Galvanic corrosion is an electrochemical deterioration process that occurs when two different metals are in direct electrical contact while submerged in a conductive liquid, known as an electrolyte. This phenomenon acts as an unintentional battery, where the difference in electrical potential drives a current. This current causes the accelerated degradation of one metal, which sacrifices itself to protect the other. It is a frequent and costly engineering problem, threatening the structural integrity of marine vessels, infrastructure, and plumbing systems. The Galvanic Series chart is the primary tool used by engineers and designers to predict and manage this destructive process.
Understanding the Galvanic Series
The Galvanic Series is a ranked list of metals and metal alloys based on their electrical potential when measured in a specific environment, usually a standard electrolyte like seawater. This chart establishes the relative reactivity of various materials and provides a clear hierarchy of their tendencies to corrode. This ranking is not absolute and can change depending on the specific environment, which is why charts are often specified for conditions such as flowing seawater or a particular industrial process.
The metals on the chart are divided into two distinct ends. Materials at the top, or the “active” end, are known as anodes and are more susceptible to corrosion because they tend to lose electrons. Conversely, materials at the bottom, or the “noble” end, are known as cathodes and are more resistant to corrosion. When two materials are joined, the one closer to the active end will preferentially corrode to protect the noble material. The physical distance between any two materials on the series is a direct indicator of the potential for galvanic corrosion; the greater the separation, the higher the risk of rapid deterioration for the active metal.
Practical Steps for Reading and Interpreting the Chart
Step 1: Locate Materials and Determine Roles
To assess the risk of a metallic connection, the first step is to locate both materials on the Galvanic Series chart specific to the intended environment. For example, connecting aluminum and copper in a marine environment requires using a series calibrated for seawater. Once located, the relative positions immediately indicate which material will act as the anode (corrodes) and which will act as the cathode (protected). The metal positioned higher on the active end will always suffer accelerated material loss.
Step 2: Determine Potential Difference
The second step involves determining the potential difference between the two materials, often listed in volts (V). A larger numerical difference signifies a stronger driving force for the electrochemical reaction and a higher corrosion rate for the anodic metal. This potential difference is the most direct measure of the severity of the galvanic reaction.
Step 3: Apply the Safe Zone Rule
A “safe zone” rule is often applied to manage risk. For metals exposed to a mild environment, the difference in potential should ideally be kept below 0.25V to minimize significant corrosion. In harsh environments, such as those with high humidity, saltwater, or industrial pollutants, the acceptable difference shrinks dramatically. This conservative threshold often does not exceed 0.15V, and staying within these tolerances is crucial for ensuring the long-term durability of the joined materials.
Beyond the Chart: Environmental and Design Factors
While the Galvanic Series provides a fundamental prediction, it does not account for real-world factors that modify the corrosion rate. The nature of the electrolyte is a dominant variable, as its conductivity directly influences the speed of the electrochemical reaction. For example, the high salt content in seawater creates a highly conductive medium that dramatically increases the reaction rate compared to less conductive media like pure fresh water or dry air.
A second factor is the surface area ratio of the coupled metals. If a small piece of the active metal (anode) is connected to a very large piece of the noble metal (cathode), the corrosion of the anode will be severely concentrated and rapid. This unfavorable ratio accelerates the breakdown of the smaller anodic component, such as an aluminum sheet held by a large stainless steel fastener, by focusing the entire corrosive current onto the tiny anode area.
The operating temperature of the system is also a significant modifier. Electrochemical reactions proceed faster as the temperature increases. Higher temperatures enhance the mobility of ions and increase the conductivity of the electrolyte. These variables explain why metals deemed acceptable in a controlled laboratory setting may fail rapidly when deployed in a hot, humid, or salt-laden outdoor environment.
Strategies for Preventing Galvanic Corrosion
The following strategies are used to mitigate galvanic corrosion:
- Careful material selection involves choosing metal pairs that are close neighbors on the Galvanic Series, ensuring a minimal potential difference and keeping the reaction rate low. Utilizing metals from the same family or those with similar electrochemical properties is a foundational step in any durable design.
- When dissimilar metals must be used, the electrical connection between them should be broken with an insulating barrier. Non-conductive materials like plastic washers, sleeves, or gaskets physically interrupt the flow of electrons, preventing the formation of the galvanic cell.
- Applying protective coatings can manage the corrosion risk, but they must be carefully applied, ideally to the more noble (cathodic) metal. Coating the cathode reduces its exposed surface area, preventing the destructive situation where a small anode is coupled with a large cathode.
- A final strategy is the use of sacrificial anodes, which intentionally introduce a highly active metal like zinc or magnesium into the system. This metal becomes the new, preferred anode, diverting the corrosive current entirely to protect the primary structure, a method known as cathodic protection.