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I was doing some repair on a tiny leak in one of my natural gas pipes, and noticed that inside the pipe was a small amount of rust. When I took the pipe off, when I turned it upside down a small amount of rust came out. I could see a thin layer of rust coating the pipe in at least a few parts.

It wasn't large enough to concern me, but it made me wonder why this happened. Do these pipes rust from the inside out, and eventually need replacement?

I'm also not completely sure how these pipes could rust inside them. The pipes themselves should always have methane in them and no oxygen or water. There was zero rust on the outside.

The house itself was built in 1924. I'm not sure when gas was put in, but this was a gas pipe in the laundry room feeding the dryer, so it might be as old as 60-70 years.

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It's entirely normal. There is always a bit of moisture in natural gas and there can also be small amounts of other impurities including oxygen. This is enough to cause small amounts of rust in black iron pipes.

Obviously there is little danger of it rusting through any time soon since, as you've noted, it's been there perhaps 60-70 years and will likely be good for another 60 years or longer.

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  • Small addition to the excellent explanation: the mysterious stub of pipe hanging down near each gas appliance is a trap. It is there to catch water and other debris that might come along through the gas pipe so that these don't cause blockages or other trouble in the appliance. – Greg Hill Oct 19 '20 at 1:14
  • That is why "dip legs" are used with gas pipe so any loose rust drops out .. – blacksmith37 Oct 19 '20 at 2:10

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