Questions
~4 questions
Difficulty
Medium
Importance
High yield for mechanical and metallurgy papers
Overview
Corrosion Engineering deals with the degradation of metals due to electrochemical or chemical reactions with their environment. In PSU exams, this topic is critical as it relates to equipment longevity in power plants and refineries, requiring a solid grasp of electrochemical potential and protection mechanisms.
Electrochemical Theory of Corrosion
Corrosion is essentially an electrochemical process involving an anode and a cathode connected by an electrolyte. Oxidation occurs at the anode (metal loss), while reduction occurs at the cathode, often involving oxygen or hydrogen evolution.
- Anode reaction: M -> M^n+ + ne-
- Cathodic reactions: O2 + 2H2O + 4e- -> 4OH- or 2H+ + 2e- -> H2
- Gibbs Free Energy criterion: Delta G must be negative for spontaneous corrosion
- Nernst equation application to electrode potentials
- Faraday's law links corrosion rate to current density
Types of Corrosion
Aspirants must distinguish between localized and uniform corrosion. Localized forms are particularly dangerous as they lead to premature, often catastrophic structural failure in pressure vessels and piping.
- Galvanic: Potential difference between dissimilar metals
- Pitting: Localized penetration due to Cl- ions
- Crevice: Occurs in stagnant environments like gaskets
- Stress Corrosion Cracking: Tensile stress + corrosive medium
- Intergranular: Chromium carbide precipitation in stainless steel
Corrosion Protection
Protection strategies focus on breaking the corrosion circuit or altering the thermodynamics of the system. Cathodic protection is the most frequently tested protection method in PSU exams.
- Sacrificial Anode (Galvanic): Zinc or Magnesium anodes
- Impressed Current Cathodic Protection (ICCP)
- Anodic protection via passivation
- Pilling-Bedworth ratio (PBR) for oxidation stability
- Inhibitors: Anodic, cathodic, or mixed types
Formula Sheet
Pilling-Bedworth Ratio (PBR) = V_oxide / V_metal
Faraday's Law: m = (I*t*M) / (n*F)
Nernst Equation: E = E0 + (RT/nF)ln(a_ion)
Exam Tip
Memorize that for corrosion to be protective, the Pilling-Bedworth ratio must be greater than 1 but less than 2.
Common Mistakes
- Confusing the roles of sacrificial anodes as being the cathode in the system
- Ignoring the Pilling-Bedworth ratio impact on the stability of protective oxide films
- Failing to recognize that stainless steel is prone to intergranular corrosion after sensitization
More Revision Notes
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