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Galvanic Corrosion: A Comprehensive Analysis
Galvanic corrosion, also called bimetallic or dissimilar metal corrosion, is an electrochemical process that occurs when two different metals come into contact with each other in an electrolyte, like saltwater or moisture.
Galvanic corrosion seriously threatens the structural integrity and functionality of metal components, affecting various sectors. This type of corrosion happens when dissimilar metals are in close proximity to each other, such as in pipelines, bridges, boats, and electrical connections. The electrolytic environment in these situations further accelerates the corrosive process.
This article offers an overview of essential concepts in galvanic corrosion and how to mitigate its effects using preventative chromium coating solutions.
Understanding Galvanic Corrosion
Galvanic corrosion is an electrochemical process where different metals or alloys in direct contact with each other create a corrosion potential when they are exposed to an electrolyte, such as water.
This phenomenon, known as dissimilar metal corrosion, allows the metal that is more resistant to oxidation (the noble metal) to serve as a cathode for the other metal (the active metal).
Meanwhile, galvanic corrosion, like fire, occurs only when all those conditions are met:
- The various metals exhibit different corrosion potentials.
- There is direct contact between the metal surfaces.
- A conductive electrolyte consistently connects the two metals.
- If any of these conditions are absent, galvanic corrosion will not occur.
Some Metals Commonly Subjected
Galvanic Series
Explanation of the Galvanic Series
The galvanic series ranks metals according to their reactivity or nobility in a specific electrolyte. Metals that are higher on the series are less likely to corrode, meaning they are more noble. In contrast, lower metals on the series are more susceptible to corrosion, making them less noble.
Importance of Relative Nobility in Predicting Corrosion
The relative nobility of metals is essential in assessing the risk of galvanic corrosion. When two different metals are connected, the less noble metal (which is lower on the galvanic series) typically serves as the anode and will corrode. In contrast, the more noble metal serves as the cathode and keeps it protected.
Effects of Anode/Cathode Area Ratio
The ratio of anode’s surface area to cathode’s surface area can influence the galvanic corrosion rate. The large anode area relative to the cathode can accelerate anode corrosion. This is because a larger anode surface can offer more sites for oxidation to happen while the cathode maintains relative protection.
Avoiding large anode-to-cathode area ratios is often recommended to minimize galvanic corrosion.
Galvanic Corrosion Chart
Table 1: The table below lists metals in the galvanic series, ordered from most reactive to least reactive.
| Metal | Galvanic Series Position | Potential (V) |
| Magnesium | Most Anodic (Most Active) | -1.75 |
| Zinc | -1.1 | |
| Aluminum Alloys | -0.90 to -1.00 | |
| Cadmium | -0.75 | |
| Mild Steel | -0.68 | |
| Cast Iron | -0.6 | |
| Chromium Steel (Active) | -0.56 | |
| Lead | -0.55 | |
| Tin | -0.49 | |
| Nickel (Active) | -0.44 | |
| Brass | -0.4 | |
| Copper | -0.34 | |
| Bronze | -0.3 | |
| Chromium Steel (Passive) | -0.2 | |
| Nickel (Passive) | -0.15 | |
| Silver | -0.15 | |
| Titanium | -0.1 | |
| Gold | -0.05 | |
| Platinum | Most Cathodic (Least Active) | 0 |
Factors Influencing Galvanic Corrosion
Several factors affect the rate and intensity of galvanic corrosion, including:
- Distance between metals: A shorter distance between metals in a galvanic pair can increase the corrosion rate due to lower electrical path resistance.
- Metal area: If the anode’s area is significantly smaller than the cathode’s area, corrosion occurs more rapidly due to the increased current density at the anode.
- Nature of metals: The difference in standard electrode potentials of metals is crucial. A more enormous difference increases the risk of accelerated corrosion at the anode.
- Electrolyte conditions: Factors such as temperature, PH, salt concentration, and oxygen presence influence the conductivity of the electrolyte, which in turn affects the corrosion rate.
