Protection macrocells

Anodic and cathodic processes may take place preferentially on separate areas of the surface of the reinforcement, leading to a macrocell. This can be established, for instance, between active and passive areas of the reinforcement. Current circulating between the former, which are less noble and thus function as anodes, and the latter, which are more noble and thus function as cathodes, accelerates the corrosion attack on active surfaces while further stabilising the protective state of passive ones. The magnitude of this current, known as the macrocell current, increases as the difference in the free corrosion potential between passive and active rebars increases, and decreases as the dissipation produced by the current itself at the anodic and cathodic sites and within the concrete increases. 

Protection effect. MacroceU currents can have beneficial effects on rebars that are polarized cathodically. This is indirectly evident for patch repair of chloride-contaminated structures when only the concrete in the corroding areas is replaced with alkaline and chloride-free mortar, but surrounding concrete containing chlorides is not removed. Before the repair, the corroding rebars behave as an anode with respect to those in the surrounding areas, which are polarized cathodically and thus are protected by the macrocell. After the repair, formerly anodic zones no longer provide protection, and corrosion can initiate in the areas surrounding repaired zones (these have been called incipient anodes) [3]. Consequences for repair are discussed in Chapter 18. 

Other macrocell effects. A special case of macrocell effects has been observed on structures contaminated by chlorides where an activated titanium mesh anode was installed in order to apply cathodic protection when the cathodic protection system is installed but is not in operation, locahzed corrosion on steel can be slightly enhanced by the presence of the distributed anode [4]. 

B. Bazzoni, L. Lazzari Macrocell effect on potential measurements in concrete cathodic protection systems , Corrosion, 1996, 52, 552-557. 

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... 

The trajectory of the potential of steel reinforcement in a concrete stracture exposed to chlorides and then protected by a cathodic protection system is shown in Figure 20.4. The initial condition is represented by point 1 where the chloride content is zero and the steel is passive. By increasing the chloride content, the working point shifts to 4, within the corrosion region. Corrosion of the steel occurs rapidly by pitting or macrocell mechanisms. Applying cathodic protection leads to 5 so that the passivity is restored or to 6 without fuUy restoring passivity. 

Figure 3.6 Strongly differentiated anode and cathode regions or macrocell effects on a jetty substructure (new reinforcement has been placed to replace corroded stirrups prior to repair and the application of impressed current cathodic protection).

An alternative approach, which reintroduces the theme of long-term corrosion monitoring, is the embedding of macrocell devices. This includes galvanic couples of different steels (Beeby, 1985) or embedding steel in high chloride concrete to create a corrosion cell, as is popular in cathodic protection monitoring systems, particularly in North America (NACE, 2000). 

The 100-150 mV criterion is straightforward to apply and is the most universally agreed criterion. Other control criteria such as the plotting of the applied current against the log of the potential (ElogI), absolute potentials, macrocell or null probe current reversals and other potential shifts have been used and are used by some cathodic protection specialists but there is some controversy about their theory and practice. 

The steel probe is sometimes embedded in an excessively salty patch. With no current applied, a macrocell current flo vs from the probe to the reinforcement if they are connected with an ammeter between them. As cathodic protection current is applied, the current reduces and then reverses. This is called the macrocell probe approach and is used to show that a very anodic area has been made cathodic. It is therefore assumed that the rest of the steel is cathodic, too, However this is... 

On disbonded areas under the protective coating, corrosion of the metal may occur as a result of the action of corrosion micro-and macrocells and stray currents. In microcells, oxidation and reduction reactions may proceed locally in the area of the defect and the disbonded surface. In the macrocell, the oxidation reaction takes place in the defect and the reduction reaction in another place in the structure, causing cathodic disbonding. Corrosion caused by stray currents is similar to corrosion in the macrocell. The oxidation reaction occurs in such a case in the area of the defect or on the disbonded... 

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