Macrocell formation

As stated in Chapter 2, corrosion proceeds by the formation of anodes and cathodes (Figures 2.1 and 2.2). In the case of chloride attack they are often well separated with areas of rusting separated by areas of clean steel. This is known as the macrocell phenomenon. Chloride induced corrosion is particularly prone to macrocell formation as a high level of water is usually present to carry the chloride into the concrete and because chlorides in concrete are hygroscopic (i.e. they absorb and retain moisture). The presence of water in the pores increases the electrical conductivity of the concrete. The higher conductivity allows the separation of anode and cathode as the ions can move through the water filled (or water lined) pores. 

Let us now examine the formation of aich microstructures. Lowe and co-workers ) consider that the formation of microcells is caused by a rupture of macrocell walls by formaldehyde vapors. Indeed, oligomer polycondensation of 2-methylolphenol in the presence of acidic catalyst may partially proceed via 2-hydroxydibenzyl ethers ... 

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. 

Figure 8-11. Increased localized corrosion of steel in concrete due to the formation of a macrocell ac-tive/passive. The current (/) is flowing from the local anode to the cathode.

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