Lessons in Galvanic Cathodic Protection Technology from Soldier
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Lessons in Galvanic Cathodic Arne P. Johnson Protection Technology John S. Lawler from Soldier Field and the Michael S. Murphy Franklin Avenue Bridge Fig. 1. Franklin Avenue Bridge, Minneapolis, Minnesota, during construction in 1922. This bridge has been recently rehabilitated. Image courtesy of Hennepin County Library. Deterioration of older reinforced concrete structures is commonly caused by one or more of four mechanisms: Design of galvanic chloride- or carbonation-induced corrosion of embed- cathodic protection ded steel, freeze-thaw deterioration of non-air-entrained systems requires concrete, and deterioration of the concrete matrix due to proper consideration deleterious internal chemical reactions. These deteriora- of five key design factors, which are tion mechanisms must be addressed in order to achieve particularly relevant successful rehabilitations (Fig. 1). for historic structures. Steel reinforcement in concrete is normally protected from corrosion by a thin oxide film (also known as a passive film) that develops around the bars as a result of the highly alkaline concrete pore solution that results from portland cement hydration reactions. As long as this passive film remains, corrosion is impeded. Chloride-induced corrosion can be initiated when chloride ions, in the presence of moisture and oxygen, accumulate to a sufficient concentration and then destroy the passive film around the reinforcing bars. Chloride ions can originate from external sources, such as deicing salts or seawater, or internal 27 APT BULLETIN: THE JOURNAL OF PRESERVATION TECHNOLOGY / 50:2–3 2019 Fig. 2. Example of typical corrosion cell in reinforced concrete. All figures by Wiss, Janney, Elstner Associates, Inc., unless otherwise noted. Fig. 3, opposite. Soldier Field, Chicago, Illinois, west grandstand and colonnade during construc- tion in 1924. Image courtesy of the Chicago Park District. sources, such as chloride-contaminated In concrete deterioration caused by Used in the 1800s to protect ships by aggregates or chloride-containing corrosion of embedded steel (either attaching billets of zinc to ferrous hulls, admixtures. Carbonation-induced chloride- or carbonation-induced), the CP was first used for reinforced concrete corrosion can be initiated when the corrosion process is an electrochemi- structures in the 1970s. In short, CP carbonation front in the concrete cal reaction that breaks down the steel involves making the reinforcing steel the reaches the level of the reinforcement. to more chemically stable iron-oxide cathode of a corrosion cell by supplying Carbonation is a natural process that compounds (also known as rust). For an alternative anode. There are two occurs when carbon dioxide in the air this reaction to occur, a corrosion cell basic types of CP: galvanic CP (also penetrates the concrete and reacts with must be formed; it consists of an anode known as sacrificial CP) and impressed the cement paste, lowering its pH from (location of oxidation reaction where current CP (ICCP). as high as 13 in its uncarbonated state electrons are lost) and a cathode (loca- Galvanic CP is a passive system in down to as low as 8.5, breaking down tion of reduction reaction where elec- which a galvanic anode, typically zinc the passive film. trons are gained) (Fig. 2). The anode in various shapes and sizes, is embed- and cathode are connected by an ionic Freeze-thaw deterioration occurs in ded in or attached to the concrete and current path (ions passing through an non-air-entrained concrete when the electrically connected to the steel. Be- electrolyte such as moist concrete) and concrete is critically saturated with cause the zinc is electrochemically more an electronic current path (electrons water and subjected to repeated freez- active than the steel, the zinc becomes passing through a metallic conductor). ing and thawing cycles. The damage the anode and corrodes preferentially The corrosion reaction results in rust first manifests in internal microcrack- to the reinforcement, which becomes at the anode that occupies a greater ing, progresses to paste deterioration the cathode and is polarized, reducing volume than the steel consumed, creat- or map-cracking visible on the surface, the rate of corrosion. A galvanic CP ing expansive pressures and eventually and culminates in disintegration of the system is usually relatively inexpensive concrete distress in the form of crack- concrete from the surface inward. to install and requires little to no main- ing, delaminating, and spalling of the Modern air entrainment avoids this tenance. The anode is consumed and concrete surface. eventually exhausted, at which time it damage mechanism by providing voids must be