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

Hmmmm - that throws more light on the subject for me
Thanks Bill !
George (down under)

Bill,
While were on the subject, I've heard that Titanium has it's own set of
problems in different environments.
Can't for the life of me think what this might be - any suggestions in
relation to engine parts ?
George (down under)

Other than high cost, some difficulty in machining, low thermal conductivity
and
relatively poor sliding wear performance (like a cam or lifter or valve
stem) it is
a pretty good engineering material.  Depending on the alloy, it can be
nearly as
strong as good steels and only slightly heavier than aluminum.  Most alloys are
extremely corrosion resistant, for example, seawater-cooled power plants use
titanium tubes in the heat exchangers!  The alloy used for this is not very
strong,
tho, and the higher strength alloys are not as corrosion resistant, but are
still
quite good.  One other potential problem is the lower stiffness compared to
steel.
Steel has a Young's modulus (measure of stiffness) of 30 million psi,
aluminum is
about 10 million and titanium is about 15 million.  If you made three
dimensionally
identical cantilever beams and loaded each the same at the tip, the aluminum
one would deflect 3 times as much as the steel, and the titanium one would
deflect twice as much as the steel.  Sometimes stiffness is critical, in flutter
applications, for instance. Stiffness has essentially nothing to do with
strength.

One example of a bad application is the titanium backpacking cooking pots
and pans offered.  They are light and strong, but the thermal conductivity is
poor, which is OK for boiling water, but poor for any other cooking.  The
heat won't spread laterally and any non-uniformity in the heat source (likely)
isn't  corrected by the pan.  Scorched in one spot, cool nearby.  Poor choice
of materials.  The Russians are desperately trying to figure out what consumer
products they can make to try to utilize a huge investiment in titanium mining,
refining and fabrication originally invested to support aerospace applications.
You can now buy a Russian titanium crowbar for about $50-75 or so, IIRC.
Wierd, but probably OK, depending on the alloy.  Materials selection is a
fairly
complex process and requires due diligence.

Titanium is MUCH cheaper today than it was 20 yrs ago, and we can machine
it a lot better (much better cutting tools and lubes), but it is still not
for everything
by a long shot.

Hope this answers your somewhat open ended question.

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

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