Tony, K1KP, asked:
> 3. Where can one buy the cathodic protection devices? What
> is a good cathode material? Zinc like on boats? Or does it
> have to be further down the electroactive series to protect
> the zinc galvanization of the guy anchor?
Zinc is normally used to protect iron (galvanizing). The only
practical materials more anodic to zinc are magnesium and
magnesium/aluminum alloys in that order.
Earlier, Don Moman, VE6JY, wrote:
> A rule of thumb is 1 amp flowing for 1 year will remove 20
> lbs of material.
This is a rule best discarded as the mass of material removed
is dependent on the molecular weight of the material and the
valence electrons of the material. Faraday's Laws of
Electrolysis govern the removal of material. One Faraday of
electricity flowing will dissolve (or deposit) one gram-
equivalent weight of a substance. The gram-equivalent weight
is obtained by dividing the gram-atomic weight of the
substance divided by the number of valence electrons required
per atom. A Faraday is 9.6501E4 ampere-seconds.
Rather than go through the math, let me present a table of
common materials [use a fixed width font].
Material ounce/amp-hr pounds/amp-year
-------- ------------ ---------------
Aluminum 0.0118 6.465
Zinc 0.0430 23.559
Iron (valence 3) 0.0245 13.423
Iron (valence 2) 0.0368 20.162
Copper (valence 2) 0.0418 22.901
Copper (valence 1) 0.0836 45.802
Silver 0.1420 77.798
To go back to Tony's question, it is possible to use an active
cathodic protection scheme instead of a passive anodic
material, i.e. one that uses a controlled voltage to
counteract the potential produced by the corroding material.
Active systems require frequent checking to stay in control.
They can also generate electrical noise. Eastman Chemical
Company used active protection in a few areas. Running low-
level instrumentation signals in the area often produced quite
high noise levels. In a buried pipeline, this noise should not
be a problem, but it might in Tony's situation.
73, Dr. Barry L. Ornitz WA4VZQ ornitz@tricon.net
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