Chloride macrocells

Electrical current flow by ion migration in concrete is important for electrochemical rehabilitation techniques such as chloride removal (Chapter 20), but also for (macrocell) corrosion processes (Chapter 8). 

The most frequent type of macrocell in reinforced-concrete structures exposed to the atmosphere is that established between more superficial rebars that have been depassivated by carbonation or chloride penetration, and internal passive rebars. Another example may be walls where chloride penetrates from one side and oxygen penetrates from the other side, which may occur in hoUow structures Hke tunnels and offshore platform legs or with ground retaining walls. 

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]. 

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... 

Structures immersed in seawater. Macrocells may form between rebars reached by chlorides and passive rebars on which, for any reason, oxygen is available. Macrocell current is then controlled by the amount of oxygen that can be reduced on the passive rebars. The galvanic coupling lowers the potential on these rebars and produces alkalinity on their surface. Therefore the macrocell contributes to maintaining the steel passive. 

Only calcium nitrite, introduced more than thirty years ago, has a long and proven track record as a corrosion inhibitor for reinforced concrete [1,4]. MFP and alkano-lamine-based organic inhibitor blends are increasingly used but unfortunately most of the commercial apphcations lack rigorous control of the inhibitor effect. One of the very few comparative field tests on chloride-contaminated concrete studied MFP and a proprietary alkanolamine inhibitor added in the side-walls of a tunnel (16). The inhibitors were applied by the producers. The measurements of macrocell currents and half-cell potential mapping revealed that both inhibitors were virtually ineffective at the chloride concentrations of 1-2 % by mass of cement present [16]. 

Fields of applicability. Figure 15.3 depicts the fields of applicability of pickled stainless steels in chloride-contaminated concrete exposed to temperatures of 20 °C or 40 °C. Fields have been plotted by analysing the critical chloride values obtained by different authors from exposure tests in concrete or from electrochemical tests in solution and mortar and taking into consideration the worst conditions [11-28]. Nevertheless, it should be pointed out that values are indicative only, since the critical chloride content depends on the potential of the steel, and thus it can vary when oxygen access to the reinforcement is restricted as well as when stray current or macrocells are present. For instance, the domains of applicability are enlarged when the free corrosion potential is reduced, such as in saturated concrete. Furthermore, the values of the critical chloride Hmit for stainless steel with surface finishing other than that obtained by pickling can be lower. 

Figure 15.4 Macrocell current density exchanged between a corroding bar of carbon steel in 3% chloride-contaminated concrete and a (parallel) passive bar of carbon steel in chloride-free concrete, 316L stainless steel in...

Very high corrosion rates in the vicinity of defects in the coating can occur in the presence of macrocells. A typical situation is that of stractures in which epoxy-coated reinforcement in contact with chloride-contaminated concrete is coupled to non-coated reinforcement embedded in concrete that is uncontaminated or contains a level of chlorides below the critical level. In this case, the passive non-coated... 

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. 

The uneven distribution of chloride ions in the concrete and at the steel surface also greatly affects the corrosion. The high level of chloride on the top layer and decreasing chloride concentration with distance results in increased corrosion rate because of macrocell corrosion. 

One of the primary causes of external corrosion is exposure to corrosive soils. The electrical and chemical characteristics of soil and water are closely related to corrosivity. Variations in soil characteristics because of soil type, fill compaction, amount of moisture, bacteria, chloride concentration help establish corrosion cells. Over a period of time, if untreated, the corrosion process can result in wall thickness reduction and can lead to leaks. The 6 o clock position of the USTs is one of the most critical locations because that is the rest point where the tank bottom touches the bottom of the hole dug for the tank. At such a location, the layer of backfill is relatively thin therefore, the soil characteristics can be different from the adjacent soil, setting up conditions for macrocell corrosion. 

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. 

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). 

An alternative is to identify the most anodic area of the zone and to isolate a short section of steel to form a macrocell probe without disturbing the concrete around the probe as discussed in SHRP-S-347 Bennett and Schue (1993) and Bartholomew et al. (1993). This is more realistic than embedding a probe in salty concrete but suffers the same problem with chloride movement with time. 

Effective corrosion inhibitors will increase the time to initiation of severe corrosion currents and detrimental concrete admixtures will reduce the time to initiation of severe corrosion. A composite curve from the ASTM round robin is given in Fig. 3. The integrated macrocell current (Coulombs) is a measurement of the total corrosion due to the macrocell. The test is not designed to evaluate materials such as silica fume that increase the resistance between the steel bars and significantly decrease chloride ingress. 

Testing procedures for cracked concrete are essentially the same as used for other laboratory specimens exposed to chloride solutions continuously or cyclically. Methods that can be used include polarization resistance, electrochemical impedance, and macrocell corrosion which were discussed above. Procedures for conducting these tests are described in Refs 27 and 28. 

One method of conducting these tests is to have two mats of reinforcing bars with or without admixed chlorides in the top mat and to pond with chloride. The macrocell current between the top and bottom mats as well as the corrosion potentials are measured according to ASTM C 876. This procediue is described in Ref 29. 

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. 

Schiessl, P, and Raupach, M., Macrocell Steel Corrosion in Concrete Caused by Chlorides, Second CANMET/ACI International Conference on Durability of Concrete, Montreal, Canada, CANMET, 1991, pp. 665-583. 

Reinforced concrete with significant gradients in chloride ion content is vulnerable to macrocell corrosion, especially if subjected to cycles of wetting and drying. 

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