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Thread: Preventing corrosion with no-hub couplings on cast iron

  1. #1
    DIY Member Gordan's Avatar
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    Default Preventing corrosion with no-hub couplings on cast iron

    Dismantled a botched PVC-to-cast iron joint that had a 3" PVC pipe inserted about 8" into a 4" cast iron pipe (not hub, but the pipe itself), and then a Fernco rubber boot over the whole kit'n'kaboodle. The end of the cast iron pipe is pretty badly corroded in places, so much so that I could crumble off bits of it. This "handywork" dates back to 1994, so it didn't take terribly long for this state of affairs to develop.

    Now, I suspect that it was the fact that there was a place for stagnant water to collect (between the PVC pipe and the cast iron) which eventually caused crevice corrosion; I've seen stainless steel crumble into nothing in a small area under similar circumstances (under a gasketed cover) with the rest of it being pristine. With a code-compliant reinforced coupling this issue may not exist, but I'm still a little leery - should I or can I apply sealant between the inner lip of the coupling and the edge of the cast iron to ensure that no water will be able to collect in that space, or between the rubber and the cast iron?

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    Retired Defense Industry Engineer jadnashua's Avatar
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    Get a piece of 4" pvc, attach that with a no-hub, then use a reducing coupler to reconnect to the 3".
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    DIY Member Gordan's Avatar
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    Thanks, Jim; I'm already planning to do that. My question had to do with sealing the rib of the coupling to the face of the cast iron pipe in order to prevent stagnant water pooling in the crack and causing crevice corrosion. As in, is it likely to help, make no difference, or make things worse by encouraging some other failure mode that I have not considered.

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    Test, Don't Guess! cacher_chick's Avatar
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    There are drain and sewer lines which are underwater below ground for much of their service life. I don't think the seam in your connection is something to be too concerned about.

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    DIY Member Gordan's Avatar
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    I'd certainly like to believe so, but I was taken aback when the cast iron crumbled under my fingers. I guess I'll chalk it up to the handyman joint that was in place.

    Crevice corrosion is a real phenomenon, and the speed and degree of damage it causes can be pretty stunning. I had no idea until I saw it, and now seeing it again does make me worry.

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    DIY Senior Member asktom's Avatar
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    When you go from 3" to 4" you need a 4' (OK, 3 1/2") cleanout. You should throw in a 4' wye w/ a cleanout plug.

  7. #7
    Test, Don't Guess! cacher_chick's Avatar
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    Quote Originally Posted by Gordan View Post
    I'd certainly like to believe so, but I was taken aback when the cast iron crumbled under my fingers.
    I worked at a building which had about 100 feet of c.i. pipe going to the street, which was diagnosed as having a "break" in the pipe. When the section was dug up, they found that one 20' stick of pipe was entirely rotted away, but the attaching pipes to each side of that stick still looked as good as new. I have to guess that there was a problem with the manufacture of one batch of pipe, and one stick of it ended up on that pallet.

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    Retired Defense Industry Engineer jadnashua's Avatar
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    Ever use some drain cleaners, and with that section of pipe not ever being 'cleansed' by subsequent flow, and it could corrode easily in that time. Some water's pH is acidic, too. A properly mated nohub should present no obstruction to flow - the 'stop' in the middle fills the gap between the pipe to the same level as the ID. Just make sure the two pieces you are joining are cut off nice and square.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    DIY Member Gordan's Avatar
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    What I wound up doing is cut off the affected cast iron section nice and square, and coated the inside of the coupling with plumber's grease before pushing the center stop nice and tight against the end of the cast iron. Hopefully that'll at least minimize the penetration of water into any crevices. The cast iron pipe was also unsupported on the close end so I put a split-ring hanger on it; it's not going anywhere now. I put the hanger on before cutting the pipe, as I didn't want to rattle it around and potentially disturb a downstream joint.

    Yes, I put a cleanout at the beginning of the 4" section (which I extended most of the way to the opposing wall as I didn't want the cleanout to be in or near any passages, and there's a doorway right above the end of the cast iron.) A wye would not fit; too close to the floor. I used a flush cleanout tee.

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