Effect Of Potential And Temperature On The Structure And Growth Mechanism Of The Surface Film On Nanocrystalline 304 Stainless Steel

Austenitic stainless steel?SS?is widely used in various fields of modern society because of its superior corrosion resistance and formability.However,the strength and hardness of the traditional coarse grained stainless steel?CC SS?is not high,which affects its application to some extent.This disadvantage can be made up by severe plastic deformation to form bulk nanocrystalline austenitic stainless steel?NC SS?.The corrosion resistance of nanocrystalline stainless steel depends on the properties of its surface passive film.Therefore,it is necessary to fully understand the effect of nanocrystalline structure on the composition and structure of the passive film on NC SS,especially the film-forming mechanism and the diffusion rate change of the key film forming element of chromium.In this paper,304 NC SS without deforming defects was prepared by equal channel angular pressing?ECAP?process.The microstructure of NC SS was analyzed by optical microscopy?OM?,transmission electron microscopy?TEM?and X-ray diffraction?XRD?.The corrosion resistance of the NC SS was investigated by using the open circuit potential,the potential polarization curve and the average activation energy.In order to fully understand the law and mechanism of nanocrystallization on the surface film,comprehensive methods,such as electrochemical impedance spectroscopy?EIS?,capacitance measurement?Mott-Schottky curve?,constant potential current-time curve?I-t curve?,constant current potential-time curve?E-t curve?and atomic force microscope?AFM?,were used to study systematically the characteristics of the surface film on NC SS at room temperature in different media?air,acidic solution?and at different film forming potential?open circuit potential,passivation area potential,over-passivation area potential?.Finally,the characteristics of the oxide film on the NC SS at different film forming temperatures with thermal stability were systematically analyzed and the effect of temperature on the structural evolution and mechanism of the oxide film on NC SS were discussed.The research results show that a bi-layer passive film composed of an inner Cr-enriched oxide layer and an outer Fe-enriched oxide layer was obtained on both the NC and CC SSs in air at room temperature.The fact that Cr enrichment did not occur in these films is reasonable,since the diffusivity of Cr at room temperature is similarly low in both matrix.The improved corrosion resistance of the NC SS is due to the higher compactness of its passive film from the instantaneous nucleation by high fraction of grain boundaries and dislocation density.The passive film formed in the acidic solution at open-circuit potential on the NC SS also has a bi-layer structure,but the chromium oxide content in the inner layer is higher than that in the CC SS.This is attributed to the selective dissolution of iron oxide and the faster dissolution rate of iron oxide with small particle size in the inner layer of the NC SS.However,the composition and structure of the outer layer passive film on the CC and NC SSs is similar largely due to their similar nucleation and growth mechanism,although the more compact passive film on the NC SS will reduce migration rate of iron ion through the inner layer to form outer layer.The passive film formed in the acidic solution at passive region potential on the NC SS also has a bi-layer structure but is more compact.This bi-layer structure formation is attributed to the more negative Gibbs free energy of Cr₂O₃ and different oxide formation potentials U_ox of Cr and Fe.The formation process of the passive film on both the NC and CC SSs consists of four distinct stages.The lower U_ox?Cr??-0.52V?makes the nucleation and growth of Cr₂O₃ predominate in Stages I and II.The Fe₂O₃ with a higher U_ox?Fe??-0.08V?starts to nucleate and grow when the surface potential increases due to the inner Cr₂O₃ passive layer formation and predominates the outer layer instead because of the higher diffusion rate of Fe than Cr to across the inner layer in stages III and IV.The Cr₂O₃-enriched inner passive film on the NC SS was more compact due to the higher nucleation and growth rate of Cr₂O₃facilitated by nanocrystalline microstructure.However,there is no compactness distinction of the Fe₂O₃-enriched outer layer on the NC and CC SSs.This results from their similar growth rate,although the nucleation rate of outer layer on the NC SSs may be lower due to the decreased migration rate of iron ion in its more compact inner film.The oxide films formed in air on the SSs grow thicker as the temperature rises,wherein the content of hydroxide decreases and the content of chromium and iron oxides increases.Compared with that of the CC SS,the thickness of the oxide films on the NC SS are slightly thinner,and the inner chromium oxide-rich layers are denser.The oxide films exhibit a bi-layer feature in the LTR?25–150??and HTR?300–450??,but show a single-layer feature in the MTR?150–300??for both the NC and CC SSs.The temperatures of 150?and 300?are thought to be two transition points.This phenomenon occurred due to different diffusion rates of iron and chromium at different temperatures,leading to a change in composition and layering of the oxide films.In the LTR?25–150??,the diffusivities of Cr and Fe in the SS matrix are very low,leading to a very thin oxide film with a bi-layer structure on SSs at low temperature.In the MTR?150–300??,the Cr-enriched oxide is replaced by the Fe-enriched oxide due to the remarkably increased diffusivity of Fe with temperature,thus a single-layer Fe oxide enriched film is present.In the HTR?300–450??,both the diffusivities of Cr and Fe increase and the difference between their diffusivities becomes small,so the Cr oxide enriched inner layer becomes obvious again and the bi-layer feature appears again in this temperature range.The oxide film on the NC SS becomes obviously thinner and more compact than that on the CC SS above 300?and a higher Cr_ox/Fe_ox ratio is observed as the temperature continues to increase.This indicates that outward diffusion of Cr from the NC matrix to the interface is promoted by the NC microstructure above this temperature,which subsequently leads to a higher Cr_ox content in the oxide film on the NC SS.Therefore the temperature 300?is deemed to be the critical temperature for the detectable Cr diffusion enhancement by the NC microstructure in the 304 SS.