Electrochemical chloride extraction

The electrolyte is the liquid in the cassette, cellulose fibre, blankets, ponds or other containment system that transport the current from the anode to the steel. Potable water is a suitable electrolyte but is liable to acidify and promote the evolution of chlorine gas. Regular replacement will be required 

The very first trials of this system were based on a rapid treatment period of 12-24 hours. Trials were done in Ohio, United States, and lab tests in Kansas Department of Transportation in 1976 (Morrison et ah, 1976, Slater et al., 1976). However, a more fundamental study carried out by the Strategic Highway Research Program (SHRP) in 1987-1992 (Bennett et al., 1993a,b) showed dangers in applying more than 4 A-m of steel or concrete surface area. Current treatment times are now measured in weeks rather than days. 

Ontario Ministry of Transportation carried out a trial of the Norwegian system on Burlington Skyway in 1989. The voltage was kept at about 40 V. The total charge passed was 610 A h-m of concrete surface area over 55 days giving an average current density of 0.462 A-m . This structure had a low steel concentration so removal was patchy, very high over the rebars but lower between the bars. They found 78-87% of the chloride removed directly above the rebars and 42-77% of the chloride removed between the rebars. 

This work was followed up by laboratory tests on concrete to rebar pull out strength and is discussed later. 

A typical system will run at no more than 4 A-m of steel surface area for a period of 2-8 weeks. 

---

Electrochemical chloride extraction and electrochemical realkalisation have been proposed as an alternative to traditional repair methods for the rehabilitation of chloride-contaminated or carbonated structures. Due to increasing field experience... 

For electrochemical chloride extraction (abbreviated CE, also called chloride removal, or desalination), a direct current is applied between the reinforcement (cathode) and an anode that is placed temporarily on the outer surface of the concrete. The anode is an activated titanium wire mesh or a reinforcing steel mesh. The anode is surrounded by tap water or saturated calcium hydroxide solution in ponds (upper, horizontal surfaces) or tanks (vertical or overhead surfaces) or as a paste that can be sprayed onto all types of surface. Chloride ions migrate from the reinforcement to the anode. Due to a relatively high current density of 1 to 2 A/m, relatively large amounts of chloride can be removed from the concrete within a relatively short time, usually 6 to 10 weeks. After that, the anode, the electrolyte and the incorporated chloride ions are removed from the stracture. The principle layout and electrode reactions involved are indicated in Figure 20.8. 

L. Bertolini, C. L. Page, W. Y. Shu, Effects of electrochemical chloride extraction on chemical and mechanical properties of hydrated cement paste . Advances in Cement Research, 1996, 8, 93-100. 

R. B. Polder, A. W. M. van den Hondel, Laboratory investigation of electrochemical chloride extraction of concrete with penetrated chloride . Heron, 2002, 47 (3), 211-220. 

C. Alonso, Mathematical modeling of electrochemical chloride extraction from concrete . Magazine of Cement and Concrete Research, 1995,... 

M. Castellote, C. Andrade, C. Alonso, Electrochemical chloride extraction influence of testing conditions and mathematical modelling . Advances in Cement Research, 1999, 11, 63-80. 

Electrochemical treatments - cathodic protection, electrochemical chloride extraction and electrochemical realkalization are designed to shift the potential of the steel. This effect may be permanent in the case of cathodic protection or temporary but quite long term in the case of the other two techniques. 

We have already determined that the chloride ion is a catalyst to corrosion (Section 3.2.3). As it is negatively charged we can use the electrochemical process to repel the chloride ion from the steel surface and move it towards an external anode. This process, called electrochemical chloride extraction (ECE), desalination or chloride removal, uses a temporary anode and a higher electrical power density than CP, but is otherwise similar (Figure 7.1). Preparation in terms of concrete repair, power supplies etc are similar to those for impressed current cathodic protection except that the power supply is temporary and may be from a temporary source such as a generator. The output is larger, up to 50 V and 2 A-m. ... 

Figure 1.20 Electrochemical chloride extraction being applied to the Burlington Skyway Ontario, Canada 1989, using cellulose fibre over a mild steel mesh. Acknowledgements Jack Bennett.

Standards and guidance for electrochemical chloride extraction and realkalization... 

Pianca, E, Schell, H., Lai, D. and Raven, R. (2003). The Ministry of Transportation of Ontario s Experience with Electrochemical Chloride Extraction (ECE) 1989-2002. NACE Corrosion 2003. Paper No. 3293. 

If chlorides are the cause of corrosion then patching (including overlaying and encasement), cathodic protection and electrochemical chloride extraction are the three most likely options. As discussed previously all techniques have different merits and limitations. This section explores them and gives some comparisons. 

Figure 6.25 tlesaIination (electrochemical chloride extraction) trial fiurlington Skyway, Ontario, Canada 1 89. Acknowledgements Ontario Ministry of Transportation, Hltech, SHKP. 

In Fig. 8-31, a schematic diagram is shown of a typical electrochemical chloride extraction installation from reinforced concrete. The principle of this method is identical to the case of the CP of reinforced con-... 

Figure 8-31. Schematic diagram of typical electrochemical chloride extraction installation [Whitmore (1996), reprinted with permission].

Figure 5.46 Bridge deck energized for electrochemical chloride extraction. (Courtesy of fector Corrosion Technolr ies)...

Electrochemical Chloride Extraction from Concrete Bridge Components. Technical Brief 2. Toronto, Canada Canadian Strategic Highway Research Program (C-SHRP), 1995. 

Fajardo G, EscadeiUas G, Arhguie G (2006) Electrochemical chloride extraction (ECE) from steel-reinforced concrete specimens contaminated by artificial sea-water. Corros Sci 48(1) 110-125.doi 10.1016/j.corsci.2004.11.015... 

Re-alkalization is another form similar to the electrochemical chloride extraction process, and is used to recreate the alkaline conditions which are lost due to carbonation. As stated in an earlier section, the ingress of carbon dioxide leads to the loss of alkalinity on the reinforcement because of the fall of pH firom 14 to 9 due to the combination of carbon dioxide with calcium hydroxide [Ca(OH)2] which means loss of alkalinity from the concrete. 

---