Designing with Metals: Dissimilar Metals and The Galvanic Series

The galvanic series is a list of metals arranged in order of their relative electrical potential. A simple version of the galvanic series is shown in Figure 16.55, page 604 of the textbook. When two metals are in contact in the presence of moisture, their locations within the series indicate the risk of corrosion due to the flow of electric current between them. The closer the two metals on the list, the less the difference in electrical potential, and the less the risk of corrosion. The further apart the two materials on the list, the greater the risk of corrosion. The following are some guidelines for working with dissimilar metals and interpreting the galvanic series.

Avoid contact between metals far apart on the galvanic series.
Virtually every student of building technology is taught this most basic fact about the galvanic series. However, when presented in its usual list form, the galvanic series provides only minimal guidance on judging the relative differences between metals and evaluating their potential incompatibility. A more complete picture of the compatibility of metals can be constructed when numeric values for the metals' electrical potential are attached to the list as well.

This chart lists common architectural metals along with their ranges of relative electrical potential. As with the galvanic series, metals are arranged in order of increasing potential, but in this case, the relative differences between various metal types are more readily apparent.

For example, in the chart above consider the aluminum bronze alloy group and the next metals listed directly above and below. We can see that between the aluminum bronze alloys and the brass alloys directly below (naval, red, and yellow brasses), the relative difference between these metals is small. On the other hand, the difference between aluminum bronzes and mild steel, cast iron, and wrought iron directly above is many times greater. In fact, one must read down the list nine or more metals below aluminum bronze before the electrical potential difference is comparable to moving up only to the first metals above.

With quantified potential differences between metals, the galvanic series can also be used to estimate the compatibility of different metals under varying environmental conditions using the following rules of thumb:

• In coastal, very high humidity, or other harsh environments, galvanic metal pairs should be limited to those with a potential difference no greater than 0.15 volts.
• In moderate environments, metal pairs should have a potential difference no greater than 0.25 volts.
• In environments with controlled humidity and temperature, potential differences as great as 0.50 volts may be acceptable.

For example, consider again the aluminum bronze alloy group. In a harsh environment, the designer may opt to limit metals to be used in contact with this alloy group to other bronze alloys, brasses of various types, copper, tin, and 400 series stainless steel. On the other hand, in a controlled environment, aluminum bronze might safely be combined with any other metal listed on the chart, with the exception of zinc and galvanized steel.

Based on these rules of thumb, metals listed in the chart have been color-coded into groups that fall within potential difference ranges of roughly 0.20 volts. Metals within each of these groups may be considered least corrosion prone when used together in normal architectural conditions.

Avoid smaller anodes in contact with larger cathodes.
On the chart above, the more negative end of the potential scale is noted as anodic or active, and the more positive end of the scale as cathodic or passive. When different metals react galvanically, an exchange of electrons takes place between the two metals, with electrons flowing from the metal with greater negative potential (the anode) to the metal with lesser negative potential (the cathode). For example, if aluminum bronze and a 300 series stainless steel are used together, the aluminum bronze has a greater negative potential and will act as the anode, donating electrons to the less negative stainless steel, the cathode. On the other hand, if aluminum bronze is used with mild steel, mild steel has a greater negative potential and will act as the anode, donating electrons to the aluminum bronze, which in this case acts as the cathode. (A note on terminology: Literature on galvanic reactions often refers to cathodic metals as noble. These two terms are synonymous.)

To a chemist, the anode’s release of electrons is termed oxidation. In laymen’s terms this is known as corrosion. In other words, with any galvanic pair of metals, the anode corrodes as the galvanic reaction takes place. Controlling the rate of corrosion of the anodic metal is an important consideration in working with galvanic metal pairs. After consideration of the electrical potential difference between the two metals, the next most important factor governing the rate of corrosion of the anode is the relative surface area of the anode in comparision to the cathode. The smaller the surface area of the anode in relation to the cathode, the more concentrated the flow of electrons, and the faster the rate of corrosion. The larger the anode's surface area in relation the cathode, the more spread out the flow of electrons, and the less the corrosion. This principal, called the area ratio, often has important architectural implications.

