Connecting a copper cable to an aluminium mast without galvanic corrosion

An aluminium mast bolted to a copper cable is one of the most reliably corroding joints in any installation — on a sailing yacht, on a telecom or antenna mast, on a wind-turbine tower. The mast is aluminium because it has to be light. The bonding, earthing or lightning conductor is copper because it has to conduct. Where the two meet, the joint quietly destroys itself.

We are writing this because we keep being asked. Two of the last three enquiries that reached us were exactly this problem: one connecting a tinned marine copper cable to an aluminium mast, one running a 4-0 AWG copper lightning conductor to an aluminium sailboat mast.

What actually happens at the joint

Aluminium and copper sit about 2 V apart in the electrochemical series. Put them in contact with any electrolyte — rain, condensation, and especially salt spray — and you have a galvanic cell. Aluminium is the less noble metal, so the mast is what corrodes, not the cable.

The corrosion product is aluminium oxide, which is a poor conductor. So the joint does not simply weaken; it loses conductivity. On a lightning ground path that matters more than it first sounds: the connection can look intact, be mechanically tight, and no longer be a usable path to ground. On a bonding system it means the protection you believe you have is gradually not there.

Salt makes it dramatically worse. A joint that would take years to degrade inland can be visibly attacked in a season on a boat.

Why a stainless washer is not the fix

The common workaround is to put a stainless steel washer between the copper cable lug and the aluminium. It helps, and it is better than nothing — but it does not solve the problem. Stainless is also dissimilar to aluminium, so you have moved the couple rather than removed it, and the assembly still needs opening and cleaning periodically to stay conductive.

Antioxidant paste has the same limitation. It slows the reaction by keeping the electrolyte out, but the dissimilar-metal couple is still there and the paste needs re-application.

Neither is wrong as a stop-gap. Neither is permanent.

What to put in the joint instead

Use a copper-aluminium bimetal transition. It is a single piece of material with an aluminium face and a copper face, roll-bonded so the Cu-Al interface is inside the part and sealed from the environment.

Assembled correctly, the aluminium face goes against the mast and the copper face against the copper cable lug. Each metal only ever touches its own kind. There is no exposed dissimilar contact for an electrolyte to bridge, so there is nothing to corrode and nothing to re-service.

Two forms cover almost every mast joint:

• A bimetal washer for a single bolted lug — M6 to M20. The two enquiries above wanted M10, which is the common size for a heavy bonding cable.
• A cut plate where a larger contact face is needed, or where the mast fitting has its own hole pattern. We cut these from 0.5–2.0 mm sheet to a drawing; 2 mm is typical for a mast terminal.

Practical notes

• Orientation matters. Fitted the wrong way round the transition achieves nothing. Aluminium to the mast, copper to the cable.
• Keep the fastener out of the couple. Use stainless hardware with insulating washers under the head and nut, so the bolt does not create a second dissimilar path around the transition.
• Clean both faces before assembly. Aluminium re-oxidises in minutes; a wire-brushed surface assembled promptly makes a materially better joint.
• Cross-section, not just size. CUPAL carries about 41.9 m/Ω·mm², so a transition needs roughly 1.41× the cross-section of a pure copper part for the same current. For a bonding or lightning path that is usually irrelevant — the area is set by the bolt and the fitting, not by ampacity — but it matters if the joint is carrying working current.
• On a boat, this is one part of a system. Bonding, lightning protection and galvanic isolation are separate problems that interact. A bimetal transition fixes the dissimilar-metal joint. It is not a galvanic isolator and it does not replace anode protection or a properly designed ground path. We supply the material and its documentation, not vessel certification — specify to your own standard and your surveyor's requirements.

What we can supply

CUPAL is a roll-bonded copper-aluminium bimetal, 70 % aluminium / 30 % copper, 4.6 g/cm³ — a little over half the density of pure copper, which is the reason it suits masts in the first place. Every delivery carries an EN 10204 2.2 test report as standard, and 3.1 on request.

Stock washers ship from M6 to M20. Anything else — a disc, a strip, a rectangular terminal plate, a custom contour — we cut from sheet to your drawing in DXF, PDF or JPG, typically 5–10 working days. If you are unsure which size fits your fitting, send a photograph and the bolt size and we will tell you. If your joint is not a mast at all — an aluminium cable end, an in-line cable joint, a busbar transition — connecting copper to aluminium names the part for each of the four cases. Ask for a quote.