolbar, click

to open the window.

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Go to the window.

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In the tree, select .

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Click in the window toolbar.

AISI 4150 steel in 5% NaCl (mat1)

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In the window for , locate the section.

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From the list, choose .

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Select Boundary 2 only.

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In the window, expand the node.

Interpolation 1 (iloc_exp)

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In the window, expand the node, then click .

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In the window for , click

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The function plot should look like this:

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Go to the window.

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In the tree, select .

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Click in the window toolbar.

AZ91D in 5% NaCl (mat2)

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In the window for , locate the section.

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From the list, choose .

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Select Boundary 4 only.

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In the window, expand the node.

Interpolation 1 (iloc_exp)

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In the window, expand the node, then click .

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In the window for , click

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The function plot should look like this:

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In the toolbar, click

to close the window.

Secondary Current Distribution (cd)

Now set up the electrochemistry. Start with selecting the reference electrode.

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In the window for , click to expand the section.

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From the list, choose .

Next set up the user defined electrolyte conductivity.

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In the window, under click .

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In the window for , locate the section.

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

Electrode Surface 1

The anode and cathode are modeled as Electrode Surface nodes, having the same electric potential in the electron conducting phase. Define the cathode first.

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In the toolbar, click

and choose .

The cathode is located at the center of the bottom boundary.

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Select Boundary 2 only.

Electrode Reaction 1

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In the window, click .

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In the window for , locate the section.

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From the iloc,expr list, choose .

Electrode Surface 2

Now define the anode surface, located on the outer part of the bottom boundary.

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In the toolbar, click

and choose .

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Select Boundary 4 only.

Electrode Reaction 1

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In the window, click .

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In the window for , locate the section.

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From the iloc,expr list, choose .

The model is now ready to solve. Add a parametric sweep to study the impact of different cathode radii.

Parametric Sweep

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In the toolbar, click

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In the window for , locate the section.

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In the table, enter the following settings:


Parameter name	Parameter value list	Parameter unit
R_inner (Radius of inner disc)	0.005 0.01 0.03	m

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In the toolbar, click

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Electrolyte Potential, 3D (cd)

A revolved plot of the electrolyte potential is added by default.

Local current density

The following steps create a plot of the electrode reaction currents. (Compare with Figure 3 in the Results and Discussion section above).

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In the toolbar, click

and choose .

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In the window for , type Local current density in the text field.

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Locate the section. From the list, choose .

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Click to expand the section. From the list, choose .

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Right-click and choose .

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Select Boundaries 2 and 4 only.

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In the window for , click in the upper-right corner of the section. From the menu, choose .

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Locate the section. From the list, choose .

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In the text field, type r.

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Click to expand the section. Select the check box.

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In the toolbar, click

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