# dielectric union

**URL:** <https://boards.straightdope.com/t/dielectric-union/98218>\
**Category:** Factual Questions\
**Created:** [March 14, 2002, 5:05am UTC](https://boards.straightdope.com/t/dielectric-union/98218 "2002-03-14T05:05:45Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![dcurt](https://avatars.discourse-cdn.com/v4/letter/d/f6c823/32.png) [@dcurt](https://boards.straightdope.com/u/dcurt)\
**Post date:** [March 14, 2002, 5:05am UTC](https://boards.straightdope.com/t/dielectric-union/98218/1 "2002-03-14T05:05:45Z")

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I am wanting to tie in to a galvanized water line with new copper pipe.the guy at the hardware store is telling me I need a dielectric union ? is there anything to this? at the start of the new line I will install a brass shut off valve,to me it seems like this will do the same thing as the union.

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**Author:** ![Gary\_T](https://avatars.discourse-cdn.com/v4/letter/g/3ec8ea/32.png) [@Gary\_T](https://boards.straightdope.com/u/Gary_T)\
**Post date:** [March 14, 2002, 5:09am UTC](https://boards.straightdope.com/t/dielectric-union/98218/2 "2002-03-14T05:09:59Z")

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If I’m not mistaken, the dielectric union insulates the two dissimilar metals from each other. If they are not separated electrically, crud will build up fairly rapidly and clog the pipes.

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**Author:** ![NutMagnet](https://avatars.discourse-cdn.com/v4/letter/n/ea5d25/32.png) [@NutMagnet](https://boards.straightdope.com/u/NutMagnet)\
**Post date:** [March 14, 2002, 6:01pm UTC](https://boards.straightdope.com/t/dielectric-union/98218/3 "2002-03-14T18:01:20Z")

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That crud is corroding metal. The two dissimilar metals cause a reaction (like a battery) which corrodes the galvanized. He’s not blowing smoke.