fasteners and galvanic corrosion 1

[Qluq]

Hi everyone,

on our carbon spar with aluminium brackets attached, we tried to prevent galvanic corrosion by applying glass separation plies between the aluminium and carbon parts. Where glue is not sufficient, the brackets are connected to the spar by titanium bolts with titanium or stainless steel washers and stainless steel nuts, some of which are silver plated. The bolt shanks contact the carbon in the spar hole, and contact the aluminium in the bracket hole, we did not put any isolation between the holes and bolt shanks.

We put a test section of spar with a titanium bolt, stainless washer and stainless nut in a thin layer of salty water and are seeing corrosion on the washer and nut. The test is too simple to draw any conclusions, but it does make us wonder...

Is this in your opinion a sound enough way of preventing galvanic corrosion, or would you recommend different material combinations, additional isolation (eg between shank and hole, how would that be done?), or some solution we have overlooked altogether? We will be flying under an experimental certification, we expect to not have a lifespan of ages, about 2 to 3 years. However, we will be flying near the sea, so salty water around plenty.

Also, what literature can you recommend on galvanic corrosion in composite metal hybrid structures?

Thanks!

 

[rb1957]

if you're flying low level over oceans … then frequent water washing would help reduce corrosion.

The brits would spray wash the Nimrod after each mission (I believe) … I've seen pix of a Nimrod taxiing over a spray.
The USN (I believe) don't wash their P3s … and have lots of corrosion.
Draw your own conclusion !

another day in paradise, or is paradise one day closer ?

 

[Qluq]

thanks, we will!

any tricks for the internal structure?

 

[Sparweb]

Sealant on fastener heads/nuts
Fillet sealant
Closure webs
Gaskets
Blade seals/wipers
Boots
Drains
4 more coats of paint...

Whatever it takes to keep the moisture out.

http://www.sparweb.ca

 

[WKTaylor]

Many Years ago I found this document that provided insight into mechanical fastening of composites.

AFWAL-TR-86-3035 DESIGN GUIDE FOR BOLTED JOINTS IN COMPOSITE STRUCTURES [AD-B108 123]

Galvanic Compatibility and Corrosion Prevention

Sample paragraph...

The main considerations in the selection of the fastener
material are its compatibility with the bolted plate material and
its mechanical properties. Galvanic corrosion is a problem when
steel or aluminum is used adjacent to graphite/epoxy composites,
especially in a salt spray atmosphere (see Table 1, Figure 15 and
Reference 12). Titanium does not corrode when it is in contact with
graphite/epoxy composites. The compatibility of other materials
with graphite/epoxy composites is rated in Table 1. Consequently,
titanium fastener& are preferred for use in bolted composite
structures. Also, a corrosion barrier is generally introduced
between bolted composite and metallic parts, if the metal is steel
or aluminum (see Figure 15).


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]

 

[tbuelna]

The silver plated cres nut installed on a titanium bolt should be avoided due to potential stress corrosion cracking.

 

[Qluq]

Thanks everyone!