Cast iron pipe rust

Do cast iron pipes rust inside to any large extent? I'm looking at replacing the galvanize waste pipes that go to my septic and they meet up at the main line and dump into a cast iron pipe. The galvanized is rusted, no question, but I wonder if the cast iron would be too? I really don't want to replace the sewer line into the septic tank.
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On Wed, 25 Jul 2007 21:46:48 -0700, "Eigenvector"

Cast iron will rust but then the rust on cast iron forms a sort of protective coating, preventing total rust through.
However, what really happens with cast iron piping is that stuff adhers to the internal rust layer, especially chemical deposits. The pipes will slowly close up to the point of clogging. Regularly snaking out drain pipes will reduce this effect. I've used power snakes with cutting blades. Even they sometimes have a hard time cutting through the hardened crud. Let the pipe almost close down and it can be impossible to snake out.
Doug
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wrote:

Doesn't sound like it reacts like galvanized then. whereas in galvanized if you have rust, that means the walls of the pipe are seriously thin and at the point of failure - reaming them out won't help, it will only hasten the failure. So it sounds like it should be okay. Then replacing the galvanized drain pipes with plastic and leave the cast iron sewer line alone.
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Eigenvector wrote:

Cast doesn't act quite like malleable iron, no, because it has more carbon.
But, the prime reason is the difference in wall thickness--there simply is a lot more material in cast iron than in Sch 40 pipe. For a 4", cast 30-lb test (the lightest common), wall thickness is 0.35" while for Sch 40 it 0.237" -- nearly 50% thicker. Consequently, it has a longer life in general. Also, owing to being used almost exclusively for waste in residential usage, it is non-pressurized whereas water service piping is under pressure, accelerating any leak.
There's a current other thread of a poster who has leaking cast mains, so it does happen.
But, I agree w/ the other poster of "if it ain't broke, it don't need fixin'" unless you've already got it torn up and just go ahead or are doing extensive work that will be in the way if there's need for repair in the future.
--
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sorry my main soil stack is 57 years old, and rusted thru leaking.
if your into it anyway replace with schedule 40 plastic. except indoors in living areas, cast iron is quiet, with plastic you hear every drop.
cast iron just fails slower
coming soon to this home, replace the main soil stack:(
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On Fri, 27 Jul 2007 03:11:38 -0000, " snipped-for-privacy@aol.com"

A life of only 57 years is rather unusual for cast iron. Does your water have a high acidity level?
I've got several houses with cast iron waste stacks. They range from 65 to 110 years old without any trouble.
You're right about schedule 40 being noisy. I've visited many a "luxury" condo where I could hear the neighbor's every toilet flush. I regard using schedule 40 plastic pipe for a waste stack as being evidence of typical cheap construction.
Doug
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It can be troublesome to pin down why cast iron sometimes prematurely corrodes because there is a long list of possible culprits, including the composition of the pipe. I've always wondered why cast iron toilet flanges often corrode like crazy when the rest of the stack is A-OK. I'm guessing maybe that galvanic corrosion ovewhelms the carbon/ graphite matrix.
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varies a lot and can be extremely long in some cases, mostly depending on the soil and other enviornment. My advice is not to disturb it at all unless there is some reason. Failure of drain systems is rarely catastrophic and if it starts to leak or clog, or if you are doing some other related work, then evaluate it and decide what to do.
Don Young
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