JNTU B.Tech III ENVIRONMENTAL DEGRADATION OF MATERIALS Previous Question Paper (March - 2021 - R18)

Corrosion is fundamentally an electrochemical process involving the spontaneous oxidation of a metal in the presence of an electrolyte. The process requires an anode (where oxidation occurs), a cathode (where reduction occurs), an electrolyte (conducting medium), and an electrical path for electron flow.

The electrochemical reactions can be summarized as:

• Anodic reaction: M → Mⁿ⁺ + ne^- (Metal dissolution/oxidation)
• Cathodic reaction: X^m+ + me^- → X (Reduction of species like H⁺ or O₂)

Thermodynamic Aspects of Corrosion:

Thermodynamics determines the spontaneity of a corrosion reaction. A reaction will proceed spontaneously if the change in Gibbs Free Energy (ΔG) is negative. The relationship between ΔG and the Electrode Potential (E) is given by the Nernst equation-derived relation:

ΔG = -nFE

Where:

• n is the number of electrons transferred.
• F is the Faraday constant (96,485 C/mol).
• E is the electromotive force (EMF) of the cell, defined as E = E_cathode - E_anode.

For corrosion to be thermodynamically favorable, the cell potential E must be positive, resulting in a negative ΔG. This is assessed using the Electrochemical Series or Standard Reduction Potentials. Metals with more negative standard reduction potentials are more active and prone to corrosion when coupled with more noble metals.

Note: While thermodynamics predicts the tendency for corrosion to occur, it does not provide information regarding the rate of corrosion. The actual kinetics are governed by factors such as polarization, activation energy, and mass transport, which determine the current density (i) and thus the metal loss rate according to Faraday's Law.

The cost of corrosion represents the total economic impact resulting from the deterioration of materials due to environmental interactions. This cost is typically categorized into two types:

• Direct Costs: These include the expenses associated with the replacement of corroded components, maintenance, protective coatings, inhibitors, and the use of more expensive, corrosion-resistant materials (e.g., stainless steel or superalloys) instead of carbon steel.
• Indirect Costs: These are often more significant and include losses due to plant shutdowns, product leakage, loss of efficiency, contamination, and catastrophic failures leading to environmental damage or loss of life.

Environmental degradation encompasses various mechanisms by which materials lose their structural integrity. The primary forms include:

• Uniform Corrosion: A generalized attack where the entire surface of the metal corrodes at a relatively constant rate, often managed through thickness allowances or protective coatings.
• Galvanic Corrosion: Occurs when two dissimilar metals are in electrical contact in an electrolyte, causing the more anodic metal to corrode preferentially.
• Pitting Corrosion: A localized, highly destructive form of attack that creates small holes or cavities in the metal surface, often occurring in passive metals like stainless steel in chloride-rich environments.
• Crevice Corrosion: Occurs in stagnant micro-environments, such as under gaskets, washers, or bolt heads, where oxygen depletion leads to a localized acidic environment.
• Intergranular Corrosion: Localized attack at the grain boundaries of an alloy, often caused by the precipitation of secondary phases (e.g., chromium carbides in stainless steel), which depletes the surrounding area of corrosion-resistant elements.
• Stress Corrosion Cracking (SCC): The growth of cracks under the combined influence of tensile stress and a specific corrosive environment, leading to sudden, brittle-like failure in ductile materials.
• Erosion-Corrosion: The acceleration of material degradation due to the relative movement between a corrosive fluid and a metal surface, which removes protective films and exposes fresh metal.
• Hydrogen Damage: Includes hydrogen embrittlement and blistering, where atomic hydrogen diffuses into the metal lattice, reducing ductility and causing internal pressure or cracking.
• Dealloying (Selective Leaching): The selective removal of one element from an alloy, such as the removal of zinc from brass (dezincification), leaving behind a porous, weak structure.