Guide
Stainless vs carbon steel flanges for data center cooling
The choice between stainless and carbon steel flanges in a data center is not a question of quality. It depends on which loop the flange is on, what touches the water, and how clean the loop has to stay.
Use stainless steel (316L or 304L) for every flange that touches the technology cooling system coolant, and at the CDU secondary side. Use carbon steel (A105, or A350 LF2 where it gets cold) for condenser water and most chilled water and facility water, unless the operator's specification calls for stainless.
Decision by loop
| Loop | Recommended | Acceptable | Avoid |
|---|---|---|---|
| Technology cooling system | 316L | 304L | Carbon steel, galvanised steel |
| CDU secondary side | 316L | 304L | Carbon steel |
| CDU primary side | Match the facility water piping | Stainless with isolation kit to carbon steel | - |
| Facility water mains | A105 or per operator spec | 304L / 316L | - |
| Chilled water plant | A105 | Stainless where specified | - |
| Condenser water | A105; AWWA C207 on large mains | A350 LF2 in cold climates | - |
Corrosion behaviour
Carbon steel relies on the water chemistry for protection. In a closed, inhibited chilled water loop with low oxygen, corrosion rates are low, which is why it is the long-standing standard for building hydronics. Stainless steel protects itself with a thin chromium oxide film. It does not need inhibitors to stay passive, and it does not shed iron oxide into the water, which is what makes it the material for loops feeding cold plates.
Stainless is not immune. Chlorides can cause pitting, and heat tint left after welding lowers corrosion resistance. 316L's molybdenum helps against chlorides, and pickling and passivation restores the surface after fabrication.
Ratings and dimensions
| Material | Group | Class 150 rating |
|---|---|---|
| A105, A350 LF2 | 1.1 | 19.6 bar (285 psi) |
| F304, F316 | 2.1, 2.2 | 19.0 bar (275 psi) |
| F304L, F316L | 2.3 | 15.9 bar (230 psi) |
Dimensions do not change with material: a Class 150 NPS 8 flange has the same bolt circle in A105 and 316L. Only the rating changes, and for data center water pressures, Class 150 has margin in every grade.
Transitions
The usual transition is at the CDU, where a carbon steel facility water main meets stainless equipment. Water conducts current between the metals, and the carbon steel near the joint corrodes faster. Many specifications put an insulating gasket kit at that flange. See how isolation kits work.
Stainless flanges cost more than carbon steel. On large stainless lines, a stub end with a lap joint flange keeps the wetted part in 316L while the loose flange can be a lower cost material.
Frequently asked questions
Is stainless steel always the safer choice?
For wetted parts on the TCS, yes. On the facility side, carbon steel with good water treatment has a long track record, and stainless adds cost without always adding benefit. Use the loop and the operator's specification to decide.
Does 316L have the same pressure rating as A105?
No. At ambient temperature a Class 150 flange is rated 15.9 bar in 316L (ASME B16.5 group 2.3) and 19.6 bar in A105 (group 1.1). Both are well above typical data center water pressures.
Can we mix stainless and carbon steel flanges in one system?
Yes, at defined transitions such as the CDU primary connection. Consider an insulating gasket kit at the joint, and never put carbon steel on the clean TCS side.