The effect of a magnetic field on the pH value in front of the electrode surface during the electrodeposition of Co, Fe and CoFe alloys

The effect of an uniform magnetic field applied parallel to the electrode surface, with flux density up to 1 T on the pH value close to the electrode surface during the electrodeposition of Co, Fe, CoFe alloys as well as for the pure hydrogen evolution reaction (HER) has been investigated. Voltammetric and chronoamperometric experiments have been carried out with the in situ pH measurements at the electrode surface. Results show that the limiting current for all the investigated systems is increased due to the magnetohydrodynamic (MHD) effect. The hydroxides formation in the metal containing electrolytes buffers the cathodic layer and the interfacial pH value increases to a lesser extent compared to the case of HER electrolyte. The increase of the pH value at the electrode surface is lower in the applied magnetic field compared to the case without a magnetic field. It was found that the interfacial pH value is a power function of the magnetic flux density. This effect is attributed to enhanced convection induced by the Lorentz force. The depletion of H+ concentration at the electrode surface is compensated by the MHD effect and the hydroxide formation is suppressed, moreover the hydroxyl products are removed faster from the electrode surface in the magnetic field resulting in better quality of the deposited layers. The pH value close to the electrode surface during CoFe alloys deposition is high enough for the precipitation of the iron hydroxide, what satisfies the model proposed by Dahms and Croll.