Corrosion in magnetic fields

The degradation of materials by corrosion limits their lifetime. This process is in many cases influenced by superimposed magnetic fields. Additionally, the corrosion might be localized by magnetic fields, what can lead to an early failure. Thus, it is important to understand the role of magnetic fields in the corrosion process. The main influence of a magnetic field on electrochemical processes is the introduction of two additional forces into the electrolyte. The Lorentz force, which acts on moving charge carriers, introduces a movement of the electrolyte (so-called magneto hydrodynamic (MHD) effect) and thus increases the mass transport. The field gradient force pulls paramagnetic ions in regions of high flux density, leading to concentration gradients at an electrodes surface. The magnetization of a ferromagnetic electrode in an external homogeneous magnetic field leads to an inhomogeneous stray field in front of the electrode as shown in the left figure [1]. This stray field and its gradients can alter the free corrosion process [2] as well as the anodic behaviour [3,4] of the electrode significantly. An increase of the diffusion-limited dissolution current density and a shift of the active-passive transition potential to more noble potentials were observed when the magnetic field was applied parallel to the electrode surface (maximum Lorentz force configuration). In contrast, in perpendicular field configuration (maximum field gradient force configuration) the diffusion-limited current density is lowered and the active-passive transition potential is shifted to less noble values [1]. A profiling of the electrode according to the flux density distribution and the Lorentz force driven electrolyte motion was observed after diffusion-controlled dissolution (right figure) [4]. Furthermore, the formally charge transfer controlled dissolution may be effected by the mass transfer of metal ions in the electrolyte by their electrostatic interaction with the hydrogen ions [3].

In summary, magnetic fields can enhance or reduce the corrosion reaction depending on the magnetic field to electrode configuration. In many configurations a localization of the material loss is induced by superimposed magnetic fields.

Magnitude of flux density at the electrode surface, Bext = 0.3 T	Corrosion pattern on an iron cylinder after diffusion-controlled dissolution (E = -400 mV vs. Hg/HgSO4(sat.), Q = 2C)

[1] R. Sueptitz, J. Koza, M. Uhlemann, A. Gebert, L. Schultz, Electrochimica Acta 54 (2009) 2229

[2] R. Sueptitz, K. Tschulik, M. Uhlemann, A. Gebert, L. Schultz, Electrochimica Acta 55 (2010) 5200

[3] R. Sueptitz, K. Tschulik, M. Uhlemann, L. Schultz, A. Gebert, Electrochimica Acta 56 (2010) 5866

[4] R. Sueptitz, K. Tschulik, M. Uhlemann, L. Schultz, A. Gebert, Corrosion Science 53 (2011) 3222