Electrochemical chloride

Saunders and Brown (38) reported a method to prepare recessed tip silver/silver chloride microelectrodes. In the first step of this procedure they electrochemically sharpened the end of a silver wire (diameter 125 /xin) by dipping it into basic sodium cyanide solution and anodizing it under voltage control. This was followed by an electrochemical chloride-coating step in 100 mM HC1 solution. 

The most frequently used methods for repair of chloride-contaminated structures are shown in Figure 18.5. When the chloride level is high, conventional repair methods usually do not give a durable result In these cases, it is necessary to consider other methods such as cathodic protection or electrochemical chloride... 

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. 

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. 

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. 

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. 

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

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. 

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. 

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. 

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... 

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... 

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