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14  Galvanic Series

 

When Corrosion?

 

Knowing the galvanic series is important when designing interconnections between different metals (e.g. for bonding of shields). If you don't take care of which two different metals you use, the interconnection may corrode within a short time (and the shielding effect is not given anymore). The galvanic series helps us to choose the right combination of metals for an interconnection, in terms of corrosion.

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When Does Corrosion Happen?

Why does corrosion happen and what are the recommendations for connections of dissimilar metals:

  • Corrosion. The less noble metal (anode) of the interconnection of two metals experiences galvanic corrosion if:

    1. The two metals have a galvanic incompatibility (voltage difference to high). A difference of hundreds of millivolts is likely to result in galvanic corrosion, but only a few tens of millivolts are unlikely to be a problem.

    2. An electrolyte (e.g., water, moisture) is present.

    3. The two metals have an electrical conducting connection.

  • Rate of corrosion. The rate of corrosion depends on the moisture of the environment, how far apart the metals in the galvanic series are (further apart leads to faster corrosion, because the ion transfer is faster), and other parameters like the type of electrolyte (pH, concentration, flow rate), temperature (rate of corrosion increased with higher temperatures), degree of aeration, humidity, pressure and even the geometry of the interconnection.

  • Recommendations. The galvanic series helps us choose the right combination of metals for interconnection in terms of corrosion. Here is a rule-of-thumb on how to choose metals depending on their potential difference with respect to the environment [14.4]:​

    • Harsh (outdoor, high humidity, salt-laden, military). Choose metals where the electrode potential difference is ≤0.15V.

    • Normal (non-temperature/humidity controlled, consumer product, indoor). Choose metals where the electrode potential difference is ≤0.3V.

    • Controlled (temperature/humidity controlled, indoor). Choose metals where the electrode potential difference is ≤0.5V.

 

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Corrosion Prevention.

Three methods for corrosion prevention of dissimilar metal connections:

  • Isolation. If no electrically conductive connection is necessary, put an insulator between the dissimilar metals to avoid direct contact.

  • Low potential difference. If an electrically conductive connection between dissimilar metals is a must, the dissimilar metals should have a low voltage difference in the galvanic series.

  • Keep interconnection dry. Another option for preventing corrosion is to keep the interconnection between the dissimilar metals dry, e.g., with a coating (no electrolyte = no corrosion).

Corrosion Prevention

 

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Standard Electrode Potential Series.

The table below shows standard electrode potentials (E) for various metals [14.5]. The standard electrode potential is defined by measuring the potential relative to a standard hydrogen electrode (SHE) using 1mol solution at 25°C at the pressure of 1atm. An electrode potential series can be derived for metals in any electrolyte solution. Be careful! The real-world corrosion rate depends on the solution conditions like: electrolyte concentration, pH, flow rate, aeration, temperature, humidity, and pressure. Therefore, it is common to use the seawater electrode potential rather than the standard hydrogen electrode potential.

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Galvanic series with standard electrode potentials
Standard Electrode Potential Series

 

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Seawater Galvanic Series.

The seawater galvanic series is often used to approximate the probable galvanic effects in other environments for which there are no data. For example: From the standard electrode potentials shown in the table above, it can be seen that aluminum (Al) should behave anodically toward zinc (Zn) and presumably would retard the corrosion of zinc in a usual coupled situation. However, the reverse is true for seawater, as can be seen from the established galvanic series of metals in seawater in the table below.

In the table below, metals are grouped [14.1][14.2]. All metals, alloys, and platings of the same group have similar electro-motive forces (EMF) within 0.05V when coupled with a saturated calomel electrode in seawater at room temperature. All members of a group, regardless of metallurgical similarity or dissimilarity, are considered compatible. Compatible couples between groups have been specified in the table (green areas) based on a potential difference of 0.25V maximum.

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Galvanic series. Seawater electrode potentials.

 

In the table below, the galvanic series of selected metals in seawater is presented [14.3]. This series can be used as a reference to minimize galvanic corrosion when selecting metals that will be in direct contact. Generally said, the closer the metals in the series, the less galvanic corrosion is expected. In a galvanic couple, the metal higher in the series represents the anode and will corrode preferentially in the environment to the cathode, which is lower in the series.

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Seawater Galvanic Series
Galvanic series of selected metals in seawater
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