Electrochemical Principles

Author: Chief Editor: Li Di
Publisher:
Publish Date: 2003-11-01
Features: This book primarily introduces the fundamental principles of aqueous solution electrochemistry. The entire book is divided into three main parts: electrochemical thermodynamics, the structure and properties of electrode-solution interfaces, and electrode process kinetics, with a focus on relatively mature basic theories. In this revision, content on the anodic processes of metals, the electrodeposition process of metals, and chemical power sources has been added, enhancing its practicality. This book can serve as a teaching material for undergraduate and graduate students majoring in electrochemical engineering, as well as a reference for scientists and engineers working in electrochemistry, corrosion and protection, electroplating, electrolysis, chemical power sources, and electroanalytical chemistry. Excerpt: The nature of the electrode here refers to its composition. Since the oxidized and reduced substances constituting the electrode differ, their abilities to gain or lose electrons also differ, resulting in different electrode potentials. This can be seen from Table 2.3.
2. Surface State of Metals
The precision of surface processing, purity of the surface layer, the presence of oxide films or other phase-forming films, and the adsorption of atoms or molecules on the surface all have a significant impact on the electrode potential of metals, causing it to change by as much as 1V. Among these factors, the influence of naturally formed protective film layers on the metal surface is particularly notable. The formation of protective films generally shifts the metal electrode potential to the positive side, whereas the destruction of these films (e.g., cracking, increased porosity) or enhanced penetration of ions from the solution into the film often causes the electrode potential to shift to the negative side. The potential change can reach hundreds of mV. Gaseous atoms adsorbed on the metal surface often have a strong effect on the electrode potential. These adsorbed gases may have originally been dissolved in the solution or may have been adsorbed onto the metal surface before it was placed in the electrolyte. For example, when iron is placed in a 1 mol/L KOH solution, the electrode potential is -0.27V with a large amount of oxygen adsorbed, and -0.67V with a large amount of hydrogen adsorbed. This difference arises from the adsorption of different gas atoms. Typically, oxygen adsorption causes the metal electrode potential to shift to the positive side, while hydrogen adsorption causes it to shift to the negative side. The influence of adsorbed gases on the electrode potential is generally tens of mV, sometimes reaching hundreds of mV.
3. Mechanical Deformation and Internal Stress of Metals
The presence of deformation and internal stress generally causes the electrode potential to shift to the negative side, but the effect is usually minor, ranging from a few mV to tens of mV. The reason for this can be explained as follows: On deformed metal, the energy of metal ions increases, and their activity also increases. When the metal is immersed in a solution, it dissolves more easily into ions. Therefore, when the interfacial reaction reaches equilibrium, the potential difference of the double electric layer formed is relatively more negative. If the protective film on the metal surface is damaged due to deformation or stress, the potential will also shift to the negative side.
4. pH of the Solution
The pH value has a significant effect on the electrode potential. Table 2.6 lists the electrode potentials of the same metal in typical acid, base, and salt solutions, such as 1 mol/dm3 HCl, KCl, and KOH. From the table, it can be seen that the influence of pH can cause the electrode potential to change by hundreds of mV.
5. Presence of Oxidizing Agents in the Solution
As shown in Table 2.4, the addition of oxidizing agents (e.g., H?O?) has a significant effect on the electrode potential. In the typical metal corrosion processes, the oxidizing agent encountered is often oxygen dissolved in the electrolyte. Oxidizing agents generally cause the electrode potential to shift to the positive side, in addition to the effect of adsorbed oxygen, they may also shift the potential to the positive side by forming an oxide film or making the original protective film more dense.
6. Presence of Chelating Agents in the Solution
When chelating agents are present in the solution, metal ions may no longer exist in the form of hydrated ions but rather as certain chelated ions, which can affect the nature of the electrode reaction and the magnitude of the electrode potential.