replaced to provide continued in the concrete into which internal Cathodic Protection Basics protection. Service life (i.e., the length water can expand when it freezes. Con- Cathodic protection (CP) is one of of time that protection is provided) crete deterioration can also be caused several technologies used to prolong depends on many factors, including the by deleterious chemical reactions such the life of historic concrete structures size of the anode and severity of the as alkali-silica reaction (ASR) and that are prone to damage from corrosion corrosion environment, but typically delayed ettringite formation (DEF). of embedded steel (i.e., CP does ranges, in a well-designed system, from Further information on all of these not address deterioration caused by 7 to 20 years. mechanisms can be found elsewhere.1 mechanisms other than corrosion). 28 Here’s where the current thinking about masonry restoration falls apart. Despite a history of failure, masonry restoration products that contain adhesive bonding agents trap moisture and salt, which cause failure – and further damage – within a few years. Our mortars are free of bonding agents. In fact, they’re custom-engineered and lab-tested to match your substrate. So they expand, contract, absorb, and discharge moisture in precisely the same way as your building. So our integrated masonry systems last a lifetime, which makes them a wise choice for your next restoration. The leader in scientific masonry restoration www.cathedralstone.com • [email protected] • 800-684-0901 LESSONS IN GALVANIC CATHODIC PROTECTION TECHNOLOGY Calcium chloride had apparently been added as an accelerator to the face mix to aid in the two-layer placement method. In the concourses, sampling and testing showed that corrosion was due principally to carbonation of the concrete.5 Cathodic protection trials. In 2001, when adaptive reuse and rehabilitation of the stadium were being contem- plated, the authors’ firm was engaged to study means to prolong the life of the historic concrete elements (the firm was not involved in the adaptive-reuse design). Trials of five corrosion-mitiga- tion systems were installed, including three galvanic CP systems: discrete zinc anodes, arc-sprayed zinc anodes, and zinc-hydrogel sheet anodes. The other methods were re-alkalization (an electrochemical treatment to address carbonization-induced corrosion) and surface-applied migrating corrosion ICCP is based on an active system in Soldier Field inhibitors (a spray-applied penetrating 6 which the anodes, powered by a DC Soldier Field was constructed between liquid intended to mitigate corrosion). power source, introduce an electrical 1922 and 1926 in Chicago, Illinois This paper focuses on the discrete current that promotes cathodic (Fig. 3). The stadium was listed on the galvanic CP system installed in the cof- reactions at the surface of embedded National Register of Historic Places fered ceilings of the colonnades con- steel. Use of a DC power supply in 1984 and designated a National sisting of equally spaced, cylindrically may allow for higher currents and Historic Landmark from 1987 to 2006. shaped anodes embedded in holes that greater control. An inert anode that Original elements of the structure that were cored from the attic into the top is not sacrificed over time is typically remain include the concourses at the of the colonnade beams and electrically utilized. ICCP systems are generally stadium perimeter and the east and connected to the reinforcement. Anodes more expensive to install and require west colonnades.4 were installed at two spacings: 22 inches more maintenance. Service life depends and 32 inches (Figs. 4 and 6). on many variables but can exceed 25 Overhead concrete in the concourses Performance evaluation. After years.2 Hybrid CP systems, which have includes large transfer girders and sec- installation, the corrosion-mitigation been introduced recently, function ondary framing for the promenades and systems were monitored for six months. as an impressed current system for a colonnades above. The coffered ceilings Monitoring consisted of measurements relatively short period of time (typically of the colonnades consist of an orthog- of corrosion potential (using copper- a few months) then revert to a galvanic onal grid of heavily reinforced concrete sulfate reference electrodes), corrosion system for the remainder of their beams that support a thin, lightly rate (using surface-contact linear service life. reinforced concrete slab (Fig. 4). The formwork for the ceilings of the colon- polarization instruments and em- This paper focuses on galvanic systems, nade was lined with a 2- to 3-inch-thick bedded corrosion-rate probes), and the type of system used in the case architectural face mix that resembles cathodic protection current (by studies presented,