When designing or maintaining chemical plant equipment, material selection is one of the most important decisions an engineer makes.
Among the most common choices are SS400 (carbon steel) and SUS304 (stainless steel) — both widely used, but with very different characteristics.
At first glance, it might seem simple:
“Use SUS304 for corrosion resistance, and SS400 when you want to save cost.”
However, in real plant design, the choice is not always that straightforward.
This article explains the practical guidelines and hidden logic behind how professionals use SS400 and SUS304 differently in chemical plants.
🔹 The Basic Difference Between SS400 and SUS304
| Property | SS400 (Carbon Steel) | SUS304 (Stainless Steel) |
|---|---|---|
| Corrosion resistance | Poor | Excellent |
| Cost | Low | High |
That’s often all you need to remember in practice.
While detailed metallurgical differences may interest researchers, what matters in a plant is how the material performs under process conditions.
🔹 Use SUS304 in Highly Corrosive Environments
If the fluid is corrosive, the choice is simple — use SUS304.
When the chemical is even stronger, you may need SUS316L, Hastelloy, or even glass- or fluoropolymer-lined equipment.
In most cases, stainless steel equipment or piping without external paint indicates the presence of hazardous chemicals inside.
Conversely, if you see painted carbon steel, it likely carries less aggressive fluids like cooling water or steam — and that’s perfectly fine.
🔹 Use SS400 for Utility Systems
For utilities such as water, steam, or compressed air, SS400 (for tanks) or SGP (for piping) is generally used.
They are affordable, easy to fabricate, and sufficient for non-corrosive services.
Even for some low-temperature brines or organic solvents (like alcohol or heavy oil), SS400 may still perform adequately.
However, for systems where fluids change frequently or may mix with process liquids, SUS304 is safer.
🔹 Use SUS304 Where Mixing with Process Is Possible
There’s one clear exception: if a utility line might mix with the process stream, always use SUS304 after filtration.
Typically, the line after a strainer (for removing rust or debris) should be made of stainless steel.
If rust from SS400 enters the process, it can not only corrode downstream piping but also contaminate the product — a serious safety and quality concern.
A common practice is to design with:
SS400/SGP before the strainer → SUS304 after the strainer.
🔹 To Save Cost: Consider Lining
If full SUS304 construction is too expensive, consider lining — for example, SS400 + SUS304 lining on the wetted surface.
This hybrid design keeps the strength of SS400 while adding corrosion resistance where needed.
However, lining is not perfect.
Under high temperature, pressure, or vibration, delamination or cracking can occur, especially in complex geometries.
So lining should only be used for relatively stable operating conditions.
🔹 Sometimes, SS400 Is the Better Choice
Interestingly, SS400 can sometimes be safer.
In wastewater systems, for instance, carbon steel corrosion is visible and gradual — while SUS304 may fail suddenly due to stress corrosion cracking (SCC) caused by chlorides.
In such systems, it’s often wiser to allow SS400 to corrode predictably and plan for replacement, rather than rely on stainless steel and face unexpected rupture.
Conclusion
In summary:
- Use SUS304 in corrosive or process-related systems.
- Use SS400 for utilities and non-corrosive services.
- Combine or line materials if needed to balance cost and reliability.
- In some special cases, deliberate use of SS400 can even improve safety.
Understanding the logic behind material selection allows plant engineers to design systems that are both cost-effective and reliable.
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