Repassivation cathodic protection

Cathodic protection. The technique of cathodic protection, which is usually ap-pHed for chloride-induced corrosion, has also been applied to carbonated structures in the presence of small amounts of chlorides. It requires the permanent application of a small direct current to the steel and it can lead to repassivation of the reinforcement because of the realkalization of the concrete around the steel [6]. This technique is described in Section 20.3. 

In chloride-contaminated structures, repassivation of steel can be achieved by replacing the contaminated concrete with chloride-free material (conventional repait), by removing chlorides from the concrete or by means of cathodic protection (Chapter 20). 

This technique requires the permanent application of a small direct current to protect the steel. It can also lead to repassivation of the reinforcement if it lowers the steel potential below the repassivation potential (Section 7.3). Provided it is applied properly, cathodic protection is able to stop corrosion for any level of chloride con-... 

T. Pastore, Repassivation of steel in carbonated concrete induced by cathodic protection . Materials and Corrosion, 2003, 54, 163-175. 

Cathodic protection (CP) is applied to structures already affected by corrosion, mainly induced by chlorides the steel is subjected to cathodic polarization, i.e. its potential is brought to values more negative than the free corrosion value, so that the corrosion rate is reduced. Corrosion can actually be stopped if a potential more negative than the value of repassivation (E, Figure 7.9) is reached. 

The experience on bridge decks shows that, in the cases in which the cathodic protection path runs according to 4-5 of Figure 20.4, the current required to maintain protection conditions (verified by the so-called four-hour 100 mV potential decay empirical criterion decreases with time, even after months or years from start up. This happens because the cathodic current can bring about repassivation of steel in active zones. When passivity is established on the entire surface of the steel, the current required to maintain passivity is reduced to a few mA/m (e. g. 2-5 mA/m ). If the CP path runs according to 4-6, the current density to fulfil the protection criterion remains high and does not decrease with the time, since passivity is not obtained. 

The propagation of crevice corrosion can also be arrested by decreasing the potential of the outside surfaces below a critical value (see earlier). The existence of a repassivation or protection potential was recognized very early, in particular by Pourbaix et al. [81] for pitting corrosion. From a practical point of view, the existence of a protection potential below which no crevice corrosion is possible is of major importance because it guarantees the immunity of passivated alloys in near-neutral chloride solutions in the absence of oxidizing species and because it makes possible the cathodic protection of stmctuies. 

Repassivation with alkaline mortar or concrete. Repassivation of steel can be obtained by replacing the chloride-contaminated concrete with chloride-free and alkaline mortar or concrete. Because of the mechanism of chloride-induced corrosion, it is not sufficient to repair the concrete in the area where the reinforcement is de-passivated. The concrete must be removed in all areas where the chloride threshold has reached the depth of the reinforcement or is expected to reach it during the design Hfe of the repair. In fact, the concrete that surrounds the zones of corrosion usually has a chloride content higher than the chloride threshold, even though the steel remains passive because it is protected by the corroding site. In fact, a macrocell forms (Figure 18.6a) that provides cathodic polarization to adjacent steel and... 

After five months, in the carbonated specimens protected at 10 mA/m, the potential difference between the two activated titanium electrodes increased significantly and reached a value up to 200 mV, suggesting that the atkahnity produced by the cathodic reaction at the steel surface induced locaHzed reaUsaUzation of the concrete in the vicinity of the rebar. For the other specimens (i. e. in carbonated concrete at 2 and 5 mA/m ) realkalisation of concrete and repassivation of steel did not occur even after five years of testing [45]. In the same study, it was found that the application of a start-up current density of 70 mA/m for 1 month proved to be an effective way for achieving repassivation of steel in carbonated concrete [46]. The tests showed that, once repassivation is induced, even a current density of 5 mA/m is sufficient to guarantee protection. 

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