How does dry or atmospheric corrosion occur, and what decides whether the oxide film protects the metal?

In one line

Dry corrosion is direct chemical attack of a metal by dry gases (mainly oxygen) forming a surface film, and the nature of this film decides whether corrosion stops or continues.

Explained simply

When a metal is exposed to oxygen (especially when hot), a thin oxide "skin" forms. If the skin is tight and non-porous like cling film, it seals the metal and corrosion stops. If it is cracked, porous or flakes off, fresh metal stays exposed and corrosion keeps going.

How it works

Oxidation corrosion mechanism:

1. At the surface: 2M → 2Mⁿ⁺ + 2ne⁻ (oxidation); \( \frac{n}{2} \)O₂ + 2ne⁻ → nO²⁻; together they form the oxide film M₂O_n.
2. For further growth, metal ions diffuse outward or oxide ions diffuse inward through the film (metal ions are smaller and usually diffuse faster).

Types of oxide film:

• Stable: fine-grained, tightly adhering, non-porous; acts as a protective barrier (Al, Cr, Sn, Pb, Cu).
• Unstable: oxide decomposes back to metal and O₂, so no net corrosion (Ag, Au, Pt).
• Volatile: oxide evaporates as soon as it forms, exposing fresh metal; rapid continuous corrosion (MoO₃ on molybdenum).
• Porous: has pores or cracks; O₂ reaches the metal and corrosion continues (alkali and alkaline earth metals, e.g., Na, K, Mg).

Key formula

Pilling–Bedworth rule:

\( \text{PBR} = \dfrac{\text{volume of oxide formed}}{\text{volume of metal consumed}} \)

• PBR < 1: oxide is porous, non-protective (e.g., Mg about 0.8).
• PBR ≥ 1 (and not too large): oxide is compact and protective (e.g., Al about 1.3).
• PBR much greater than 1 (above about 2): stresses build up, the film cracks and spalls, so protection is lost.

Other dry corrosion: by other gases (Cl₂ on Ag forms a protective AgCl film; dry Cl₂ on Sn forms volatile SnCl₄; H₂S on steel forms porous FeS) and liquid-metal corrosion (e.g., molten sodium in nuclear coolant systems attacking containers).

Quick example

Aluminium window frames stay bright for years because a thin compact Al₂O₃ film forms instantly and stops further attack, even though Al is a very reactive metal.

Common mistakes

• Thinking a reactive metal must always corrode fast; the film matters more (Al vs Mg).
• Reversing the PBR ratio (it is oxide volume over metal volume).
• Saying dry corrosion needs moisture; it occurs without a liquid electrolyte.

How MU asks it

"Explain the mechanism of dry corrosion" or "Explain oxidation corrosion with the Pilling–Bedworth rule" (5 marks). For full marks give reactions, a sketch of metal/oxide/air layers with ion diffusion arrows, the four film types with examples, and the PBR rule.