elctrical bond between faying surfaces

[jallenc]

When applying the BMS595 sealant to the electrical faying surface, do you also cover the electrical bond area.
My interpetation of Boeing Spec BAC5117-6 Cat 2 is that you apply the sealant to the entire faying surface of one part. I'm dealing with an aluminum assy and there is a bracket to be installed with a small "bare" area on the bracket and the mating part.

 

[IRstuff]

Can you meet your electrical bonding requirement with the sealant covering everything?

TTFN

FAQ731-376

 

[jallenc]

Yes! we meet the requirement, but I'm confused as to why there would be a bare area covered by sealant. Why not have chem film and primer on all surfaces and let the fasteners establish the contact.

 

[WKTaylor]

jallenc...

1. Fay sealing is for fluid exclusion and corrosion resistance [isolation] between fay-surfaces of parts (works in conjunction with the individual fay-surface corrosion-protective finishes). Fay sealing deliberately intends to reduce/eliminate electrical activity between surfaces.

2. The electrical bond actually occurs between sheets thru the fasteners and bare fastener-holes.

Driven solid rivets are ideal since they forge/deform to fit irregular hole-walls with fairly high interference forces and remain relatively tight/sealed for life.

Pin-collar, bolt-nut and blind fasteners are less-ideal for electrical bonding due to relatively poorer fit with holes [RE: driven rivets]; fastener finishes vary from non-conductive SFL to cadmium to aluminum-IVD to aluminum pigmented resin coatings (and sometimes installed wet with sealant for corrosion protection); and potential for hole/fastener corrosion [increased resistance] over time.

Large numbers of metallic fasteners thru holes in layered metallic materials generally provides an adequate electrical bonding path between parts… but NOT always.

3. Where electrical bonding gets tricky is adhesive bonded metallic structure: every part is actually isolated from the other by primer and adhesive.

Electrical bonding rivets through layers, wire-jumpers between parts, etc must be deliberately employed to electrically bond parts together.

NOTE. I have seen adhesive-bonded parts blown-apart due to poor electrical bonding and moisture intrusion when hit with lightning. Likewise I have seen aluminum bonding rivets disintegrate [vaporize] when high by a high current surge [lightning, high-power shorts, etc].

4. NOTE. “Conductive adhesives” and “conductive sealants” have relatively poor adhesive qualities and have been known to induce corrosion if not perfectly sealed from electrolyte intrusion.

5. Electrical bonding of composite structure is a whole different issue.


Regards, Wil Taylor