Cathode

2H20 water

Rust formation

electrons

4OH-

oxygen electrons hydroxyl ions (from the air)

iron ions hydroxyl ions

> Fe2O3.H2O iron hydroxide rust

Overall summary

iron

3O2 + oxygen

2H2O water

2Fe2O3.H2O rust

The key factors which accelerate the rate of corrosion are the presence of two dissimilar metals in mutual contact and the degree of pollution within any moisture surrounding the metals. If the more base metal is small in quantity compared to the more noble metal, then rapid corrosion of the more base metal will occur. Figure 5.34 shows which pairs of metals commonly used in construction should not generally be allowed into contact. Within a single metal the following may all cause accelerated corrosion: minor surface variations, such as crystal grain boundaries; the effects of cold working or welding; the presence of impurities or alloying components within the metal; variable cleanliness or access to aerial oxygen. Figure 5.35 illustrates the effect of corrosion between an extruded aluminium gutter and a steel rainwater pipe. The aluminium corroded producing a white deposit near the point of contact between the two metals; this was followed by rapid corrosion of the steel.

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