Iron dissolution, stray currents

Electrical conductivity is of interest in corrosion processes in cell formation (see Section 2.2.4.2), in stray currents, and in electrochemical protection methods. Conductivity is increased by dissolved salts even though they do not take part in the corrosion process. Similarly, the corrosion rate of carbon steels in brine, which is influenced by oxygen content according to Eq. (2-9), is not affected by the salt concentration [4]. Nevertheless, dissolved salts have a strong indirect influence on many local corrosion processes. For instance, chloride ions that accumulate at local anodes can stimulate dissolution of iron and prevent the formation of a film. Alkali ions are usually regarded as completely harmless, but as counterions to OH ions in cathodic regions, they result in very high pH values and aid formation of films (see Section 2.2.4.2 and Chapter 4). 

Above the passivity range, that is for potentials above about +600 mV, the steel is brought to conditions known as transpassivity. oxygen may be produced on its surface according to the anodic reaction of oxygen evolution 2H2O —> O2 + 4H + 4e, which produces acidity. Steel reaches these conditions only in the presence of an external polarization (for example in the presence of stray currents). Since the anodic reaction is oxygen evolution, dissolution of iron and consequent corrosion of the steel does not take place (i. e. the passive film is not destroyed). Nevertheless, if these conditions persist until the quantity of acidity produced is sufficient to neutralize the aLkaHnity in the concrete in contact with the steel, the passive film will be destroyed and corrosion will initiate. This aspect wiU be dealt with in Chapter 9. 

In principle, the cathodic process can be supported also from interference currents or stray currents. If the concrete is not degraded, iron is passive, and the anodic process is not the iron dissolution but the oxygen evolution without corrosion of the iron itself. If chlorides are present at a concentration sufficient to depassivate iron, oxygen evolution does not occur, but there is iron corrosion at much lower potential differences. 

---

See also in sourсe #XX -- [ ]

---