1) With reference to electrochemical corrosion, explain the following phenomena:
i) Anodic Polarization
ii) Cathodic Polarization
2) What are two degradation mechanisms that are the responsible for the loss in molecular weight of polymeric materials?
1) Electrochemical corrosion occurs when a metal interacts with its surrounding environment, leading to a degradation of the metal. This type of corrosion involves two primary phenomena: anodic polarization and cathodic polarization.
i) Anodic Polarization: Anodic polarization occurs when the metal becomes an anode in an electrochemical cell. In this case, the metal undergoes oxidation, losing electrons and dissolving into its ionic form. The process of anodic polarization involves the following steps:
- Formation of anodic sites: Anodic polarization begins with the formation of localized areas on the metal's surface, known as anodic sites. These sites have a higher tendency to undergo oxidation than the surrounding metal surface.
- Metal oxidation: At the anodic sites, metal atoms lose electrons and go into their ionic state. For example, iron atoms can oxidize to form Fe2+ or Fe3+ ions, depending on the corrosion environment.
- Ion migration: Once the metal atoms are in their ionic form, these ions can migrate away from the anodic sites into the surrounding electrolyte, causing the metal to dissolve. This process is commonly known as metal ion corrosion.
ii) Cathodic Polarization: Cathodic polarization occurs when the metal becomes a cathode in an electrochemical cell. In this case, the metal is reduced and gains electrons, which protects it from further corrosion. The process of cathodic polarization involves the following steps:
- Formation of cathodic sites: Cathodic polarization starts with the formation of localized areas, called cathodic sites, typically near the anodic sites. These cathodic sites act as electron-receiving regions.
- Electron reduction: At the cathodic sites, certain species from the electrolyte (such as oxygen or hydrogen ions) are reduced by accepting electrons from the metal surface. For example, in the presence of oxygen, the reduction reaction can produce hydroxide ions (OH-) that can react with metal ions to form metal hydroxides, providing a passivation layer.
The combination of anodic polarization and cathodic polarization creates a corrosion cell where metal dissolution occurs at the anode (anodic site) and corrosion protection occurs at the cathode (cathodic site). The flow of electrons between these sites drives the electrochemical corrosion process.
2) The loss in molecular weight of polymeric materials can be attributed to two main degradation mechanisms:
i) Thermal Degradation: Thermal degradation occurs when the polymer is exposed to high temperatures, causing the chemical bonds within the polymer chains to break. This process can result in a reduction in molecular weight. The high temperatures can be generated by external heat sources, processing conditions, or the activation of the degradation process by other factors such as light or radiation.
ii) Chemical Degradation: Chemical degradation refers to the degradation of polymeric materials caused by reactive species present in the environment or chemical agents that the material comes into contact with. Some common factors leading to chemical degradation include exposure to oxidizing agents, acids, bases, or reactive species like free radicals. These agents can attack and break the chemical bonds in the polymer chains, causing a decrease in molecular weight.
Both thermal degradation and chemical degradation can lead to a loss in molecular weight, which results in a decrease in the polymer's mechanical properties and overall performance. Understanding and controlling these degradation mechanisms are crucial in maintaining the service life and durability of polymeric materials.