A battery you did not intend to build
When two dissimilar metals are in contact in the presence of an electrolyte, the one lower in the galvanic series becomes the anode and corrodes. The joint behaves like a short-circuited battery: no external current is needed, and the reaction continues as long as moisture is present.
Steel, aluminium and magnesium are far apart in that series. So a steel bolt in a magnesium die-casting is not a marginal case — the driving force is large, and the magnesium, which is the expensive part, is the anode.
Why a coating only half solves it
A plated steel bolt slows the reaction because the coating separates the two metals. But the separation is physical, not electrochemical: any damage to the coating — a scratch at the thread, a chip at the head, a cut edge — exposes bare steel and restarts the cell at that point.
That is why coated steel fasteners in light-alloy structures still show corrosion at the damaged spots. The coating is a barrier, and a barrier is only as good as its weakest point.
Option one: remove the couple
An aluminium bolt is metallurgically close to the magnesium or aluminium structure it clamps, so the potential difference at the interface is small and the corrosion driving force is correspondingly low. The joint does not depend on a coating staying intact, because there is no strong cell to suppress.
In the high-strength aluminium bolt programme documented in our material, a 7-series bolt averages 629 MPa tensile and 510 MPa yield with 8.21% elongation — comparable to a class 8.8 steel bolt at roughly one third of the density.
Option two: isolate the interface
Where a steel fastener must be used, the interface can be isolated with an anti-galvanic adhesive. In the application documented in our material, this is used when a high-strength aluminium bolt is connected to a magnesium die-casting: the small potential difference does most of the work, and the adhesive coating on the bolt head adds physical isolation.
The important point is that isolation has to be designed as part of the joint. A washer added at the bench is not the same as a coating applied in a controlled process, and the bolt threads are usually the part of the interface that gets forgotten.
- Match the fastener material to the structure where the joint will see moisture
- If a steel fastener is unavoidable, isolate the full interface, including the thread
- Do not rely on a barrier coating as the only protection — damage restarts the cell
- Check the thermal expansion match at the same time; the same material choice addresses both