Case Studies Show Galvanic CP Effective on Corroded Reinforced Concrete Structures
February 10, 2022 •Corrosion CONTROLLED, Corrosion Essentials, CP
Chloride-induced corrosion of reinforcing steel in concrete is a major problem worldwide. Chlorides can be introduced into concrete through deicing chemicals, seawater, soil, or as contaminants in the concrete mix. This can lead to pitting corrosion and concrete deterioration.
Localized repairs are often performed to address concrete corrosion damage. When a repair is completed, new corrosion sites can form just outside the repaired area and are driven by the residual chloride contamination and the difference in potential between the steel in the chloride- contaminated and chloride-free sections. When incipient anodes form, further repairs will be required. The patch repair process may be repeated several times over the remaining life of the structure.
Galvanic Cathodic Prevention (CP)
Localized corrosion can be addressed by the installation of Type 1 discrete galvanic anodes around the perimeter of repairs (see figure). One of the oldest monitored projects (1999) in which alkali activated, Type 1 anodes were installed was monitored for 20 years. The project’s anodes had a high-purity zinc core cast around steel tie wires to provide a durable zinc-to-steel connection and to prevent dissimilar metal corrosion in the electrical connection between the wires and the reinforcing steel. The zinc cores were cast in high alkalinity mortar with a pH >14 to ensure the anode remains electrochemically active and does not passivate over time. After 20 years, the alkali-activated anodes remained active and continued to produce sufficient galvanic current to meet ISO 126964 cathodic prevention requirements and avoid corrosion initiation in the vicinity of the repairs.
Figure: Alkali-activated galvanic anodes during installation on a bridge pier cap.
Case Studies Help to Verify Galvanic CP's Long-term Performance
Historically, many practitioners believed CP of reinforced concrete structures required an impressed current power supply to provide enough current to protect the reinforcing steel. This is no longer the case as properly designed galvanic encasements using high output, long life, distributed alkali-activated galvanic anodes can provide sufficient CD to polarize the reinforcing steel and meet NACE CP criteria for many years.
Long-term CP requires the galvanic anodes to remain electrochemically active and the electrical connections between the zinc and the steel to remain intact over the desired service life. Laboratory tests are good for verifying short-term performance. Long-term performance of galvanic CP systems, however, is best verified by monitoring systems installed on real structures over time.
Long-term galvanic CP was provided to and then studied on a bridge substructure of the Ohio Department of Transportation (DOT), a bridge deck of the Ontario Ministry of Transportation (MTO), and a tidal/splash zone of the New York State DOT marine columns.
1. Alkali-activated Type 1A discrete galvanic anodes can significantly extend the service life of concrete repairs.
• Anodes installed in the 1990s remained active and provided corrosion prevention (cathodic prevention) CD to reinforcing steel for 20 years. CP current resulted in passivation of the steel over time.
• Examination of anodes removed from the structure verified that there was enough zinc remaining to last 25 to 30 years.
2. Galvanic CP systems utilizing alkali-activated anodes can be designed to remain active and produce sufficient current to meet NACE CP criteria for 20 to 40 years in a range of environments including:
• Bridge substructures in temperate, deicing salt environments
• Bridge decks in temperate, deicing salt environments
• Bridge piles in marine tidal/splash zones
• Bridge columns in tropical, marine environments above the tidal zone
• Florida DOT Marine Substructures (Above Tidal Zone)
See AMPP’s Cathodic Protection Program -- courses and certifications for success in the field.
Source: materialperformance.com; article based on a paper by David Whitmore, Vector Corrosion Technologies, Ltd., and Matthew Miltenberger, Vector Corrosion Services, Inc. titled “Galvanic Cathodic Protection of Corroded Reinforced Concrete Structures” presented at CORROSION 2019.
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