Polarization macrocells

A further example, which confirms the necessity of evaluating the resistivity of the medium very carefully, concerns the corrosion of rebars in reinforced concrete. In this caae the intensity of the current flowing between the anodic and cathodic zones of a macrocell depends on the resistivity of the concrete and the extent of the region involved. To determine the concrete resistivity various methods have been developed, which can be applied in the laboratory [14] as well as in the field [15]. It should be noted, however, that in the latter case most researchers have pursued the approach suggested by Wenner [16] for the evaluation of the resistivity of soils. The contribution of the ohmic drop to the electrode overvoltage cannot be neglected when the values of the corrosion rate of the rebars are appreciable, even if the current intensity is small within a given polarization potential interval, because under such conditions the interpretation of experimental results could be completely distorted. 

Protection effect. MacroceU currents can have beneficial effects on rebars that are polarized cathodically. This is indirectly evident for patch repair of chloride-contaminated structures when only the concrete in the corroding areas is replaced with alkaline and chloride-free mortar, but surrounding concrete containing chlorides is not removed. Before the repair, the corroding rebars behave as an anode with respect to those in the surrounding areas, which are polarized cathodically and thus are protected by the macrocell. After the repair, formerly anodic zones no longer provide protection, and corrosion can initiate in the areas surrounding repaired zones (these have been called incipient anodes) [3]. Consequences for repair are discussed in Chapter 18. 

It can be seen in Figure 8.4 how the corrosion rate induced on the active rebar (measured by J depends on the value of the macrocell current (i), even though the increase in corrosion rate (/, - is lower than this current. In fact, the anodic polarization causes a decrease in the cathodic current from I cor to 1 ... 

To evaluate polarizations and thus determine conditions of corrosion due to the macrocell, it is necessary to consider current densities exchanged at the anodic and cathodic surfaces as well as the macrocell current, I. 

Repassivation with alkaline mortar or concrete. Repassivation of steel can be obtained by replacing the chloride-contaminated concrete with chloride-free and alkaline mortar or concrete. Because of the mechanism of chloride-induced corrosion, it is not sufficient to repair the concrete in the area where the reinforcement is de-passivated. The concrete must be removed in all areas where the chloride threshold has reached the depth of the reinforcement or is expected to reach it during the design Hfe of the repair. In fact, the concrete that surrounds the zones of corrosion usually has a chloride content higher than the chloride threshold, even though the steel remains passive because it is protected by the corroding site. In fact, a macrocell forms (Figure 18.6a) that provides cathodic polarization to adjacent steel and... 

The corrosion rate is probably the nearest the engineer will get with currently available technology to measuring the rate of deterioration. There are various ways of measuring the rate of corrosion, including AC Impedance and electrochemical noise (Dawson, 1983). However, these techniques are not suitable for use in the field for application to the corrosion of steel in concrete so this section will concentrate on linear polarization, also known as polarization resistance or LPR, and will discuss various macrocell or galvanic current measurement techniques. 

As stated in Section 4.12, corrosion rates can be measured by linear polarization or by galvanic or macrocell techniques. Both have their merits and limitations as discussed in the Sections 5.2.2 and 5.2.3. 

An alternative approach to the linear polarization technique, which reintroduces the theme of long-term corrosion monitoring, is the embedding of macrocell devices. This includes galvanic couples of different steels (Beeby, 1985) or embedding steel in high chloride concrete to create a corrosion cell, as is popular in cathodic protection monitoring systems, particularly in North America (NACE, 1990). 

The macrocell measures the Faraday current from metal dissolution, the same as in the linear polarization technique. There are two major... 

The second issue is how representative macrocell currents are of the true corrosion currents in the steel. The microcell currents may be more important than macrocell current flows. In a comparison with linear polarization (Berke ef aL, 1990) the macrocell technique underestimated the corrosion rate sometimes by an order of magnitude. As this was using the ASTM prism technique, it should be considered the most accurate use of the macrocell technique, so if it is an order of magnitude out, field use of macrocell techniques is probably even le.ss accurate. 

Testing procedures for cracked concrete are essentially the same as used for other laboratory specimens exposed to chloride solutions continuously or cyclically. Methods that can be used include polarization resistance, electrochemical impedance, and macrocell corrosion which were discussed above. Procedures for conducting these tests are described in Refs 27 and 28. 

Berke, N. S., Shen, D. F., and Sundberg, K. M., Comparison of the Polarization Resistance Technique to Macrocell Corrosion Technique, Corrosion Rates of Steel in Concrete, ASTM STP 1065, Berke, Chaker, and Whiting Eds., ASTM International, West Conshohocken, PA, 1990. 

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