Smitty Two wrote:
In article ,
"Morgans" wrote:


"Lou" wrote in message
groups.com...

Ok, but why do I want to coat the aluminum? Is this a process just for
metal planes?
should i be doing this on my aluminum hinges attatched to my wooden
spars?


Aluminum will oxidize if left to it's own means. What the etch - alodine
coat process does is prevent the process form continuing on it's own.



Aluminum protects itself. When it rusts (aka oxidizes,) which it does
very very quickly, it forms aluminum oxide (duh.) A coating of aluminum
oxide is about the hardest, most impervious thing you could put on a
piece of metal. The "process" does not "continue on its own." As soon as
a microscopically thin layer forms, your aluminum is protected from
further degradation.


Bingo! You are 100% correct Smitty. The oxide is impervious to water
and is very resisant to abrasion. Many metals form an insoluable oxide,
with the most notable exception being steel. Rust is soluable AND
hygroscopic, so the rust just goes straight through once it starts.

The wild card here is acid. Acid will eat right through the oxide layer
without stopping to ask questions, exposing another microscopic layer,
which then oxidizes. Unfortunately, our culture has spent the last
200years burning tons of coal and dumping all sorts of sulfur in the
air. This sulfur rains back down on us (and our airplanes) as very
dilute sulfuric ACID. That's why the Statue of Liberty needed to be
refubished. The acid rain had worn the copper thin.

Put something on that aluminum, not to protect it from water which is
actually rather benign to aluminum for the reason you state, but to
protect it from the acid that you'll be flying through.

--
This is by far the hardest lesson about freedom. It goes against
instinct, and morality, to just sit back and watch people make
mistakes. We want to help them, which means control them and their
decisions, but in doing so we actually hurt them (and ourselves)."