Replies

1. 
   peteduffy | Feb 25, 2006 08:07pm | #1

   Is this plumbing for water supply, waste, or gas supply?  Since it's black pipe, I'm guessing gas supply.  Also, are you sure they are copper fittings and not brass or bronze?

   Anytime 2 dissimilar metals (like steel and copper) are in contact and moisture is present, the connection will corrode.  This is called galvanic corrosion.  It actually creates an electrical cell, on a really small scale, but enough to transfer electrons from one metal to the other.

   If it as a brass fitting on black pipe, I have seen that on gas supplies and it shouldn't be a problem.  Quite common, actually, as long as all the pieces are rated for their intended use.

   If it is for water supply, it will corrode rather quickly.  Black pipe has no corrosion resistance, like galvanized does.

   Hope this helped.  Good luck.

   Pete Duffy, Handyman

1. 
   PatchogPhil | Feb 25, 2006 08:21pm | #3

   Might also be on a hot water heating system.  In a closed-loop system like hydronic heating where no new fresh water is introduced on a regular basis,  there will be very minimal (i.e. basically microscopic) oxidation.  Once the system heats up and air (oxygen) bubbles out and leaves via the airvent,  there's no worry for any corrosion.

1. 
   BillHartmann | Feb 25, 2006 09:01pm | #4

   "Might also be on a hot water heating system. In a closed-loop system like hydronic heating where no new fresh water is introduced on a regular basis, there will be very minimal (i.e. basically microscopic) oxidation. Once the system heats up and air (oxygen) bubbles out and leaves via the airvent, there's no worry for any corrosion."No corrosion on the black pipe from oxidation (rust).But the water is still conductive, Won't you still have problem with electroylsis?

1. 
   peteduffy | Feb 25, 2006 10:16pm | #5

   Yep, the electrolysis (galvanic corrosion) will still occur regardless of the presence of oxygen.  Galvanic corrosion is different than oxidation.  It depends on the differerence in the reactivity of the two metals.  Metals close together on the periodic chart of elements will react less with each other than those farther away from each other.

   e.g.  Nickel and Copper are right next to each other on the chart, so there will be less reactivity than say, iron and aluminum.  I'm sure many of us have had to deal with with a frozen aluminum nut on a steel bolt or vice versa.  Coating this connection with grease helps keep water out, slowing down the galvanic reaction.

   Magnesium is a highly reactive metal, and that is why it is used as a sacrificial anode on boats.  It's also used in underground pipes.  They put magnesium in contact with the steel or other metal pipes, therefore sacrificing the magnesium.  The magnesium corrodes so the other metal doesn't.Pete Duffy, Handyman

2. 
   plumbbill | Feb 25, 2006 08:18pm | #2

   To add on pete's post

   If it's waste or vent then it is normal practices.

   If it's heating there should be an isulater installed, this can be done a few different ways.

   Dielectric union which has a plastic washer & a rubber washer to seperate the two.

   Brass nipple atleast 6" long

   or a clearflow¯ nipple which has a plastic liner inside the nipple to seperate the two.