Comparative study of plasma oxynitriding and plasma nitriding for AISI 4140 steel

SEM images of corroded specimens. (a) PN: Vair = 0L/min; (b) PON: Vair = 0.3L/min. (a) clearly shows that there exist many corrosion pits distributed on the surface due to the pores existed in the compound layer. While there exists no corrosion pits in the image of PON treated sample. (b) due to the protection from chemically stable and compact Fe3O4 formed in the compound layer, corresponding to the optimum corrosion resistance.630