[Antennas] Re: Metal Reactions (Galvanic Corrosion)

Wed, 16 Apr 2003 11:15:13 -0700

Barry:
As usual you have authoured a great letter.
Thank you.
Chris opr VE7HCB

At 03:04 AM 2003-04-16 -0400, you wrote:
>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     [email protected]
>
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