JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech III Year ENVIRONMENTAL DEGRADATION OF MATERIALS Previous Question Paper (September - 2021 - R18)

Corrosion is the destructive attack of a metal by chemical or electrochemical reaction with its environment, resulting in the degradation of its physical and mechanical properties. It is essentially the natural tendency of metals to revert to their stable, lower-energy oxidized states, such as oxides, hydroxides, or sulfides, which are found in nature as ores.

Corrosion is broadly classified into two main categories:

1. Dry Corrosion (Chemical Corrosion): Occurs in the absence of moisture, typically involving direct chemical attack by gases (e.g., O2, Cl2, H2S) at high temperatures. It involves the formation of a surface film of corrosion products.
2. Wet Corrosion (Electrochemical Corrosion): Occurs in the presence of an aqueous electrolyte. It involves the formation of electrochemical cells with distinct anodic and cathodic regions on the metal surface, leading to the flow of electrons and the dissolution of the metal at the anode.

Thermodynamically, the feasibility of corrosion is determined by the Gibbs Free Energy change (ΔG) of the reaction. For a spontaneous corrosion process to occur, the total change in free energy must be negative (ΔG < 0).

Where:

• n is the number of electrons transferred.
• F is the Faraday constant (96,500 C/mol).
• E is the electromotive force (EMF) or cell potential.

A positive cell potential (E > 0) indicates a spontaneous reaction. According to the Nernst equation, the potential depends on the concentration of metal ions and the activity of the environment. If the potential of the metal-environment system is higher than the potential required to maintain the metallic state, the metal will thermodynamically favor oxidation. While thermodynamics predicts the possibility of corrosion, it does not dictate the rate, which is governed by kinetic factors such as polarization, surface films, and mass transport.

Corrosion is fundamentally an electrochemical process involving the transfer of electrons between a metal and its environment. It occurs through the simultaneous operation of anodic and cathodic reactions on the surface of the metal, typically in the presence of an electrolyte.

The anodic reaction involves the oxidation of the metal, where metal atoms lose electrons and enter the electrolyte as ions: M → Mⁿ⁺ + ne^-. The cathodic reaction consumes these electrons, typically through the reduction of dissolved oxygen (in neutral or alkaline solutions) or the evolution of hydrogen (in acidic solutions). For the process to continue, there must be a potential difference between the anode and cathode, an electrical path (metallic conductor), and an ionic path (electrolyte).

Environmental degradation manifests in various forms:

• Uniform Corrosion: The most common form, characterized by a general, even thinning of the metal surface across the entire exposed area.
• Pitting Corrosion: A highly localized form of attack that results in small holes or cavities in the metal, often difficult to detect but leading to rapid structural failure.
• Galvanic Corrosion: Occurs when two dissimilar metals are in electrical contact in an electrolyte, causing the more active (anodic) metal to corrode preferentially.
• Crevice Corrosion: Occurs in shielded areas such as under gaskets, washers, or bolt heads where stagnant solutions create a localized chemical environment distinct from the bulk fluid.
• Stress Corrosion Cracking (SCC): The growth of cracks due to the combined influence of tensile stress and a specific corrosive environment, often occurring at stress levels below the yield strength.
• Intergranular Corrosion: Localized attack at or adjacent to grain boundaries, often caused by the precipitation of secondary phases (e.g., chromium carbides in stainless steels) that deplete the surrounding region of alloying elements.
• Erosion-Corrosion: The acceleration of material degradation due to the relative movement between a corrosive fluid and a metal surface, which mechanically removes protective surface films.
• Hydrogen Embrittlement: The loss of ductility and load-bearing capacity of a metal due to the absorption of atomic hydrogen, often leading to brittle fracture.