What influences the rate of corrosion?

Arthur C Bark, West Malling

In answers to ealier questions here and here, I looked briefly at how corrosion is an electrochemical process, in which thermodynamics says what reactions are possible on an electrode (metal) surface and which metals corrode preferentially to others when coupled together. However, thermodynamics says nothing (or very little) about the rate of the corrosion process. For this, we have to look to the kinetics of the reactions.

In the real world, the rate of corrosion of metals and alloys in an environment is governed above all by the presence of surface layers and their protectiveness (This applies to high temperature oxidation, as much as to aqueous corrosion). Some alloys form a naturally protective, microscopically thin surface oxide on contact with air. These, so-called passive layers, are formed on stainless steels and aluminium alloys. Other metals, in the right conditions in water, can form passive oxide layers, which slow down the corrosion rate by several orders of magnitude.  A good example of this is mild steel in low oxygen-containing water, which forms a passive layer of Fe3O4 (magnetite) on the surface.

Although some metals and alloys do not form these passive layers, the corrosion rate in water is still governed by the formation of thicker macroscopic layers. The corrosion rate of galvanised coatings on steel is controlled by zinc oxide/hydroxide corrosion product layers. The corrosion of copper pipework in drinking water systems is greatly reduced by the formation of copper oxide and copper carbonate patinas. 

It is when these protective layers either do not form properly in the first place or become disrupted for some reason that problems occur.

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