Side Effects and Galvanic Anode Cathodic Protection

Side Effects and Galvanic Anode Cathodic Protection

By Hamid Khan, Product Segment Specialist – Repairs & Grouts at FOSROC (ANZ)

Accelerated deterioration of buildings and other reinforced concrete structures during the first ten years of life is a major concern to asset owners. Premature deterioration can result in reduced service life of buildings. ?Over the past few decades, the desire to extend the useful service life of concrete infrastructure has become of paramount significance. Reinforced concrete structures in marine environments are subject to even more aggressive chloride exposure and can show signs of corrosion after short service periods.

Due to limitations in external power availability, budgetary restraints, and lack of resources, an ICCP system is not always feasible. Galvanic anodes eliminate any requirement for ongoing external AC power thereby significantly reducing maintenance costs. The use of Galvanic cathodic protection (GCP) in reinforced concrete structures has increased due to ease of application, high installation productivity, low monitoring, and maintenance requirements, reduced overall system cost, suitability for prestressed concrete elements where the naturally maintained protection potential diminishes the risk of hydrogen embrittlement.

Zinc galvanic anodes corrode to protect the steel reinforcement. It is one of the most studied galvanic materials for concrete structures. When considering galvanic anodes, some of the questions that may arise in the mind of the user are, how long will the anodes last in atmospheric zones; how do I determine aging or depletion of the anodes; what would be the relative cost/benefit of installing anodes in chloride contaminated structures vs typical patching; and is there any long-term monitoring data available by field or laboratory testing.

Galvanic anodes balance electrochemical incompatibilities: To ensure a durable repair system will meet the desired service life it should be compatible with the concrete substrate (Figure 1).?While we often discuss the importance of dimensional compatibility, bond compatibility, structural, mechanical, and chemical compatibility for long term integrity of the repair system, it is also equally important to ensure electrochemical compatibility. Electrochemical compatibility relates to the capacity of the repair system to avert corrosion both within the repair cavity and within adjacent non-repaired areas. Installation of galvanic anodes in chloride contaminated concrete balances electrochemical incompatibilities which would otherwise exist.

Figure 1

Galvashield discrete anodes range are embedded in concrete repairs patch to provide corrosion prevention or corrosion control to adjacent areas of reinforced concrete elements, thereby reducing the possibility of a new active corrosion of the surrounding rebar.

Is halo or anodic ring effect a fad? Corrosion of steel generates iron oxides and hydroxides, resulting in the volume 5 to 8 times its original size. Since the corrosion products occupy more volume than the steel reinforcement, expansive forces within the concrete around reinforcement increase over time leading to concrete cracking, and spalling. Localized patch repairs are commonly used to address this concrete damage. A traditional patch repair is conducted by saw cutting the edges of the repair area, removing the deteriorated, spalled concrete; cleaning the rebar locally; and applying a repair mortar. It is well documented that traditional patch repair of chloride-contaminated concrete can cause electrochemical imbalances that lead to brisk corrosion rates of the steel reinforcement adjacent to the patch repair. This phenomenon is known as incipient anode effect, ring anode effect, or the halo effect.

Concrete spalling and cracking occur at the anodic site in a corrosion cell since this is the location where steel is corroding, and corrosion products will accumulate. After removing the chloride contaminated concrete and filling the repair cavity with a chloride free repair mortar, the previously corroding anodic site will be re-passivated by the new alkaline repair mortar. Removing the chloride ions from the repair area reverses the potential difference, creating a new corrosion cell outside of the repaired area. This is the incipient anode effect. The steel in the repaired section that was previously anodic (more active) to surrounding areas will now act as a cathode. ?The surrounding area that was previously cathodic to the spalled or cracked patch area has now become anodic (more active), relative to the new repair resulting in incipient anode formation. The redistribution of the anodic and cathodic sites forms new corrosion sites just outside the repaired area. These corrosion sites are driven by the chloride contamination in the concrete which is not removed and the driving potential difference between the steel in the chloride-free newly repaired cavities and the adjacent areas. (Figure 2A)

Figure 2A: Incipient anodes or Halo effect. Illustration of Corrosion Around Concrete Patch Repair. Diagram reprinted with permission from Vector Corrosion Technologies

The halo effect can induce corrosion of reinforcing steel surrounding of the patch repair cavity. Cracking and spalling may occur within a short period of time, for instance 5 years or even quicker.?

Sacrificial anode – How it works? Localised embedded galvanic anodes with a higher potential will corrode sacrificially in preference to the more noble metal (reinforcing steel). Galvanic anodes prevent the incipient anode effect, by replacing the original anode (i.e., the corroding steel), with a new, powerful sacrificial anode (i.e., the Zinc anode), therefore protecting the reinforcement outside of the patch repair due to its greater radius of influence and throwing power. If the anode has an effective throw of say 0.5m and the repair area is 1m2 then the ‘’effective repair’’ becomes 4m2 for the price of a 1m2 repair plus the anode cost (the installation cost is minimal). The increase in the effective area does not mean that the site of the anodes will be moved outside the throw of the anode. Previously these areas were not corroding i.e., they were cathodic, the influence of the anode is to ensure that they remain so. (Figure 2B, 3)

Figure 3: Effective repair area is increased

Performance of anodes in dry environments (improved anode current) has been enhanced with the use of non-corrosive humectants in the anode. Protection current density and consequently the reinforcement polarization both depend on the reinforcement surface area, corrosivity of the environment and anode design spacing (number of anodes per unit length of patch perimeter).

