A Mach-Zehnder interferometer is employed to study the effect of magnetic field on the local corrosion process of a ferromagnetic metal. The local corrosion of a pure iron electrode in the 0.1 mol/L NaCl solution is investigated with the combination of traditional electrochemical method and potentiodynamic polarization measurement. The video images of interference fringes in the electrode/solution interface are first recorded by CCD, then these fringes are analyzed by Fourier transform and the adaptive bandpass filter, and finally the dynamic process of local corrosion in the electrode/solution interface is reproduced by the dynamic phase reconstruction of interference fringes. The results show that, without the external magnetic field, the local corrosion appears only in the middle area of the electrode/solution interface at first, but the local corrosion area of the electrode/solution interface soon expands with the increase of voltage, and finally the entire electrode/solution interface is activated and dissolved. With the external magnetic field, the corrosion degree of the electrode surface is positively correlated with the magnetic induction, and an obvious edge corrosion phenomenon occurs. In other words, the external magnetic field can accelerate the electrochemical corrosion of ferromagnetic metals.