Passivation is the formation of an extremely thin, dense oxide film on a metal surface that stops the metal corroding further. In construction it almost always refers to the passivation of reinforcing steel in concrete: the strongly alkaline pore water of concrete forms an oxide skin on the steel that protects it for decades.
How it works in concrete
Fresh concrete has pore water with a pH of around 12.5 to 13. In that strongly alkaline environment a stable oxide layer a few nanometres thick forms on the reinforcement. As long as the film is intact the steel barely rusts, even with water and oxygen present.
What destroys the passive film
- Carbonation — carbon dioxide from the air slowly penetrates the concrete and reacts with the calcium hydroxide, lowering the pH. Once the pH falls below about 9, the passive film is no longer stable and the steel can rust. The carbonation front advances over years to decades, at a rate that depends on concrete quality, water-cement ratio and moisture conditions.
- Chlorides — chloride ions from de-icing salt or seawater break the passive film locally, even at high pH. That produces pitting corrosion: deep, localised loss of section that is barely visible from the surface.
| Carbonation | Carbon dioxide from the air lowers the pH; below about 9 the passive film is no longer stable. The front advances over years to decades. |
|---|---|
| Chlorides | Chloride ions from de-icing salt or seawater break the film locally, even at high pH, causing pitting corrosion that is barely visible from the surface. |
Rusting reinforcement expands to many times its original volume. That pressure pushes the cover off — the familiar picture of spalling concrete with brown rust staining.
Applications and prevention
- Adequate cover — concrete cover to the reinforcement is the first line of defence. How much is needed follows from the exposure class in the applicable concrete standard; always use spacers.
- Dense concrete — a low water-cement ratio and proper curing slow both carbonation and chloride ingress.
- Repair — in concrete repair loose rust is removed and the steel is treated with an alkaline repair mortar or a corrosion-inhibiting coating so that passivation is restored.
- Other metals — stainless steel and aluminium also owe their resistance to a spontaneously formed passive oxide layer. Stainless steel is often chemically re-passivated after welding.
Related terms
- Carbonation
- Reinforcement
- Concrete cover
- Spalling
- Corrosion
- Stainless steel
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