Common Examples of Galvanic Corrosion
Galvanic corrosion poses significant challenges across various industries, often resulting in material failures and, in some situations, catastrophic consequences. Common examples of galvanic corrosion are listed below, along with historical instances and applicable preventative measures：
Marine Environments
Saltwater is one of the most corrosive environments, creating good conditions for galvanic corrosion. In marine applications, dissimilar metals—such as aluminum hulls and stainless steel fittings—often come into contact with each other, which can lead to accelerated galvanic reactions due to the electrolyte-rich environment. This is a significant concern for the marine industry, as propellers, propellers, and submerged components are particularly vulnerable to metal corrosion in saltwater.
- Historical Example: In 1963, the sinking of the U.S.S. Thresher demonstrated the vulnerabilities of submarines to dissimilar metal corrosion. Investigations uncovered that contact corrosion appeared in piping systems where various metals interacted under high-pressure saltwater conditions.
- Preventative Measure: Coatings such as Armoloy Thin Dense Chrome (TDC) provide a protective barrier that prevents the electrolyte from contacting the base metal. This reduces the risk of bimetallic corrosion and extends the lifespan of marine components.
Medical Equipment
Medical devices and surgical instruments often incorporate titanium, stainless steel, and other metals to fulfill specific functional requirements. Meanwhile, galvanic corrosion presents a significant challenge in the medical field, as these devices are often exposed to saline solutions, moisture, or blood, which create an electrolyte-rich environment. Bimetallic corrosion can lead to equipment failure, significantly impacting patient safety.
- Historical Example: In the 1990s, corrosion at the connections between stainless steel and titanium in orthopedic implants caused failures that necessitated revision surgeries. This highlighted the need for improved materials and coatings to stop metal corrosion in medical devices.
- Preventative Measure: Armoloy TDC coatings are biocompatible and effective at preventing galvanic reactions in medical equipment, guaranteeing safety and long-term functionality.
Electronics
When different metal alloys come into contact with each other in moisture or humidity within electrical systems, galvanic corrosion can happen. This type of corrosion can compromise the functionality and lifespan of electronic components, resulting in reduced conductivity, increased resistance, and even absolute failure of the device.
Manufacturers use various techniques, including coatings, to reduce galvanic corrosion in electronics to stop degradation and enhance corrosion resistance in electro-galvanized iron wire and other electrical components.
Plumbing Systems
In both residential and commercial construction, galvanic corrosion often occurs at the junctions where copper pipes connect with galvanized steel fittings. Water acts as an electrolyte, which accelerates the corrosion of the metals and leads to material degradation over time. This type of corrosion is a notable challenge in infrastructure projects, primarily when mixed materials are used.
- Historical Example: During the 1950s and 60s, multi-family housing developments experienced significant plumbing issues because of bimetallic corrosion occurring at copper-steel connections. These issues led to leaks and costly repairs.
- Preventative Measure: Dielectric unions and protective coatings can reduce galvanic action by splitting dissimilar metals. Using a galvanic corrosion table during the design phase guarantees better material selection to stop corrosion.
Renewable Energy Systems
The renewable energy systems, like wind turbines and solar panels, utilize various materials, including metals like steel and aluminum, to transmit energy efficiently. These systems are typically designed for outdoor environments that are exposed to moisture and fluctuating weather conditions. Therefore, galvanic corrosion can be a significant concern.
For instance, aluminum frames supporting solar panels can come into contact with steel mounting hardware, leading to galvanic corrosion of the solar panel hardware(such as Solar Triangle Connector). Over time, this corrosion can compromise the panels’ structural integrity and the ability to transmit energy efficiently.
Automotive and Aerospace Applications
In the automotive and aerospace industries, galvanic corrosion poses significant challenges due to the use of various material combinations in harsh environments. Aircraft and automobiles frequently combine aluminum with carbon fiber composites to reduce weight. However, this pairing creates a high-risk scenario for galvanic action, especially when moisture infiltrates the interfaces. This moisture can initiate corrosion and compromise the structural integrity of these components.
