Potentiodynamic Polarization and Potentiostatic Polarization Behaviors for Iron in Sulfuric Acid Solution with or without a Magnetic Field
Authors: Li Hongjuan , Chen Zhen , Xiao Qian , Lu Zhanpeng 
Published: February 2020

The behavior of iron during the corrosion process has been already studied in great detail due to its importance as a main constituent of many metallic construction materials. Magnetic field generated by electromagnetic facilities such as power generators, motors and solenoid valves and electric transmission lines can affect the corrosion of these facilities and their surrounding materials. Effects of imposed magnetic fields on the anodic behavior of iron has been investigated and discussed with respect to the modulation of the mass transport due to the Lorentz-force-driven magnetohydrodynamic (MHD) effect and magnetic field gradient force (MFGF) effect. Hydrogen evolution reaction (HER) on different metals/alloys in acidic or alkaline media is one of the most investigated cathodic reactions in the field of corrosion electrochemistry.  Meanwhile, hydrogen is a potential energy source for fuel cells and other applications and the HER supplies the highly pure hydrogen. Its reaction kinetics of HER has been generalized. The effect of magnetic field on the kinetics and mechanism of HER needs investigation in details.

In the present study, the effect of a magnetic field on the cathodic reaction of iron in a sulfuric acid solution was studied. Potentiodynamic polarization curves with or without a 0.4 T magnetic field and potentiostatic polarizations under the intermittent imposing or withdrawing of the magnetic field were used to determine the magnetic field effect on hydrogen evolution reaction. An electromagnet with a direct current source was used. The magnetic field was horizontal and oriented parallel to the electrode surface. Results of potentiostatic polarization curves show that the cathode current density at high acthodic over-poentials increased due to the presence of a 0.4T magnetic field while the shape of the polarization curve was not significantly changed by magnetic field, as shown in Fig. 1. The phenomenon has been described in previous studies that the magnetic field generates a new overpotential which leads to accelerating the hydrogen reaction.

Figure 2 shows the impact of an applied magnetic field on the current vs. time curves under potentiostatic polarization for iron in sulphuric acid solution. The effect of magnetic field on cathodic current was related to the polarization sequence and the sequence of applying magnetic field. When the iron electrode has been polarized after many cycles of imposing or withdrawing the magnetic field, the magnetic field could cause a sudden decrease of the current density and then the current density maintained at a relatively low value, and withdrawing the magnet caused a sudden increase of the current density and then the current density maintained at a relatively high value. It has been found that if the magnetic field was applied at the start of polarization, withdrawing the magnetic field for the first time caused to a sudden but not big decrease of the current density and then the current density maintained at a relatively low value.

The results the applied magnetic field effect on cathodic hydrogen evolution could be different depending on the cathodic polarization mode and the test sequence. One possible reason is that iron electrode would be subject to hydrogen charging that will change the properties of the iron electrode itself thus making the magnetic field effect different. Another possible reason can be the memory effect of magnetic field on solution properties or cathodic reaction. Systematic measurements are required for clarifying these issues.

Figure 1