Method and apparatus for determining the thickness of a chromium depleted zone of a surface region of a steel member

The invention provides a method and apparatus for monitoring subsurface chromium depletion from a steel member, such as a pyrolysis pipe. In the harsh conditions of a pyrolysis furnace, chromium within the pipe 16 migrates towards the pipe surface which results in the formation of a chromium depleted layer 14 . This layer can provide useful data about the condition and operation of the furnace. The degree of chromium depletion is measured by using a magnetic source of known strength to create a magnetic field in the surface region of the pipe 16 . An estimate of the thickness of the chromium depleted layer 14 is determined from the resultant magnetic flux, which can be measured by a hall element arranged at substantially 45° to the longitudinal axis of the magnet.