Figure 2B: Installed galvanic anode. Illustration of Localized Galvanic Protection Protecting steel in the Remaining Concrete Adjacent to Concrete Repair. Diagram reprinted with permission from Vector Corrosion Technologies

Figure 4: Installation of Galvashield XPT sacrificial galvanic anodes on bridge pier

Figure 5: Embedded galvanic anodes during installation on a bridge pier cap late 1990’s. One of the first monitored installations in the UK. Photo reprinted with permission from Vector Corrosion Technologies

Galvanic anode long term track record: Sacrificial galvanic anodes for mitigation of corrosion of steel in concrete have been monitored on some projects for more than 20 years (Figure 5). Even after 20 years anodes remain active and continue to pass current to the reinforcing steel. This impressive long track record as well as field and laboratory data has enabled detailed analysis and a much better understanding of long-term anode behaviour. ?Long term field performance data can be used to design galvanic cathodic protection systems to provide specifiable long-term current density and polarization of the steel.

Galvanic anode applications and service life:?Discrete galvanic anodes are most often used to mitigate ring anode formation (halo effect) in patch repair applications. Applications include balconies, columns and beams, parking garages, bridge widening, slab replacements, piers and wharfs, expansion joint repairs and other, interfaces between new and existing concrete, repair of prestressed and post-tensioned concrete, repair chloride contaminated or carbonated concrete and extending the life of concrete repairs. Discrete galvanic anodes can provide long term economic solutions with service life of 10 to 30 years and 20 to 40+ years for suitably designed distributed anode systems such as the Galvashield DAS anodes. Galvanic anodes do not require external power, maintenance, or monitoring; thereby improving the durability and reliability of repair. ?Installation is easy and fast and does not require any specialized equipment or tools.

Figure 6: Galvanic anodes Cutaways

Targeted Proactive approach: Galvanic anodes have been developed for corrosion control applications where the concrete is still intact, and repair is not yet required but the condition assessment survey indicates high corrosion potentials or the presence of corrosive conditions. ?These galvanic anodes are used to control on-going corrosion and to prevent the initiation of new corrosion activity in concrete structures. The cylindrical discrete anodes are available in a variety of standard sizes. ?Quick and easy installation into concrete that is mechanically sound but has ongoing corrosion activity is what makes them unique. Once installed, the zinc anode corrodes preferentially to the surrounding rebar, providing galvanic corrosion control to the adjacent reinforcing steel.

?These anodes (Fig. 7 and 8) are designed to fit in drilled holes and are installed in a grid configuration connected together to provide protection over large areas of un-damaged concrete. A non-destructive half-cell corrosion potential survey can identify areas of high-risk. After mapping the high-risk areas, galvanic anodes can be installed in the identified areas as a more proactive and targeted approach.

Figure 7:?Proactive approach – top left corner, cylindrical anodes to be drilled in areas of highly negative half-cell potentials but active corrosion and spalling has not yet initiated.

Figure 8: Proactive approach - cylindrical anodes designed to be inserted in an array of drilled holes in concrete

Conclusion: Asset owners who desire to extend the service life of their assets such as buildings, car parks, bridges, and other structures, should consider applying this proactive, targeted and holistic approach. Spalled corroded areas can be protected by installing galvanic anodes such as Galvashield XP range (Fig. 6) in broken out areas. Whilst sound concrete areas where highly negative half-cell potentials were found can receive proactive protection with discrete anodes as Galvashield CC installed in drilled holes. To counter the undesirable side effects of incipient anode corrosion, galvanic anodes provide an ideal solution. Discrete anodes allow targeted protection of high corrosion risk areas and incorporating galvanic anodes into structure modifications such as slab replacements, interfaces between new and existing concrete, expansion joints and critical areas in new construction will reduce the need for future repairs. ?

About the Author:

Fosroc’s Hamid Khan is a forensic engineering specialist and industry-recognised specialist in structural concrete repairs.?As the Product Segment Specialist (ANZ) Hamid’s role is to harness high-performing technologies to respond to complex repairs and applications with 24 years of experience in the industry.

Working in the Fosroc International team in Dubai for 14 years, Hamid was able to see repair products in action on a vast range of projects in the Gulf, Middle East, Europe, and Asia.

Hamid’s trusted expertise is demonstrated by his regular speaking roles at industry conferences and seminars, and numerous published articles. He is a director on the board of the Australasian Concrete Repair & Remedial Building Association (ACRA) and a past president of ACRA. Hamid played an active role in HB84:2018 Guide for Concrete Repair and Protection (A Joint publication of ACRA, CSIRO, -and Standards Australia) and other industry courses.

Hamid holds a bachelor’s degree in Civil Engineering, and a double Master’s in Business and Strategy from the University of Wollongong. He is certified in Concrete Technology and Construction by the City & Guilds of London Institute (UK).

References

1.??????Whitmore, D. Miltenberger, M. “Galvanic cathodic protection of corroded reinforced concrete structures”. Nace International Corrosion conference and expo. Paper No. 13085 (2019)

2.??????G. Sergi, G. Seneviratne & D. Simpson “Monitoring Results of Galvanic Anodes in Steel Reinforced Concrete over 20 Years” J. Construction & Building Materials (2020).

3.??????Whitmore, D. G. Sergi, G. “Long-term monitoring provides data required to predict performance and perform intelligent design of galvanic corrosion control systems for reinforced concrete structures” Nace International (2021).

4.??????Simpson, D. “Multi-story car park repair and maintenance: A holistic approach for corrosion control” Concrete Repair Bulletin, International Concrete Repair Institute (2013).

5.??????Rincon, O. et, al. “Galvanic anodes for reinforced concrete structures: A Review” Nace International, Corrosion—Vol. 74, No. 6 (2018).

6.??????Khan, H. (2018), Corrosion conundrum – durability risks and protection to bridge structures. Corrosion and Materials, Vol 43 No 4, pp 62-64

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