Galvanic corrosion between aluminium and steel happens when the two metals touch while an electrolyte – seawater, condensation, wet insulation – connects them. Aluminium is the less noble metal, so it becomes the anode and corrodes, while the steel is protected. On ships and offshore structures, where aluminium superstructures sit on steel hulls and decks, this is one of the main reasons aluminium and steel are never simply bolted together without precautions.
Why aluminium corrodes when it touches steel
Every metal has its own electrochemical potential in a given electrolyte. In seawater, aluminium alloys sit well below carbon steel in the galvanic series, and far below stainless steel and copper alloys. Connect them and a small battery forms: electrons flow from the aluminium to the steel, and the aluminium dissolves at the contact zone.
| Metal in seawater | Position in the galvanic series | Role when coupled with aluminium |
|---|---|---|
| Magnesium, zinc | More active than aluminium | Corrode first (used as sacrificial anodes) |
| Aluminium alloys (5083, 6082) | Active | – |
| Carbon steel, cast iron | More noble than aluminium | Cathode: aluminium corrodes |
| Copper alloys, copper-nickel | Clearly more noble | Cathode: strong attack on aluminium |
| Stainless steel (passive), titanium | Most noble of these | Cathode: strong attack on aluminium |
What makes it worse
- Electrolyte: seawater and splash zones are far more aggressive than dry indoor air. Trapped moisture in crevices, under insulation or behind sealant that has failed keeps the cell running.
- Area ratio: a small aluminium part fastened to a large steel surface corrodes fastest, because a large cathode drives a high current through a small anode. The reverse – small steel fasteners in a large aluminium plate – is far less critical.
- Potential difference: the further apart the metals are in the galvanic series, the faster the attack. Aluminium against stainless steel or copper is worse than against carbon steel.
- Crevices and contact: bolted laps and overlapping plates hold water and oxygen differences, adding crevice corrosion to the galvanic effect.
How to prevent galvanic corrosion between aluminium and steel
- Break the electrical contact. Insulating washers, sleeves and gaskets in bolted connections stop the current, as long as they stay intact and the bolts do not bridge the two metals.
- Keep the electrolyte out. Seal and paint the joint area so water cannot bridge the metals. When coating only one metal, coat the more noble one (the steel) or both – coating only the aluminium concentrates attack at every pinhole.
- Choose the area ratio. Keep the aluminium part large relative to the steel in contact with it.
- Use a transition joint instead of a bolted lap. A welded transition joint removes the crevice and the bolts altogether: the aluminium structure is welded to the aluminium side, the steel structure to the steel side, and the joint is painted as part of the coating system.
- Add cathodic protection in immersed zones, with sacrificial anodes that are more active than aluminium.
Transition joints: the welded alternative to bolting
Bolted or riveted aluminium-to-steel connections need insulation, sealing and inspection for the life of the ship. An explosion-welded transition joint such as Triplate® replaces them with two ordinary welds: steel to steel, and aluminium to aluminium. There is no crevice and no fastener, and the bonded interface is a continuous metallurgical bond rather than a contact surface.
Triplate® adds two details aimed at corrosion. The aluminium side is alloy 5083, which resists seawater, and the vacuum-welded interface is 100% dense, so there is no porosity where moisture could enter. Like every transition joint, it is painted together with the adjacent structure – the military specification MIL-J-24445A requires a suitable coating for exactly this reason. For aluminium-to-stainless connections there is Inox-Triplate®.
More background: how to weld aluminium to steel and joining dissimilar metals. Need joints for a project? Send the steel grade, aluminium alloy and member thicknesses through our contact page.
Frequently asked questions
Does aluminium corrode when it touches steel?
Yes, when an electrolyte such as seawater or condensation connects them. Aluminium is less noble than steel, so it becomes the anode and corrodes while the steel is protected.
How do you stop galvanic corrosion between aluminium and steel?
Break the electrical contact with insulating washers and gaskets, keep water out with sealant and paint (coat the steel or both metals, not only the aluminium), keep the aluminium area large, or replace the bolted joint with a welded transition joint.
Is galvanic corrosion worse with stainless steel than with carbon steel?
Yes. Passive stainless steel is more noble than carbon steel, so the potential difference with aluminium – and the corrosion rate – is higher.
Why are transition joints used on ships?
A transition joint lets the aluminium superstructure be welded to the aluminium side and the steel deck to the steel side. It removes bolts and crevices, and the joint is painted with the rest of the structure.
Do transition joints need coating?
Yes. The aluminium–steel interface must be protected by the paint system, as the US Navy specification MIL-J-24445A also requires.
Written by Joeri Hofstede, CEO of Shockwave Metalworking Technologies — vacuum explosion welding specialists since 1985.