Dissimilar Metals [RE-wrenches]

[graham at solarexpert.com]

Dear Wrenches,

I have no idea why zinc cannot come in contact with aluminum, but here 
is a quick note on my experience with electrolysis caused by dissimilar 
metals in direct contact.

I recall a few times being called to fix leaking DHW solar panels on 
apartment building roofs.  On one building in particular I noticed that 
almost all 30 solar panels had a white powdery substance under the 
glazing on the absorbers.  I knew from experience that this white 
substance was caused by electrolysis, but I had never seen such a bad 
case.  Upon closer examination of the entire water heating system I 
found that all of the connections on top of the row of solar storage 
tanks were copper pipes connected directly to galvanized nipples.  This 
system which was about 15 years old was beyond repair.

I do not know what this white substance is but it is a clear indication 
of electrolysis.  Another example was a residential system that had this 
powder all over the pipes on the solar loop of plumbing.  The homeowner 
had made an electrical connection to ground on the copper solar pipes in 
his basement.  This system had dielectric unions that did not provide a 
path to ground due to the Teflon and rubber between the dissimilar 
metals. Brass unions can also be used to connect dissimilar metals.

Solar heating systems that have a propensity to leak from pin holes in 
the sides of copper piping is also an indication that electrolysis is 
occurring.  It seems to take about 10 years or more for the negative 
effects and damage to appear.

I hope this proves helpful.

Graham Owen
GO Solar Company


Jim Hartley wrote:
> 
> Baran Galocy wrote:
> > Wrenches,
> > 
> > Would someone be so kind as to educate me on why it is not advisable to 
> > use
> > hot-dipped zinc galvanized products in direct contact with aluminum.  
> > I've
> > been told it is a no-no, but I've not been told the long-term results of
> > such an action.
> 
> The problem of galvanic corrosion is precisely why plumbers use 
> dialectric unions.  The union employs a non-metallic seal that isolates 
> the two dissimilar metals from one another.  You can accomplish the same 
> 
> thing by using non-metallic spacers prohibiting
> dissimilar metals contact.  Ideally, the spacers should be shouldered
> so that a sleeved portion penetrates through the hole in both joined 
> members.  Your bolt would slide through this sleeve having isolation 
> from the surrounding metal surfaces.  Enabling the correct fitted  
> tolerances and positioning may be a bit of extra work, but certainly 
> do-able.  Usually such dialectric spacers/washers are made of Teflon or 
> some similar material. I suspect you can find such things in a good 
> hardware store.  If not, you may have to order them from a fastener 
> supply house.  In a pinch, you can use just flat non-metallic surface 
> washers and dialectric barrels or sleeving cut down to the right length 
> which passes through the holes of the two joined metal elements.  
> Ingenuity will get you through this problem.
> 
> Jim Hartley
> http://www.homestead-specialties.com
> JamesHartley at homestead-specialties.com
> Direct Email: grail at inil.com
> 630-584-8780
> 630-584-9092 FAX
> 
> 
> 

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