Certify66
Start free
← Back to Materials & Hardware
Section 6.4

Corrosion

The electrochemical process, forms of corrosion, the galvanic series, prevention and treatment.

Notes

Corrosion — summary notes

Free
Main ideas
  • Corrosion is the electrochemical degradation of a metal — it needs an anode, a cathode, an electrolyte (moisture) and a metal path. Most corrosion is oxidation returning the metal toward its natural ore.
  • Common forms: uniform/surface, pitting (localised, in aluminium and stainless), intergranular (along grain boundaries) and its severe layered form exfoliation, galvanic (dissimilar metals), stress-corrosion cracking, filiform (under paint), fretting and microbial (in fuel tanks).
  • The galvanic series runs from noble/cathodic (gold, stainless, titanium) to active/anodic (aluminium, zinc, magnesium). When two dissimilar metals touch with an electrolyte present, the MORE ANODIC (less noble) metal corrodes, and the further apart in the series, the faster.
  • Prevention: choose compatible materials, apply protective finishes (primer/paint, anodising, cadmium or zinc plating, Alclad), insulate/seal dissimilar-metal joints, ensure drainage, and apply corrosion-inhibiting compounds — keeping surfaces clean and dry is the single biggest factor.
  • Treatment: remove all corrosion products back to sound metal, assess the remaining material against the allowable damage limits, then restore the protective finish and record the work.
  • ⚠ Exam trap: in a galvanic couple it is the more anodic (LESS noble) metal that sacrifices itself — aluminium corrodes when coupled to stainless steel; a zinc/magnesium sacrificial anode protects by being deliberately anodic. Exfoliation is the layered form of intergranular corrosion.
Solved examples

  1. A cadmium-plated steel bolt is fitted directly into a magnesium casting with no protection. Which part corrodes, and how should the joint have been made?

    Magnesium is far more anodic than cadmium-plated steel, so the magnesium casting corrodes preferentially around the bolt. The joint should use compatible platings and be sealed/insulated (wet-assembly with sealant, insulating washers, correct primer) to break the galvanic circuit between the dissimilar metals.

Mind map

Corrosion concept map

Free

Corrosion

Quiz

Corrosion quiz

Pro

This quiz is part of Engineer

Upgrade to unlock every quiz, mock exam and advanced simulation across all modules. Notes and mind maps stay free.

Unlock with Engineer
← Back to Materials & Hardware