The pitting corrosion behaviour of a 17-4PH martensitic stainless steel (MSS) manufactured by power bed laser beam melting (LBM) was compared to that of a wrought MSS. More noble pitting potentials were measured for LBM samples, probably due to a smaller size of NbCs as compared to wrought MSS. The metastable pits were less numerous, but had a higher nucleation rate and longer life time for the LBM samples compared to the wrought MSS. This was explained by assuming a decrease in the repassivation ability of LBM samples due to small gas pores.

Combined XPS / TEM study of the chemical composition and structure of the passive film formed on additive manufactured 17-4PH stainless steel. (2021) Surfaces and Interfaces, 22. 100874.

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. References [1] O. Gharbi, A. Dizon, M. Orazem, M.T.T. Tran, B. Tribollet, V. Vivier, From Frequency Dispersion to Ohmic Impedance: A New Insight on the High-Frequency Impedance Analysis of Electrochemical Systems. Electrochimica Acta, 320 (2019) 134609.

Conventionaly manufactured, i.e. wrought, 17-4PH martensitic stainless steel (MSS) is widely used in a large variety of applications, going from biomedical ustensils to large turbine blades. Recently, it appeared as a good candidate for additive manufacturing processes, e.g. laser beam melting (LBM), which is certainly the most common and developed process at this time. The specific microstructures generated by LBM have been well discussed in the literature for many types of materials. Nevertheless, these peculiarities can also affect the durability of the LBM-manufactured parts and specifically their corrosion behavior. The main goal of this work was to propose a detailed study of the pitting corrosion behavior of a 17-4PH MSS manufactured by LBM. In this framework, LBM parts were built from 17-4PH MSS powder, using optimized machine parameters. Samples of a commercial counterpart (CM –MSS) machined from a wrought cylindrical bar were studied as references. The pitting corrosion behavior of both MSSs was studied in chloride-containing solutions by combining linear potentiokinetic polarization associated with the determination of pitting potentials, and a statistical analysis of the current transients observed during potentiostatic experiments for metastable pitting. Significant differences were observed between the LBM and CM MSSs. They were explained considering the passive film composition, structure and morphology evaluated by combining transmission electron microscopy (TEM), scanning TEM (STEM) using the high-angle annular dark field (HAADF) mode, and Energy dispersive X-ray Spectroscopy (EDS) analyses. Results were related to the specific microstructure of the LBM samples as compared to that of the CM MSS, both microstructures being characterized by TEM, STEM and EDS.