Pitting of stainless steel under the influence of a magnetic field

 

In a new concept of hydroelectric power generators, permenent magnets are part of the runner which is fully submersed in water. Encasing of the magnets by a non magnetic material is required as protection against corrosive and mechanical stresses, and for hydrodynamic considerations. For fresh water applications, the paramagnetic austenic stainless steels are considered as a candidate material in freshwater applications. Consequently, a possible effect of a magnetic field on the corrosion behaviour, in particular on pitting, has to be considered. In order to learn more on this unusual corrosion system, some experimental work has been carried out.

Classical potentiodynamic tests were carried out in cylindrical cells in chloride solution, with and without a magnetic field applied by a permanent magnet. The follwing effects were observed:

(i) there was no effect of the magnetic field on the open circuit potential.
(ii) the potential of passivity breakdown was also not influenced by the field.
(iii) under the influence of the field, irrespective of its direction, the potential of repassivation was ca. 65 mV lower compared to the experiments without field.
(iv) a vortical flow of the electrolyte developed in the experiments with field, its direction depending on the field orientation. (see videos from experiments with indicator for Fe-ions in the electrolyte).
(v) a uniform distribution of large pits was observed in the experiments without field. With field, a radial pit pattern developed with large pits at a low density in the center and a small pits at high density in the periphery of the electrode.

In order to understand the mechanism of pit pattern formation and the shift in the repassivation potential, the experimental setup was modified. The working electrode was split to a ring-disc-electrode allowing to record each current seperately. Moreover, additional potential sensing electrodes were placed at positions with increasing radius. Again, measurements with and without field were carried out.


 

 

 

Video without magnetic field (2.5Mb)

Video with magnetic field (1.7Mb) 

 

 

   

Quelle: VA TECH HYDRO GesmbH.

  

  

 

From the data received, the following conclusion could be drawn: As long as the metal is passive, the magnetic field has no influence on the system. As soon as pitting has started and significant current flows, the electrolyte starts to flow vortically according to the magnetohydrodynamic (MHD) effect. The driving force is the Lorentz force caused by the interaction of the ions carrying the current with the magnetic field. This flow interacts with the pitting process and leads to position dependant repassivation. By this delicate process, potentials in the periphery reach higher values and lead to much higher densities of early repassivated pits. In contrast, pits in the center, once initiated, remain active and grow continuously.

Details on this project are provided in:

Electrochemical investigation of chloride induced pitting of stainless steel under the influence of a magnetic field
P. Linhardt, G. Ball, E. Schlemmer
Proceedings of the 4th Kurt Schwabe Symposium Mechanisms of Corrosion and Corrosion Prevention, June 13-18 2004, Helsinki University of Technology, Finland, p.42-49.

Electrochemical investigation of chloride induced pitting of stainless steel under the influence of a magnetic field
P. Linhardt, G. Ball, E. Schlemmer
Corr. Sci. 47 (2005), 1599-1603.