A couple of more questions...
Is a fiberglass-epoxy or boron-epoxy cloth isolation layer bonded to the fay-side that will remain in-contact with the aluminum? Also what finish is applied over fay/mating surface with the aluminum. Non-corrosion or corrosion-inhibited primer???
Is the aluminum shot-peened, followed by PAA, SAA, CAA or BSAA anodic coatings, followed by an adhesive bond primer or a general purpose corrosion protective primer [epoxy of course]???
Does the joint rely on: (a) adhesive bonding + fasteners + sealant over-coat; or (b) sealant + fasteners + sealant over coat [IE: black-aluminum Assy practices]?
I am unfamiliar with SAA of titanium: what spec are these fasteners coated per? NOTE: I reviewed may Ti Hi-Lok pin specs and did not find any reference to SAA coatings.
As for the NUT you specified [A-286 passivated + SFL]...
What material does the nut bare-up-against: aluminum? GRP?
I sure hope You are planning to install a dissimilar-material and friction/bearing isolation WASHER underneath the Nut... or are You planning to install a nut [A286+SFL] with an integral A286 washer!!????? Regardless, the washer alloy and coating(s) must be compatible with material it is bearing against.
The A-286 Nut passivated + SFL coating will be fully compatible with Ti-Bolt/pin threads.
General/random thoughts
Primary Ti fastener [bolts, lock-bolts] coatings I'm familiar with are...
NONE [bare, cleaned]
IVD aluminum
Organic + aluminum Pigment
Phosphate-fluoride conversion coating.
Baked solid film lubricant applied over bare or Phos-fluoride-treated surfaces
Anodized Ti finishes appear to be reserved for structural parts, not fasteners.
Virtually all Ti fasteners I reviewed are either bare, have an aluminum pigmented organic coating or are phos-fluoride CC treated; and may/may not have a lubricant or SFL coating applied.
Generally speaking, the IVD-A, aluminum pigmented organic coating and SFL coatings are best suited for aluminum and certain other metals; but are relatively hostile/useless in contact with graphite fibers.
The next best coating suitable for contact with aluminum [whit added isolation coatings] , and to a high degree graphite, is probably the phos-fluoride CC or bare.
Note...'bare/cleaned titanium is generally inert; however it is less desirable dissimilar-couple against aluminum.
NOTE. MIL-STD-1568 MATERIALS AND PROCESSES FOR CORROSION PREVENTION AND CONTROL IN AEROSPACE WEAPONS SYSTEMS makes the following statements
5.9.1 Detail requirements.
All permanently installed fasteners except as noted in 5.9.3 (all fasteners not normally removed
for regular access or servicing) used in areas up to 1070C (2250F) shall be wet installed with
either a corrosion inhibiting sealant conforming to MIL-S-81733 or an epoxy primer conforming
to MIL-P-23377, Type I, Class 1 or 2, or a MIL-P-85582, Type I, Class 2 material which does not
contain water. In high temperature areas, exceeding 107oC (225oF), Type I, Class 1 or 2 epoxy
primer, or a sealant which is suitable for the thermal environment shall be used. Fasteners in
integral fuel tanks shall be installed with wet sealant conforming to MlL-S-8802 or MlL-S-83430.
The use of sealant or corrosion inhibiting coatings not addressed by this paragraph must be
approved by the procuring activity.
5.9.5 Fasteners in graphite composites.
Fastener materials for use in graphite composite structures shall be titanium or A286. Cadmium
plated fasteners and aluminum fasteners shall not be used. Fasteners shall be wet when
installed using sealants as specified in 5.9.1.
Hmmmm... gotta-go-back-to-work.
Regards, Wil Taylor
o Trust - But Verify!
o We believe to be true what we prefer to be true.
o For those who believe, no proof is required; for those who cannot believe, no proof is possible.
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion"]
o Learn the rules like a pro, so you can break them like an artist. [Picasso]
