Pits, stray current and bacterial corrosion

The chemistry of pitting is quite complex and is explained in most chemistry text books. However, the principle is fairly simple, especially where chlorides are present. At some suitable site on the steel surface (often thought to be a void in the cement paste or a sulphide inclusion in the steel) the passive layer is more vulnerable to attack and an electrochemical potential difference exists that attracts chloride ions. Corrosion is initiated and acids are formed, hydrogen sulphide from the MnS inclusion and HCl from 

A pit forms, rust may form over the pit, concentrating the acid (H ), excluding oxygen so that the iron stays in solution preventing the formation of a protective oxide layer and accelerating corrosion. 

We will return to the subject of pitting corrosion later. It is related to the problems of coated reinforcement and to the black rust phenomenon discussed earlier. 

Corrosion is often local, with a few centimetres of corrosion and then up to a metre of clean passive bar, particularly for chloride induced corrosion. This indicates the separation of the anodic reaction (2.1) and the cathodic reaction (2.2) to form a macrocell . Chloride induced corrosion gives rise particularly well defined macrocells. This is partly due to the mechanism of chloride attack, with pit formation and with small concentrated anodes being fed by large cathodes. It is also because chloride attack is usually associated with high levels of moisture giving low electrical resistance in the concrete and easy transport of ions so the anodes and cathodes can separate easily. 

In North America, this is used as a way of measuring corrosion by measuring macrocell currents . This is discussed later in Section 4.12.6. 

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