Electrochemical inspection techniques

The electrochemistry of corrosion and the basics of electrochemical potentials and corrosion rate have been discussed in Chapter 7. Here the principles and application of electrochemical inspection techniques for reinforced-concrete structures are given. The different techniques will give different types of information (Figure 16.3). 

Electrochemical Inspection Techniques 279 Table 16.1 Potentials vs. NHE for reference electrodes [9, 10]... 

Other workers have published improved procedures for inspecting both reinforced concrete and prestressed concrete structures with regard to determination of the embedded steel components [110]. A prototype ultrasonic procedure was developed to determine the condition of prestressed and pretensioned tendons in concrete. The application of electrochemical surface-mounted systems for estimating the rate of corrosion of reinforcing steel and other embedded steel components in large concrete structures was described using this technique. 

It should also be observed that, because of the absence of electrical connection between the individual coated bars, if the coating is not effective in protecting the bar, the application of electrochemical techniques (Chapter 20), such as cathodic protection, is not possible in practice. Even the inspection of structures is difficult, e. g. potential mapping (Chapter 16) cannot be appHed if bars are disconnected. To avoid this problem, for pre-cast elements in Europe the coating has also been appHed on welded mesh or complete reinforcement cages, so that electrical cormection of bars was guaranteed [46]. 

The popularity of the cychc voltammetry (CV) technique has led to its extensive study and numerous simple criteria are available for immediate anal-j sis of electrochemical systems from the shape, position and time-behaviour of the experimental voltammograms [1, 2], For example, a quick inspection of the cyclic voltammograms offers information about the diffusive or adsorptive nature of the electrode process, its kinetic and thermodynamic parameters, as well as the existence and characteristics of coupled homogeneous chemical reactions [2]. This electrochemical method is also very useful for the evaluation of the magnitude of imdesirable effects such as those derived from ohmic drop or double-layer capacitance. Accordingly, cyclic voltammetry is frequently used for the analysis of electroactive species and surfaces, and for the determination of reaction mechanisms and rate constants. 

As shown above, galvanic cell reaction is a convenient method for fabricating SHSs. It does not require electrical source, and the reaction condition is also mild. A disadvantage of this technique is that a longer reaction time is needed to form a surface with large WCA (usually several days), compard with other electrochemical methods. Furthermore, since no electric source is applied, inspecting and controlling the reaction become less convenient. 

Aqueous corrosion is electrochemical in nature. It is therefore possible to measure corrosion rate by employing electrochemical techniques. Two methods based on electrochemical polarization are available The Tafel extrapolation and linear polarization. Electrochemical methods permit rapid and precise corrosion-rate measurement and may be used to measure corrosion rate in systems that cannot be visually inspected or subject to weight-loss tests. Measurement of the corrosion current while the corrosion potential is varied is possible with the apparatus shown in Figure 1.4. 

In addition to the major techniques used for evaluation of corrosion of the reinforcing steel described above, non-destructive techniques such as computed tomography, impact echo and magnetic field disturbance, electrochemical noise and other techniques have also been used for inspection and detection of corrosion of reinforcing steels. Reference on these techniques along with the references on major techniques are cited at the end of this chapter. 

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