For example, consider a sheet metal copper roof fastened with Type 304 stainless steel screws. The potential difference between the two metals is in the range 0.2 to 0.3 volts, so some corrosion effects may be expected under exterior conditions. In this galvanic pair, copper has a higher negative potential and will act as the anode and stainless steel will act as the cathode. However, since the surface area of the anode (the copper roof metal) is large in comparison to the surface area of the cathode (the stainless steel fasteners ), the corrosive effect on the copper is distributed over a relatively large area and greatly mitigated. In this case little if any long-term negative effect is anticipated. In fact, in practice, stainless steel screws are an accepted method of attachment for copper roofing.

As a counter example, consider a stainless steel sheet metal roof fastened with copper nails. In this case, the surface area of the anode (the copper fasteners) is very small in relation to the surface area of the cathode (the stainless steel roof metal), the flow of electrons from the anodes is highly concentrated, and rapid corrosion of the fasteners is expected.

In fact, the rate of corrosion of the anode in a galvanic metal pair is directly related to the numeric area ratio of the two metals. That is, if the surface area ratio of cathode to anode is doubled, the rate of corrosion of the anode is also doubled. Likewise, if the area ratio is halved, the rate of corrosion of the anode is halved.

Avoid Fasteners Acting as Anodes
The previous examples illustrate an important guideline, that fasteners should generally be selected to avoid taking on the role of anode in a galvanic reaction. Due to their normally small surface area in relation to the materials being fastened, such fasteners will be at risk of rapid corrosion. Thus when fastener and base metal differ, the fastener metal should be selected to be cathodic in relation to the base metal. When two dissimilar metals are joined with a third fastener, the fastener should be cathodic in relation to at least one of the other metals, so that it does not take on the role of anode in a galvanic reaction between the three.

There are many finer points regarding the selection of metal fasteners in relation to metals being joined, the building environment, and the particular application. This topic will be addressed more fully in a later article in this Designing with Metals series.

Avoid Rainwater Runoff From Cathode To Anode
Consider a sheet metal copper roof with a galvanized steel gutter. Rainwater flowing over the roof metal will pick up copper in solution and carry this dissolved metal into the gutter. When the dissolved copper and galvanized steel come into contact, these metals will react as a galvanic pair. Since the galvanized steel is anodic to copper, the gutter will be corroded. Alternatively, consider a galvanized steel roof and a copper gutter. In this case, dissolved zinc (from the galvanized coating on the steel) is carried into the copper gutter, where the zinc will act as the anode in the galvanic pair. In this case the copper gutter acts as the cathode and is not threatened with corrosion.

As a general rule with metal roof and wall systems, care should be taken that rainwater does not flow from metal surfaces that are relatively cathodic to others that are relatively anodic.

Treat Plated Metals According To Their Plating
When using galvanic series charts with plated metals, read from the position of the plating, not the base metal. For example, a cadmium plated mild steel fastener reacts according to the electrical potential of cadmium, not mild steel. Galvanized steel (a zinc metal coating on steel) reacts according to the electrical potential of zinc. Lead coated copper sheet will react according to the electrical potential of lead, not copper. Etc.

Understand Your Project Particulars
In practice, there are additional considerations that can influence the severity of the galvanic reaction between metals. For example, coastal environments tend to produce salt-laden precipitation that can significantly accelerate the reaction between galvanic metal pairs in comparison to non-coastal areas. Urban and industrial environments, with their relatively high concentrations of air pollutants, produce precipitation that is more acidic and conducive to corrosion than precipitation further from such areas. Perhaps less obvious is the potential for accelerated corrosion in some agricultural environments, where certain fertilizers are a known source of corrosive air pollutants.

