Rebar corrosion potential difference

The corrosion potential of passive reinforcement (Ecorr) is determined by the availability of oxygen at the surface of the rebars. The maximum and minimum values of potential taken on by passive reinforcement under different environmental conditions are, respectively, +100 mV in aerated concrete, and —1 V in the total... 

Anodic and cathodic processes may take place preferentially on separate areas of the surface of the reinforcement, leading to a macrocell. This can be established, for instance, between active and passive areas of the reinforcement. Current circulating between the former, which are less noble and thus function as anodes, and the latter, which are more noble and thus function as cathodes, accelerates the corrosion attack on active surfaces while further stabilising the protective state of passive ones. The magnitude of this current, known as the macrocell current, increases as the difference in the free corrosion potential between passive and active rebars increases, and decreases as the dissipation produced by the current itself at the anodic and cathodic sites and within the concrete increases. 

Presence of different metals. Rebars of carbon steel in certain cases can be connected to rebars or facilities made of stainless steel or copper. This type of coupling, which in other electrolytes would provoke a considerable degree of corrosion in carbon steel by galvanic attack, does not cause problems in the case of concrete any different from those provoked by coupling with normal passive steel. In fact, the corrosion potential of passive carbon steel in concrete is not much different... 

Corrosion potential. Any embedded reference electrode allows the electrochemical potential of the adjacent rebars to be measured. This allows depassivation of the rebars or of any other steel sensor element put at different depths to be detected by a drop in half-cell potential. The corrosion potential will be influenced by concrete humidity and oxygen content (Chapter 7). The depassivation of the steel probe located in the outermost cover concrete will present an early warning and suitable in-depth distribution of a set of steel probes allows the corrosion risk to be evaluated or the time of depassivation of the rebars to be calculated. 

The proof of protection is more difficult to establish in this case for two reasons. First, the object is to restore passivity to the rebar and not to render it virtually immune to corrosion. Second, it is difficult to measure the true electrode potential of rebars under these conditions. This is because the cathodic-protection current flowing through the concrete produces a voltage error in the measurements made (see below). For this reason it has been found convenient to use a potential decay technique to assess protection rather than a direct potential measurement. Thus a 100 mV decay of polarisation in 4 h once current has been interrupted has been adopted as the criterion for adequate protection. It will be seen that this proposal does not differ substantially from the decay criterion included in Table 10.3 and recommended by NACE for assessing the full protection of steel in other environments. Of course, in this case the cathodic polarisation is intended to inhibit pit growth and restore passivity, not to establish effective immunity. 

Principle. Corroding and passive rebars in concrete show a difference in electrical potential of up to 0.5 V, thus a macrocell generates and current flows between these areas (Chapter 8). The electric field coupled with the corrosion current between corroding and passive areas of the rebars (Figure 16.4) can be measured experimentally with a suitable reference electrode (half-cell) placed on the concrete surface, resulting in equipotential lines (potential field) that allow the location of corroding rebars at the most negative values [5-8]. 

Theoretical considerations and practical experience on a large number of structures (Elsener and Bohni, 1990 Elsener etal., 1996) demonstrate that the results of potential mapping need careful interpretation. It has been found that there are no absolute potential values to indicate corrosion hazard in a structure - in contrast to the interpretation given in the ASTM C876-91 that relies on a fixed potential value of -0.35 V CSE. Depending on moisture content, chloride content, temperature, carbonation of the concrete and cover thickness, different potential values indicate corrosion of the rebars in different structures (Fig. 8-18). Thus the gradient between corroding and passive areas is more important than the absolute... 

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