A copper-nickel flange bolts to a carbon steel flange without complaint. Dimensionally it is a perfect fit. Electrically it is a battery, and the steel is the part that dissolves. This comes up on nearly every copper alloy job, because copper alloy is almost never the whole system.
How the couple works
Put two different metals in electrical contact in an electrolyte, and the more noble one becomes the cathode while the less noble one becomes the anode. The anode corrodes, and it corrodes faster than it would have on its own. Seawater is an excellent electrolyte, which is why this is a marine problem above all others.
The relevant ordering for copper alloy flange work, from less noble to more noble:
| Position | Material | Behaviour against copper-nickel |
|---|---|---|
| Least noble | Zinc, magnesium, aluminum | Corrode sacrificially. Used deliberately as anodes |
| Carbon steel, cast iron | Corrode. This is the common problem joint | |
| Aluminum bronze | Close to copper-nickel. Mild couple | |
| Copper, 90/10 Cu-Ni, 70/30 Cu-Ni | Reference | |
| Monel 400, passive 316, superduplex | Copper-nickel becomes the anode and corrodes | |
| Most noble | Titanium, graphite | Copper-nickel corrodes strongly |
Area ratio is the whole story
The nobility ordering tells you which metal corrodes. The area ratio tells you how fast, and it is the part people skip.
The corrosion current spreads over the anode surface. A large cathode driving a small anode concentrates all that current into a small area, and the local corrosion rate is severe. The reverse is comparatively harmless.
- Bad: carbon steel bolts in a copper-nickel flange system. Small anode, large cathode. The bolts go first, and quickly.
- Bad: a short carbon steel spool tying into a long copper-nickel header.
- Tolerable: a copper-nickel valve in a long carbon steel line. Small cathode, large anode. The steel corrosion is spread thin.
Practical rule. Never let the sacrificial material be the small part, and never let it be the part that is hard to replace. A gasket is cheap to change. A buried header is not.
Breaking the couple properly
An insulating gasket kit is the standard answer, and it is frequently installed wrong. A complete kit has three components, and it only works if all three are present:
- A non-conductive gasket, which separates the two flange faces.
- Insulating sleeves through every bolt hole, which stop the stud touching either flange bore.
- Insulating washers under the nut at each end, with a steel backing washer outboard of them to spread the load.
Fit the gasket and skip the sleeves, and the studs simply carry the current from one flange to the other. The joint is electrically continuous and the kit has achieved nothing. This is the single most common installation error on isolated flange joints.
After assembly, the isolation should be verified with a resistance check across the joint, before the line is filled. It is a two minute test that prevents a five figure repair.
Other approaches
- Transition spool. Run a length of the same alloy through the transition so the dissimilar joint happens at an accessible, inspectable, above-ground point rather than somewhere awkward.
- Sacrificial anodes. Fit zinc or aluminum anodes near the joint and let them be the anode instead of your steel. Standard practice in seawater systems, and it needs a replacement schedule.
- Coating the cathode. Counter-intuitive but correct. Coating the noble metal reduces the cathode area and so reduces the driving current. Coating the anode is risky, because a holiday in the coating concentrates all the attack into a pinhole.
- Design it out. Cheapest of all. Keep the seawater-wetted system to one alloy family where the budget allows.
Bolting
Do not default to A193 B7 studs on a copper alloy flange in a marine environment. They are the small anode in a large cathode system, and they are also exposed to the atmosphere. Monel K-500, superduplex, and coated or fully sleeved alloy steel are the usual options, each with its own consequences. Whichever is chosen, it belongs on the specification explicitly.
See also marine and seawater applications for joint design in context.