Surface Characterization and Electrochemical Behavior of AISI 316l Stainless Steel Machined with Green Supercritical CO₂ Coolant

Cutting fluids are usually applied during milling to reduce the friction and to protect the tool and the material from corrosion. These fluids are associated with toxicity and environmental problems. Moreover, the waste management of cutting fluids entails large expenses. The need to reduce cutting fluids has fostered the use of alternative coolants such as supercritical (sc) CO₂, alone or with minimum quantity lubrication (MQL). sc CO₂ and sc CO₂ + MQL coolants have been studied for face milling of a cold worked (CW) AISI 316L stainless steel (SS), evaluating their effect on the residual stresses generated in the surface, in the outermost microstructure of this material, and the corrosion performance. Furthermore, they are compared with those caused by traditional face milling and with a manually ground-generated surface. Ultrafine grain (UFG) layers of about 1 μm and passive layers (of similar chemical compositions) are identified for all the surfaces under study. The three milling processes under study generate a deformation layer under the UFG layer that does not appear below ground surfaces. Moreover, the preexistent compressive stresses created by the CW process change into tensile, being higher for the alternative green machining processes than for the traditional one. The probability of undergoing pitting (studied with cyclic polarization curves) appears to be linked to the nature and structure of the passive layer (characterized by Auger spectroscopy and Mott–Schottky analyses, respectively). Electrochemical impedance spectroscopy studies also confirm similar electrochemical performances for all analyzed surfaces. The active-to-passive transitions of the SS, which have been characterized by electrochemical potentiodynamic reactivation tests, appear to be related to the stresses and deformation state of the deformed layers. Passivation on the alloy in acid media appears to be favored after the sc CO₂ and sc CO₂ + MQL alternative milling processes than after traditional face milling and grinding.