Understanding Galvanic Corrosion: Causes and Effects
Galvanic corrosion is also called bimetallic corrosion or dissimilar metallic corrosion, which occurs in several industries, notably the painting and coating industry. The electrolyte used may be in the form of water between two dissimilar metals where corrosion occurs by the electrochemical reaction between them, causing corrosion in one metal than in the other.
How does galvanic corrosion happen?
The main source of galvanic corrosion is the flow of electric current between two different metals. When exposed to an electrolyte, either water or moisture in the air, and with these metals being in electrical contact, it becomes an electrochemical cell. This cell has a metal that will corrode, or be the anode, and another metal that won’t corrode, or will be the cathode. The corrosion is driven by the potential difference between the two metals.
For galvanic corrosion to occur, three conditions must be present:
- Two dissimilar metals must be in contact with each other.
- They need to be in contact with an electrolyte like water, salt, or moisture.
- There should be an electrical contact between the two metals.
What is steel?
Steel is one of the most used construction, machinery, and other industries used materials. On the other hand, steel lacks resistance against corrosive substances in the environment-a fact of being affected by moisture, salt, or even air, leading to corrosion demonstrated as red rust formed when steel reacts with oxygen and water.
Steel usually gets protected from corrosion through coating with zinc. The corresponding steel, due to the protective layer formed by zinc, remains free from corrosion. This process is known as galvanization, which not only protects the steel from rusting but also retracts it from galvanic corrosion against other metals.
What is corrosion?
Corrosion is a chemical reaction in an environmental interaction with metals that causes gradual degradation of the metals. Generally, corrosion involves oxidation; where the metals lose their electrons to agents in their environment, such as water, oxygen, and acids. The two stages in which corrosion occurs are: First, the making of an electrical circuit between the metal and the electrolyte then, the chemical reaction that causes the breakdown of the metal.
In galvanic corrosion, an accelerated chemical reaction takes place, in which a flow of electric current between the two metals serves to speed up corrosion. This simply means that anodic metal, easily corroded compared with the other less anodic, or rather cathodic metal, will naturally deteriorate before the latter one does.
What Causes Galvanic Corrosion?
The main cause of galvanic corrosion is a difference in electrochemical properties of metals. This is because, depending on their electrochemical properties, all metals are more reactive as compared to others. For instance, there is the likelihood of the metals, such as aluminum or zinc, corroding as compared to metals, such as stainless steel or copper.
When these dissimilar metals are in contact, the more reactive metal loses electrons, which flow through the electrical connection to the less reactive metal. This process accelerates the corrosion of the anode. Environmental factors such as moisture, saltwater, and temperature can further speed up this process.
Galvanic Corrosion Examples
- Aluminum and Steel: The most common example of galvanic corrosion is the association of aluminum and steel. Aluminum being more reactive is the anode and corrodes faster in the presence of steel, the cathode. It is commonly found in construction when steel fasteners or bolts are used with aluminum panels or frames.
- Copper and Steel: A perfect example is that of copper with steel. Copper is noble compared to steel because it is corrosion resistant. In contrast, the latter rusts. If such a mixture gets in contact with water, the iron may corrode considerably quickly, especially around the contact points.
- Zinc and Copper: Zinc, primarily used as an overlay of steel, can also suffer galvanic corrosion from copper. Some plumbing systems clearly show this example where pipes containing zinc-coated are coupled together with pipes carrying copper.
How to Prevent Galvanic Corrosion?
Galvanic corrosion can be prevented by taking several preventive measures:
Insulating Materials:
The most effective way to prevent galvanic corrosion is by using insulating materials between the dissimilar metals. This prevents the metals from making electrical contact, breaking the electrical circuit, and stopping the flow of current. For example, rubber washers or plastic spacers can be used to separate the metals.
Choosing Metals Wisely:
The difference can be minimized further by choosing two metals that appear close to each other in the galvanic series. The lesser the distance on the series between the two metals chosen, the smaller the chance is of galvanic corrosion from contact.
Avoid Small Anodes and Large Cathodes:
Sometimes, galvanic corrosion is made worse by having a small anode and a large cathode area. For instance, attaching a small steel part to a large copper surface makes the steel corrode faster. One should pay attention to the size and area of the metals to reduce corrosion risk.
Coating Metals:
Protecting metals by adding layers of paint or galvanizing may prevent corrosion to a certain extent. This is particularly useful in cases where the dissimilar metals cannot be separated entirely.
Routine Maintenance and Inspection:
Equipment and infrastructure should be checked regularly; part of this will have alerts to certain signs of corrosion, allowing for early repairs and replacement.
Conclusion
Galvanic corrosion is a very serious problem affecting many industries, especially in the construction, marine, and manufacturing sectors. To ensure the durability and safety of metal structures and equipment, one needs to know how it happens, why it happens, and how to prevent it. It can be prevented by using insulating materials, selecting compatible metals, and applying protective coatings.
FAQs on Galvanic Corrosion
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What is galvanic corrosion?
Galvanic corrosion occurs when two dissimilar metals come into electrical contact in the presence of an electrolyte. The more anodic metal corrodes faster due to the flow of electric current between the metals.
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How does galvanic corrosion happen?
It occurs in the presence of a potential difference between two metals. The current flows from the anode, which is the more reactive metal, to the cathode, the less reactive metal, thereby accelerating corrosion at the anode.
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What are some examples of galvanic corrosion?
Examples include:
- Aluminum and steel where aluminum corrodes
- Copper and steel where steel corrodes
- Zinc and copper where zinc corrodes
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How do you prevent galvanic corrosion?
Prevention: Preventive measures are the application of insulating materials, use of metals close together in the galvanic series, avoiding small anodes and large cathodes, and application of protective coatings on metals.
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What is the function of zinc as a preventive of galvanic corrosion?
Zinc is frequently applied as a coating for steel as a sacrificial protection where the zinc corrodes first, and therefore the steel remains free from rusting.
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