Corrosion is an enemy to numerous industries like oil and gas, leading to massive financial losses, environmental degradation, and potential safety hazards. One common type of corrosion that warrants particular attention is galvanic or dissimilar metal corrosion. This occurs when two metals with different electrochemical potentials come into contact in a common environment. In this blog post, we delve into different types of corrosion but will focus on what galvanic corrosion is, the factors that influence it, and various prevention strategies. So, let’s arm ourselves with the knowledge to battle this industrially significant issue.
Understanding Different Types of Corrosion
Bimetallic corrosion performance
The severity of metal-based corrosion depends on the materials involved (see corrosion metal), relative surfaces, and the conductivity of the electrolyzed substance. In most situations, bimetallic corrosion is more intense in a submerged environment than in an atmosphere. In a climatic context, for example in an atmosphere a level of bimetallic corrosion can be low if the proportion of the pore area is low in anodic or cathodic materials; in addition, frequency and duration are reduced.
Pitting Corrosion
A common form of corrosion pitting is difficult to detect and prevent. It occurs when local anodic/cathodic points form a corrosion cell with the surrounding material. These pittings often create holes or gaps that penetrate material vertically down towards its base. During a corrosion attack, it can result from damage to the protective coating or rust or from non-uniformity. This hazardous corrosion form can result in structural breakdown in the absence of relatively minimal metal loss.
Intergranular Corrosion
Intergranular corrosion occurs at grain borders, where impurities form during solidification. Alternatively, it could happen through the decomposition or enrichment of the alloy at grain limits. It can cause damage to the metals despite being in close proximity to it affecting their mechanical property.
Uniform Corrosion
Uniform corrosion is the most common form of corrosion and is generally spread evenly on large surfaces of material. It occurs when materials are exposed to corrosive environments and can be very challenging to detect. Generally speaking, uniform corrosion is caused by a combination of factors including temperature and humidity, as well as the material’s chemical composition. In some cases, the presence of oxygen and other dissolved gases in water or air can also contribute to its development.
Galvanic Corrosion
Galvanic corrosion occurs when two metals with differing electrochemical potentials are physically or electrically connected in the presence of an electrolyte, like water with dissolved salts. In this setup, the metal with a higher corrosion potential becomes the cathode and resists corrosion, while the metal with a lower potential acts as the anode and corrodes. This type of corrosion is common in structures that use more than one type of metal, such as ships, pipelines, and buildings. It’s a widespread concern in various industries because it can lead to rapid deterioration of structural components.
Dissimilar Metal Corrosion
Dissimilar metal corrosion, often considered a subset of galvanic corrosion, specifically refers to the corrosion that occurs between two dissimilar materials/different kinds of metal or alloy, rather than between two metals of different electrochemical potentials. Even metals that individually resist corrosion well can corrode quickly when connected to a metal of a different t type. This is especially problematic in complex systems where various kinds of metals are used for different components. The risk of dissimilar metal corrosion is notably high in environments that are humid, salty, or acidic.
There are various types of corrosion. Both galvanic and dissimilar metal corrosion involve similar principles and often the same preventive measures, such as careful material selection, the use of protective coatings, and the employment of insulating barriers. However, the key difference lies in the types of metals involved: galvanic corrosion can occur between different pieces of the same metal if their electrochemical potentials differ, while dissimilar metal corrosion specifically involves two different types of metal or alloy.
Factors Influencing Galvanic Corrosion & Prevention Measures
Environment
The environment plays a critical role in determining the extent of galvanic corrosion, with factors like humidity, temperature, and chemical exposure all coming into play.
- Humidity and Moisture: A wet or humid environment serves as an electrolyte, facilitating the flow of electric current between the anode and cathode, which is essential for galvanic corrosion to occur. In dry conditions, the absence of an electrolyte halts this flow, thereby preventing galvanic corrosion.
- Temperature: Elevated temperatures can accelerate corrosion by increasing reaction rates. Conversely, colder temperatures slow down the electrochemical processes, reducing the rate of corrosion.
- Chemical Exposure: The presence of corrosive agents like acids, salts, or industrial chemicals can also significantly increase the risk. These substances can act as electrolytes and accelerate corrosion.
- Oxygen Levels: Higher levels of oxygen can make an environment more corrosive, particularly for metals like steel that form oxide layers.
It’s crucial to understand these elements when designing an anti-corrosion strategy. For instance, in a humid, saline environment, extra precautions like protective coatings and high-resistance materials may be necessary.
Distance
In galvanic corrosion, distance is more than just a matter of inches or centimeters. The closer the two metals are at the junction, the more aggressive the corrosion tends to be. This is due to the reduced resistance for the flow of ions and electric current, leading to accelerated reaction rates at the junction.
