Is it advisable to run electric cable inside a steel pipe?
Wires are routinely run in steel conduit, NOT pipe. Old water pipes would be rough and may have standing seams that can cut the insulation. The electrical code in the U.S. would not allow this because the water pipe is not listed for wiring purposes. Used water pipe for a wire way is asking for problems. Added:
  I currently work in a lumber mill. A few months ago the owner bought almost 1k' of used conduit big stuff. We cut off most of the rusty threads and tossed the rusted conduit. To make a tough week+ of pulling 500kcm 3 phase we had a short in the pipe on the first pull then 2 on the second . This was only ~200' after the 2nd pull and close to 600' of 500 being damaged. We now don't use rusty conduit. This has happened in just the last few years with used conduit. Water pipe is much worse.