Guide
Isolating dissimilar metal flanges in water loops
Where a carbon steel facility water main meets a stainless CDU, the two metals and the water between them form a galvanic cell. The carbon steel near the joint corrodes faster. An insulating gasket kit breaks the circuit.
At carbon steel to stainless steel flange joints in water service, use an insulating gasket kit (an insulating gasket plus a sleeve and washers on every bolt) where the specification calls for isolation, most often at CDU and heat exchanger connections.
How galvanic corrosion works
Galvanic corrosion needs three things: two metals with different electrochemical potentials, an electrolyte connecting them, and an electrical path between them. In a flanged joint between carbon steel and stainless steel, the water is the electrolyte and the bolts complete the electrical path. Carbon steel is the more active metal, so it corrodes faster than it would on its own, mostly close to the joint. The effect is worst when a small area of carbon steel is connected to a large area of stainless.
| Pair | Risk | Usual practice |
|---|---|---|
| Carbon steel to stainless steel | Carbon steel corrodes faster near the joint | Isolate where the specification requires, often at CDUs |
| Carbon steel to copper or brass | Carbon steel corrodes faster | Dielectric unions or isolating flanges |
| Stainless steel to copper alloys | Low; close in the galvanic series | Normally not isolated on TCS loops |
| Aluminium with copper | High | Avoid on liquid cooling loops unless approved |
Insulating gasket kits
A kit has three parts. The insulating gasket separates the flange faces. An insulating sleeve lines each bolt hole so the bolt does not touch either flange. Insulating washers, backed by steel washers, separate the nuts from the flange. Without the sleeves and washers, the bolts carry current around the gasket and the kit does nothing.
| Style | Description | Use |
|---|---|---|
| Full face | Covers the whole face, with bolt holes | Flat face flanges; best at keeping debris out of the bolt holes |
| Raised face (ring) | Fits inside the bolt circle | Raised face flanges |
| Ring joint | Insulating oval or octagonal ring | RTJ flanges; rare in data centers |
Insulating joints interrupt electrical continuity in the piping. Coordinate them with the electrical bonding and grounding design for the building.
Specifying it
- Mark isolation joints on the P&ID and the flange schedule.
- Give both flanges at the joint the same standard, class and face, for example ASME B16.5 Class 150 RF A105 to 316L.
- Choose gasket and sleeve materials rated for the fluid and temperature, and check them against the coolant supplier's list on the secondary side.
Frequently asked questions
Do stainless steel and copper need isolating on the TCS?
Usually not. Stainless steel and copper alloys are close in the galvanic series and both are standard wetted materials on technology cooling loops. Follow the coolant supplier's approved materials list.
Is an isolation kit needed between carbon steel and stainless steel on chilled water?
It depends on the specification. In treated, closed loops the risk is lower, but many specifications isolate the transition at CDUs and heat exchangers to protect the carbon steel.