PRIMER
BY ELISE M. LOVE, P.E. Wiss, Janney, Elstner Associates, Inc. Galvanic Corrosion At the detail design stage, careful consideration must be given to the compatibility of building materials or systems used in structural and architectural applications.
One potential consequence of incompatibility (termed ‘galvanic action’). The more ‘active’ A phenomenon known is accelerated corrosion of metals, which can metal will support the oxidation reaction and occur if dissimilar metals are combined in corrode (becoming the anode, where metal as galvanic corrosion certain environments. This is galvanic will be consumed), while the more ‘noble’ occurs when dissimilar corrosion. Evaluations of compatibility are metal will support the reduction reaction metals, subjected to increasing in importance, as the array of (becoming the cathode, where metal will not potential building materials is ever-expanding. be consumed). the same environment, Fortunately, durability issues related to comprised of a conducting galvanic corrosion can be minimized or The Galvanic Series prevented with proper design. solution, are in direct Compatibility of different metals can be electrical contact. assessed, relative to the potential for galvanic Galvanic Corrosion Defined corrosion, with the use of charts depicting In general, corrosion in metals is an the galvanic (or electromotive force) series in electrochemcial reaction with the different environments. The galvanic series surrounding environment consisting of two indicates which dissimilar metal will tend to partial reactions: an oxidation reaction and corrode (anode) and which dissimilar metal a reduction reaction. When different metals, will tend to support the reduction reaction or metal alloys, are placed in a corrosive (cathode), when the requirements for galvanic environment, each metal develops a potential corrosion are met. All other things being for corrosion that is unique to both the metal equal, a greater separation between the and to the environment. Corrosion will occur metals in the galvanic series will result in at different rates when different metals are greater galvanic action. A chart depicting placed in the same corrosive environment, the galvanic series for some common metals and likewise, the same metal will corrode at in a frequently encountered conducting Location: different rates when placed in different solution, seawater, is included in Figure 1. DETROIT OFFICE corrosive environments. When different Though the order of metals in a galvanic 30700 Telegraph Road metals develop potentials for corrosion that series remains the same in most conducting Suite 3580 are significantly different, these metals are solutions, some electrolytes can affect this Bingham Farms, MI 48025 termed ‘dissimilar.’ A phenomenon known order. While galvanic series are most Contact: as galvanic corrosion occurs when dissimilar commonly presented for seawater, seawater T: 248.593.0900 metals, subjected to the same environment, is a severely corrosive environment and E: [email protected] comprised of a conducting solution (usually a does not necessarily represent the service water-based solution in which an electrolyte, environment or localized conditions to which such as sodium, potassium, chloride, calcium, metals will be subjected. The potential for www.wje.com or phosphate, is dissolved), are in direct galvanic corrosion is greater in other highly (electrical) contact. When the requirements corrosive environments (urban or industrial), for galvanic corrosion are met, one dissimilar and lower in less corrosive environments metal will corrode preferentially to the other (rural); metals that are relatively compatible 1 PRIMER
Galvanic Corrosion (CONTINUED)
in one environment are not necessarily compatible in other environments. ACTIVE/ANODE (CORRODED)
Magnesium Most Common Occurrences of Zinc Galvanic Corrosion Aluminum Examples of common areas for occurrence Tin of galvanic corrosion in buildings and other Lead structures include the following: Iron/Steel Brass Where a fastener metal is dissimilar from Bronze the metal of the connected elements Copper (such as structural connections, fasteners Stainless Steel for metal roofing and facade elements, fasteners for metal roof edge terminations and copings, strap and clip NOBLE/CATHODE (PROTECTED) anchors for metal gutters/downspouts/ FIGURE 1 FIGURE 2 piping, etc.): Galvanic Series in Seawater. Galvanic corrosion of carbon steel clip angle — Stainless steel screws and aluminum connecting copper downspout to substrate window components (Photo by James Chiropolos of WJE). — Carbon steel clip anchors with copper downspouts (see Figure 2) — Galvanized steel fasteners and aluminum kerf anchors (see Figures 3 and 4) — Galvanized steel hurricane clip roof tile fasteners and copper eave flashing — Stainless or galvanized steel fasteners and aluminum termination bar or aluminum flashing
