Résumé
Stainless steel, because of their high chromium content, naturally presents a native oxide layer that protects them against aggressive environment. In a first approach, this passive film when in contact with aqueous environment can be seen as composed of two layers (bi-layer) : An internal one, which is in direct contact with the metallic substrate, formed of ferrous and chromium oxides types [(Fe, Cr)2O3] and an external one essentially composed of hydroxide as Cr2(OH3)nH2O. In a certain way, from studies, it generally admitted that stability of the passive state is mainly conditioned by the film composition and that it can be locally limited, by the presence of chloride ions or when breaking of the passive film is observed. In such a case, chloride ions are adsorbed on the passive layer, and their penetration leads to pitting. It's thus generally observed that pitting corrosion starts at non metallic inclusions (MnS). Surface mechanical attrition treatment, among other SPD processes, is at present known to be efficient to form a nanocrystalline layer on stainless steel through successive grain refinement mechanisms. More recently, it has been shown that combined with a co-rolling process, thick structures alternating nanostructured layers, with coarse grained layers, can be developed and lead to rather interesting mechanical properties. However, never electrochemical corrosion properties of such a nanocristallised structures have been study. In particular, because the volume fraction of grains boundaries, their structure and their compositions change after reduction of the grain size up to nanometer scale, the nature and the distribution of non metallic inclusions and of chromium is affected leading to drastic change of the corrosion resistance. In addition, it's proven that SMAT induce phase transformation and some roughness that have also effects on corrosion resistance. In this work, some clarifications of the electrochemical aspects of austenitic stainless steel after SMAT and co-rolling are given in relation with the observed microsctrure. In particular, after TEM and DRX characterizations of the microstructural changed observed in SMA treated samples, potentiondynamic polarization experiments with reverse scan were performed as well as measurement by means of Electrochemical Impedance Spectroscopy. The re-passivation capability of the SMA treated austenitic stainless steel and an approximate thickness of the oxide layer were calculated. It is shown that austenite phase transformation observed at the extreme surface has the tendency to degrade the corrosion resistance in presence of chloride ions whereas removing of these regions lead to higher corrosion resistance than austenitic stainless steel alone. The multiple diffusion channels produce through grains refinement seems to be the reason for this aspect. From literature, it's not well understood and often contradictory why nanostructuration can increase the corrosion properties of materials (case of Ni, Zn and stainless steel) whereas it can degrade them for others. It's thus rather interesting to lead such a study in case of surface mechanical attrition treatment.