- Spatial Arrangement: If the metals are layered or placed so that the distance between them increases gradually, the rate of corrosion tends to diminish correspondingly.
- Localized Corrosion: A red flag for galvanic corrosion is the presence of localized corrosion near the metal junction. This is often the first visible symptom and a warning sign that prevention measures should be considered immediately.
Area of Section
The relative sizes of the anodic and cathodic areas significantly influence the corrosion process:
- Small Anodic Area: When the anodic area is small relative to the cathodic area, the anode corrodes at a faster rate. This is because the same amount of current will disperse over a smaller area, leading to a higher “corrosion current density.”
- Large Cathodic Area: A large cathodic area can also function like a “sink,” drawing more current from a smaller anode and exacerbating its degradation.
- Design Implications: The concept of anodic and cathodic areas is critical when considering sacrificial anodes, commonly used in marine environments to protect ship hulls and underwater structures. Here, the sacrificial anode (often made of zinc) is deliberately designed to be smaller, so it corrodes instead of the more valuable structure it’s protecting.
By deeply understanding the impact of environmental conditions, distance between metals, and relative sizes of anodic and cathodic areas, engineers and designers can better strategize to prevent or mitigate the destructive power of galvanic corrosion.
Strategies to Prevent Galvanic or Dissimilar Metal Corrosion
Material Selection
Choosing the right materials is the first line of defense against galvanic corrosion. Opt for metals that are close together on the galvanic series to reduce the potential difference, thus mitigating the extent of corrosion. If you must use dissimilar metals, consult with experts on the compatibility and electrochemical properties of the materials. Sophisticated tools like corrosion rate simulators can also provide predictive insights, allowing you to forecast the lifespan of different metal combinations in specific environments. Special alloys with improved corrosion resistance are sometimes worth the higher upfront cost, considering the potential long-term savings from reduced maintenance and replacements.
Avoid Metal Interconnections
In cases where you have the liberty to choose the materials involved in construction or fabrication, avoiding metal interconnections can save you from the headaches of galvanic corrosion later on. Try to design your project in a way that minimizes the direct contact between dissimilar metals. If such a design is not feasible, consider using insulating barriers like rubber or plastic gaskets between the two metals. These barriers can disrupt the electrochemical pathway, significantly reducing the corrosion rate.
Employ Protective Layers
Protective coatings serve as an additional barrier to halt or slow down galvanic corrosion. Types of coatings include paint, galvanized layers, or specialized anti-corrosion films. When applying a protective layer, focus on coating the cathode, as doing so will create a non-conductive barrier that disrupts the electrochemical cell formation. New advances in nanotechnology offer even more robust coating solutions, such as self-healing coatings that repair themselves when damaged, providing longer-lasting protection.
Consider Cross-Sectional Areas
When it comes to galvanic corrosion, size matters. A small anode coupled with a large cathode will lead to accelerated corrosion of the anode. Therefore, pay attention to the cross-sectional areas of your metals when designing your project. Aim to balance these areas, or, if that’s not possible, make the anode larger than the cathode. In doing so, you disperse the electrical current over a greater area, which can reduce the corrosion rate significantly.
Incorporate Inhibitors
Chemical inhibitors are substances that can be added to the environment to suppress the corrosive activity. These are often used in situations where mechanical changes are not feasible or as a supplement to other prevention methods. Some inhibitors work by forming a protective film on the metal surface, while others neutralize the acidity of the environment. However, it’s crucial to understand the specific conditions you’re dealing with, as using the wrong inhibitor could exacerbate the problem.
Well-Designed Replacements
Realistically, no prevention method will make your setup completely immune to galvanic corrosion. Therefore, make sure to design your system for easy maintenance and replacement of anodic parts. Use materials that are readily available so that repairs can be performed quickly to minimize downtime. Also, consider increasing the thickness of the anodic material to extend the operational life of the component. This might increase the initial investment but will pay off in the long run due to reduced maintenance and part replacement costs.
Preventing galvanic or dissimilar metal corrosion is not just good practice, it’s essential for safeguarding assets, the environment, and human life. The road to mitigation starts with a comprehensive understanding of influencing factors like environment, distance, and relative areas of anode and cathode. By carefully selecting materials, avoiding risky metal interconnections, applying protective layers, balancing cross-sectional areas, and using corrosion inhibitors, industries can significantly reduce the risks associated with severe corrosion.
If you’re dealing with corrosion issues or have questions about the best prevention methods, don’t hesitate to reach out to Pittsburgh Mineral & Environmental Technology (PMET). Our team of experts is always ready to provide you with solutions tailored to your specific needs. Contact PMET today for a consultation.