The electrical potential of metals may vary depending on the medium in which the galvanic reaction takes place. In fact, most galvanic series, including the chart in this article, are based on the electrical potential of metals when immersed in flowing sea water. The designer should keep in mind that metals buried in soil, exposed to highly corrosive industrial solutions, or otherwise exposed to atypical environments may react quite differently from what is predicted by the standard galvanic series.

As one example, consider a 300 series stainless steel angle buried in mud. Due to a lack of free oxygen in such a soil condition, the stainless steel may not be able to maintain the passive layer that normally protects the underlying metal from corrosion. In this condition, the stainless steel surface can become more electrochemically active and assume a location in the galvanic series close to mild steel, a change in its electrical potential of approximately -0.5 volts. (For a discussion of stainless steel alloys and its active and passive conditions, see the related article Stainless Steel and Corrosion Resistance.) As another example, curiously, zinc may become cathodic to iron when immersed in hot tap water.

The particulars of a metal detail or assembly can also influence the rate of corrosion. For example, low-slope metal roofs, in which standing water can accumulate, may exhibit higher rates of corrosion in comparison to steeper roofs that shed water more rapidly and therefore remain dryer. Details that capture and trap water can also lead to accelerated corrosion in localized areas.

Approach Insulating Strategies Cautiously
One strategy for mitigating corrosion between metals is to insulate the metals from each other so that the electrochemical reaction can not take place. While this strategy is theoretically sound, in practice it must be approached with caution.

For example, consider a copper roof fastened with galvanized steel anchor clips. This is not a recommended assembly since the galvanized steel is anodic to the copper. Given the relatively small surface area of anchor clips in relation to roof metal, the anchor clips are expected to corrode rapidly. One way to attempt to overcome this problem might be to apply a non-conductive coating, such as asphalt mastic, to the anchor clips, thereby preventing electrical contact between the two metals. However, in practice, it must be assumed that some gaps will appear in the coating, either due to imperfect application or due to wear and tear over time as the metals expand and contract. In either case, where gaps occur, the galvanic reaction will proceed. Furthermore, given the even smaller area ratio between just these exposed areas on the anodes and the larger area of cathodic roof metal, the reaction will proceed in these areas at an even more accelerated rate than it would have otherwise.

In other words, applying insulating coatings to only the anode in a galvanic pair is strongly discouraged, as it may actually increase the risk of corrosion. When insulating coatings are used to prevent electrical conduction between galvanic pairs, the cathode should always be coated, whether the anode is coated or not.

As another example, consider a black rubber washer between a fastener and roof of different metals. If the washer contains a high percentage of carbon black (used to color the washer), it may be sufficiently electrically conductive to allow the galvanic reaction to proceed between the two metals. Furthermore, even with an insulating washer, the two metals remain in contact where the fastener penetrates the roof sheet, and the galvanic reaction can still proceed through this juncture.

Conclusion
The galvanic series is a powerful tool for evaluating the potential risk of corrosion between metals. However, to be used effectively, it must be applied knowledgeably and with consideration of factors that may influence the risk of corrosion between dissimilar metals. Wherever possible, past experience and local knowledge should be included in these considerations.

This is the second article in an occasional series on architectural metals.
Stainless Steel and Corrosion Resistance
Dissimilar Metals And The Galvanic Series (this article)
Next: Selecting Metallic Fasteners

For more information:
ASTM International's ASTM G 82 Standard Guide for Development and Use of a Galvanic Series for Predicting Galvanic Corrosion Performance includes the galvanic series, guidelines for its interpretation, and information on the deriviation of electrical potential values.
Corrosion Doctors and CorrosionSource.com web sites provide extensive information on many aspects of the corrosion of metals. See for example The Galvanic Series, Introduction To Design and Corrosion, Prevention of Galvanic Corrosion By Design, and Galvanic Compatibility
Department of Defense, Military Specification Finishes for Ground Based Electronic Equipment, MIL-F-14072D(ER) provides some useful guidelines and technical details for working with the galvanic series.