"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.
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.
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.
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.
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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