Galvanized stell pipe

Galvanized should never be used for drain and waste lines (although I've run into a lot in that era that was, too).

You're only real cure is to replace it.

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Iron pipe will eventually corrode and clog. It has to be replaced. There is no way to clean out the insides of it. A snake may get you some temporary relief, but it does not fix the real problem.

Galvanised pipe is not normally used for drain lines. Cast iron or plastic is all I have seen in my long years. Cast does not corrode over a normal lifetime of a house but may build up blockages. If they are are cast iron, a rotorooter may be able to clear them. If they indeed are galvanised, your only choice is replacement as they do corrode.

Harry K