Many older homes use galvanised pipes in their water systems -- steel pipes that contain an outer coating of zinc to resist rust and corrosion. Over time, the coating my wear away, which allows the piping to corrode on the inside and around connections. This is problematic if you must unscrew the galvanised pipe for servicing. If your galvanised pipes are corroded, loosen them quickly by following a few simple steps.
Put on eye protection such as safety glasses or goggles to protect your eyes from chemical spray.
Spray the galvanised pipe connection with a lubricant such as WD-40, Liquid Wrench or PB Blaster.
Allow the lubricant to work for the time recommended on the can. Usually it takes only a few minutes.
Place one pipe wrench on the fitting, and another on the pipe. The wrenches should face opposite directions.
Hold steady pressure on the pipe wrench located on the fitting, and turn the other pipe wrench counterclockwise to loosen the pipe. Continue to turn the wrench counterclockwise until the pipe is unscrewed.
As an alternative to a lubricant, heat the fitting with a propane torch. Touch the connection with candle or paraffin wax. The wax will pull into the connection and allow you to unscrew the rusty pipe in the same manner as above.