Numerous reinforced concrete structures worldwide, such as bridges and offshore platforms, are exposed to aggressive chloride environments and show evidence of corrosion after short service periods. The cost of repairing or replacing deteriorated structures has become a major liability for highway agencies—it is estimated to be more than $20 billion and increasing at $500 million per year. The corrosion of steel reinforcing bars due to chlorides is the primary cause of this deterioration—cracking, delamination and spalling. Deicing chemicals and seawater are the two main sources of chlorides.
According to Oladis Troconis de Rincón, FNACE, of Universidad del Zulia (Maracaibo, Zulia, Venezuela); Alberto A. Sagüés, FNACE, of University of South Florida (Tampa, Fla.); Andrés Torres-Acosta of Instituto Mexicano del Transporte (Sanfandila, Querétaro, Mexico); and Miguel Martinez-Madrid of Instituto Mexicano del Transporte, authors of an article on galvanic anodes for reinforced concrete structures published in Corrosion, many existing structures currently have a considerable amount of corrosion in progress. Since the mid-1970s, research efforts have addressed various techniques to prevent corrosion from chloride exposure. The consensus is that cathodic protection (CP) is the only substantiated method to arrest chloride-related corrosion of metals embedded in concrete. According to the U.S. Department of Transportation’s Federal Highway Administration, CP is the only rehabilitation technique that has been proven to stop corrosion in salt-contaminated bridge decks, regardless of the chloride content of the concrete.
Much of the research on CP systems for steel-reinforced concrete structures has focused on impressed current CP (ICCP) systems based on the assumption that concrete’s high electrical resistance does not allow the use of sacrificial anodes. However, the authors note, in locations where there are no power lines and where maintenance and control of rectifiers is overly expensive, sacrificial anode CP systems would be highly desirable. They add that the use of sacrificial anodes for CP in reinforced concrete structures has increased in recent years due to the ease of installation, low maintenance requirements and the desire to decrease the risk of hydrogen embrittlement of the reinforcing steel for prestressed concrete structures.
Zinc-based alloys are among the most evaluated sacrificial anode materials for concrete structures, particularly in the United States. These anodes can be utilized in many ways and in various forms. The protection capacity of zinc alloy anodes can be limited, however, based both on laboratory and on-field application studies. The authors shared their findings during a symposium presentation at Corrosion 2017 in New Orleans, where they reviewed the research work done in both the laboratory and the field as part of a study to identify situations where sacrificial anode use may be a practical option for steel-reinforcement protection in concrete structures.
Thermal spray anodes, installation of embedded anodes with moisturizing agents to promote sustained electrochemical activity, jacketed zinc anodes embedded in a Portland cement mortar cover and small localized “point” anodes are the zinc-based anode usages studied the most.
As an alternative galvanic CP system, thermal spray metallic coatings are low-cost and simple to implement. These coatings, typically approximately 50 µm (2 mils) thick, are generally produced from zinc- and/or aluminum-alloy wires that are melted with an electric arc and sprayed using pressurized air. Installation requires the delaminated concrete to be removed and the reinforcing steel to be exposed by pressurized sandblasting before the molten anode alloy is sprayed. Generally, the electrical connection is achieved directly by the applied zinc on the exposed reinforcing steel.
Thermal spray galvanic anodes using zinc- or aluminum-based alloys can provide effective protection of the rebar in marine environments; however, the authors note the protection is limited mainly to the tidal and splash zones, provided the concrete there is a good conductor. System life there can be limited by self-corrosion. Some benefit may also be attained in the atmospheric zone, but it may be due to the sprayed metal acting as a physical barrier against aggressive agents and oxygen.
