Joining pipes... Help I'm Confused!

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I realise this question has been asked numerous times in various forms, but I am still confused. The water enters my house through a metal pipe of some description (not copper and not lead). How can I determine what this pipe is made of? It is around 7/8" in external diameter (I can't measure the internal diameter as I haven't dismantled it yet although from other posts I am assuming it will be 3/4") and screws together along it's length.

Anyway, my questions are as follows:

1. I am guessing the pipe is made of steel of some variety? Is there an easy way to determine whether it is galvanized or not?

2. I am confused as to whether I require a dieletric union at the join between this pipe and the rest of the pipework in the house which is copper. Is it true that such a union is only required in systems where the water circulates, i.e. a central heating systems? If I do need to fit a dielectric union to prevent corrosion I assume I will need to bridge the union with some kind of earth bonding to ensure earth continuity. However surely doing this will defeat the object of installing it in the first place?!

PLEASE HELP ME!!!

Many thanks,

Ben.
 

Kes

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This sounds like 1/2" steel pipework with BSP threads. If the ext dia is 3/4" then the internal will be 1/2", giving a wall thickness of 1/8". It probably was galvanised once. How long has it been installed? Long time probably = fragile, so don't be too robust with it. Danger points are where it contacts the ground, can you see any corrosion there? Don't know about sacrificial nodes. I would call in a plumber to advise.
 
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Many thanks for your reply. I have re-measured the outer diameter of the pipe and it definately seems to be nearer 7/8". I guess I won't know for sure until I have dismantled it though. Unfortunately I can't get to where the pipe enters the house so I cannot assess how corroded it is. I suspect that I don't want to know. The house was built in 1936 and I doubt that part of the plumbing has been updated since then.

Does the zinc coating wear away? Looking on the net it seems that it is chemically bonded to the iron molecules in steel as part of the galvinisation process. If the zinc does wear away I am guessing that the rate of corrosion would increase dramatically.

What is the standard parctice when connecting copper pipes to steel or galvanized steel pipes? What is taught in plumbing school? I probably know the answer to that.... Replace the inlet pipe with MDPE. Is it acceptable practice to join steel (galvanised or not) to copper using a Yorkshire Kuterlite Female Coupler 612 15mm x 1/2" or 15mm x 3/4" or similar or will I end up with leaking/blocked pipes a couple of years down the line?

Many thanks,

Ben.

P.S. I would get a plumber in but last time I did that I got charged £50 to replace a stopcock. A job which took no longer than 10 minutes!
 
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3/4 in pipe is 1.050in OD. 1/2 in pipe is 0.840in OD.
galvanising is done by 'hot dip'.
Zinc applied chemically is 'electro plated'.
It does erode over time, the time depends on the conditions. Of course the pipe ends where it is cut and threaded will not be galvaised.
Zinc can also be ' metal sprayed'
I have a few 'plain iron' fittings on my boiler fitted to 'bronze' fittings with no sign of corrosion yet, after 34 years.
 
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