Corrosion of Steel in Mortar

Due to both carbonization and penetration of chloride ions, steel will pass from a passive to an active condition and (consequently) may corrode. If the mortar is completely surrounded by water, oxygen diffusion in wet mortar is extremely low so that the situation is corrosion resistant because the cathodic partial reaction according to Eq. (2-17) scarcely occurs. For this reason the mortar lining of waste pipes remains protective against corrosion even if it is completely carbonated or if it is penetrated by chloride ions. 

The cathodic effectiveness of the passive steel in cement mortar can be seen in Fig. 5-13. The cell current is measured between a mortar-coated DN 100 pipe section and an uncoated steel ring 16 mm broad as anode. It can be clearly seen that the cell current immediately falls and after 100 days goes toward zero. The same result is obtained by removing the specimens and aerating the mortar coating and repeating the experiment with the same components [51]. 

Cathodic protection can be used to protect steel in concrete (see Chapter 19). There is no fear of damage by H2 evolution due to porosity of the mortar. Local corrosion attack can be observed under extreme conditions due to porosity (water/ cement ratio =1) and polarization (U = -0.98 V) with portland cement but not with blast furnace cement, corresponding to field IV in Fig. 2-2 [53]. However, such conditions do not occur in practice. 

Anodic polarization can occur in the presence of stray currents. Oxygen is evolved on the passive steel according to  

The anodically produced acid is neutralized by the alkaline mortar (CaO). Corrosion is then possible only if the supply of alkali at the steel surface is consumed and the steel becomes active. This process is possible only under certain circumstances after a very long incubation period. Apparently in steel-concrete foundations the possible current densities are so small that this case never arises. The possibility of danger has to be verified with thin outer coatings where deliming has been noticed on the steel surface. 

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Handbook of Cathodic Corrosion Protection 5.3.2 Corrosion of Steel in Mortar... 

FIGURE 6.22 Corrosion of steel in mortar beam and ECC beam. (After Sahmaran, M. et al., ACIMaterials Journal, 105(3) 243-250, 2008. With permission.)... 

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