Isolating Dissimilar-Metal Fasteners with Nylon Washers and Sleeves

Formosa Fasteners · 2026-09-06

Quick answer: A nylon (or other dielectric) washer and sleeve combination breaks the electrical path between a fastener and a dissimilar base metal, but only if the whole current path is actually interrupted — a washer alone often isn't enough.

Nylon washers and insulating sleeves are the standard low-cost method for isolating a fastener from a dissimilar base metal to prevent galvanic corrosion, but a partial isolation detail is a common reason this approach fails to actually prevent the corrosion it was specified to prevent.

Isolation has to break the entire current path, not just one contact point

A galvanic cell needs an electrical path between the two metals in the presence of an electrolyte (usually moisture) — a nylon washer under the fastener head isolates that one contact point, but if the fastener shank still directly contacts the hole wall of the dissimilar metal, or if the nut on the other side sits directly against dissimilar metal without its own isolating washer, the current path remains intact through those other contact points. Full isolation typically requires a washer under the head, an insulating sleeve through the hole, and a washer under the nut — a partial kit that only addresses one of these frequently gives a false sense of protection.

Material choice for the isolator matters for the application

Nylon is common and inexpensive but has limits on temperature range and long-term creep under sustained clamp load, which can allow preload loss over time in ways a metal washer wouldn't; other dielectric materials (certain fiber composites, for example) are used where higher temperature or lower creep is needed. The isolator material should be selected against the joint's actual service conditions, not defaulted to nylon in every case.

Verify isolation didn't get value-engineered out during assembly

Because isolating hardware is inexpensive relative to the fastener and easy to omit without an obvious visual difference (especially the sleeve, which is hidden inside the hole), it's worth spot-checking assembled joints rather than assuming the full isolation kit was actually installed as specified — this is a detail that's easy to miss during general visual inspection.

If your assembly mixes dissimilar metals and needs isolation hardware specified correctly, send us the joint detail through our contact form.

Frequently Asked Questions

Is a nylon washer under the bolt head enough to prevent galvanic corrosion?

Usually not by itself — full isolation typically requires a washer under the head, an insulating sleeve through the fastener hole, and a washer under the nut, since any remaining metal-to-metal contact point keeps the galvanic path intact.

Can nylon isolating washers lose effectiveness over time?

Yes — nylon can creep (slowly deform) under sustained clamp load, particularly at elevated temperature, which can reduce clamp force over time; this should be considered for joints with high temperature exposure or where long-term preload retention is critical.

How can you tell if isolation hardware was actually installed correctly during assembly?

Visual inspection alone often can't confirm the sleeve is present since it sits inside the hole; a sample teardown inspection or requiring photo documentation during assembly is a more reliable verification method for critical joints.