Phosphorus surface segregation was measured by Auger electron spectroscopy on a 17-4 PH martensitic stainless steel at 450, 550 and 600C. Surface segregation was shown to be much faster than expected which was 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 coefficient 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.

Ultra-Fast Grain Boundary Diffusion and its Contribution to Surface Segregation on a Martensitic Steel. Experiments and Modeling. F.Christien, R.Le Gall: Surface Science, 2011, 605[17-18], 1711-8