On Dec 10, 2007, at 6:41 AM, R.C.Macaulay wrote:
Howdy Charlie,
Surprisingly, your thoughts parallel a view of a mental comparison
we hold about the cavitation " needle pitting" we observe on the
surface of our high speed rotating member in vacuum inducting
chlorine gas in certain applications usually with seawater or
brackish water infiltration. The rotating member is made of UHMW
high density polyethene and the cavitation attack can be as deep
as 3/8 inch. According to our experience with cavitation phenomena
NO pitting should occur with UHMW. Bronze and non ferrous metals
love to be attacked.. but .. UHMW.. no way , shouldn't happen..
unless. . a process at the atomic level exists so perhaps your
explanation has greater merit.
I am curious as to why you would not expect pitting to occur with
UHMW. I would expect a cavitation attack to be worse on plastics
than metal - at the same rate of cavitation bubble formation on the
surface that is. Plastics might ward off bubble formation in some
circumstances via their flexibility and softness, and thus have a
tendency to prevent vacuum formation. This tendency would have
limited effect on cavitation however, and would also be temperature
sensitive. Unfortunately, due to increased flexibility at higher
temperatures, plastic rotors tend to go a bit limp and lose their
pumping capacity as things warm up. (I've seen this first hand in a
plastic impeller driven marine engine cooling heat exchange system.)
In any case, cavitation bubbles, once formed on a surface, erode
based on their temperature, so plastics, having a lower specific
heat, should be more vulnerable to them than metals, true?
Horace
