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Question DetailsAsked on 5/20/2017

The cast iron waste pipe in my basement has a powdery buildup on the TOP side that has been developing for years. I

The buildup is oddly similar to efflorescence that can occur on damp basement walls but the thickness is as much as one half inch and crusty hard. My pipe has not leaked any in its 43 year history and it flows okay. What is going on and do I need to worry? (Also, the buildup occurs in numerous locations)

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1 Answer


Common causes of this:

1) minor leaks - split pipe or leaking joints - the efflorescence can travel many feet along the pipe, commonly 2-4 feet at least each way from the leak spot but I have seen it as much as 6-8 feet from a pinhole leak, because it wicks through the mineral buildup and then evaporates at the free surface and end, gradually extending itself laterally, This is most likely what you are seeing, especially if that thick a buildup - a lot of little pinholes or leaking joints causing buildup of the minerals from the liquid. Here are some pictures of it (though first one is discolored with a lot of rust from the water heater housing) -

Usually a pipe with this has it a good portion of the way around it and usually forming stalagtites on the underside at the leak as well, and commonly indicates (if a lot of them) general corrosion in the pipe. You can try wire brushing and look for corroded metal underneath - commonly the only visible pipe damage will be right at the leak point. (Course, if you wire brush and the pipe is pretty well shot, you may form holes clear through it).

Can also for from efflorescence of the mortar in walls the pipe penetrates - sometimes spreading a number of feet from the wall.

2) caking of dust on the pipe - cakes because the pipe (assuming unheated basement) gets cold enough that condensation forms on it, making the dust cake on - though that is usually grayish (native soil color in persistent wind dusty/desert areas), would be pretty uniform over entire length of horizontal pipe runs, and while it does come off in "slabs" there would be no unusual corrosion underneath or significant sticking to the pipe - brushes off pretty clean.

3) drywall dust settling on it from overlying ceiling, either from unpainted drywall or very rarely from termites boring through it - though the dust would also be on the ground/floor around it, and would be pretty distributed from the source area. Rehardens (to a soft consistency like hardened drywall joint compound) with wetting from condensation.

4) depending on your pipe manufacturing process, some cast iron pipes have a storage rust preventative treatment on the surface which eventually corrodes into a white zinc-based powder - though I have never seen it only on top, though I guess temperature differences could result in first condensation there.

If it wire brushes off clean without exposing pitting and corrosion of the pipe unearneath, I would not be concerned. You could try cleaning a few spots and see if it grows back from a specific point, which might be the source of a pinhole leak.

Otherwise, have a plumber in - or if you don't do your own plumbing, have the next plumber who comes to fix something look at it.

Answered 3 years ago by LCD

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