Repair and protection (C) – electrochemical methods
Galvanic electrochemical treatments
Galvanic electrochemical treatment/application of reinforced concrete consists most commonly of various configurations of discrete zinc anodes including but not limited to discrete zinc ‘pucks’, zinc cylinder arrays, or zinc strips. These types of anodes are typically encased in a medium that is formulated to maintain the electrochemical activity of the zinc (since Zn is prone to passivation from oxides from its own corrosion products) (Holloway et ah, 2012).
Galvanic discrete zinc anodes typically do not provide CP in accordance with the criteria of Standards such as discussed at Section 9.9. They do not output sufficient cathodic current, for reasons as discussed below, to effect CP per se but rather provide electrochemical treatment and a reduction in corrosion rate.
It should also be noted that some galvanic anode suppliers, contractors, and conflicted consultants are misleadingly using the acronym ‘CP’ to describe ‘corrosion protection’ that may be offered by these systems rather than cathodic protection. CP of reinforced concrete is achieved when the protection criteria of Standards such as AS 2832.5 (Standards Australia, 2008) or ISO 12696 (BSI, 2016) are met on a continuous basis. If the protection criteria of concrete CP Standards are not met, then CP is not afforded by discrete zinc anodes.
Discrete zinc anodes in patch repairs
One of the first of the galvanic electrochemical treatment systems developed consists of Zn encased in a high alkalinity (pH >14) lithium hydroxide- based mortar so as to maintain the activity of the Zn (Sergi & Page, 1999), see Figure 10.1.
This type of system is installed by directly connecting the zinc anode to the reinforcing steel within a concrete patch repair, see Figure 10.1. When
Figure 10.1 Examples of discrete zinc anodes encapsulated in lithium hydroxide- based mortar for installation in patch repairs. (Courtesy ofVector)
installed as part of a patch repair system it is claimed that the primary function of the anode is to delay the onset of incipient anode formation at the reinforcing steel immediately adjacent to the patch repair.
The phenomenon of ‘incipient anode’, ‘ring anode’ or ‘halo’ effect (Concrete Society, 2011) within concrete patch repairs has been previously discussed at Section 8.4.5. Page and Treadaway (1982) were the first to suggest that the mechanism of incipient anode formation in chloride contaminated concrete is the concept of macro-cell development (the formation of spatially separated anodes and cathodes with the anodes being the areas adjacent the repair and the cathodes being the repair itself).
As advised at Section 8.4.5, when it comes to incipient anode management, Green et al. (2013) report that the there is a perception in the industry that the only way to manage such is by the insertion of galvanic anodes within patch repairs. However, that is not the case. Green et al. (2013) point out that it should be remembered that conventional concrete patch repair utilising a reinforcement coating system (zinc-rich epoxy, epoxy or resin modified cementitious), a bonding agent (acrylic, styrene butadiene rubber, epoxy or polymer modified cementitious) and a polymer modified cementitious repair mortar (hand-applied, poured, sprayed or combinations thereof), effectively ‘isolate’ the patch such that macro-cell activity is minimised and incipient anode formation managed. By the use of reinforcement coating systems, cathodic reactions are severely restricted on the steel surfaces within the patch. Bonding agents provide a restriction to ionic current flow out of the patch into the surrounding parent concrete and polymer modified cementitious repair mortars have electrical resistivities which further restrict ionic current flow between the patch and surrounding parent concrete.
Manufacturers of electrochemical treatments for patch repairs continue to modify the size and configuration of the anode systems in an attempt to improve their current output and lifetime (e.g. Allan et al., 2000; Ball &c Whitmore, 2008; Sergi, 2009) and refer to Figure 10.2.
Figure 10.2 Examples of different configurations of discrete zinc anodes encapsulated in formulated mortar (to maintain zinc activity) for installation in patch repairs, ((a) Courtesy ofVector) and ((b) Courtesy of BASF)
Figure 10.3 Examples of a 'rolled Zn sheet system encased in a slightly acidic pH environment’ for installation in patch repairs. (Courtesy of SRCP)
More recently, a paper by Giorgini and Papworth (2011) introduces a rolled Zn sheet system encased in a slightly acidic pH environment, refer to Figure 10.3.
Discrete zinc anode systems that are installed within the parent concrete around the perimeter of patch repair areas are shown at Figure 10.4. These systems include an integral connecting titanium wire and are installed into drill holes and embedded in ‘a specially formulated backfill mortar’ (SPA, 2020).
Figure 10.4 Example of a discrete galvanic anode system embedded within drill holes using a pliable viscous backfill mortar of pH <12.4. (Courtesy of Duoguard)