Corrosion Resistance of 3D Printed Ti6Al4V Gyroid Lattices with Varying Porosity

Abstract

Corrosion of medical implants is a possible failure mode via induced local inflammatory effects, systemic deposition and corrosion related mechanical failure. Cyclic potentiodynamic polarisation (CPP) testing was utilized to evaluate the effect of increased porosity (60% and 80%) and decreased wall thickness in gyroid lattice structures on the electrochemical behaviour of LPBF Ti6Al4V structures. The use of CPP allowed for the landmarks of breakdown potential, resting potential and vertex potential to be analysed, as well as facilitating the construction of Tafel plots and qualitative Goldberg analysis. The results indicated that 60% gyroid samples were most susceptible to the onset of pitting corrosion when compared to 80% gyroid and solid samples. This was shown through decreased breakdown and vertex potentials and were found to correlate to increased lattice surface area to void volume ratio. Tafel plots indicated that despite the earlier onset of pitting corrosion, both gyroid test groups displayed lower rates of corrosion per year, indicating a lower severity of corrosion. This study highlighted inherent tradeoffs between lattice optimisation and corrosion behaviour with a potential parabolic link between void volume, surface area and corrosion being identified. This potential link is supported by 60% gyroid samples having the lowest breakdown potentials, but investigation into other porosity ranges is suggested to support the hypothesis. All 3D printed materials studied here showed breakdown potentials higher than ASTM F2129′s suggestion of 800 mV for evaluation within the physiological environment, indicating that under static conditions pitting and crevice corrosion should not initiate within the body.

Comparative Analysis of Bone Ingrowth in 3D-Printed Titanium Lattice Structures with Different Patterns

“…We investigated the following shapes in our animal experiments: gyroid, cube, cylinder, tetrahedron, double pyramid, and Voronoi ( Figure 1 ). Some of these shapes were chosen for comparison with those examined by several authors [ 6 , 8 , 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 ]. The degree of bone growth was determined using rivet-like cylindrical implants with a diameter of 6 mm and a rounded shape; these were used in the animal experiments.…”

Corrosion and Wear Behavior of Additively Manufactured Metallic Parts in Biomedical Applications

“…They also concluded that the AM-manufactured Ti samples with larger porosity showed increased corrosion values; the optimum range of porosity was revealed to be between 40% and 60% [111]. In another study, Sharp et al [112] investigated the corrosion performance of AM-produced Ti6Al4V gyroid lattices with varied porosity values (60-80%). It was shown that the structure with 60% gyroid was susceptible to pitting corrosion in comparison to 80% gyroid and solid samples, mainly due to a higher lattice surface area to void volume ratio.…”

“…It can also be observed that corrosion attacks mainly initiate and focus on special locations such as corners and raised edges. The solid sample manifested a very interconnected form, while the porous samples had distinct and isolated patches [112].…”

Corrosion, fatigue and wear of additively manufactured Ti alloys for orthopaedic implants

“…There has been very few studies involving corrosion of additively manufactured Ti-6Al-4V lattices in the literature. Sharp et al [114] studied the effect of porosity on LPBF Ti-6Al-4V Gyroid structures and found out that a lower porosity lattice is more susceptible to pitting corrosion. This suggests a trade-off between lattice optimisation and corrosion behaviour and thus a potential link between void volume, surface area and corrosion.…”