Cathodic macrocells

A further example, which confirms the necessity of evaluating the resistivity of the medium very carefully, concerns the corrosion of rebars in reinforced concrete. In this caae the intensity of the current flowing between the anodic and cathodic zones of a macrocell depends on the resistivity of the concrete and the extent of the region involved. To determine the concrete resistivity various methods have been developed, which can be applied in the laboratory [14] as well as in the field [15]. It should be noted, however, that in the latter case most researchers have pursued the approach suggested by Wenner [16] for the evaluation of the resistivity of soils. The contribution of the ohmic drop to the electrode overvoltage cannot be neglected when the values of the corrosion rate of the rebars are appreciable, even if the current intensity is small within a given polarization potential interval, because under such conditions the interpretation of experimental results could be completely distorted. 

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]. 

The action of macrocells in structures buried in the soil or immersed in water is different from that of structures exposed to the atmosphere two circumstances promote macrocell effects while another reduces them. First, concrete is wetter than in aerated structures and its resistivity is lower, particularly in structures immersed in seawater. This reduces the ohmic drop in the concrete and increases the size of the effective cathodic area in relation to the anodic one. Secondly, the soil or the seawater around the concrete is an electrolyte of low resistivity, and the macro-cell current can also flow outside the concrete. This further reduces the ohmic resistance between the anodic area and passive reinforcement. Thirdly, there is, however, a mitigating aspect. Oxygen can only diffuse with great difficulty through wet concrete and thus it hardly reaches the surface of the embedded steel. Depletion of oxygen at the surface of the rebar that is observed in this case makes initiation of corrosion very difficult, and, even when corrosion initiates, the driving voltage for the macrocell is very low. 

Differential aeration in buried structures. A clear example of macrocell action was documented in diaphragm walls in Berlin, illustrated in Figure 8.1 [6]. In this case, anodic areas had formed at the lower, non-aerated parts of the reinforcement at the ground side, while steel on the free side and higher up acted as cathode. Large amounts of corrosion products were found inside the concrete at various distances from the anodes and in the soil, suggesting that relatively soluble iron(II) oxides had formed that were able to move away from the anodes. Chlorides originated... 

Figure 8.4 Schematic representation of the macrocell between a passive and an active rebar and determination of dissipation occurring at the anode at the cathode ( )J and in the concrete...

It can be seen in Figure 8.4 how the corrosion rate induced on the active rebar (measured by J depends on the value of the macrocell current (i), even though the increase in corrosion rate (/, - is lower than this current. In fact, the anodic polarization causes a decrease in the cathodic current from I cor to 1 ... 

To evaluate polarizations and thus determine conditions of corrosion due to the macrocell, it is necessary to consider current densities exchanged at the anodic and cathodic surfaces as well as the macrocell current, I. 

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

Steel can be obtained due to the small cathode/anode ratio. This macrocell monitoring system has been installed since 1990 into tunnels, bridges, foundations and other structures exposed to aggressive environments [5]. Similar systems have been developed and are on the market (6). 

Figure 17.6 Schematic view of macrocell current measurements between an Isolated piece of rebar (anode) and the surrounding rebar network (cathode). In the open-circuit condition the potential difference and the resistance between the two electrodes can be monitored [15]... 

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. 

Corrosion is often local, with a few centimetres of corrosion and then up to a metre of clean passive bar, particularly for chloride induced corrosion. This indicates the separation of the anodic reaction (2.1) and the cathodic reaction (2.2) to form a macrocell . Chloride induced corrosion gives rise particularly well defined macrocells. This is partly due to the mechanism of chloride attack, with pit formation and with small concentrated anodes being fed by large cathodes. It is also because chloride attack is usually associated with high levels of moisture giving low electrical resistance in the concrete and easy transport of ions so the anodes and cathodes can separate easily. 

As stated in Chapter 2, corrosion proceeds by the formation of anodes and cathodes (Figures 2.1 and 2.2). In the case of chloride attack they are often well separated with areas of rusting separated by areas of clean steel. This is known as the macrocell phenomenon. Chloride induced corrosion is particularly prone to macrocell formation as a high level of water is usually present to carry the chloride into the concrete and because chlorides in concrete are hygroscopic (i.e. they absorb and retain moisture). The presence of water in the pores increases the electrical conductivity of the concrete. The higher conductivity allows the separation of anode and cathode as the ions can move through the water filled (or water lined) pores. 

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 steel probe is sometimes embedded in an excessively salty patch. With no current applied, a macrocell current flows from the probe to the reinforcement if they are connected with an ammeter between them. As CP current is applied, the current reduces and then reverses. This is called the macrocell or null probe approach and is used to show that a very anodic area has been made cathodic. It is therefore assumed that all of the rest of the steel is cathodic too. However it is dependent upon the amount of salt added to the patch, and if the salt diffuses away then it may no longer be the most anodic area after a few years. 

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. 

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... 

A characteristic feature for the chloride induced corrosion of steel in concrete (pitting) is the development of macrocells, that is the coexistence of passive and corroding areas on the same rebar forming a short circuited galvanic element with the corroding area acting as anode and the passive surface as cathode (Fig. 8-11). The cell voltage. 

Figure 8-11. Increased localized corrosion of steel in concrete due to the formation of a macrocell ac-tive/passive. The current (/) is flowing from the local anode to the cathode.

AU, in this macrocell equals the potential difference between corroding and passive steel and may attain as much as 0.5 V. The resulting current flow, I, (which is directly proportional to the mass loss of the steel) is determined by the electrical resistance of the concrete and the anodic (7 ) and cathodic (/ c) reaction resistance ... 

At the anode, iron dissolution is accelerated, while hydroxide ions are produced at the cathode and the macrocell stabilizes itself. A key factor controlling the corrosion rate is the electrical resistivity of the concrete, governed mainly by temperature, conductivity of the pore solution and porosity of the concrete. 

---