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Section 6.4

Corrosion

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

Notes

Corrosion, summary notes

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.

Simulation

Galvanic corrosion & the area effect

Galvanic Corrosion & the Area EffectFull screen ↗
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Mind map

Corrosion concept map

Corrosion

Quiz

Corrosion quiz

Corrosion, quiz

Ref 6.48 of 16Pass 75%

  1. 1. Corrosion is fundamentally a(n):

    Electrochemical process
    Purely mechanical process
    Thermal process only

  2. 2. In a galvanic couple, the metal that corrodes is the:

    More anodic (less noble) one
    More noble one
    Harder one

  3. 3. Aluminium in direct contact with stainless steel (with moisture) will:

    Corrode preferentially
    Protect the stainless by corroding it
    Not corrode at all

  4. 4. The layered corrosion that lifts a metal into flakes is called:

    Exfoliation
    Filiform
    Fretting

  5. 5. A sacrificial anode (e.g. zinc) protects a structure by:

    Being deliberately more anodic and corroding first
    Being the most noble metal present
    Sealing out all moisture

  6. 6. Pitting corrosion is characterised by:

    Localised deep holes with little surface spread
    Uniform thinning over the whole surface
    Cracking along grain boundaries

  7. 7. The single most effective everyday defence against corrosion is:

    Keeping surfaces clean and dry with intact finishes
    Leaving surfaces bare
    Storing in a humid hangar

  8. 8. Correct treatment of corrosion is to:

    Remove products to sound metal, assess to limits, restore the finish
    Paint straight over it once cleaned, sealing the affected area away from further moisture and oxygen
    Ignore light corrosion, on the basis that surface attack stabilises itself and does not progress inward

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