I need help understanding the phenomenon of anode solution polarization

This post was last edited by sdhklh on 2010-11-29 09:06. The original poster may find it useful as a reference; it might provide some help: http://www.zgdiandu.com.cn/technology/common%20sense/Commonview.aspx?id=2537. Polarization refers to the phenomenon where the electrode potential deviates from its equilibrium potential when current flows through the electrode. Cathode polarization causes the cathode electrode potential to shift in the negative direction ； Anodic polarization shifts the potential in the positive direction. The greater the current density passing through the electrode, the larger the absolute value of the deviation of the electrode potential from the equilibrium electrode potential. This deviation can be expressed by the term overpotential or electrode potential difference ΔE. Generally, overpotential is represented as a positive value; thus, there is a cathode overpotential and an anode overpotential. The reason for polarization of the electrodes when electricity is applied is due to the slow rate of a certain step in the electrode reaction process. Taking the cathodic reaction process in which metal ions are reduced to metal at the electrode as an example, this reaction process consists of the following three consecutive steps: ① Metal hydrated ions move from within the solution to the cathode interface — the step of mass transfer in the liquid phase ； ②Metal ions gain electrons at the cathode and at the interface, being reduced to metal atoms – an electrochemical process ； ③Metal atoms arrange themselves into a specific structure to form a metal crystal – the step of forming a new phase. These three steps occur sequentially, but the speed of each step varies; therefore, the speed of the entire electrode reaction is determined by the slowest step. Polarization that occurs because the diffusion rate of reactants or products near the electrode surface is slower than the rate of electrochemical reactions is known as concentration polarization. The polarization that results from the electrochemical reaction rate at the electrode being slower than the speed of electron movement in the external circuit is known as electrochemical polarization or activation polarization. (1) Concentration polarization: In the electrode process, the individual step in which reaction particles are transported from within the solution to the electrode surface is known as the mass transfer step in the liquid phase. When the electrode process is controlled by the liquid-phase mass transfer step, concentration polarization occurs at the electrode. The liquid-phase mass transfer process can be accomplished through three mechanisms: electromigration, convection, and diffusion. In an acidic zinc plating solution, when no electricity is applied, the concentration is uniform throughout all parts of the solution. After power is applied, the reactant that is consumed first in the plating solution should be the zinc ions present in the liquid layer near the cathode surface. As a result, the zinc ion concentration in the liquid layer near the cathode surface continuously decreases, creating a concentration difference with the main body of the plating solution. At this point, the zinc ions in the solution itself should diffuse to the vicinity of the electrode surface to compensate, so that the concentrations become equal. Since the diffusion rate of zinc ions cannot keep up with the rate at which they are consumed in the electrode reaction, the ion concentration in the liquid layer near the electrode surface decreases further. Therefore, even if the reaction rate of Znz++2e—Zn keeps up with the rate of electron transfer, the lack of zinc ions near the electrode surface still leads to an accumulation of electrons at the cathode, causing the electrode potential to become negative and resulting in polarization. Since a decrease in zinc ion concentration inevitably occurs in the liquid layer near the electrode at this time, creating a concentration difference with the bulk solution, this phenomenon is known as concentration polarization. The same is true for concentration polarization at the anode: the zinc ions that dissolve from the zinc anode into the solution cannot diffuse inward into the solution in a timely manner, resulting in an increase in the concentration of zinc ions in the liquid layer near the anode surface. This causes the electrode potential to shift in the positive direction, leading to concentration polarization at the anode. In the cathodic region, the current density at which the current increases to such an extent that the concentration of pre-deposited metal ions approaches zero is called the limiting current density, and a plateau appears on the electrochemical polarography curve. When the cathode area reaches its limiting current, the extreme shortage of pre-plating ions leads to H+ discharge and substantial hydrogen evolution, causing the cathode area to become highly alkaline. At this point, a large amount of hydroxides are incorporated into the coating, resulting in a rough, porous, sponge-like electroplated layer; this phenomenon is known as \"charring\" in electroplating processes. (2) Electrochemical polarization: The change in electrode potential resulting from the slow progression of the electrochemical steps during the cathodic reaction is known as electrochemical polarization. This change in electrode potential can also be considered as a change in the activation energy of the electrode reaction, thereby affecting the rate of that electrode reaction. During the galvanizing process, when no current is flowing, the cathode in the plating solution is in an equilibrium state, with its electrode potential at a constant level. After power is applied, assuming that the rate of the electrochemical steps is infinite, it is still possible to allow zinc ions to undergo reduction at the cathode while maintaining a constant equilibrium potential, even though the cathodic current density is high (i.e., many electrons are supplied to the electrode per unit time). In other words, all the electrons that flow in from the external circuit are immediately consumed by the reduction reaction of zinc ions as soon as they reach the electrode surface; as a result, there is no accumulation of excess electrons on the electrode surface. The charge of the electrode remains the same as when it is not powered, and the original double layer does not change either. In other words, the electrode potential remains unchanged, and the electrode reactions continue to occur at the equilibrium potential. If the rate of the electrode reaction is limited, meaning that the reduction of zinc ions requires a certain amount of time to complete, but the amount of electric charge supplied to the electrode per unit time is extremely small (i.e., the cathodic current density is extremely low), then the zinc ions still have sufficient time to combine with the electrons on the electrode. As a result, there is no accumulation of excess electrons on the electrode surface, and therefore the electrode potential remains unchanged, staying at its equilibrium value. However, in reality, neither of these two assumptions holds true. During electroplating, the speed at which charge flows toward the electrode (i.e., the current) is not infinitesimally small, nor is the speed at which zinc ions are reduced on the electrode infinitely high. Due to the electrode reactions involving the gain or loss of electrons, certain resistance is always present. Therefore, after an external power source supplies electrons to the electrode, the zinc ions cannot be reduced immediately, and the electrons supplied by the external power source cannot be completely consumed. As a result, an excess of electrons accumulates on the electrode surface (compared to the equilibrium state when no electricity is applied), causing the negative charge on the electrode surface to increase compared to before electrification. This leads to the electrode potential shifting in the negative direction, resulting in polarization. By the same principle, since the rate at which zinc atoms release electrons at the anode is slower than the rate at which electrons flow from the anode into the external power supply, an excess of positive charge accumulates at the anode (in the form of zinc ions), causing the anode potential to deviate from its equilibrium value and become positive; this is what constitutes electrochemical polarization of the anode. From the discussion of the electrode polarization process above, it can be seen that polarization of the electrode occurs essentially because the rates of electrode reaction, electron transfer, and ion diffusion are not in harmony with each other. Cathodic concentration polarization occurs because the rate of ion diffusion is slower than the rate at which ions are consumed in the electrode reaction, whereas cathodic electrochemical polarization results from the rate of electron transfer being faster than the rate at which electrons are consumed in the electrode reaction.