Chloride removal electrochemical

Electrochemical chloride removal. This method is based on the application of direct current to the reinforcement from a temporary anode placed on the surface of the concrete. The treatment may last up to several months. Chloride ions are removed from the concrete since they migrate towards the surface of concrete due to the applied current. This technique is described in Section 20.4.1. 

Electrochemical techniques appHed for controlling corrosion of steel in concrete are cathodic protection, cathodic prevention, electrochemical chloride removal and electrochemical reaUcalization. 

Electrochemical realkalization (RE) and electrochemical chloride removal (CE) can be applied to structures in which corrosion has not or has already initiated. They are techniques aimed at modifying the composition of concrete that is carbonated or contains chlorides, in order to restore its original protective characteristics. 

Alkali-aggregate reaction. The increase in alkalinity produced at the cathode can cause damage if the concrete contains aggregates potentially susceptible to al-kah-sihca reaction (Section 3.4). In practice, this may happen only for current densities weU over 20 mA/m [33]. Therefore it may involve electrochemical chloride removal or electrochemical realkahzation, which in fact cause large increases in pH [34, 35, 36] at the surfaces of the reinforcement, but not cathodic protection, and much less, cathodic prevention. 

J. Tritthart, Changes in the composition of pore solution and solids during electrochemical chloride removal in contaminated concrete , Proc. 2 CANMET/ACI International Symposium on Advances in Concrete Technology, Las Vegas, SP154-8, 127-143, 1995. 

M. Schmid, Electrochemical chloride removal site example , SIA tec21 127 issue 31-32, 21-23,... 

R. B. Polder, Electrochemical chloride removal from reinforced concrete prisms containing chloride penetrated from seawater . Construction and Building Materials,... 

J. E. Bennett, T. J. Schue, "Electrochemical chloride removal from concrete a SHRP contract status report , NACE, Corrosion 90, Paper 316, 1990. 

Altering the concrete environment to make it less corrosive. One way would be to extract the chlorides from concrete such as electrochemical chloride removal. 

Several factors in the explanation given in this section are important and will be used later to explain how we measure and stop corrosion. The electrical current flow, and the generation and consumption of electrons in the anode and cathode reactions are used in half-cell potential measurements and cathodic protection. The formation of protective, alkaline hydroxyl ions is used in cathodic protection, electrochemical chloride removal and realkalization. The fact that the cathodic and anodic reactions must balance... 

Bennett, J., Kuan, F.F. and Schue, T.J. (1993a). Electrochemical Chloride Removal and Protection of Concrete Bridge Components Field Trials, Strategic Highway Research Program, SHRP-5-669, National Research Council, Washington, DC. 

One of the major issues facing any consultant or owner of a structure suffering from chloride or carbonation induced corrosion is what form of repair to undertake. As we have seen from the previous sections there are coatings, sealants, membranes and enclosures, specialized patch repair materials, options for total or partial replacement, impressed current and galvanic cathodic protection, electrochemical chloride removal, realkalization, electro-osmosis and corrosion inhibitors. These can be applied to structures suffering different degrees of corrosion due to chloride attack or carbonation or a combination of these two. Each treatment will have implications for the future maintenance requirements, time to next major intervention and ultimate service life of the structure. 

Until recently membranes were not considered compatible with cathodic protection as the anode would have to be under the membrane to pass current, and the anode generates gases that must escape. Membranes and cathodic protection have been used in car parks on thin slabs with some success. Some of the newer probe anodes have venting tubes and so they can be used under membranes. Galvanic anodes can be used as they do not generate gasses electrochemical chloride removal (or realkalization) could be done before the replacement of the membrane. 

The structures programme was entirely concerned with corrosion of reinforced concrete bridges suffering from salt induced corrosion. Its work on structures covered physical assessment, cathodic protection, electrochemical chloride removal, physical and chemical methods of rehabilitating bridge components and a methodology of bridge... 

Evaluation ofNorcure Process for Etectrochemicat Chloride Removal. Provides an analysis of the rate and total amount of chloride removed, the corrosive state of the steel before and after the process, the effects on the concrete, and other aspects of the installations. Comparisons are made to slabs used in other SHRP research on electrochemical chloride removal and protection of concrete bridge components. 31 pages. SHRP-C-620... 

