Phosphorus surface segregation was measured using Auger electron spectroscopy of 17-4 PH martensitic stainless steel at 450, 550 and 600C. Surface segregation was much faster than expected; attributed to a high contribution of phosphorus diffusion along the former austenitic grain boundaries. A model of surface segregation was developed following the

Darken–Du Plessis approach and taking account of both bulk and grain boundary solute diffusion. The phosphorus grain boundary diffusion in 17-4PH was described by:

D(cm2/s) = 6.2 x 104exp[-157(kJ/mol)/RT]

It was found to be more than three orders of magnitude higher in 17-4 PH steel than in α-iron.

F.Christien, R.Le Gall: Surface Science, 2011, 605[17-18], 1711-8