Industries involving liquids such as chemical/petrochemical, oil and gas, and water/wastewater, are constantly challenged by corrosion. Understanding the different types of corrosion and how they occur is the first step in protecting your process instruments from its damaging effects.
As a leading authority in pressure and temperature instrumentation, Ashcroft® offers valuable insights into corrosion and its impact on businesses. This article explores the different types of corrosion, the factors influencing corrosive environments, and the significance of selecting compatible materials for process instrumentation. Additionally, discover key considerations for choosing pressure instrumentation capable of withstanding harsh corrosive conditions.
How corrosion occurs.
Corrosion can occur in any application that involves a liquid (from water to chemical). For example, if you imagine a block of iron, steel, or any other component in your process, coming into contact with a drop of water, a chain reaction happens:
- The iron will react with the water, leading to the formation of iron oxides and releasing electrons in the process.
- The electrons react to oxygen in the atmosphere and water to form hydroxide ions.
- These ions then combine with the iron already in solution to create iron hydroxides
- The iron hydroxides eventually dehydrate and transform into iron oxides, commonly known as rust.
If you can ensure the iron or other metal remains dry, you can effectively prevent this type of corrosion. In chemical applications, the liquid itself is likely to be corrosive.
Three factors that can affect corrosion.
When it comes to selecting materials for pressure and temperature instruments, their unique properties play a crucial role. The intended use and functional characteristics determine which material is best suited for the job. Here are three key considerations for choosing the right material:
Composition
Alloys are created by combining two or more metallic elements to form a new metal with specific properties. It's essential to note that certain elements in an alloy can make it vulnerable when exposed to harsh substances. For instance, alloys containing copper can be attacked by concentrated ammonia, while high-carbon steel is susceptible to the hydrogen sulfide found in sour gas and sour crude. Specifying engineers can rely on available data from standards and corrosion guides to determine the compatibility of alloys with corrosive substances.
Temperature
Temperature plays a significant role in chemical reactions, either accelerating or slowing them down. In some cases, certain temperatures may prevent reactions altogether. For example, artifacts retrieved from the frigid depths of the Great Lakes often exhibit minimal corrosion due to the cold water temperature.
While cold temperatures can inhibit corrosion, they can also affect physical properties, making metals more prone to brittleness. Conversely, extreme heat can expedite corrosion reactions. Therefore, the choice of alloy may vary depending on the climate, with installations in different locations requiring specific considerations.
Physical properties
Strength, hardness, density, malleability, and melting point are essential physical properties of metals that must be taken into account. These properties are crucial in combating corrosion, as they may restrict the use of non-corrosive materials lacking other necessary attributes. For instance, although Hastelloy® C276 boasts excellent corrosion resistance in various environments, it may not be suitable for electrical contacts due to its poor conductivity.
Different types of corrosion.
Understanding the nuances of different corrosion types is crucial for maintaining the integrity and safety of structures and equipment in various industries. Note, the first two listed below are not necessarily classified as corrosion, but rather a 'wear' phenomena that can set the stage for corrosion:
- Fretting. This begins the minute contact between metals happens at their surfaces, potentially leading to accelerated corrosion when wear occurs. For example, wear can occur at the point where two flanges meet, either through compression of the bolts or from the movement of the flange facings, causing the material to corrode a lot faster.
- Erosion. This can be caused by abrasive particles in a solution that can scratch process piping or instrumentation and result in premature failure.
- Galvanic or electrochemical. This occurs when dissimilar metals interact in the presence of an electrolyte, leading to oxidation of the less corrosion resistant, or less noble material to suffer the majority of the corrosion. The other. more corrosion-resistant or noble metal will stay predominantly free from damage.
- Pitting. This deterioration poses a threat on a smaller scale by breaching the passive layer of a material such as stainless steel, which forms depressions or holes through the metal surface. It is sometimes difficult to see because it can be covered by surface rust but can compromise the strength of the metal and lead to premature failure.
- Crevice. This is often the result of a design or manufacturing defect that allows corrosive media to accumulate and get trapped in tight spaces. Prolonged exposure causes the corrosive substance to erode the metal. This is a common occurrence in areas where welds have not fully adhered or are porous.
- Stress corrosion cracking. This is a critical concern in the chemical, petrochemical and oil and gas industries. It refers to the cracking caused by the combined effect of tensile stresses (from either internal sources or external loads) and a corrosive environment. For example, in the case of a Bourdon tube for pressure measurements, the tube flexes and experiences stress, creating an ideal environment for stress corrosion cracking to occur, especially if the metal is not resistant to this type of corrosive process.
- Chemical. This is sometimes referred to as general or uniform corrosion. It follows a similar process as all of the other types of corrosion, but it happens at a much larger scale.
Protecting your process instruments from corrosion.
The most common and effective way to protect your pressure and temperature instruments from corrosive substances is to use a diaphragm seal. These seals act as a protective barrier between your instrument and the process media.
The diaphragm, which is made of a thin metal piece, is designed to flex in response to changes in pressure. This component, along with the top housing that connects to your instrumentation, allows for the system to be filled with a pressure-transmitting media. Once all components are assembled, the entire setup is clamped together securely.
Figure 1: Ashcroft® Diaphragm Seal.
Advantages of a diaphragm seal
Diaphragm seals can accommodate a wide range of wetted materials. In cases where particularly corrosive substances are involved, a diaphragm and lower housing can be constructed from more exotic materials.
In addition to protecting instruments from corrosive media, diaphragm seals can help dissipate heat in instances where high temperature can damage a gauge, and are suitable for use in applications involving suspended solids or requiring an added layer of protection.
How a diaphragm seal functions
The diagram shows a diaphragm seal connected to a gauge, with the fill fluid highlighted in red. To ensure proper operation of the diaphragm seal, it is essential to fill the system correctly by initially creating a vacuum on the seal and then backfilling to remove any air bubbles.
During operation, pressure is applied to the lower housing, where the diaphragm seal interacts with the process. Whether using a threaded or flanged lower housing, the diaphragm seal reacts to pressure variations, transferring this pressure to the upper housing and the Bourdon tube. The pressure travels through the seal and into the Bourdon tube, causing it to flex and indicate pressure changes.
Figure 2. Diaphragm Seal operation.
Selecting appropriate materials for your corrosive process.
Before choosing a diaphragm seal for your corrosive application, you need to ensure that the wetted material of your seal is compatible with your process. You can do this by talking to a product expert or using the online Ashcroft® Material Selection Tool for this purpose. This interactive resource allows you to input your specifications and obtain detailed information regarding material compatibility for your specific application.
Ready to learn more about protecting your pressure and temperature instruments?
Now that you know the basics about pressure gauges and how they work, you likely have more questions. Here are a few helpful resources to help you take the next step in building your knowledge on the subject:
- NACE-compliant pressure gauges for sour gas and crude oil applications
- What to consider when choosing material for your diaphragm seal
- How to safely select a diaphragm seal for high-temperature applications
- Isolation ring assemblies for water/wastewater
Or, to speak with someone directly, feel free to contact one of our product experts with any questions you have. In the meantime, download our solutions guide for chemical/petrochemical industries.