Could be condensation on the pipe that's causing the rusting. You can solve it by painting over the pipe. I'd use something like yellow paint since it's probably a gas pipe. Your relative humidity in the basement could be high enough that the temperature difference between the gas in the pipe and your basement causes condensation on the pipe that then corrodes the metal.
I'm not a plumber, gas fitter, or anything professional, so take my advice for what it's worth - which is nothing!
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