Where structural systems interface FIGURE 3 FIGURE 4 with architectural systems or different Galvanized steel fastener through aluminum kerf Fractured galvanized steel fasteners that were structural systems (such as facade anchor (Photo by Fiona Aldous of WJE). used with aluminum kerf anchors (Photo by and curtainwall connections to Fiona Aldous of WJE). structural framing): — Wrought iron framing and copper Where dissimilar metal pipes are conditions as discussed previously. Metals cladding (Statue of Liberty) directly connected: that are further apart in a galvanic series, — Galvanized steel railings set in stainless — Copper pipe and galvanized steel pipe such as aluminum and stainless steel in the steel sleeves galvanic series example provided in Metals that are very close together in a Figure 1, are more commonly affected by — Stainless steel access ladders connected galvanic series can usually be used in direct galvanic corrosion. to structural weathering steel contact without a problem, however — Aluminum and stainless steel space consideration must be made for metals frame components used in more corrosive environments and
2 PRIMER
Galvanic Corrosion (CONTINUED)
Factors Affecting Galvanic Corrosion steel members. Even when metals are very (cathode) and limits steel corrosion. Occurrence and Extent close together in a galvanic series, the area Cathodic protection of steel in water, soil, Aside from the separation between the effect can result in accelerated corrosion or concrete is often achieved in a similar metals or metal alloys in the galvanic series, when the requirements for galvanic fashion, with the use of sacrificial anodes the occurrence and extent of galvanic corrosion are met. Accelerated corrosion of made of zinc or magnesium. In many corrosion can be affected by a number of the zinc coating and fracture of galvanized instances, when these sacrificial anodes are factors related to the conducting solution steel fasteners has been observed when corroded away, they can be replaced and environment and its properties. These the fasteners were used to attach facade cathodic protection of the steel restored. factors include the solution temperature, panels through aluminum kerf anchors due temperature gradients, humidity, volume, to the large surface area of the aluminum Summary solution height above the metals (if kerf anchor (noble/cathode) to the zinc The compatibility of metals in structural submerged), flow rate across the surface of galvanizing coating on the steel fastener and architectural applications, especially the metals, oxygen level, pH level, pollutant (active/anode). at connections where dissimilar metals level, corrosivity, and conductivity. can come into direct contact, must always Prevention of Galvanic Corrosion be considered to prevent accelerated Another factor that can possibly affect the The simplest approach for prevention corrosion of metals due to galvanic action. occurrence and extent of galvanic corrosion of galvanic corrosion is to use the same The occurrence and extent of galvanic is the method used to join the two or compatible materials (for that corrosion will be affected by the relative dissimilar metals or metal alloys. This environment) throughout the structure. separation, in the galvanic series, of the method could be direct contact between a However, using the same or compatible metals or metal alloys, the conducting fastener and the main component, a metals for all aspects of an assembly is solution environment, and the electrical welded area creating a transition from one not always possible or practical. Thus, connection between the dissimilar metals. material to the other, or any number of tactics for reducing or preventing galvanic Rapid galvanic corrosion will occur with a other possible methods of joining the two corrosion, including application of coatings high cathode to anode area ratio. Galvanic metals or metal alloys. at points of contact or the use of neoprene corrosion can be reduced or prevented or other inert washers to isolate dissimilar with the use of metals that are close Rate of Attack metals, are often employed. together in the galvanic series or by The rate of galvanic attack will vary directly electrically isolating the dissimilar metals. with the ratio of cathode area to anode Galvanic Action as Protection Fortunately, galvanic corrosion can often area. In general, a high cathode to anode The mechanism of galvanic action can be avoided in the design phase. When area ratio will result in rapid corrosion, be used to provide corrosion protection. designing or reviewing construction plans while a high anode to cathode area ratio will A commonly recognizable example of this and details relative to the interaction of result in slowed corrosion; this is commonly is the galvanization of steel for ‘cathodic metal elements, the potential detriments termed the ‘area effect.’ An example of the protection,’ where a zinc coating is applied of galvanic action should be considered. area effect is the rapid rate of corrosion in over the steel as a ‘sacrificial anode’; steel fasteners when used in copper thus, the zinc coating (anode) corrodes members, and the negligible corrosion in preferentially to the underlying steel the steel when copper fasteners are used in
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