With embedded zinc anodes, byproducts generated by the dissolved zinc—such as oxides and hydroxides—can accumulate at the zinc/concrete interface and eventually cause delamination and flaws in the metallic coatings, which over time will decrease the galvanic CP system’s protective capabilities. The authors comment that both laboratory and field investigations have shown that the current provided by the galvanic system increases when humidity at the zinc/concrete interface is increased, and this also contributes to the redistribution of the dissolving coating’s byproducts into the concrete. Studies were conducted on the use of substances (moisturizers) that promote high humidity at the anode/concrete interface and enable the anodes to retain their protective capability. Lithium bromide (LiBr), lithium nitrate (LiNO3), and potassium acetate (KC2H3O2) were moisturizers investigated. The results proved that lithium salt-based moisturizers improved the zinc anodes’ behavior, with LiBr showing the best results. They commented that the use of moisturizing agents increases zinc alloy activity in splash and atmospheric zones, but not enough to maintain it for long periods of time. Zinc-hydrogel anode applications, where a zinc sheet is embedded in an ionically conductive adhesive, also were seen to increase the zinc anode activity, and improved performance was reported.
Jacketed anode systems hold zinc-based anodes in place against a reinforced concrete surface with a jacketing material, and they have been used to protect bridge piles. The authors say that placing zinc-based anodes in a jacket system provides efficient long-term steel protection for the tidal region, where there is appreciable chloride ion ingress into the mortar and the influence of the submerged anode is greatest. Unlike thermal sprayed anodes, these jacketed systems allow better monitoring of the sacrificial anode system because they can be installed with monitoring systems that allow measurement of the current and potentials for both the anode and the steel reinforcement using electrodes embedded in the protected element.
The Florida Department of Transportation (FDOT) started using a new version of jacketed anodes in 1994 with a configuration similar to one used for an ICCP system with jacketed titanium mesh anodes. According to the authors, this system is comprised of expanded mesh of nearly pure zinc alloy with small additions of elements intended to improve its formability and anodic performance. The anode mesh is mounted in a glass fiber jacket that provides an annular space between the mesh and the protected structure. The space is filled with a mortar mix of Portland cement, fine aggregate and sand. The jacket is placed around the pile, starting at the lowest tide point and extending up to approximately 1.8 meters above low tide.
Because there are no activating admixtures in the jacket filler, any activation of the anode must rely on the absorption of seawater by the filler after the jacket is placed in service. Studies have shown that zinc readily passivates in normal Portland cement concrete, and that a high concentration of chloride ions in the concrete is needed for zinc to be consistently active and able to protect the embedded rebar. Since the fill mortar is chloride-free, the zinc alloy in the jacket tended to passivate in the splash and atmospheric zones, and protection there is limited to physical barrier effects. This system has become a standard method of pile repairing for FDOT, which has repaired hundreds of piles in numerous bridges with good results.
In applications such as patch repairs and other general sacrificial CP installations, localized “point” anodes are used. According to the researchers, it is established that repairs of chloride-contaminated or carbonated concrete can create electrochemical incompatibilities between the “new” and “old” concrete that may lead to the accelerated corrosion of the steel reinforcement in the concrete near the patch repair; this is known as the “halo” effect. The resulting corrosion can induce cracking that may require extending the patching repair after a short time period (e.g. three to five years).
The idea of the localized CP anode application is that small galvanic point anodes installed in the patch repair will sacrificially corrode and reduce the possibility of a new active corrosion zone on the surrounding rebar, the authors comment. There are various versions of these anodes. Some, with an overall diameter of approximately 60 mm and a height of approximately 30 mm, may utilize a single zinc alloy core (with a mass of several tens of grams) surrounded by a cylindrical active matrix of cementitious components. Others, with larger dimensions, may incorporate multiple zinc alloy cores embedded in a common activating matrix. The zinc alloy cores are usually connected to rebar tie wires that can be easily tied to the reinforcing steel in the area to be patched before casting the repair concrete. The overall result, according to the findings of some investigators, is an appreciable extension of service life in patch repairs. The authors caution, however, that the service range of those anodes may be limited and it should be carefully evaluated against other options.