Designing Fasteners for Magnesium: The Galvanic Corrosion Playbook

When a magnesium part fails corrosion testing, the post-mortem almost never blames the magnesium. It blames a steel bolt somebody torqued directly into a magnesium boss — the most anodic structural metal in the assembly wired straight to a cathode, with the clamp joint as the circuit.

High-purity AZ91D with a conversion coat and e-coat passes automotive salt-spray requirements. The alloy is not the risk. The joint is. Here is the stack that removes it.

The four-layer defence

• Layer 1 — alloy purity — Hold Fe, Ni and Cu to high-purity spec. Tramp iron in the casting seeds corrosion no coating can save.

• Layer 2 — the coating stack — Chromate-free conversion coat plus e-coat, validated to your program's salt-spray hours — specified as a system, not as two line items.

• Layer 3 — isolation at every joint — No bare steel touches magnesium. Isolation washers under heads, coated or polymer-sleeved inserts in bosses, and compatible-coated fasteners (Geomet-type, not plain zinc).

• Layer 4 — drainage — Water finds low points. Add drain paths and avoid pockets where electrolyte can sit against a joint — a dry galvanic couple is an open circuit.

Take the answer into your design review

“It corrodes” will be said in the room. The engineer who answers with the four layers — and the galvanic mechanism behind them — ends that conversation in one slide.

Reach out to us if you need a part to be reviewed for DFM. https://www.exclusivemagnesium.com/contact-us