Understanding Corrosion: The Polarization Characteristics in Electrochemical Reactions

Corrosion is a complex process that involves various electrochemical reactions, particularly anodic and cathodic reactions. A useful method for analyzing these reactions is to display their polarization characteristics on a single diagram. This approach allows for a comprehensive understanding of how these reactions interact within a specific corroding system, such as iron in neutral aerated water.

To begin, the equilibrium potentials for possible reactions are determined using the Nernst equation. This equation takes into account the prevailing activities of different species in the solution. For instance, in the case of iron corrosion, the reactions can be represented as the dissolution of iron ions (Fe²⁺ + 2e⁻ = Fe) and the reduction of oxygen (O₂ + H₂O + 2e⁻ = 2OH⁻). With conditions assumed at pH 7 and saturation with oxygen, the equilibrium potentials can be calculated, yielding critical values that inform the overall corrosion behavior.

Once the equilibrium potentials are established, a diagram is constructed with axes labeled for potential (E) and current density (i). In this diagram, the anodic and cathodic currents are plotted, indicating how they vary with respect to the applied potential. The point at which the total anodic and cathodic currents are equal defines the corrosion potential (E_CORROSION), which is essential for understanding the rate at which metal dissolves.

Interestingly, in some situations, the anodic reaction of metal dissolution can be supported by multiple cathodic reactions. For example, in an aerated dilute acid, both the reduction of oxygen and the discharge of hydrogen can significantly contribute to the total cathodic current. In such cases, the corrosion potential is reached when the anodic current from metal dissolution equals the combined currents from these cathodic reactions.

In practical applications, by analyzing specific data and applying the Tafel equation, one can derive the anodic and cathodic current densities. This analysis not only reveals the dominant reactions sustaining corrosion but also helps predict how various factors, such as changes in pH or oxygen concentration, may influence the corrosion process. The examination of polarization characteristics thus serves as a critical tool in understanding and managing corrosion in various environments.