(Invited) Analysis By Electrochemical Impedance Spectroscopy of Passive Film Properties of Additive Manufactured 17-4PH Stainless Steel

Abstract

Additive manufacturing is currently one of the most innovative processes to build metallic components, in particular because it allows to manufacture parts with new design out of reach of conventional processes. Among the various additive manufacturing processes, power bed technologies such as laser beam melting (LBM), also called selective laser melting (SLM), are certainly the most common and developed process at this time. The specific microstructures generated by LBM and the associated mechanical properties of the final parts have been well discussed in the literature for many types of materials. Nevertheless, these peculiarities can also affect the durability of such parts and specifically their corrosion behaviour.The main goal of this work was to analyse the electrochemical impedance measurements performed on a 17-4PH martensitic stainless steel (MSS) manufactured by LBM. In this framework, LBM parts were built from 17-4PH MSS powder, using optimised machine parameters. Samples machined from a wrought cylindrical bar were studied as references. Differences between the impedance of the LBM samples and the references samples were observed; they appeared as a small shift in frequency on the Bode plots. The impedance of the passive layer was analysed in detail by using the Young impedance model; in this model it is also necessary to take into account the ohmic impedance [1] to perform the analysis in the entire frequency range. In agreement with TEM observations, an increase of about 40% of the film thickness was obtained for the LBM samples; but, for both samples the resistivity distribution was similar. In conclusion, electrochemical impedance spectroscopy appears as a powerful method to study passive film and better understand the corrosion behaviour of LBM 17-4PH stainless steel.