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by Jeff Hopkins, on 9/4/19 9:00 AM
It takes the right materials and the right training to keep corrosion at bay
"Finding a proper materials solution means starting at the source of the problem."
- From Corrosion Types
Just about every metal corrodes under certain circumstances. Even stainless steel tubing is vulnerable to chemical reactions with the fluid inside and the environment outside. If corrosion gets bad enough, fluid can escape and cause a safety hazard. Ignore the problem long enough, and it could lead to a catastrophic system failure.
So it's important to understand the various types of corrosion and how to prevent them. Primarily, the two forms of corrosion that affect stainless steel tubing: pitting and crevice corrosion.
Pitting corrosion
Stainless steels and nickel alloys come with a thin chromium-rich oxide film. When that protective layer breaks down, the exposed metal atoms can give up their electrons easily. This electrochemical reaction creates corrosion in the form of tiny pits.
Left alone, these tiny pits will grow deeper, eventually even penetrating a tube wall entirely. Pits can also lead to cracks in stressed components.
When monitoring for corrosion, look for reddish brown iron oxide deposits and for pits that may have formed in a metal surface.
Crevice corrosion
Similarly, crevice corrosion starts with the breakdown of the protective oxide film and continues with the formation of shallow pits. Rather than occurring in plain sight, crevice corrosion occurs in crevices - a metal surface in contact with another surface.
Variations on a theme
- Galvanic corrosion occurs when two dissimilar metals come into contact with each other in the presence of an electrolyte, an electrically conducting fluid such as seawater. Galvanic corrosion leads to pitting and material loss of the lesser of the two metals.
- Stress corrosion cracking can be a problem in certain alloys susceptible to chloride-ion induced stress corrosion cracking, such as when they are in salt water environments. The chloride ion interacts chemically with the material where tensile stresses are highest, such as at the tip of a crack. This failure mode is dangerous because it can destroy a component at stress levels below the yield strength of an alloy.
Stop it early
To keep a lid on corrosion, first consider the choice of materials for tubing applications, from the tubing to tube supports and tube clamps.
Normally, 316 stainless steel tubing works well as long as it is kept clean and temperatures are not excessively high. In hot climates, especially in locations where salt deposits readily form and cannot be washed off, you are more likely to see corrosion. Likewise, 316 stainless steel is more vulnerable in installations where rust from carbon steel structural beams and floors accumulate on stainless steel surfaces.
For these situations, consider using tubing made from superaustenitic or superduplex stainless steel. The higher yield and tensile strength of superduplex stainless steel also make it easier to build systems that must be rated to a high maximum allowed working pressure.
When installing tubing, avoid using tube support strips. The relatively large crevice contact area makes it easier for a corrosive crevice solution to form. Try tubing clamps instead. They minimize clamp-to-tubing contact and make it easier to visually inspect the tubing.
When you are designing a fluid system, minimize crevices and the contact of non-compatible metals susceptible to galvanic corrosion. One way is to avoid placing tubing directly against walls or against each other.
Keep an eye out
Regular inspections, and knowing what to look for, are as important as good design. Swagelok's classroom instruction and hands-on training can be invaluable for operators and technicians. Even if you have a corrosion expert, it's smart to build a basic understanding of corrosion - what it looks like, where it occurs, and for what reasons - among those who work with tubing systems every day.
Just ask
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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