Macrocells currents

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. 

Structures immersed in seawater. Macrocells may form between rebars reached by chlorides and passive rebars on which, for any reason, oxygen is available. Macrocell current is then controlled by the amount of oxygen that can be reduced on the passive rebars. The galvanic coupling lowers the potential on these rebars and produces alkalinity on their surface. Therefore the macrocell contributes to maintaining the steel passive. 

Where oxygen access is low, it can be seen that the macrocell current tends to diminish in time because of oxygen depletion at the surface of the passive steel. The potential of passive steel consequently decreases in time until it reaches a value similar to that of corroding bars. 

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. 

Only calcium nitrite, introduced more than thirty years ago, has a long and proven track record as a corrosion inhibitor for reinforced concrete [1,4]. MFP and alkano-lamine-based organic inhibitor blends are increasingly used but unfortunately most of the commercial apphcations lack rigorous control of the inhibitor effect. One of the very few comparative field tests on chloride-contaminated concrete studied MFP and a proprietary alkanolamine inhibitor added in the side-walls of a tunnel (16). The inhibitors were applied by the producers. The measurements of macrocell currents and half-cell potential mapping revealed that both inhibitors were virtually ineffective at the chloride concentrations of 1-2 % by mass of cement present [16]. 

Figure 15.4 Macrocell current density exchanged between a corroding bar of carbon steel in 3% chloride-contaminated concrete and a (parallel) passive bar of carbon steel in chloride-free concrete, 316L stainless steel in...

Although the macrocell currents measured with the anode-ladder system can detect the time-to-depassivation, no information regarding the corrosion rate of the... 

Figure 17.6 Schematic view of macrocell current measurements between an Isolated piece of rebar (anode) and the surrounding rebar network (cathode). In the open-circuit condition the potential difference and the resistance between the two electrodes can be monitored [15]... 

In North America, this is used as a way of measuring corrosion by measuring macrocell currents . This is discussed later in Section 4.12.6. 

Figure 4.22 Schematic ofASTM GI09 macrocell current prism.

The second issue is how representative are macrocell currents of the true corrosion currents in the steel. The microcell currents may be more important than macrocell current flows. In a comparison with linear... 

The steel probe is sometimes embedded in an excessively salty patch. With no current applied, a macrocell current flows from the probe to the reinforcement if they are connected with an ammeter between them. As CP current is applied, the current reduces and then reverses. This is called the macrocell or null probe approach and is used to show that a very anodic area has been made cathodic. It is therefore assumed that all of the rest of the steel is cathodic too. However it is dependent upon the amount of salt added to the patch, and if the salt diffuses away then it may no longer be the most anodic area after a few years. 

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. 

Effective corrosion inhibitors will increase the time to initiation of severe corrosion currents and detrimental concrete admixtures will reduce the time to initiation of severe corrosion. A composite curve from the ASTM round robin is given in Fig. 3. The integrated macrocell current (Coulombs) is a measurement of the total corrosion due to the macrocell. The test is not designed to evaluate materials such as silica fume that increase the resistance between the steel bars and significantly decrease chloride ingress. 

One method of conducting these tests is to have two mats of reinforcing bars with or without admixed chlorides in the top mat and to pond with chloride. The macrocell current between the top and bottom mats as well as the corrosion potentials are measured according to ASTM C 876. This procediue is described in Ref 29. 

Zero-resistance ammetry. The macrocell current measured between embedded rebar probes has been used for monitoring the severity of corrosion. This principle has been widely used, as part of the ASTM G102-92 laboratory corrosion test procedure, with current flow between probes located at different depths of cover. For the monitoring of actual structures, a similar approach has been adopted. Here, current flow has been measured between carbon steel probe elements strategically positioned at... 

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