Magnetic field effects

A magnetic field can influence electrochemical processes in two ways. The first is via the Lorentz force which acts on the current density j in the cell to give a body force density:

FL = j × B. (1)

Whenever a field is applied parallel to the electrode of an electrochemical cell, this force leads to convective stirring of the electrolyte. The transport of ions to the cathode, where they are reduced to metal, is governed by the concentration gradient ∇c, where c is the ionic concentration in moles per cubic metre. The current density j = D∇c, where D is the diffusion constant and ∇c = c₀/δ with δ the thickness of the diffusion layer, a region a few hundred micrometres wide near the cathode where the ionic concentration falls from the average concentration c0 in the bath to zero at the cathode surface. The stirring action of the Lorentz force reduces the thickness of the diffusion layer, and it therefore increases the mass-transport-limited current density. For typical plating current densities j = 1 mA mm⁻², the Lorentz force in 1 T is 10³ N m⁻³. Corrosion currents, which flow from cathodic to anodic sites some micrometres apart on the surface of a corroding electrode can be similarly influenced by magnetic fields.
The other way magnetic fields can influence the action in electrochemical cells is by means of field gradients. The force on an electrolyte containing a concentration c mol m⁻³ of ions of susceptibility χ_mol , which follows  when demagnetizing fields are negligible, as they always are in the solutions used in electrochemistry. The field-gradient force can be much enhanced at ferromagnetic microelectrodes, where significant forces are exerted on paramagnetic ions in solution. Values of ∇B can be as high as 10⁵ T m⁻¹.