Galvanic corrosion, also known as bimetallic corrosion or dissimilar metal corrosion, is an electrochemical process that occurs when two different metals are in contact with each other in the presence of an electrolyte, such as saltwater or moisture.
Present across a variety of sectors, including industrial, marine, and automotive, galvanic corrosion poses a significant risk to the structural integrity and functionality of metal structures, components, and devices. Where dissimilar metals are in proximity — pipelines, boats, bridges, and electrical connections — the electrolytic environment exacerbates the corrosive process.
One common scenario is in marine environments, where ships and boats are constructed using various metals, such as steel hulls with bronze or stainless steel fittings. The seawater acts as the electrolyte, facilitating galvanic corrosion between these different metals, particularly in areas where they are in direct contact or proximity. Another prevalent scenario occurs in plumbing systems, where copper pipes are connected to brass or galvanized steel fittings. In the presence of water or moisture, galvanic corrosion can occur at these junctions, compromising the integrity of the plumbing network over time.
In this post, we’ll provide an overview of the key concepts involved in galvanic corrosion and how to mitigate the effects with preventative chromium coating solutions.
What is Galvanic Corrosion: Key Concepts
Galvanic corrosion stems from the fundamental principle of electrochemical potential difference, where dissimilar metals in contact with an electrolyte undergo a spontaneous redox reaction. This reaction occurs due to the variation in the metals’ tendencies to lose or gain electrons, known as their electrochemical potentials. Understanding galvanic corrosion depends on two essential concepts: the galvanic series and the area effect.
Galvanic Series
The galvanic series is a list of metals in order of decreasing electrochemical potential, and it describes whether one metal is anodic or cathodic to another metal in a particular environment. That last part is important; these relationships are only true in the specific environment used for testing. The most common galvanic series to see in print is for seawater, but many others are available.
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- When two metals with different electrochemical potentials come into contact, an electrical potential difference is established between them. The metal with a higher tendency to lose electrons (the more “active” metal) becomes the anode, undergoing oxidation and releasing electrons.
- Conversely, the metal with a lower tendency to lose electrons (the more “noble” or “passive” metal) becomes the cathode, accepting electrons and undergoing reduction. This flow of electrons constitutes an electrical current, leading to the corrosion of the anodic metal and the protection of the cathodic metal.
- Noble metals, such as gold, platinum, and stainless steel, exhibit high corrosion resistance and typically serve as cathodes in galvanic corrosion scenarios. In contrast, less noble metals, such as aluminum, zinc, and magnesium, are more prone to corrosion and act as anodes.
We consider two metals compatible if their emf values fall within 0.25V of each other on the galvanic series. If their difference exceeds 0.25V, we expect the metal with the higher Anodic Index, the more anodic metal, to corrode.
Anodic Index Chart for Metal Groups
Figure 1: The galvanic series for selected alloys in seawater at 25°C (77°F)
Surface Area Effect
The relative surface area of the anode to the cathode impacts galvanic corrosion rates. The surface area effect tells us that if the area of the anode is much smaller than that of the cathode, then corrosion of the anode will be much worse. If the area of the anode is much larger, corrosion will be less severe.
An unfavorable ratio of cathode area to anode area could make couples that we normally expect to work fine incompatible. This becomes especially important when we discuss metallic coatings for corrosion.
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- Armoloy TDC works against corrosion by functioning as a barrier coating, preventing the environment (electrolyte) from contacting the base metal.
- Its ability to prevent corrosion depends entirely on the coating being free from pinholes and other defects the expose the base metal.
A Visual Perspective
Consider the case illustrated in Figure 2 (below), where Armoloy TDC is applied to a steel substrate. A pinhole creates a situation where we have a very large surface area of TDC and a very small surface area of steel exposed to the environment. Looking at the galvanic series in Figure 1, we see that TDC has an emf of 0.5V and steel an emf of 0.75V.
Under normal circumstances, these two are 0.25V apart and should have moderately good compatibility. With the unfavorable area ratio though, the exposed steel will quickly corrode. Conversely, the area effect would work in our favor when TDC is applied to metals that appear higher in the galvanic series like brass and bronze.
Figure 2: Illustration of the surface area effect as it applies to galvanic corrosion. A large cathode-to-anode ratio will result in accelerated corrosion.
Figure 3: Illustration of how pinholes in the coating can create an unfavorable ratio of cathode area to anode area. Here, the area of exposed steel is very small (bottom of the pinhole) while the area of exposed TDC is much larger.
Additional Factors Influencing Galvanic Corrosion
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- Conductivity: Metals with higher conductivity facilitate the flow of electrons in the galvanic cell, accelerating the corrosion process.
- Electrolyte composition: The composition of the electrolyte, including pH level, salinity, and presence of other corrosive agents, significantly affects galvanic corrosion rates.
- Temperature: Higher temperatures generally increase the rate of corrosion reactions, including galvanic corrosion.
- Environmental conditions: Humidity, atmospheric pollutants, and exposure to environmental chemicals can also influence galvanic corrosion rates.
Galvanic Corrosion Effect on Materials
This type of corrosion poses a significant threat to materials whenever two dissimilar metals come into contact with an electrolyte. For instance, pairing aluminum with stainless steel, stainless steel with carbon or alloy steel, and copper with carbon or alloy steel can lead to galvanic corrosion. In these combinations, the difference in electrochemical potentials between the metals initiates a corrosive process, accelerating the degradation of the less noble metal.
The impact of galvanic corrosion extends beyond mere material degradation. It can severely compromise the structural integrity, aesthetics, and functionality of components or structures.
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- Structural Integrity: Galvanic corrosion can compromise the strength and stability of structural elements, leading to unexpected failures in critical infrastructures such as bridges, pipelines, and offshore platforms.
- Aesthetics: Galvanic corrosion can result in unsightly surface blemishes, discoloration, and pitting on metal surfaces, therefore affecting the visual appeal of architectural structures, automobiles, and consumer goods.
- Functionality: In functional applications, such as electrical connections or mechanical joints, galvanizing corrosion can impair performance and reliability.
Industries Impacted by Galvanic Corrosion
Galvanic corrosion is a significant concern across various industries.
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- Marine industry: Ships, boats, and offshore structures are exposed to saltwater. Galvanic corrosion affects hulls, propellers, and submerged components.
- Construction and infrastructure projects: Buildings, bridges, and utility systems, especially in coastal or industrial areas, experience galvanic corrosion due to environmental exposure.
- Automotive and Aerospace industry: Significant challenges in preventing galvanic corrosion in automotive and aerospace components are due to diverse material combinations and exposure to harsh operating conditions and demanding environments.
Preventive Measures to Minimize Galvanic Corrosion
Several strategies can be implemented early in the engineering design process to mitigate the damaging effects of galvanic corrosion:
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- Material Selection: Opt for metals close together in the galvanic series when possible.
- Cathodic Protection: Introduce an external anode thus making the metal act as a cathode.
- Coatings: Apply protective coatings like Armoloy thin dense chrome to shield metal surfaces from the environment and prevent the electrolyte from making contact.
- Control Electrolyte Composition: Alter the composition or use inhibitors to reduce corrosion rates.
- Design Considerations: To decrease corrosion rates, ensure anodic areas are larger than cathodic areas.
- Regular Inspections: Detect early signs of galvanic corrosion and employ techniques for assessing corrosion severity.
Working with Armoloy
With a focus on precision and reliability, Armoloy coatings are engineered to withstand harsh environments and provide long-lasting protection against corrosion. By combining expertise with innovation, we deliver tailored solutions that extend product and component lifespan.