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[TowerTalk] Guy Anchor Cathodic Protection - Where?

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Subject: [TowerTalk] Guy Anchor Cathodic Protection - Where?
From: (Barry L. Ornitz)
Date: Sat May 3 20:59:17 2003
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

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