How do the galvanic series position, metal purity and nature of corrosion product affect corrosion rate?

In one line

Corrosion is faster when the metal is more active (higher in the galvanic series) or coupled with a very different metal, when it contains impurities, and when the corrosion product is porous, soluble or volatile.

Explained simply

These are the "nature of the metal" factors. A metal that strongly wants to lose electrons corrodes faster, especially if joined to a metal that readily accepts them. Impurities create many tiny batteries on the surface. And a corrosion product that forms a tight protective layer acts like a seal, while a loose one does not.

How it works

1. Position in the galvanic series: the galvanic series ranks metals and alloys by their actual corrosion tendency in a given environment (usually sea water). When two metals are in electrical contact in an electrolyte, the one higher (more active, anodic) corrodes; the lower one (noble, cathodic) is protected. The greater the separation, the faster the corrosion of the anodic metal.
   Simplified order in sea water (active → noble): Mg, Zn, Al alloys, Cd, mild steel, cast iron, stainless steel (active), Pb, Sn, brass, Cu, bronze, Ni, stainless steel (passive), Ag, Ti, graphite, Au, Pt.
2. Purity of metal: impurities (e.g., Fe or Pb in Zn) create local galvanic cells, with the metal acting as anode; rate rises with impurity level. Very pure Zn corrodes far more slowly in dilute acid than commercial Zn.
3. Nature of corrosion product: if the product is insoluble, non-porous and adherent (Al₂O₃, Cr₂O₃, PbSO₄ on lead in H₂SO₄), it protects the metal. If it is soluble, porous or volatile, corrosion continues.

Key formula

No formula. Remember the rule: greater potential difference between coupled metals → stronger galvanic cell → faster corrosion of the anodic metal.

Quick example

Zinc and aluminium: in the electrochemical (EMF) series Al is more active than Zn, but in the galvanic series in sea water Zn lies above Al because Al gets passivated by its oxide film. This is why the galvanic series, not the EMF series, is used for practical predictions.

A brass fitting screwed into a steel pipe makes the steel corrode near the joint.

Common mistakes

• Treating the galvanic series and the electrochemical series as identical. The galvanic series includes alloys and passivity effects.
• Saying the noble metal corrodes in a galvanic couple. The more active metal corrodes.
• Thinking any corrosion product slows corrosion. Only a compact, adherent, insoluble one does.

How MU asks it

"Explain any three/four factors affecting the rate of corrosion" (5–6 marks), or "Explain the effect of galvanic series on corrosion" (3 marks). For full marks: name the factor, explain the reason in 2–3 lines, and give one example each. Mention why the galvanic series is more practical than the EMF series.