[Antennas] Re: Metal Reactions (Galvanic Corrosion)

[Barry L. Ornitz]

Kurt, KD7JYK, wrote:

> I have an idea for masting, but am concerned about possible 
> reactions between galvanized pipe and the following:
> 
> Copper
> Brass
> Stainless Steel
> 
> Any problems with these?  The supports would be galvanized 
> pipe and only one of these materials.

I have written about galvanic corrosion in this mail reflector 
(and others) and in the Usenet antenna newsgroups in the 
past.  The archives for this group should bring up some 
relevant articles.

To summarize quickly, when two dissimilar metals are 
electrically connected, corrosion can occur through galvanic 
action.  Typically one metal, called the anodic metal, will 
corrode preferentially to the other, the cathodic metal.  In 
the case of galvanized pipe, zinc is the anodic metal which 
protects the underlying iron or steel.

In the case you have described, the electrochemical reaction 
is between the zinc and the other metals.  To determine the 
possibility of galvanic action between the metals, you need to 
consult a table of the galvanic series of metals.  I believe 
that I have listed one on this mail reflector in the past.  If 
you cannot find it or another suitable one, email me 
personally and I can send you one.  Such a table list metals 
in order from the most anodic (sometimes called least noble) 
to the most anodic (most noble).  To find the most compatible 
materials, look to metals close to the one you are considering 
in the table.  The farther apart they are in the table, the  
more galvanic corrosion that will occur.  Better tables list 
the actual electrochemical potential between the metals.

In this case zinc is near the top of the table and is very 
anodic.  Most brasses behave similarly to copper, but there 
are slight differences depending on the particular alloy.  
Copper is fairly cathodic to zinc so galvanic corrosion will 
occur.  And since copper is more cathodic than the steel, once 
the zinc is dissolved away, the iron will preferentially 
corrode over the copper.

Stainless steels occur in several places in most complete 
galvanic series tables.  There are, of course, hundreds of 
stainless alloys, and there are at least a dozen popular 
standard alloys falling into the three major variations - the 
austenitic, ferritic, and martensitic alloys. 

The martensitic alloys can be heat treated to increase their 
hardness.  The ferritic alloys cannot be heat treated and 
their harness can only be increased by cold working.  The most 
chemically resistant stainless alloys are the austenitic 
alloys.  [I am most familiar with these from the chemical 
industry.]

To give some examples, type 410 SS is martensitic, 430 SS is 
ferritic, and 302, 308, and 316 SS are austenitic.  Most of 
the small stainless nuts and bolts sold in hardware stores 
falls in the 308 or 316 variety if it is not magnetic. [308 
can become magnetic after cold working, and many stainless 
steels are quite magnetic.  This becomes an issue when 
stainless is used as an antenna conductor.]  The martensitic 
and ferritic alloys may be called stainless, but they are far 
from completely rust proof!

The problem with stainless steels in galvanic corrosion is 
that their properties are very dependent on the surface 
treatment of the material.  Typically the surface of stainless 
is considered active or passive.  This depends on how oxidized 
the surface is and on what thermal history the surface has 
seen.  It is possible for a single piece of stainless to be 
both active and passive in different places.  Thus galvanic 
corrosion can occur between two different areas on a single 
piece of stainless.

Going back to the original question, active stainless surfaces 
of 410 or 430 alloy are slightly more anodic than copper and 
active stainless surfaces of most other alloys are slightly 
less anodic than copper.  However, passive stainless surfaces 
are generally quite a bit more cathodic than copper.  So from 
a galvanic corrosion standpoint, the stainless steels may be 
slightly more compatible than copper with the galvanized pipe 
or they may be much less compatible than copper with the 
galvanized pipe.  Murphy's Law should be considered here! 

A better choice of material for use with galvanized pipe might 
be an aluminum alloy.  Most aluminum alloys are only very 
slightly more cathodic than zinc.  If the other metal is to be 
used as a fastener (bolt, screw, etc.), it is best if these 
are made of the same metal or one that is more cathodic.  To 
minimize the corrosion, make sure that the area of the exposed 
galvanizing is much more than the area of the exposed more 
anodic metal.

If you must use dissimilar metals, consider protecting the 
joint from moisture.  Paint or other waterproofing methods 
will help prevent the corrosion.  The two metals when covered 
with water form the electrodes of a battery.  The circuit is 
completed when the metals are joined.  If you can keep the 
resistance of the loop high, for example by increasing the 
length of the liquid path between the electrodes, you will 
slow the corrosion.  Please remember than a dirty surface with 
adsorbed moisture from the air can be a conducting path too.  
Paint or other waterproofing insures a longer conductive path 
so it can slow the corrosion.

I hope this has helped.  If you have additional questions on 
the specifics of your application, feel free to contact me 
directly.

        73, Barry L. Ornitz, Ph.D.     WA4VZQ     ornitz@tricon.net