Pitting chloride-containing concrete

DC stray currents may have more serious consequences in chloride-contaminated concrete. On passive reinforcement in concrete containing chloride in a quantity below the critical content and thus in itself insufficient to initiate localized corrosion, the driving voltage AE required for current to flow through the reinforcement is lower than in chloride-free concrete and decreases as the chloride content increases (Figure 9.7). This is a consequence of less perfect passivity, and in particular a lower pitting potential. 

Initiation of pitting corrosion takes place when the chloride content at the surface of the reinforcement reaches a threshold value (or critical chloride content). A certain time is required from the breakdown of the passive film and the formation of the first pit, according to the mechanism of corrosion described above. From a practical point of view, the initiation time can be considered as the time when the reinforcement, in concrete that contains substantial moisture and oxygen, is characterized by an averaged sustained corrosion rate higher than 2 mA/m [8], The chloride threshold of a specific structure can be defined as the chloride content required to reach this condition of corrosion. 

Concrete containing chiorides. Inside corrosion pits that can develop in chloride-... 

Field studies (exposure tests) in marine or simulated marine environments demonstrated the much better corrosion resistance of stainless steels in concrete. After 4.5 years in natural marine conditions no cracking and no pitting corrosion occurred on an Fe-11% Cr alloy (Hewitt and Tull-min, 1994). Under accelerated chloride ingress the same alloy showed some pitting corrosion after one year, whereas specimens with plain carbon steel had already cracked. A 9.5 years exposure program on steels embedded in concrete containing up to 3.2% chloride additions with respect to the cement content showed that ferritic stainless steel with 13 % Cr showed corrosion at chloride levels over 1.9% (Treadaway etal., 1989). 

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