Module 6.4 Corrosion

Corrosion questions appear in Modules 6 and 7 for every category; B1 and B3 need level 3 on types and causes.

Knowledge levels

Item	A	B1 / B3	B2 / B2L
6.4(a) Chemical fundamentals; formation by galvanic action, microbiological contamination, mechanical stress	1	1	1
6.4(b) Types of corrosion and their identification; causes; material susceptibility	2	3	2

Chemical fundamentals

• Corrosion is the electrochemical deterioration of a metal by reaction with its environment, returning it towards its natural oxide or salt.
• Four conditions are needed - a corrosion cell:
  1. an anode (the metal that corrodes - loses electrons, oxidation);
  2. a cathode (receives electrons, reduction);
  3. an electrolyte (moisture, salt water, condensation, acids);
  4. an electrical path between anode and cathode.
• Remove any one condition and corrosion stops - the basis of protection (coatings exclude the electrolyte; insulation breaks the path).

Key fact: The anode corrodes; the cathode is protected.

Galvanic series (from anodic to cathodic, simplified)

More anodic (corrodes)
Magnesium and its alloys
Zinc, cadmium
Aluminium alloys (pure Al, 5052, 6061, 7075, 2024...)
Steel, cast iron
Lead, tin
Brass, copper, bronze
Nickel, Monel
Stainless steel (passive)
Titanium
Carbon/graphite, gold, platinum
More cathodic (protected)

The further apart two metals are in the series, the greater the potential difference and the faster the anodic metal corrodes. A small anode with a large cathode is the worst combination (e.g. aluminium rivets in a stainless panel).