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Selection of appropriate material for washers

Selection of appropriate material for washers

Selection of appropriate material for washers

(OP)
In my daily work repairing aircraft, I often fine myself wondering how certain hardware stack-ups will be effected by or produce effects on the aircraft in regard to their corrosion properties.
Going through school, we were taught about anodes and cathodes and dissimilar metals and their suseptability to corrosion. I was told that a general 'rule-of-thumb' was to use aluminum washers against the skin of the aircraft regardless of the fastener type to prevent dissimilar metal corrosion.  However, while working in a manufacturing setting not too long ago and aircraft building from engineered blue-prints, when installing machine screws or small AN bolts we used a washer of the same material as the fastener unless we were fastening a clamp in which case the washer was the same material as the clamp.  Further, when stacking washers for an electrical bonding jumper there is a specific stack of both steel and aluminum washer arranged to prevent corrosion of the aircraft skin.  So, obviously there is some consideration given to corrosion prevention when arranging a washer stack-up for fasteners. So, when installing things like steel panel screws or small, steel AN3/AN4 bolts, or even an electrical bonding jumper, where should the material consideration lie, with the surface of the mating materials or the material of the fastener being used?? I am not concerned with specific, stress critical applications. Only general fastening or panels, clamps or otherwise non-critical applications.   I hope I have made myself clear.
Thanks

RE: Selection of appropriate material for washers

foglights,

That's a very broad question, and normally aircraft mechanical systems/structural engineers give these galvanic compatibility issues lots of thought. Usually, any metallic interface that might be subject to galvanic corrosion gets sealed or uses some sort of corrosion protection (anodize, plating, primer, etc.) on the base materials.  In my experience, electrical bond surfaces are always sealed/potted.

Attached is a page regarding corrosion from NASA's fastener manual.  It briefly discusses the issue of which component will be more active (anodic) and thus will eroded.

Hope that helps.
Terry

RE: Selection of appropriate material for washers

Foglights...

Washers get-no-respect... but they are important buggers... more-so than most people believe.

See MIL-STD-1515 [now NASM1515] Requirement 209 for a fairly good discussion and listion of AN/MIL/MS/NAS washers.

Regards, Wil Taylor

Trust - But Verify!

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RE: Selection of appropriate material for washers

RE my previous posting...


NAS1587 are NOT appropriate for 160-180 KSI fasteners: WAYYY too "soft" [low yield... except for extremely high temp use]. They crush is service [rela-world experience].

Missing from this list are these important washers...

MS20002 WASHER, COUNTERSUNK AND PLAIN, HIGH STRENGTH [for 160--180 ksi fasteners]

MS21206 WASHER, COUNTERSUNK AND PLAIN, FOR USE WITH BOLTS AND NUTS UP TO AND INCLUDING 220 KSI FTU

NAS1149 WASHER, FLAT

MS21299 WASHER, COUNTERSUNK AND PLAIN, FOR USE WITH BOLTS AND NUTS UP TO AND INCLUDING 260 KSI FTU

Regards, Wil Taylor

Trust - But Verify!

We believe to be true what we prefer to be true.

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