Electrochemical Chloride Removal and Protection of Concrete Bridge Components Laboratory Studies. Investigates the feasibility of electrochemical removal and concurrent protection as a rehabilitation option for concrete bridge structures. Chloride removal process procedures were developed, and the effects of the process on structure concrete integrity and reinforcing steel were studied. 201 pages. SHRP-S-657... 

Electrochemical Chloride Removal and Protection of Concrete Bridge Components—field Trials. Discusses the results of field validation trials based on laboratory procedures for electrochemical chloride removal completed on a bridge deck, column substructures, and a bridge abutment within North America. 149 pages. SHRP-S-669... 

In all electrochemical restoration techniques a direct current is applied between the reinforcement (cathode) and an external anode in electrolytic contact with the concrete (Fig. 8-22). Cathodic protection (CP) is a permanent installation with design currents below 10 mA m , electrochemical chloride removal (ECR) and electrochemical realkalization (ER) are applied only on a temporary basis and use currents up to 2 A m. Both the electrochemical reactions at the rebars and ion migration in concrete are important for the treatments. 

The feasibility of electrochemical restoration has been demonstrated in the laboratory for both techniques, chloride removal and realkalization. For electrochemical chloride removal all studies show a reduction in the total chloride content of between 40 and 60% (SHRP, 1993 a Tritthart, 1996), the most pronounced decrease in chloride content being found in the cover concrete (Fig. 8-23). Chlorides beneath the first layer of the reinforcement are removed more slowly. As is further shown in Fig. 8-23, the expected increase in OH" content at the rebars does actually take place. Chloride removal is most efficient in the early stages of the treatment (Bennet, 1990).Prolonging the treatment to more than 1500 A h m" is found to be ineffective because nearly all of the current flow is transported by hydroxyl... 

Figure 8-23. Chloride and hydroxide concentrations at different depths in the concrete before and after electrochemical chloride removal (ECR) (after Tritthart, 1996).

The residual chloride content in the concrete after electrochemical chloride removal, determined on cores removed after treatment, can be taken as acceptance criteria. On structures with inhomogeneous chloride contamination (frequently the case) it may be difficult to obtain representative information. It is necessary to analyze several cores and only a statistical treatment of the data can provide the overall efficiency of the treatment (Elsener et al., 1993). The limits of acceptable chloride concentration can be higher than the thresholds for chloride initiation due to the hydroxide concentration in the pore solution around the rebars. 

Tritthart, J. (1996) Electrochemical chloride removal from reinforced concrete (in German), Bundesmi-nisterium fiir wirtschaftliche Angelegenheiten, Strassenforschung Heft 459, Wien and references therein. 

When the carbonated front reaches sufficient depth, the steel reinforcement corrodes due to loss of passivity. The process of re-alkalization is, therefore, employed to recreate the alkaline conditions on the reinforcement by attracting Ca " ions and Na+ ions (see below) which keeps it in a passive state. The vicinity of reinforcement is re-alkalized and a new passive layer is thus regenerated. The re-alkalization process is a consequence of the electrochemical chloride removal process. The anodes used are the same as the one used for chloride removal (titanium or steel). Sodium carbonate is used as an electrolyte. The function of Na2C03 is to introduce alkali metal ions from an external source, however it would not be necessary to use Na2C03 if alkali reactive a egates are present in the concrete in which case water is used as an electrolyte. The introduction of alkaline metal ions prevents the occurrence of re-carbonation. At the anode oxidation takes place according to... 

Re-alkalization is more economical and simpler than electrochemical chloride removal process. In the electrochemical chloride removal process, the evolution of chlorine gas and its removal prevents ingress of hydrogen into reinforcement making the process more complex compared to re-alkalization process. The re-alkalization technique is gradually gaining popularity in Europe and the Middle East. 

Beimett, J. and Thomas, J.S. (1993). Evaluation of NORCURE processes for electrochemical chloride removal from steel reinforced concrete bridge components. Washington, D.C National Academy of Sciences, No. HRP-C-620. 

Electrochemical chloride removal Electrochemical extraction to reduce chloride extraction near reinforcement. It is achieved by using an external anode powered by a DC source. The negatively charged chloride ions... 

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