2.3 Galvanic Series & Electromotive Force (EMF) Series

Galvanic Series & Electromotive Force (EMF) Series

Predicting Galvanic Corrosion

When two different metals are electrically connected and immersed in a common electrolyte, a galvanic corrosion cell is formed. One metal will become the anode and corrode faster than it would alone, while the other will become the cathode and corrode slower (or not at all). To predict which metal will act as the anode and which will act as the cathode, corrosion professionals rely on organized lists of metals based on their electrochemical activity. The two most common lists are the Electromotive Force (EMF) Series and the Galvanic Series.

Understanding the difference between these two series and how to apply them is a core competency for CP personnel, as the principles of the Galvanic Series govern the operation of galvanic (sacrificial) anode cathodic protection systems.

The Electromotive Force (EMF) Series

The EMF Series is a theoretical, thermodynamically derived list of pure elements arranged according to their standard electrode potentials.

To create the EMF Series, scientists measure the potential of a pure metal in a solution containing its own ions at an activity of 1.0 (a very specific, standard concentration), at a standard temperature of 25°C. These potentials are measured relative to a Standard Hydrogen Electrode (SHE), which is arbitrarily assigned a potential of 0.000 Volts.

In the EMF Series:

• Elements with a more negative standard potential (e.g., Magnesium, Zinc) are considered more active or anodic.
• Elements with a more positive standard potential (e.g., Gold, Platinum) are considered more noble or cathodic.
• If two pure elements are coupled under these standard conditions, the one with the more negative potential will be the anode.

While the EMF Series provides the fundamental thermodynamic basis for corrosion, it has limited practical application in the field. This is because:

1. Engineers rarely use pure metals; they use alloys (like carbon steel, stainless steel, brass).
2. Real-world electrolytes (soil, seawater) are rarely standard solutions of the metal's own ions at unit activity.
3. Surface conditions, such as oxide films and passive layers, significantly alter a metal's potential in the real world.

The Galvanic Series

To address the limitations of the EMF Series, practical corrosion engineers developed the Galvanic Series.

The Galvanic Series is an empirical list of metals and commercial alloys arranged according to their measured corrosion potentials in a specific, real-world electrolyte. The most common and widely published Galvanic Series is based on flowing seawater at ambient temperature.

In a Galvanic Series (such as the one for seawater):

• The metals at the top of the list (most negative potentials) are the most active and will act as anodes.
• The metals at the bottom of the list (most positive potentials) are the most noble and will act as cathodes.
• When any two metals on the list are electrically connected in that specific electrolyte, the one higher on the list will become the anode and corrode, protecting the one lower on the list.

Key Differences from the EMF Series

• Alloys are Included: The Galvanic Series includes common construction materials like Type 304 Stainless Steel, Monel, and various bronzes, which do not appear on the EMF Series.
• Environment Specific: There is no single Galvanic Series. There is a Galvanic Series for seawater, one for brackish water, one for specific soils, etc. A metal's relative position can change depending on the environment.
• Active/Passive States: Some alloys, notably stainless steels, can exhibit two very different potentials depending on whether their protective oxide film is intact (Passive) or broken down (Active). The Galvanic Series often lists these alloys twice to reflect these two states.

Using the Galvanic Series in Practice

The Galvanic Series is an invaluable tool for preventing galvanic corrosion and for designing cathodic protection systems.

1. Avoiding Galvanic Corrosion

When designing a system, engineers use the Galvanic Series to select materials that are close to each other on the list. The closer two metals are in the series, the smaller the potential difference between them, and the weaker the galvanic corrosion cell will be.

For example, if you must connect a brass valve to a pipeline, connecting it to a copper pipe (close in the series) is much safer than connecting it directly to a carbon steel pipe (far apart in the series). If widely separated metals must be used, they should be electrically isolated using dielectric insulating kits to break the metallic path of the corrosion cell.

2. Designing Cathodic Protection Systems

The principles of the Galvanic Series are the foundation of galvanic (sacrificial) anode CP systems. To protect a carbon steel pipeline, we must connect it to a metal that is significantly more active (higher) on the Galvanic Series.

Looking at a typical Galvanic Series for soils or seawater:

1. Magnesium (Mg)
2. Zinc (Zn)
3. Aluminum (Al) Alloys
4. Carbon Steel
5. Copper (Cu)

Because Magnesium, Zinc, and Aluminum are higher on the list than Carbon Steel, they will act as anodes when connected to the steel. They will sacrifice themselves (corrode) to supply electrons to the steel, forcing the steel to become the cathode. This is why we use Mg, Zn, or Al as sacrificial anodes to protect carbon steel structures. If we mistakenly connected copper to the steel, the steel would be higher on the list, becoming the anode and corroding faster to protect the copper.

When reviewing a Galvanic Series chart, the distance between two metals indicates the potential driving voltage. A larger separation means a higher driving voltage and a potentially higher galvanic corrosion rate if the two metals are coupled.