Introduction: The distinctive layer-wise fashion in which additive manufacturing (AM) technology produces parts enables production of near-net-shape components with intricate geometrical features [1]. For instance, laser powder bed fusion (L-PBF) shows promise in the fabrication of next-generation femoral stems for total hip replacement (THR) [2]. Because L-PBF is characterized by fast solidification rates, usually there are significant microstructural differences between wrought (WT) and L-PBF materials [3]. These microstructural dissimilarities can lead, for example, to variations in corrosion behaviour [4]. Acknowledging the key role corrosion resistance plays on the lifespan of an implant, it is imperative to understand the link existing between microstructure and corrosion performance of the additively manufactured Ti-6Al-4V alloy due to its relevance in the production of femoral stems. Therefore, this study aims to examine the role of material microstructure and texture on corrosion behaviour of L-PBF and WT Ti-6Al-4V in simulated physiological conditions.

Methodology: Corrosion behaviour was investigated using a 3-electrode electrochemical cell via open circuit potential (OCP), cyclic potentiodynamic polarisation (CPP) and electrochemical impedance spectroscopy (EIS) tests in phosphate buffered saline solution (PBS) at 37 °C. Microstructural features were assessed by means of scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), energy dispersive X-ray (EDX) and X-ray diffraction (XRD).

Results: Upon immersion for 2 hours, the OCP of L-PBF and WT specimens continuously increased, indicating the formation of a spontaneous oxide surface film. Both samples (i.e., L-PBF and WT) exhibited a passivation behaviour over a wide range of potentials from 0 to 1500 mV vs. Ag/AgCl, which demonstrates a strong protective nature of the oxide film formed on the samples’ surface. WT sample displayed a higher corrosion current density (0.039 ± 0.008 μA/cm2 against 0.021 ± 0.011 µA/cm2 of L-PBF), which could pinpoint to a difference in the nature of the oxide film formed amongst samples. Absence of localized corrosion (i.e., pitting) was confirmed by the breakdown potential being absent as well as a negative hysteresis loop in the CPP curves. SEM and XRD diffractograms results demonstrated that the L-PBF sample revealed a needle-shaped α’ phase whilst, a duplex α + β phase was found in the conventionally manufactured counterpart. Furthermore, EDX results showed that whilst there is a clear segregation of alloying elements in the WT sample, the L-PBF exhibited a homogeneous distribution of Ti, Al and V.

Discussion: Results from this study indicated that for Ti-6Al-4V alloy, manufactured using LPBF or conventional manufacturing route, the main distinction in the corrosion behaviour is in the passive domain, suggesting the oxide layer formed varies as a function of the microstructure. A poorer corrosion behaviour of WT samples when compared with L-PBF might be due to a galvanic coupling established between α and β phases within the material. Similar findings were reported in [5], where a comparison of the corrosion performance between EBM and L-PBF Ti-6Al-4V samples was carried out. Further experiments will be conducted aimed at tackling the kinetics and nature of the oxide layer formation in order to elucidate the role of microstructure on passive film kinetics.