Recently I came across a peculiar case of magnetism in a gas pipeline that is in use since 50 years.It is a 40",0.375"thk,API 5LX52 carbon steel pipe,14KM long,laid underground,with cathodic protection.
Corroded sections of this line were being replaced with new flanged pipe segmets,for which flanges had to be welded to the cut ends of the existing old pipe.Theline was cut using cold cutting saw.
We experienced strong magnetic field at the cut ends,making it impossible to weld the flanges on the cut ends of the old pipe.Surprisingly,the magnetic field was found only upto about 6" from the cut ends; beyond this the line was not magnetised.The cathodic protection was already disconnected and hence we presumed that the only source of the mag field induction could be the cold cutting saw.In order to confirm this,one of the cut ends was futher cut with gas torch;but,still the portion upto 6" from the cut end showed strong magnetism as before.
CAN ANYONE PLEASE EXPLAIN WHY THE MAGNETIC FIELD IS CONFINED ONLY TO THE CUT ENDS(UPTO ABOUT 6" FROM THE END)? IS IT THE RESIDUAL MAGNETISM INDUCED BY THE CATHODIC PROTECTION - IF SO, WHY IT IS CONFINED TO THE CUT ENDS AND NOT ALL THROUGH?