In applications involving conductive fluids such as tap water, seawater, ammonia and condensation from air and gases, safeguarding pressure instruments against galvanic corrosion is a critical concern. The corrosive nature of these systems can significantly compromise the functionality and longevity of your instrumentation, underscoring the importance of selecting the most appropriate options for your specific requirements.
Typically, people focus on media compatibility as it relates to the wetted materials of the individual components that will be used in a system such as pressure instruments. However, it is also critical to identify potential areas of galvanic corrosion throughout the entire system and only use materials that are corrosive-resistant and compatible with galvanic conditions.
Ashcroft has a long and successful history in pressure and temperature instrumentation, including those used in harsh applications such as saltwater and chemical environments. Read this article to better understand galvanic corrosion, where and how it occurs, and why media and material compatibility are so important when selecting the best pressure measurement solution for water-based applications.
You will also be directed to additional resources that can help you take the next step in your instrument-purchasing journey.
What is galvanic corrosion?
Galvanic corrosion, as explained in the ASM Handbook: Volume 13A: Corrosion: Fundamentals, occurs when different metals are in contact with each other in an electrolyte, such as acids, bases, or salts dissolved in liquids like water or alcohol. For example, in seawater, a highly corrosive electrolyte, combining a more anodic metal (one that oxidizes and loses electrons in the presence of an electrolyte) with a more cathodic metal (one that remains stable in the presence of an electrolyte) accelerates the corrosion of the anodic metal. In this example, the anodic metal bears the brunt of the corrosion, while the cathodic metal remains mostly unaffected.
Figure 1. Galvanic corrosion example.
How quickly does galvanic corrosion occur?
The rate and severity of corrosion will depend on the electrochemical potential properties of the metals. To help put it into perspective, the Galvanic series table below from Corrosionpedia shows metals from most active when in the presence of an electrolyte (anodic) on top of the list to the least active (cathodic) at the bottom of the list. In other words, metals at the top of the table will act as anodes and corrode first when coupled with a cathode metal.
To help prevent damaging corrosion, use materials that are close to each other in the galvanic table and be careful to check for galvanic compatibility of each metal to get the best mix of materials.
Figure 2. Galvanic series.
Source: Corrosionpedia.com
What materials are best suited to prevent or reduce corrosion from affecting your instruments?
Understanding the compatibility of instrument materials is the first and most critical step for ensuring accurate and reliable measurements in corrosive environments. Seawater, for example, with its high salinity and corrosive nature, can damage instrument components, affecting their durability and reliability. To minimize the risk of corrosion and ensure long-term performance, it is essential to select pressure instruments with high media compatibility.
When it comes to the wetted parts and case material on your instruments, the following materials are commonly used in water systems because of their corrosion resistance properties that can prevent galvanic corrosion and maintain the reliability of your instruments:
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Stainless steel is a popular choice because of its excellent resistance to rust and corrosion in a variety of environments. However, it's important to choose the right grade. For example, the Ashcroft® S1 Pressure Transducer is available with 316L stainless steel, which has higher corrosion resistance than other common 300 series stainless steel due to the addition of molybdenum in the alloy. Another option is the Ashcroft® 1009 Pressure Gauge, which features laser welded wetted 316L stainless steel components to ensure system integrity for corrosive media. Note: When the application is unknown, all stainless steel is a safe choice.
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Titanium is another material that is known for resisting corrosion in water. It forms a passive oxide layer on its surface that protects it from various forms of corrosion. This makes titanium an ideal candidate for long-term use in harsh marine environments where both strength and corrosion resistance are paramount.
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Nickel alloys, such as Monel® and Inconel®, also offer high corrosion resistance, especially in saltwater applications. These alloys can withstand extreme conditions and are often used in critical applications where failure is not an option. Monel®, for instance, is highly resistant to water corrosion and is often used in marine engineering, pumps, and valves.
When selecting the right material for your pressure instruments, it’s also essential to consider the entire system and its components, not just the process media. For instance, ensuring that all parts, such as diaphragms and seals, are made from the same corrosion-resistant materials like Hastelloy® or Tantalum can further protect the integrity of the instrument.
Ideally, all components used in a water-based application would use the same materials to mitigate the potential for galvanic corrosion. However, if any of those materials are combined with a second material on either side of the galvanic table they will eventually experience galvanic corrosion as well. It just depends on how long it will take.
A famous example of galvanic corrosion.
One of the most notable examples of galvanic corrosion involves the Statue of Liberty. This iconic monument, presented by France to the United States in 1886, features a copper exterior supported by an iron framework.
According to the National Institute of Standards and Technology (NIST), engineers were aware of the need to separate the copper and iron to prevent corrosion and added a layer of shellac-coated asbestos in the statue's design. However, over time, the harsh conditions of the New York harbor caused the shellac to deteriorate. Without the protective layer, the asbestos became brittle and porous. This degradation allowed moisture to become trapped, facilitating seawater electrolyte contact and accelerating galvanic corrosion.
In 1986, in time for the 100th anniversary of her arrival to the U.S., the National Park Services completed an extensive restoration project on the monument to stop the damaging effects of galvanic corrosion. This involved replacing the iron with stainless steel, a more corrosion-resistant material. This example illustrates the importance of material compatibility and the necessity of choosing corrosion-resistant materials in marine environments.
Other ways to keep your instruments performing properly in water applications
Maintenance and regular inspection practices are also crucial in reducing the risks of galvanic corrosion. Regularly inspecting the instruments for signs of corrosion, such as discoloration or pitting, allows for timely intervention and preventive measures. Cleaning and maintaining the instruments according to manufacturer guidelines can help extend their lifespan and ensure accurate performance.
By considering the selection of appropriate materials, applying protective coatings, and implementing proper maintenance and inspection practices, pressure instruments used in water applications can be effectively protected against galvanic corrosion.
To assist with selecting the right materials for corrosive applications, speak to an Ashcroft instrument expert.
Ready to learn more?
Now you know that galvanic corrosion can compromise the structural integrity of your pressure instruments, causing instrument leaks, inaccurate measurements, and premature failure. To learn more, below are a few articles that provide answers to some of the common questions we get about selecting instrumentation for your specific needs.
For personalized assistance, feel free to reach out directly to one of our experts.
In the meantime, download our guide, Avoid Pressure Equipment Failure.
