Corrosion
Corrosion questions test whether you can identify the electrochemical cell, name the controlling step, and choose a mitigation method that matches the mechanism rather than the symptom.
Core rules
- Four requirements for aqueous corrosion: an anode, a cathode, an electrolyte and an electron path. Removing any one stops the cell.
- The EMF series is thermodynamic and refers to pure metals in their own ions at unit activity; the galvanic series is practical and environment-specific (seawater, soil). Use the galvanic series for engineering decisions.
- Area ratio matters: a small anode coupled to a large cathode concentrates anodic current density and causes rapid local penetration. Never paint the anode and leave the cathode bare.
- Passivation (stainless steels, aluminium, titanium) gives low corrosion rates but creates vulnerability to local breakdown - pitting, crevice attack and chloride-induced stress corrosion cracking.
- Pourbaix diagrams map immunity, corrosion and passivation against potential and pH, and define cathodic protection windows.
- Cathodic protection: sacrificial anodes for small or remote structures, impressed current for large ones. Overprotection causes hydrogen embrittlement and coating disbondment.
Worked example
A 20 cm² steel coupon (ρ = 7.87 g/cm³) loses 0.35 g in 720 h. Rate = 3.45 × 10⁶ × W/(ρ·A·t) = 3.45 × 10⁶ × 0.35/(7.87 × 20 × 720) = 10.7 mpy. That is roughly 0.27 mm/year, acceptable for a structural member but not for a thin tube.
Traps that cost marks
Using the EMF series for a real environment. Mixing uniform and localised rates. Treating underdosed anodic inhibitors as safe - they can accelerate pitting. Forgetting that CO2 and H2S in oilfield fluids drive distinct mechanisms (carbonic acid corrosion versus sulphide stress cracking).