Hello all,
My question is what is the ideal material for a oil supply line?

I work for a coal power plant and we our having a problem with out aux turbine oil. The aux turbine oil supply line is within the return line and the supply line is becoming corroded. This corrosion is contaminating the oil and not allowing control devices to work properly. The piping on the return and supply are both made out of carbon steel. Is carbon steel a bad choice for this application. At the moment we are thinking of replacing it with a stainless steel.

Locating supply oil piping within the return piping is fairly common in industry, it saves space in tight insatllations and helps regulate temperatures. The return piping is generally not fully flooded and there is an air space above the liquid level. Reservoirs are typically fit withan air scavenging system which puts the system under a slight negative air pressure to prevent oil migrating thru labrynth seals and helps air to seperate from the oil in the reservoir. If you have steam carry over thru the labrynth seals then moisture enters the piping and can cause corrosion as you describe in uncoated carbon steel piping. Stainless might offer more corrosion resistance as would coating the inside of piping. You have to select a proper coating carefully so the oil doesn't get under it and cause it to flake off which can cause real problems with filters. Also distillate is very corrosive in itself as distilled water likes to leach minerals out of whatever it contacts. Seek out advice from specialists in Steam Turbine systems on materials to use.
If you do not have an offline oil conditioning unit such as a centrifuge or vacuum dehydrator I would suggest investing in one and use it religiously.