Dissimilar Metals and The Galvanic Series
The galvanic series is a list of metals arranged in order of their relative electrical potential. 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.
The above 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 comparison 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.