Inscrutable materials compatibility issue... Your comments needed.

[WKTaylor]

I think a part number error may have created a major materials compatibility issue.

1. Here is the material/finish combinations that I expected to see... which are on the drawing and noted for 'for later assemblies'...

Low alloy steel bolt with a Cd + Cr2O3 finish [cadmium plating W/ post plating chromate treatment]... assembled with a clad-aluminum 7076-T6 washer [~0.063 thick] and a low alloy steel nut with a Cd + Cr2O3 finish... and assembled 'wet' with chromated PS Sealant.

Variations for this installation would be a the nut would be thus...

alloy steel nut with a Cd + Cr2O3 + solid-film lubricant [SFL] finish... and assembled 'wet' with chromated PS Sealant.
(or)
A286-HT160-KSI Nut + passivation + SFL finish... and assembled 'wet' with chromated PS Sealant.

NOTE The SFL finish noted above is intended for adhesion to PS sealant.

2. My problem... real-world.

I have a low alloy steel [LAS] bolt with a Cd + Cr2O3 finish [cadmium plating W/ post plating chromate treatment]. The mechanics tech data then installs a clad-aluminum 7076-T6 washer [~0.063 thick] and an A286-HT160-KSI nut with silver-plating.

I am aware that Al and Cd are widely dissimilar to Ag... and that LA steel is widely dissimilar to A286 CRES... and that that Ag is not suitable for LA steel. Also, these parts are assembled 'wet' with chromated polysulfide sealant... and all parts have good adhesion with the sealant... except the silver-plating which has very poor adhesion to the PS sealant.

My concerns...

Sealant will rapidly degrade on the silver-plate allowing moisture to enter the joint.

In the presence of moisture, cadmium will sacrifice to silver which exposes bare steel and forms red-rust on the exposed bolt [especially steel in the presence of a high-nickel alloy A286 CRES] nut.

Aluminum will sacrifice to silver forming corrosion... and the presence of red-rust will also aggravate aluminum corrosion.

NOTE.
Joints with this combination have been together less-than 3-years... and no-one is specifically looking for damage.

The PN NUT I expected to see is MS21042-3 [LAS+Cd+Cr2O3] or MS21042L3 [LAS+Cd+Cr2O3+SFL]

The PN NUT designated by Tech data is MS21043-3 [A286 + Silver-plating].

What is(are) Your experience(s)/knowledge-of this weird material combo?



Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[EdStainless]

So you expect to see a steel bolt + Al washer + A286 nut?
I don't think that the Cd+chromate will prevent it (but the sealer may slow it down) and I doubt that the Ag will make things worse.
I don't know how you would ever prevent the Al and steel from corroding.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[WKTaylor]

Ed...

Finishes I expected to see are in Para 1. These are compatible/typical for aero fastening.

The A286 + Silver plate, Para 2, is wild card when mated with steel+Cd/Cr2O3 and aluminum [clad only] combo.





Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]