At 06:20 AM 6/9/2011, Robert Fisher wrote:
>So, is the lesson here that a movable line will likely wear through a
>stationary line?

No, actually.  It depends on lots of things.  But it's quite typical
that when you have a hard metal and a soft one rubbing together, the
hard one will take more wear because particles will embed in the soft
one but scour the hard one.  That's part of why bearings are made of
babbitt metal, incidentally; it's better for tiny particles to embed
than scour both surfaces.

Also the more corrodible side may take more wear because of a process
called fretting corrosion, where corrosion is mechanically knocked of
leaving a fresh corrodible surface at the point of contact.  This is
a microscopic process and caused lots of trouble to the electronic
connector industry when gold got expensive around 1980 and there was
a rush to substitute tin plating.  I think some fairly heavy research
went into developing contact systems that were less subject to
micro-motion with thermal cycling.  It's a stinker not only because
it's a rapid process, but because the corrosion points are invisibly
tiny.  See
http://www.connectorsupplier.com/tech_updates_BM_Degradation2_6-19-07.htm
.  On a larger scale you can get problems like this:
http://corrosion.ksc.nasa.gov/fretcor.htm .

Yours,
David