On Jan 22, 8:00*pm, David Nebenzahl wrote:
On 1/22/2011 1:08 PM RicodJour spake thus:
On Jan 22, 3:38 pm, wrote:
On Sat, 22 Jan 2011, RicodJour wrote:
On Jan 22, 1:59 am, David Nebenzahl
wrote:
Now *that* makes my BS meter twitch just a mite: why would
there be corrosion? Galvanized pipe is zinc over steel, so
galvanized + steel (black) should be no problem, right?
Time to recalibrate that meter, kimosabe. *Zinc is used as a
sacrificial anode in boats for the very reason that it will
corrode first and protect the iron.
http://www.mikeholt.com/mojonewsarch...alvanic-Action....
YOur BS meter needs recalibrating. The ZINC may corrode, but that
is not an issue here. It is a very thin coating on one surface that
is beinf threaded to another (identical) surface. NOT an issue.
Period.
Why would the zinc corroding not *be an issue?
If you have a galvanized pipe and the coating is compromised in any
way, the corrosion will be concentrated at that point, right?
This still makes no sense to me. (And just for the record, whenever I've
plumbed for gas I've used black pipe, not galvanized.)
I mean, with black pipe, the entire goddamned pipe and every fitting is
subject to corrosion, right? Except for the mill scale, or whatever
constitutes the "black" on black pipe, it's completely unprotected.
That's why we use galvanized for water. So if you use a galvanized
fitting on a run of black pipe, it sure as hell is not going to corrode
any faster than the rest of the pipe, so why sweat it *on that basis*?
There may be other reasons not to use galvanized with gas lines: all the
pros I've talked to about this lately tell me it makes no difference.
Nonetheless, most people still use black for gas.
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To the above I would add that the comparison to the boat use of zinc
as a
sacrificial anode is invalid, unless the pipe in question is being
used in
an environment where it's EXPOSED TO AN ELECTROLYTE. The
electrolyte essentially completes a circuit that allows current to
flow. In
the case of boats, that electrolyte is water, or even worse, salt
water.
With the boat, you have sacraficial zinc plates electrically bonded to
the
underwater metal parts, ie shafts, props, struts, etc. The circuit
looks
like two different metals, ie bronze and zinc, sitting in a beaker of
salt
water. Very similar to the experiment kids can do, using a lemon, a
dime, and a penny to make a battery. As the current flow, the zinc
erodes and slowly disappears.
With galvanized pipe installed on a gas line indoors, you don't have
an
electrolyte. If it were buried, then you could have an issue. But
buried
you have an issue with black steel too. I think it's still being
allowed in
some areas. But it's supposed to be coated and or wrapped with tape
to prevent corrosion. A process that is far from perfect. I've
seen
black steel pipe fail underground in as little as five years. It
looked
like swiss cheese, hard to believe. But it was obvious what had
ocurred. You could see where the installers had used black pipe and
then poured the tar like coating over the top of it after it was
installed
in the trench. The bottom portion did not get coated and that is
where the failure ocurrred. I remember at the time, mid 90s, we had
the gas company involved in recommending what to do at a 120
unit condo. At that time even they were undecided which was better
for that application, either black pipe correctly installed or
galvanized.
Today, for corrosion issues, plastic is now widely used underground.
Over the years gas companies have looked at ALL the issues, including
not only what the pipe is carrying, but where it is being used. The
only
potential problem with galvanized that I've ever seen documented in
any way is that which DPB brought up, which is the
concern that certain impurities in the gas could lead to the zinc
flaking
off and ultimately clogging orifices, etc. I don't believe it was
ever
an issue of the pipe failing, leaking, etc. If galvanic corrosion
were a
real issue with gas pipe in an application inside a house, the gas
companies that allow it would be pretty stupid, no? As would the fire
rating, safety, insurance companies, etc.