Incipient anodes

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. 

Most proprietary, pre-bagged mixes carry guarantees of the materials particularly for carbonation repairs. Manufacturers and applicators will be more cautious with chloride repairs and cannot guarantee that all chloride is removed in areas adjacent to the patch. The incipient anode problem (see Section 6.2.1) is far more prevalent in chloride contaminated structures so corrosion will continue around the patches. 

We have already seen that patch repairing is not usually adequate to stop further deterioration in the presence of chloride attack. If a structure with extensive chloride attack is to be patch repaired then it must be recognized that patching the corroding areas can accelerate corrosion elsewhere. When we stop the anodic reaction (2.1) we stop the generation of hydroxyl ions at the cathode (equation (2.2)). Therefore, areas previously protected from corrosion because they were made cathodic by the proximity of the anode (now repaired) will rise above the critical chloride/hydroxyl ratio and corrosion will be initiated. This often occurs around the new patch as shown schematically in Figure 6.4. This incipient anode problem is avoided by applying an electrochemical rehabilitation technique. 

Figure 6.4 Incipient anode schematic showing how anode is displaced to the edge of the repair by the formation of a new cathode in the patch repair.

Figure 6.5 (a) Patch repair with surrounding spalling due to incipient anodes on a building, (b) Incipient anode showing repair (left side) and corrosion in original concrete. 

Sprayed concrete, shotcrete (dry sprayed) or gunite (wet sprayed) are methods of rapidly applying concrete to soffits or vertical surfaces. It can be used over patches and to overlay metal mesh anodes for cathodic protection as described in Section 7.6. It is sometimes applied as a temporary cosmetic repair in the Northern USA and Canada when concrete has spalled or is in danger of spalling. As it does nothing to slow the corrosion rate it is comparable in effectiveness to patch repairing, and may suffer from the same incipient anode problem. 

Figure 7.7 Installation of a galvanic anode in a patch repair to prevent incipient anode induced corrosion around the repair. Courtesy of Fosroc Ltd.

It must conduct ionically not electronically. There must be no conductive filler such as carbon or a metal. Zinc is used in some patch materials to minimize the incipient anode effect, they must not be used for any electrochemical process repairs. Metals or carbon fibres must not be used either. 

They do not give rise to incipient anode problems. 

In the case of chlorides, patch repairs are only effective if chloride ingress is local and the chlorides can all be removed. Just patching up damaged areas is a short-term palliative not a long-term rehabilitation. This is due to the incipient anode effect discussed in Section 6.2.1. It was found from corrosion monitoring of bridges with sealers and pore blockers or even just sealed joints after years of chloride ingress that there was an increase in resistivity and a move towards more passive half cell potentials for beams where sheltered under the deck. 

Incipient anode An area of steel in a corroding structure that was originally cathodic due to the action of a local anode (q.v.). When the local anode is treated by patch repairing, the incipient anode is no longer protected and starts to corrode. 

Figure 5.5 The formation of incipient anodes after patching.

The advantage of inhibitor application is that they can reduce the incipient anode effect around patches if they are incorporated into the repair mix and then diffuse into the concrete. Their disadvantage is the co.iit of the materials, difficulty of application and unknown lifetime and effectiveness. However, with proper evaluation and monitoring, we may learn a lot more about these materials over the next few years,... 

Some degree of patching will probably be required whatever other repair is required. The merits and limitations of patch repairs have been discussed in the previous chapter along with the needs for different depths of repair for cathodic protection or chloride removal. It may also be possible to use less stringent repairs if corrosion inhibitors are being applied in the patch repair material and to the concrete. The issue of corrosion induced around the patch has also been discussed (the incipient anode. Figure 5.5). 

Zinc disks in a proprietary high alkaline mortar jacket embedded in patch repairs to prevent the ring anode or incipient anode effect discussed earlier in this section (Fig. 13.22). 

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