A Computational Approach to Understand Corrosion Under an Externally Applied Magnetic Field
Authors: Oliveira Aloadir , Camargo Paulo , De Oliveira Adilson , Pereira Ernesto Chaves 
Published: February 2020

The rate at which a electrochemical reaction takes place on an electrode is, in some cases, a function of the concentration of ions present in the interface which, then, depends on the transport of these ions from the solution. To control mass transport in the solution convection can be used, for example, stirring the liquid or rotating the electrode. A different form which leads to convection in the solution is the application of an external magnetic field. This is called magnetohydrodynamics, or simple MHD. Together with the electric field due to the interface they give rise to a Lorentz force, according to equation 1:

FLorenz = q (E + v x B) . (1)

This force is proportional to the velocity of the ions. Thus, the higher the velocity, the greater the relevance of the action of the magnetic field.

Usually the effect of the magnetic field is compared to that of the rotating disk electrode, but the magnetic field have a vector character, and, then, the Lorentz force is proportional to the vector product of its direction and the velocity of the particles. Furthermore, the geometric shape of the electrode is also an important factor, since the magnetic density along the material depends on this geometry.

Numeric simulations where performed using the finite elements methods to understand how the Lorentz force acted on the electrolyte under an externally applied magnetic field. The geometry used in the simulation was a disk of steel AISI 1020 with 3 mm of radius and 0.5 mm of height, and with an electric potential of 0.5 V. This disk represents a work electrode in an electrochemical experiment. The disk was centered in a homogenous magnetic field parallel to its surface, and immersed in air.

The external magnetic field interacts with the ferromagnetic steel, and its generated a resultant magnetic field. Besides that, the electric potential of the disk generates an electric field around it. Considering that, in an electrochemical experiment, the ions move towards the electrode, it was used the perpendicular direction of the surface of the disk as the direction of the velocity of the ions. For the speed of the ions, the value of as magnitude of . It was calculated the Lorentz force using the magnetic and electric fields of the simulation, and the velocity described.

The simulations were compared to physical experiments of corrosion of AISI 1020 steel samples in solutions of 1 M in open circuit experiments, at room temperature, during 600 seconds in a homogeneous applied magnetic field of 5000 Oe. The direction of the applied magnetic field was parallel to the surface of the work electrode. After each experiment, images of the surface of the electrodes were captured using an optical microscope. Figure 1 compare the Lorentz force calculated in the simulations with the images of the corrosion experiments.

The simulations showed that the Lorentz force is asymmetric with respect to the surface of the electrode. In one side the magnetic field has the same direction of the electric field, while in the other side they have opposite directions. This asymmetry also appears in the image of the surface of the electrode after the electrochemical experiment. The corrosion has a crescent form, that only appears on one side of the surface, corresponding with the side of the simulation that has a greater intensity in the Lorentz force.

Figure 1 – a) Simulation results for the Lorentz force and b) image of the surface of the work electrode after the open circuit experiment in an external magnetic field.

Figure 1