2. Understanding Corrosion

2.8 Concentration Polarization [1/2]

Under conditions of irreversibility (departure from equilibrium), a concentration gradient is built up between the electrolyte/electrode interface and the bulk electrolyte. To exemplify, on anodic polarization of Zinc, Zn2+ cations would be collected around the anode, if they are not removed from the vicinity of the anode as they are formed. The accumulation of positive ions would drift the potential to more positive values.

Similarly, if during a cathodic reaction (2H+ + 2e H2), the rate of discharge of H+ ions are brought to the electrode/electrolyte interface, the potential of the electrode would shift to more negative value. A concentration gradient would be built between the electrode/electrolyte interface and the bulk electrolyte, which would lead to a slowing down of the reduction process. This process is called "Concentration Polarization". It is to be noted that in cathodic polarization, the concentration of ions at the cathode is less than in the bulk of the solution, and in anodic polarization, the concentration of ions at the anode is larger than in the  bulk of the solution. Polarization can never reach infinity in practice. Instead another reaction establishes itself at a more active potential.