- Historical Example: The incident involving Aloha Airlines Flight 243 in 1988 revealed vulnerabilities where corrosion from dissimilar metals contributed to the deterioration of the fuselage over time.
- Preventative Measure: High-performance coatings such as Armoloy TDC provide a strong barrier against electrolytes, reducing the galvanic effect and prolonging the lifespan of essential aerospace and automotive components.
Transportation Infrastructure
Bridges, railways, and other transportation infrastructures frequently involve connections between different types of metals, such as steel and aluminum. Coastal and humid environments can worsen contact corrosion, especially when the materials are exposed to moisture. In these conditions, the galvanic effect is intensified, particularly when the anodic material has a smaller surface area.
- Historical Example: In 1967, the collapse of the Silver Bridge, primarily caused by stress corrosion, emphasized the necessity of addressing galvanic corrosion risks in structural connections.
- Preventative Measure: Applying protective coatings such as Armoloy TDC to fasteners and joints can decrease galvanic action and enhance the durability of infrastructure. Engineers can also utilize the anodic index to ensure material compatibility.
Oil and Gas Industry
Metals can experience uniform or galvanic corrosion in the oil and gas fields, especially in pipelines, storage tanks, and both sour and sweet well conditions. This corrosion occurs when high nickel alloys, carbon steels, and other metal alloys come into contact with electrolytes like moisture, crude oil, or saltwater. Galvanic corrosion during oil and gas production can result in pipeline leaks, equipment failures, and expensive maintenance issues.
Monuments and Sculptures
Outdoor sculptures and monuments can experience contact corrosion when stainless steel fasteners are used with bronze or copper materials. Exposure to atmospheric moisture and pollutants intensifies this galvanic effect, leading to discoloration and potential structural damage. This issue is common in both construction and restoration projects.
- Historical Example: The Statue of Liberty underwent significant renovations in the 1980s to fix bimetallic corrosion between its iron armature and copper skin, which had worsened over the decades.
- Preventative Measure: Utilizing insulated fasteners and applying protective coatings can help prevent galvanic corrosion, thereby preserving monuments’ aesthetic and structural integrity for future epochs.
The Importance of Understanding Galvanic Corrosion
Understanding galvanic corrosion is essential for preventing damage to metal structures and guaranteeing their long-term durability. Ignoring galvanic corrosion can lead to significant problems:
- Equipment Malfunction: Corrosion can negatively affect the performance of machinery and equipment.
- Structural Failure: Weak metal can lead to structural failure, which poses safety risks.
- Environmental Impacts: Corrosion can release harmful pollutants into the environment.
- Increased Maintenance Costs: Repairing or replacing corroded components can be costly.
How to Prevent Galvanic Corrosion
Several strategies can be applied early in the engineering design process to reduce the harmful effects of galvanic corrosion:
- Material Selection: Choose metals that are close together in the galvanic series whenever possible.
- Control Electrolyte Composition: Modify the composition or apply inhibitors to lower corrosion rates.
- Cathodic Protection: Introduce an external anode, which will cause the metal to function as a cathode.
- Design Considerations: To reduce corrosion rates, make sure that the anodic areas are larger than the cathodic areas.
- Coatings: Apply protective coatings to safeguard metal surfaces from environmental elements and prevent electrolytes from contacting them.
- Regular Inspections: Identify the early signs of corrosion and use techniques to evaluate the severity of the corrosion.
Conclusion
In conclusion, galvanic corrosion poses a significant challenge in materials and engineering because it can cause the degradation of metals that come into contact with each other in the presence of an electrolyte. This article has explored the causes and influences of galvanic corrosion. While it can lead to unsightly damage, galvanic corrosion can also be utilized for beneficial purposes, such as using sacrificial anodes to protect more noble metals. However, if not managed appropriately, galvanic corrosion can damage the structural integrity of objects.
Prevention is key, whether it involves selecting compatible metals, utilizing insulating materials, or operating corrosion inhibitors. Finally, understanding and effectively addressing galvanic corrosion is important for keeping the longevity and performance of diverse metal components and equipment.
