The Proliferation Of Mechanical Nano-steel Surface And Corrosion

Diffusion and corrosion are very important properties for materials.Surface mechanical attrition treatment(SMAT) is a promising and widespread technique that can greatly improve the mechanical properties of materials.Because of its special grain boundary structure,the nanocrystalline surface layer has special properties of diffusion and corrosion.To clarify the mechanism of diffusion and corrosion in nanostructured materials will have great impact on both science and practice.In this paper,AI diffusion in nanocrystalline iron at lower temperatures was studied quantitatively,and the electrochemical corrosion behaviour of nanocrystalline microstructure in the surface of 316 stainless steel(316SS) was also investigated.By means of surface mechanical attrition treatment(SMAT) to pure iron and 316SS plates,a nanometer-grained surface layer was fabricated.The mean average grain size of the surface was characterized by X-ray diffraction(XRD).The technique of hydrogen embrittlement was first employed to obtain the nanocrystalline structure on the surface.Al diffusion kinetics in the nanocrystalline iron was measured by second ion mass spectrometry(SIMS).The effects of SMAT and following annealing process on corrosion behavior of 316SS were investigated by potentiodynamic polarization curves and potentiostatic critical pitting temperature measurements.The main results are followed:1.A nanometer-grained surface layer was fabricated on the SMAT pure Fe plate.The average grain size in the top surface layer(of 5 pm thick) is about 31nm.A nanometer-grained surface layer was fabricated on the SMAT 316SS plate.The average grain size in the top surface layer(of 5 pm thick) is about 19nm.2.Within a temperature range of 300～380℃,the diffusivity of Al in the nanocrystalline iron is 6-7 orders of magnitude higher than that in iron lattice.The activation energy of Al diffusion in nanocrystalline iron is 134kJ/mol which is smaller than that for the lattice diffusion.The enhanced diffusivity of Al may originate from a large volume fraction of nonequilibrium grain boundaries.3.The corrosion resistance of 316SS decreased after SMAT because of lots of defects on the surface.Nevertheless,after annealing treatment,the corrosion resistance of the nanocrystalline surface had been improved greatly.The corrosion resistance became better with annealing temperature increased.