Keep An Eye On Corrosion And Save Big Money
by Jeff Hopkins, on 11/20/19 9:00 AM
Damage to tubing systems is pervasive, problematic and preventable
"Feeling confident that your systems will perform, even in the most demanding locations, involves not only product design but also materials of construction, application-specific corrosion issues, and unique oil field environments."
Just about every metal used to build our world corrodes, but only under certain circumstances. If you know how to identify it and where to look for it, you can minimize risk at the refinery and prevent a lot of costly damage.
Let's go through some of the most common forms of corrosion.
Pitting of a metal occurs when its protective layer breaks down, allowing the exposed metal atoms to give up their electrons easily, resulting in corrosion.
A typical coating on stainless steels and nickel alloys
is a thin chromium-rich oxide film.
This electrochemical reaction causes tiny pits to form. Corrosion products accumulate in the pits, causing the pits to grow deeper. Eventually they may penetrate a tube wall entirely. Pits can also make it easier for cracks to form in stressed components.
When monitoring for corrosion, look for reddish brown iron oxide deposits and for pits, which may have formed, in a metal surface.
Crevice corrosion likewise starts with the breakdown of the protective oxide film and continues with the formation of shallow pits. But instead of occurring in plain sight, crevice corrosion does what its name implies: It occurs on a metal surface that is in intimate contact with another surface.
A common place for crevice corrosion would be where stainless steel tubing is in intimate contact with a tube clamp. You would be able to see it only when a tubing clamp is removed from the installed tubing. It is important to remember that crevice corrosion can occur at lower temperatures than pitting corrosion.
Galvanic corrosion occurs when two dissimilar metals come into contact with each other in the presence of an electrolyte, an electrically conducting fluid. Generally speaking, the risk of galvanic corrosion is typically small if the difference in electrochemical potential between the two metals in contact does not exceed 0.2 volts.
Stress corrosion cracking
Certain alloys are susceptible to chloride-ion induced stress corrosion cracking. The chloride ion interacts chemically with the material where tensile stresses are highest, such as the very tip of a crack, making it easier for the crack to propagate. This failure mode is dangerous because it can destroy a component at stress levels below the yield strength of an alloy. While in progress, this failure mode can be difficult to detect, and final failure can occur suddenly.
Some alloys are considerably more prone to stress corrosion cracking than others. Three groups of materials are highly resistant to stress corrosion cracking: carbon steels, nickel-based alloys, and duplex stainless steels.
Corrosion often can be minimized by giving your workforce some basic knowledge. First, consider the choice of materials for tubing applications, including tube supports and tube clamps.
Our 316 stainless steel tubing works well in many installations so long as it is kept clean and temperatures are not excessively high.
Corrosion of 316 stainless tubing is more likely in hot climates and in installations where rust from carbon steel structural beams and floors accumulates on stainless steel surfaces. For these situations, superaustenitic or superduplex stainless steel offers much better corrosion resistance.
The use of tube support strips should be avoided because the relatively large crevice contact area. The industry is now moving more toward the use of tubing clamps with a design based on minimizing clamp-to-tubing contact, which also facilitates visual inspection of tubing.
Careful system design practices help prevent corrosion by minimizing locations where crevice corrosion can occur and minimizing the contact of non-compatible metals susceptible to galvanic corrosion. Avoid placing tubing directly against walls or against each other.
Beyond these simple measures, maintain a regular and robust corrosion-monitoring program and make sure your workforce gets in-depth training. Swagelok provides classroom learning and hands-on experience that can be invaluable for operators and technicians. Building a basic understanding of corrosion can prevent all manner of problems. Start saving by taking a closer look at your fluid systems—especially where it’s hard to see. Then learn more about our training including our Materials Science Training.
Swagelok Northern California has a great deal of exposure to all aspects of fluid system design and engineering. Whether you have a simple question or a complex challenge, we're glad to hear from you.
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