Corrosion Theory and Degradation Mechanisms
Core mechanism
Nearly all aqueous corrosion is electrochemical: an anodic reaction releases electrons through metal oxidation, a cathodic reaction consumes them through oxygen reduction, hydrogen evolution, or deposition, and an electrolyte plus a metallic path close the circuit. Candidates must distinguish the anode, which corrodes and is negative in a galvanic couple, from the cathode, which is protected, and must know that electrode potential is measured against a reference electrode. The galvanic series ranks alloys in a specific environment; the standard emf series ranks pure metals under standard conditions and is not a direct substitute.
Kinetics and damage forms
Equilibrium potential predicts tendency, not rate. Polarization, passivity, and film stability control actual rate. Passivation can break down locally, producing pitting and crevice corrosion; differential aeration creates an oxygen-concentration cell. Recognize mechanistic signatures: uniform thinning, pitting, crevice attack, galvanic attack, erosion-corrosion, microbiologically influenced corrosion, stress-corrosion cracking, hydrogen embrittlement, and sour-service cracking. Environmental variables such as pH, oxygen, chloride, temperature, flow, and microbial activity change both thermodynamics and kinetics.
Traps
Questions often reverse anode and cathode, treat the galvanic series as universal, or assume the more active metal always corrodes fastest. Area ratio matters: a small anode coupled to a large cathode concentrates anodic current density and accelerates penetration. A protective potential also does not guarantee protection against every cracking mechanism.
Worked example
A 1 cm² anode is coupled to a 100 cm² cathode, and the couple passes 10 mA. Anode current density is 10 mA / 1 cm² = 10 mA/cm², while cathode density is 0.1 mA/cm². The small-anode/large-cathode geometry is severe. Rule of thumb: avoid small anodes in large cathodes, and treat measured potential as a tendency indicator, not a corrosion-rate prediction.