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Materials Engineering DISCUSSION
I am fixing a 6 mm aluminium bracket to a galvanised steel frame outdoors, near the coast, using M8 bolts. I have read that aluminium and steel in contact corrode each other, and also that stainless bolts in aluminium are even worse, yet I see both combinations used everywhere.
Which metal actually corrodes in each case, how do I judge whether a combination is acceptable, and what is the right way to assemble the joint so it lasts?
Galvanic corrosion needs three things at once: two metals with different electrochemical potentials, a metallic connection between them, and an electrolyte bridging both. Take away any one and it stops. The metal with the more negative potential becomes the anode and corrodes faster than it would alone; the other becomes the cathode and is protected.
In seawater, measured against a calomel reference, zinc sits at about -1.0 V, aluminium alloys at roughly -0.75 to -1.0 V, carbon steel around -0.6 to -0.7 V and passive 316 stainless close to -0.05 V. So aluminium is the anode against both steel and stainless, while a zinc coating protects the steel under it and sits close to aluminium.
The area ratio matters as much as the potential difference. The corrosion current is limited by the cathode area and is concentrated on whatever anode area is exposed. A small cathode on a large anode, such as a stainless bolt in a big aluminium bracket, spreads a small current over a lot of aluminium, which is why that combination usually works. The reverse, an aluminium rivet in a stainless or steel sheet, fails quickly.
This also leads to a rule about paint: if you coat only one metal, coat the cathode. Painting only the anode leaves any scratch as a tiny anode facing a large bare cathode, and it pits rapidly at that point.
For your joint: galvanised steel against aluminium is one of the more compatible pairings, because zinc and aluminium are close in the series. The trouble starts when the zinc is consumed and bare steel is left against aluminium in salt water. Practical measures are a non-conductive gasket or tape between bracket and frame, nylon or similar sleeves and washers under the bolt head and nut, and a sealant or jointing compound that keeps water out of the crevice.
Design the joint so water drains and dries. A joint that stays dry has no electrolyte and barely corrodes, whatever the metals are.
On judging acceptability, a commonly quoted guide is to keep the potential difference between the two metals below about 0.15 V for harsh or marine exposure, around 0.25 V for ordinary outdoor use, and up to about 0.5 V indoors where it stays dry. Treat it as a screening rule rather than a law, since area ratio, coatings and how long the joint stays wet change the outcome.
By that guide, aluminium to passive stainless in salt spray is a large gap, so use isolation and sealant there. Hot-dip galvanised or zinc-flake coated steel bolts are the lower-risk choice against aluminium near the coast, and stainless with isolating washers is the usual answer when the higher corrosion resistance of the fastener itself is needed.
Inspection tells you whether it is working. Galvanic attack on aluminium shows as white, bulky corrosion product and pitting right next to the fastener or the contact face, not evenly across the part. Check after the first wet season and look under the washers.
Also keep copper and brass away from the aluminium, including runoff from copper fittings above it; copper ions deposited on aluminium set up small local cells and cause pitting even without direct contact between the parts.
