JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech III ENVIRONMENTAL DEGRADATION OF MATERIALS Previous Question Paper (August - 2022 - R18)

Corrosion is the electrochemical deterioration of a material due to its reaction with the environment. Key types include:

• Uniform Corrosion: The most common form, characterized by a chemical or electrochemical reaction that proceeds uniformly over the entire exposed surface. It results in a general thinning of the metal.
• Galvanic Corrosion: Occurs when two dissimilar metals are in electrical contact in a corrosive electrolyte. The more anodic metal corrodes preferentially, while the cathodic metal remains protected.
• Pitting Corrosion: A localized form of attack that produces small holes or pits in the metal. It is highly dangerous as it often goes undetected until structural failure occurs.
• Crevice Corrosion: Occurs in confined spaces where the electrolyte becomes stagnant, such as under gaskets, washers, or bolt heads. The oxygen concentration gradient leads to localized acidification and metal dissolution.
• Intergranular Corrosion: Localized attack along the grain boundaries of an alloy, often resulting from the precipitation of impurities or alloying elements (e.g., chromium carbides in stainless steel).
• Stress Corrosion Cracking (SCC): The growth of cracks due to the combined effects of tensile stress and a specific corrosive environment, leading to brittle failure in otherwise ductile materials.

Both methods aim to prevent corrosion by making the protected metal the cathode of an electrochemical cell, but they differ in application and mechanism.

Sacrificial Coatings: This involves applying a layer of a more active metal (anode) onto the base metal. The most common example is galvanizing, where steel is coated with zinc. If the coating is scratched, the zinc corrodes preferentially, protecting the exposed steel through galvanic action. The coating is consumed over time.

Cathodic Protection: This is an active system used to prevent corrosion of large structures like pipelines or ship hulls. It involves making the entire structure the cathode. This is achieved in two ways: 1) Sacrificial Anode Cathodic Protection (SACP): Attaching blocks of a highly active metal (e.g., magnesium or aluminum) to the structure. The anode corrodes instead of the structure. 2) Impressed Current Cathodic Protection (ICCP): Using an external DC power source to force electrons into the structure, maintaining it at a potential where corrosion is thermodynamically impossible. Unlike sacrificial coatings, ICCP requires an external power supply and inert anodes (e.g., graphite or platinum).

In summary, sacrificial coatings are passive surface treatments that provide localized protection, whereas cathodic protection is a system-level approach that can be actively controlled to protect large, submerged, or buried structures.