By breaking the electrical contact between two dissimilar metals at the bearing face - but only if the contact is broken everywhere.

Galvanic corrosion needs two dissimilar metals in electrical contact with an electrolyte, usually just moisture. Remove the contact and the mechanism stops.

An insulating washer under the head does that where the fastener bears on the component.

The bolt shank still touches the inside of the hole, so a proper isolation detail uses a sleeve as well as a washer.

The most common case is a steel fastener in an aluminium assembly, where the aluminium is the metal that suffers.

Keeping water out of the joint helps as much as the isolation, because without an electrolyte the couple does nothing.

Until the elastomer is visibly compressed and just begins to spread - no further.

The seal works by the elastomer flowing into the surface irregularities under controlled compression.

Under-tightened, there is not enough contact pressure and the joint leaks despite looking assembled.

Over-tightened, the elastomer extrudes out from under the metal cap and is permanently damaged, so the seal fails immediately or shortly after.

Power tools make over-tightening the usual outcome, which is why depth-setting nose cones exist for roofing screws.

On profiled sheet, fixing on the crown rather than in the valley matters more for watertightness than the washer does.

Creep is the slow relaxation of a polymer under sustained load, and it matters wherever the joint has to stay tight.

A metal joint holds preload almost indefinitely. A joint clamped through a polymer washer loses some of it as the polymer flows.

Temperature accelerates it substantially, so a joint that is stable indoors may relax quickly in a hot environment.

In practice the bolt has to be retightened, and any seal it was compressing has stopped sealing.

Materials differ a great deal: some engineering polymers and laminates resist creep far better than commodity plastics.

Where preload is critical, a metal washer with a separate isolating sleeve, or a design that tolerates relaxation, is the sounder approach.

The one whose temperature range, chemical resistance and electrical properties suit the environment - they are not interchangeable.

Nylon is the general-purpose choice: tough, cheap, insulating, and limited in temperature and by moisture absorption.

PTFE resists almost everything chemically and tolerates high temperature, but is soft and creeps readily under load.

Fibre and phenolic laminates are stiffer, resist creep better and are used where the washer must carry real compressive load.

Bonded elastomer-and-metal washers are chosen where sealing is the point, with the elastomer selected for the exposure.

Every one of these has a stated service temperature range, and exceeding it is the most common cause of unexpected failure.

No - it deforms at far lower pressure, so it is chosen for what it does rather than for how much it carries.

Compressive strength is a fraction of steel's, and the material yields progressively rather than holding its shape.

That is acceptable when the washer's job is isolation, sealing or cushioning under a modest clamping force.

It is not acceptable where the washer has to spread a heavy load over a soft substrate, which is a metal washer's job.

Combinations are common: a metal washer for load spreading with a thin insulating layer for isolation.

Torque figures developed for metal washers do not transfer, because the joint's stiffness is completely different.

Many are, provided the material is rated for ultraviolet exposure and the temperature range - and several common plastics are not.

Ultraviolet degradation embrittles unstabilised polymers, and the washer eventually crumbles at the exposed edge.

Stabilised grades and darker pigments resist this considerably better, which is why outdoor washers are commonly black.

Elastomer sealing washers for roofing are formulated for exposure and have a stated service life, often shorter than the sheet they fix.

Thermal cycling loosens joints clamped through polymers more than metal joints, because the polymer's expansion is much greater.

Where the fixing is inaccessible for its whole life, the washer's durability should match that expectation rather than the assembly's.

For electrical isolation, yes wherever contact would otherwise occur; for sealing, only on the side being sealed.

Isolation fails at any remaining path, so a washer on one side with metal contact on the other achieves nothing.

A full isolation kit is normally a washer at each end plus a sleeve through the bore, which is how flange isolation kits are supplied.

Sealing washers go on the weather side, since that is where water is being kept out.

Cushioning washers go against the fragile material, whichever side that is.

Doubling up where it is not needed adds another creeping interface and makes the joint less stable.