Chloride stray currents

Electrical conductivity is of interest in corrosion processes in cell formation (see Section 2.2.4.2), in stray currents, and in electrochemical protection methods. Conductivity is increased by dissolved salts even though they do not take part in the corrosion process. Similarly, the corrosion rate of carbon steels in brine, which is influenced by oxygen content according to Eq. (2-9), is not affected by the salt concentration [4]. Nevertheless, dissolved salts have a strong indirect influence on many local corrosion processes. For instance, chloride ions that accumulate at local anodes can stimulate dissolution of iron and prevent the formation of a film. Alkali ions are usually regarded as completely harmless, but as counterions to OH ions in cathodic regions, they result in very high pH values and aid formation of films (see Section 2.2.4.2 and Chapter 4). 

Consequences of DC stray current in reinforced concrete change according to the properties of the concrete (alkahne, carbonated or contaminated by chlorides), to the duration of the current circulation and to the current density. It is therefore necessary to distinguish concrete structures not contaminated by chlorides and not carbonated from those contaminated by chlorides in quantities insufficient to initiate corrosion and, finally, from those that already have corroding rebars because of chlorides or carbonation. 

Figure 9.3 Schematic representation of electrochemical conditions in the cathodic and anodic zones of reinforcement in non-carbonated and chloride-free concrete that is subject to stray current...

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. 

Interruptions in the stray current. Stray currents produced by rail traction systems are non-stationary, and thus the effect of interruptions of the current should be taken into consideration. In fact, gradients of ionic concentration in the pore solution near the steel surface, produced by the depletion of alkalinity due to the anodic reaction and increase in chloride concentration due to migration, can be mitigated during the interruption of current. Therefore, interruptions in the stray current may have a beneficial effect, as shown in Figure 9.6 where results of tests with continuous appHcation of the current are compared with tests with circulation of current at alternated hours (lon-loff). The periodical interruption of current had a beneficial effect, since it increased the charge required for initiation of corrosion. This effect was remarkable in cement pastes with chloride contents lower than 0.4 % by mass of cement... 

Protection that concrete offers to steel against stray current ceases when corrosion of the reinforcement has initiated, e. g. due to carbonation, chloride contamination, or the stray current itself In this case, any current flowing through the steel will increase the corrosion rate at the anodic site, similarly as in buried steel structures. Figure 9.8 shows that even small driving voltages can lead to an increase in the corrosion rate on the anodic area (from to Furthermore, it has been observed that if steel is subjected to pitting corrosion in chloride-contaminated concrete, the anodic current increases the size of the attacked area [5]. 

We have seen that stray current can hardly induce corrosion on passive steel in non-carbonated and chloride-free concrete. However, the potential adverse effects of stray current on concrete structures may become increasingly important with the increased use of underground concrete construction. Stray-current effects are rarely recognised as such. The importance increases further due to the increase of the required service lives (i. e. 100 y or more). 

The best protection against stray current is, therefore, provided by concrete. Those methods that can improve the resistance of concrete to carbonation or chloride contamination, which are illustrated in Chapters 11 and 12, are also beneficial with regard to stray-current-induced corrosion. It should be observed that this may not be the same for preventative techniques, since conditions leading to corrosion initiation due to stray current are different, in terms of potential, from those leading to corrosion initiation due to carbonation or chloride contamination. For instance, the use of stainless steel or galvanized-steel bars, which improves the resistance to pitting corrosion in chloride-contaminated concrete (Chapter 15), does not substantially improve the resistance to stray current in chloride-free and non-carbonated concrete [4]. In any case, a high concrete resistivity will reduce the current flow due to stray current. 

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. 

In principle, the cathodic process can be supported also from interference currents or stray currents. If the concrete is not degraded, iron is passive, and the anodic process is not the iron dissolution but the oxygen evolution without corrosion of the iron itself. If chlorides are present at a concentration sufficient to depassivate iron, oxygen evolution does not occur, but there is iron corrosion at much lower potential differences. 

Even in the absence of chlorides, if the current flows at a sufficiently high density for enough time to acidify the anodic area, corrosion of the iron can be sustained by potential differences considerably lower than those that are necessary for its initiation. If the concrete is carbonated, current density and times required for acidification of the anodic zones are obviously reduced however, these are quite exceptional conditions, which may in fact not occur in the case of stray currents but occur only in conditions of stationary interference currents that the design must plan to avoid. 

The problem of corrosion of steel in concrete was first ascribed to stray current flows from trams and DC railway systems Hime, 1994). Once chloride, in the form of deicing salt, was identified as the major culprit (when trams disappeared but corrosion increased), an enterprising engineer in the California Department of Transportation (Caltrans) took a standard pipeline cathodic protection design and flattened it out on a bridge deck. 

The purpose of the electrolyte is to provide a conducting medium, and at the same time, it must not corrode the cathode tool. The cheapest material commonly used is sodium chloride (NaCl) at about 30% by weight. In some cases, additives such as alcohols, amines, thiols, and aldehydes are used to inhibit stray currents, which results in overcuts. Other electrolytes such as Na2Cr207, NaN03, and NaClOs at 50-250 g/L have also been used, but the choice is limited primarily by cost. 

It is common to add up to 3% calcium chloride to concrete, in order to accelerate setting and thus avoid freezing in the cold of winter. This leads to a substantial decrease in the resistivity of concrete adding 3% calcium chloride leads to a drop in resistivity by a factor of 1000. Since calcium chloride is hygroscopic, it may keep the concrete from drying. In the presence of humidity, the conductivity of the medium increases, and electrical corrosion reactions are facilitated. In the absence of humidity, stray currents and contact with steel, adding chlorides does not significantly modify the resistance of aluminium in concrete [7]. 

As in the soil, alternating or direct stray currents can lead to corrosion of aluminium (and other metals) that are embedded in concrete. The higher the level of humidity and chlorides, the stronger this effect is. For alternating currents of industrial frequency (50 or 60 Hz), a threshold exists below which no corrosion of aluminium is observed (Figure G.4.3). 

Stray capacitances associated with micropipettes (Figure 4.10) can be lumped into Cj. Alternating-current electrode polarization in recording electrodes sometimes can be virtually eliminated by using a silver-silver chloride connection to the electrolyte in the lumen of the micropipette. When this is done, then Figure 7.5 is essentially a valid representation for a glass micropipette system. 

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