Investigation Of Relatonship Between Martensite And Pitting Susceptibility Of Austenitic 304 Stainless Steel

The relationship between deformation-induced martensite transformation and pitting susceptibility of 304 stainless steel in 3.5% NaCl solution was investigated. By potentiokinetic scanning and galvanostatic polarization tests, the effect of martensite on pitting initiation was studied. The simulated occluded corrosion cell and simulated occluded solution method were employed to study the effect of martensite on pitting propagation. Electrochemical impedance spectroscopy (EIS) characterized the surface film and the X-ray photoelectron spectroscopy (XPS) was used to study the composition and structure of the material surface film in occluded solution. The main conclusions were reported below:The austenitic 304 stainless steel was induced martensite phase transformation at low temperature. Martensite content was increased with increase of deformation degree. Meanwhile a great deal of dislocation was mainly concentrated in martensite phase.When the martensite content was below 5%, Eb, Ecorr, Enp were changed more negative with the increase of martensitecontent. When the martensite content was in the range of 5-16.5%, Eb, Ecorr, Enp became positive a little. But when the martensite content was above 16.5%, the Eb, Ecorr, Enp values dropped down again. It meant that the material possessed vary corrosion resistance regulation, which was decreased, increased and then decreased again.It has been found that the pH value fall quickly in occluded solution and the more Cl" migrated into occlude cell, the corrosion potential of specimen in occluded solution changed negative, corrosion current density increased with the increase of the martensite content. So martensite accelerated pitting propagation of 304 stainless steel.Form the EIS spectra, the relation between electrochemical parameters and martensite content was investigated, the equivalent circuit has been proposed, and the mechanism of effect of martensite on pitting initiation and propagation has been brought out.By XPS technology, it was proved that the tightness of surface film was demolished by martensite transformation. The structure and composition of material surface film was changed by Cl" absorption, which led to pitting susceptibility increase.