We need to make sure that we have a full ply or even possibly two
of glass (BID) over the hub to insulate the carbon from the steel.
I think the most serious corrosion issues have been with aluminum
fasteners, which try to "protect" the carbon and due to the large
surface area disparity, the fasteners go away rapidly.  This same
problem occurs with steel plates riveted with aluminum rivets.
The steel won't corrode much around the rivets, but the rivets sure
go fast in a high corrosion environment.

Bill Freeman
Long EZ builder & pilot
BSME, MSME
EAA Technical Counselor

Hello Bill,
Just on that issue, the feed back from a composite boat builder of some
experience has informed me that carbon attacks steel as well (even 316
stainless steel) isolation is one method (as you have suggeated) he has
suggested it's best to use titanium ( about the only thing that really
stands up). However this may be in the more agressive salt water corrosion
environment.
George (down under)

George,
The carbon doesn't actually attack the steel, but the steel provides
protection to the carbon by corroding, the same way zinc protects
steel in something that is galvanized and the galvanized is scratched
thru. The two materials set up a corrosion cell (like a battery) in the
presence of an electrolyte. One corrodes preferentially to protect
the other.  This is why your boat friend clamps zinc electrodes onto
a boat prop shaft to reduce corrosion as a sacrificial anode.
Fresh water is a poor (but workable) electrolyte.  Salt water is a
great electrolyte.  So, the carbon may not actually attack the steel
but, the net effect is the same.  The steel corrodes (anode) and the
carbon (cathode) is OK.  You are absolutely correct that the true
solution is to use titanium, but it is VERY expensive and hard to machine.

I believe that we can survive in the expected low to moderate
corrosion environment with proper glass isolation.  I would not
even attempt it in a marine environment (salt water).  In that environment
titanium would be essentially mandatory, altho some stainless
steels might work reasonably well if isolated with a glass layer.
Might not, too.

This whole discussion plays back into why my brother (test pilot
with a major airframe mfgr) thinks that corrosion played a role in
the FLT 587 crash.  No evidence other that the fact that carbon
structures can cause corrosion in aluminum or steel adjacent structures
if not engineered properly and maintained properly.

Bill Freeman
Long EZ builder & pilot
BSME, MSME
EAA Technical Counselor

 
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