Material Durability: corrosion - waterline, stress, pitting, galvanic; protection, cathodic, anodic, coatings, e-coat

Corrosion Types & Protection: Cathodic, Anodic, Coatings, E-coat

1. What corrosion is

Electrochemical eating of metal: anodic site dissolves ($M \to M^{n+} + ne^-$), cathodic site consumes electrons ($O_2 + 2H_2O + 4e^- \to 4OH^-$ in neutral medium; $2H^+ + 2e^- \to H_2$ in acid). Both must exist + electrolyte connected. Rust: $4Fe + 3O_2 + 2H_2O \to 2Fe_2O_3\cdot H_2O$.

2. Four named types

• Galvanic: two different metals in contact in electrolyte — more active metal (higher in galvanic series) corrodes faster. Steel screw in copper plate: steel eats away. Prevention: insulate joint, choose close metals, sacrificial anode.
• Pitting: tiny deep holes under broken passive film (Cl⁻ attack on stainless steel). Small anode + large cathode = rapid perforation; most dangerous because invisible until leak.
• Waterline: just below water surface gets less $O_2$ → anodic vs oxygen-rich cathodic zone above → ship hulls/metal tanks corrode along the line (differential aeration cell).
• Stress (SCC): tensile stress + corrosive medium → sudden cracking (boiler steel in alkali — caustic cracking; brass in ammonia — season cracking). No visible thinning before failure.

3. Protection methods

(a) Cathodic protection: make whole structure the cathode.

• Sacrificial anode: attach Mg/Zn/Al block (more active) — it corrodes, structure safe. Pipelines, ship hulls, water heaters.
• Impressed current: external DC source drives protective current; for large buried structures.
  (b) Anodic protection: for passive-film metals (stainless steel, Ti) — small applied anodic current keeps protective oxide stable. Needs potentiostat control.
  (c) Coatings:
• Metallic: galvanizing (Zn on steel — sacrificial + barrier), tinning (Sn — barrier only; if scratched, Sn is cathodic so steel corrodes faster!), electroplating (Ni/Cr decorative + hard).
• Inorganic: anodizing (thick $Al_2O_3$ on Al), phosphating/chromating primer.
• Organic: paints, varnishes, polymer linings — barrier against $O_2$/moisture; need surface prep + primer + topcoat system.
  (d) E-coat (electrophoretic/electro-coating): car bodies dipped in paint bath; DC voltage deposits uniform charged paint particles even in recesses → baked. Gives even, corrosion-proof primer coat — why modern cars rust far less.

Exam Points

• Galvanic vs pitting vs waterline vs stress mechanism + one example each (5 marks favourite).
• Sacrificial anode vs impressed current with sketches (3 marks); galvanizing vs tinning contrast — what happens when coating breaks (asked almost every year).
• E-coat steps and why uniform (2–3 marks).