Marine Bonding & Masts
Salt spray turns a copper-to-aluminium joint from a slow problem into a seasonal one. CUPAL washers and plates take the couple out of the mast, the bonding system and the lightning path.
The problem you need to solve
An aluminium mast and a copper conductor is the most reliably corroding joint on a boat, and it is not an unusual one — it is what every bonding system, every lightning path and most mast-step terminations are made of. The mast is aluminium because it has to be light. The bonding, earthing or lightning conductor is copper because it has to conduct. Aluminium and copper sit roughly 2 V apart in the electrochemical series, and salt water is an excellent electrolyte, so the couple is driven hard. The aluminium is the less noble metal, which means the mast corrodes, not the cable.
What makes it dangerous rather than merely ugly is that the corrosion product — aluminium oxide — is an insulator. The joint does not announce itself by falling apart. It stays mechanically tight and quietly stops being a conductor. On a lightning path that means the bonding you believe you have is, by the time it matters, not there; on a DC negative or bonding circuit it shows up as drifting readings and intermittent faults nobody can locate. A joint that would take years to degrade inland can be visibly attacked in a single season on a mooring.
In short
A copper bonding, earthing or lightning conductor bolted directly to an aluminium mast forms a galvanic cell that salt spray drives hard; the aluminium corrodes and the joint turns into an insulator while still looking sound. A CUPAL bimetal washer or plate puts aluminium against the mast and copper against the lug, so neither dissimilar pair is ever in contact.
- ~2V
- Al–Cu potential difference
- M6–M20
- Washer thread sizes from stock
- 2mm
- Washer thickness
- EN 10204 2.2
- Certificate with every order
How CUPAL solves it
A CUPAL transition is one piece of material with two metals in it, joined by diffusion welding — not a coating and not a plating. Fitted into the joint, the aluminium face meets the mast and the copper face meets the copper lug, so each contact is metal against its own metal and there is no potential difference to drive. The critical Cu/Al interface lives inside the composite, sealed away from spray and condensation, where an electrolyte cannot reach it.
That is the whole mechanism, and it is why it does not need maintenance. Antioxidant paste slows the same reaction but leaves the couple in place and has to be re-inspected and re-applied; a stainless washer does not remove the couple either, it only adds a third metal that is also cathodic to aluminium. The bimetal removes the cause rather than managing the symptom, so the inspection that matters becomes mechanical — is the bolt still tight — rather than electrochemical.
For a bolted lug the M6–M20 washers cover most of what a boat needs and ship from stock. Where a whole face meets copper — a mast step plate, a chainplate landing, a busbar in a switch panel — a CUPAL plate cut to the footprint is the right part, and we cut those from a drawing.
Numbers that decide the joint
Where the joint actually sits
Four places a copper conductor meets aluminium on a boat or a mast, and what each one needs.
Bonding or lightning cable to an aluminium mast
The problem you need to solve. A copper bonding conductor — often tinned, which changes nothing at the crimp — is bolted to a lug on the mast wall or at the mast step. Spray and condensate run down the mast and sit at exactly that bolt. Within a season or two the aluminium under the lug is powdery and the path resistance has climbed.
How CUPAL solves it. A CUPAL washer under the lug, aluminium face to the mast, copper face to the lug. For a heavy conductor (4-0 AWG and up) a cut CUPAL plate spreads the contact over the whole lug footprint instead of concentrating it at the bolt.
Mast step to the deck or keel bonding point
The problem you need to solve. A deck-stepped or keel-stepped mast lands on a plate that is bonded back to the common point with copper. The step is the wettest place on the rig — it collects everything that runs down the spar — and the joint is usually bolted and then never opened again.
How CUPAL solves it. A CUPAL plate cut to the step footprint, sized from the drawing. The full bearing face becomes a corrosion-free transition rather than a ring of oxide forming around each bolt.
Copper lug onto an aluminium busbar in the panel
The problem you need to solve. Distribution panels and battery switch panels mix copper cable lugs with aluminium bars. The cabin is humid, salt gets everywhere, and rising contact resistance at a high-current joint means heat before it means a fault.
How CUPAL solves it. A CUPAL washer between lug and bar, chosen so the outer diameter covers the whole lug foot rather than just the bolt. Contact resistance stays low and stable for the life of the panel.
Antenna, radar or telecom mast ashore
The problem you need to solve. The same joint without the boat: an aluminium mast, a copper earthing conductor, and a coastal or industrial atmosphere. Here the consequence is a lightning path that measures fine on the day it is installed and drifts out of specification without anybody re-testing it.
How CUPAL solves it. CUPAL strip wrapped at the transition, or a cut transition plate where the down-conductor meets the earthing mesh. See lightning protection for the EN 62561 component-test side of the same joint.
Compared to what is usually fitted
Four ways this joint gets made, and how each one ages in salt air.
| Approach | What it does to the couple | Maintenance | Life in salt air |
|---|---|---|---|
| Copper lug bolted straight to aluminium | Nothing — full Al/Cu couple | Re-inspect and re-make | A season to a few years |
| Antioxidant / jointing paste | Slows the reaction, couple remains | Re-inspect and re-apply | Extends it, does not remove it |
| Stainless washer between the two | Adds a third metal, still cathodic to Al | Annual inspection | Delays it only |
| CUPAL bimetal washer or plate | Removes the couple — like meets like | Mechanical check only | Design life of the fitting |
What people ask us about this joint
Recommended CUPAL products
Applicable standards for this field
Knowledge Base — CUPAL Technical Resources
9/8/2026
Connecting Copper to Aluminium: Which Part for Which Joint
Copper meets aluminium in five different joints, and each one takes a different part — including one where the answer is not us. Which lug, splice, washer or cut plate fits, in what size, and which way round it goes.
Read the full article →9/6/2026
Connecting a copper cable to an aluminium mast without galvanic corrosion
Bolting a copper earth or bonding cable straight to an aluminium mast starts galvanic corrosion within a season. What actually fails, why a stainless washer only delays it, and what to put in the joint instead.
Read the full article →3/1/2026
Can Copper & Aluminium Touch? Galvanic Corrosion Explained
Yes, copper and aluminium can touch — but their ~2 V potential difference drives galvanic corrosion. A CUPAL bimetal transition layer stops it (IEC 61238).
Read the full article →Get free expert advice
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