ABSTRACT
The target of electrochemical chloride extraction (ECE) is to reduce the chloride content in reinforced concrete non-destructively down to a level which is not critical for chloride induced corrosion activity. Within a short time (usually 4 to 8 weeks for single treatments) corrosion affected structural parts can be rehabilitated, and the corrosion protection of concrete for the embedded reinforcement can be re-established. This is different from cathodic protection (CP), which is meant to shift the reinforcement permanently into an immune state, where it cannot corrode regardless of environmental conditions. The feasibility and the efficiency of this treatment depend on many factors and can vary over concrete surfaces. Thus, they have to be evaluated thoroughly by an extensive condition survey and the experience of the ECE designer prior to an application. 8.1 Work principle
The effect of chloride removal is caused by an electrical field between the reinforcement and an external, non-permanent electrode (see Figure 8.1). This electrical field is controlled by the voltage between the electrodes, and all ions dissolved in the pore solution are moved – negatively charged ions such as chloride or hydroxyl ions towards the outside anode; positively charged ions (mainly sodium in case of de-icing salt attack) towards the reinforcement, which acts as cathode. The process requires wet concrete, and since the number of water molecules around cations is larger than around anions, more water will be moved into the concrete than out of it during such a treatment. The higher the voltage that can be set, the more intensive the chloride movement will be. Usually, 40 V is chosen for a good chloride extraction progress under still-safe work conditions.The migration of ions is a physical process which is forced along the field lines between the electrodes: indeed it can happen only within the capillary and shrinkage pores, which take other directions than the established field lines. Due to the different size, specific movability (Elsener, 1990) and concentration of the dissolved ions, they obtain varying percentages of the total ion movement. Practically the portion of chloride in the anion
movement is largest at the beginning of the ECE and decreases over the duration of the treatment, whereas the portion of hydroxyl ion migration is increasing at the same time.Depending on the type of cement used for the concrete, some of the total chloride may be bound – mainly by the C3 A as Friedel’s salt, but also by C3S or C4AF. The bound chloride does not cause corrosion activity. With the removal of free chloride from the pore area, bound chloride will be dissolved and gets free, since there is a dynamic relationship between free and bound chloride. The importance of this effect is related to the binding capacity of a concrete and is under discussion.Electrochemical reactions take place on the reinforcement surface: the reduction of oxides, oxygen and water. All of them are related to the current and electrical charge which is impressed into the reinforcement/cathode by the ECE. Typical current densities (related to the reinforcement surface), range between 0.5 and 2 A/m², but can be much higher during the first hours/days of a treatment. Normally, the main process will be the reduction of oxygen and water, and according equation (8.1a) it will generate hydroxyl ions, which raises the alkalinity of the concrete in the reinforcement vicinity and is the main target of a related method – the electrochemical re-alkalisation:
Both processes – chloride migration and reduction of oxides, oxygen and water – run at the same time, but do not depend on each other in predictable terms. Whereas chloride migration depends on the applied voltage, the cover thickness and permeability of concrete and the water content, the current is controlled by the voltage, the temperature, the resistance of concrete (as a sum parameter of concrete cover, permeability, soluble ions and water content) and the charge transfer resistances on anode and reinforcement. This does not correlate with some other publications, e.g. (Polder et al., 1993), but has a practical backup when high amounts of chloride can be removed at quite low current densities and charges as well as only slow desalination progress being observed at high current densities and total charges.So the ECE not only reduces the chloride content of the concrete but also raises its alkalinity as a result of the reduction reactions. This improves the corrosion protection additionally, since with a high OH-content also an increased chloride content can be present in the concrete without triggering reinforcement corrosion activity.On the external anode, which is usually of a dimensionally stable material, we find other electrochemical reactions that lead to very acidic conditions: the oxidation of water (8.2a), hydroxyl ions (8.2b) and chloride as well as the formation of chlorine (8.2c). According to (Elsener et al., 1993) the reaction of water and chlorine can also cause an acidic environment (8.2d). Equation 8.2 Possible oxidation reactions on the anode.
