Comparative Analysis of the Corrosion Resistance of Titanium Alloys Intended to Come into Direct or Prolonged Contact with Live Tissues

Abstract

The evaluation of the biological safety and degradation of materials is quite important for risk assessment in various biomedical applications. In this study, two procedures were followed to characterize the corrosion resistance of different Ti-based alloys. The first one consisted of performing specific electrochemical tests (open circuit potential, linear resistance polarization, Tafel plots, potentiodynamic polarization) in order to highlight their behavior to the general and localized corrosion. The static and dynamic fatigue cycles combined with crevice corrosion conducted on a new prototype have completed the study. The second procedure followed was a cations extraction investigation (by inductively coupled plasma mass spectrometry) in order to verify the ionic permeability of the oxides layers formed on the surfaces. Optical and scanning electron microscopy were used for surface analysis. It was noticed that in these two electrolytes, the bulk Ti-based alloys presented an almost similar general corrosion behavior. The small differences of behavior for Ti6Al4V scaffolds were correlated to the surface oxidation and roughness (owing to the selective laser melting process). The Ti alloys presented no traces of localized corrosion at the end of the test. The fatigue cycles revealed that a strong and adhesive oxides film was formed during the static cycles (difficult to remove even during the depassivation steps). The concentration of cations released was at the detection limit, revealing very good passivation films, in adequacy with the all the other results.

Keywords: Ti13Nb13Zr; Ti45Nb; Ti6Al4V; cations release; fatigue corrosion; generalized corrosion; localized corrosion; metallic scaffolds; nanostructure; titanium alloys.

Figure 1

Alloy scaffolds’ structures samples: cylinders (a) 1000 μm and (b) 750 μm received from KUL Leuven, Belgium.

Figure 2

Alloy nanostructured samples: discs (1000 μm and 750 μm) received from UniVieVienna, Austria.

Figure 3

(a) Rotating electrode technique; (b) electrochemical cell used for the electrochemical tests.

Figure 4

Sample used for the crevice corrosion tests.

Figure 5

(a) The fatigue corrosion device; (b) detail of the electrochemical cell.

Figure 6

Schematic image of the two types of mechanical movements (F_T—traction strength).

Figure 7

Alloy microstructure (#1): longitudinal section (a) optical microscopy (OM) and (b) scanning electron microscope (SEM) images; transversal section (c) OM and (d) SEM images.

Figure 8

Ti45Nb alloy microstructure (#3): longitudinal section (a) OM and (b) SEM images;(#4): transversal section (c) OM and (d) SEM images.

Figure 9

Ti13Nb13Zr alloy microstructure: longitudinal section (a) OM and (b) SEM images; transversal section (c) OM and (d) SEM images.

Figure 9

Ti13Nb13Zr alloy microstructure: longitudinal section (a) OM and (b) SEM images; transversal section (c) OM and (d) SEM images.

Figure 10

X-ray diffraction (XRD) patterns of the #2 (a) and #7 (b) 0 mm [γ = 0], (c) 7mm [γ = 220], and (d) 13 mm [γ = 408] from the center of the sample. Shear strain due to high pressure torsion (HPT) deformation was calculated using the following equation: γ = 2πrN/t, where r = distance from center to point of interest, N = number of rotations, and t = final thickness.

Figure 11

Ti6Al4V alloy scaffold structures with 1000 μm (a,b) and 750 μm (c,d) pore size.

Figure 12

Ti13Nb13Zr nanostructured alloy (a) OM and (b) SEM images.

Figure 13

Open circuit potential vs. immersion time curves for bulk Ti-based alloys in 9 g/L NaCl electrolyte. SCE: saturated calomel electrode.

Figure 14

Potentiodynamic polarization curves recorded in 9 g/L NaCl electrolyte for theTi-based alloys (semi-logarithmic scale).

Figure 15

Potentiodynamic polarization curves recorded in 9 g/L NaCl electrolyte for the bulk Ti-based alloys (linear scale).

Figure 16

Open circuit potential vs. immersion time curves for bulk Ti-based alloys in artificial plasma bone.

Figure 17

Potentiodynamic polarization curves recorded in artificial plasma bone for bulk Ti-based alloys (semi-logarithmic scale).

Figure 18

Potentiodynamic polarization curves recorded for the Ti6Al4V scaffold samples in 9 g/L NaCl and artificial plasma bone (semi-logarithmic scale).

Figure 19

Polarization current densities recorded for different alloys, during 15 min, at preselected potentialsand above E_oc, in a solution of 9 g/L NaCl: (a) 316L alloy [43] and Ti-based alloys: (b) #1, (c) #3 and (d) #4.

Figure 19

Polarization current densities recorded for different alloys, during 15 min, at preselected potentialsand above E_oc, in a solution of 9 g/L NaCl: (a) 316L alloy [43] and Ti-based alloys: (b) #1, (c) #3 and (d) #4.

Figure 20

Potentiostatic polarization plots, current value recorded after 15 min, versus the preselected potentials for bulk Ti-based alloys.

Figure 21

Potentiostatic curves recorded for the level of 600 mV/SCE for #1 in 9 g/L NaCl corresponding to (a) static and (b) dynamic mechanical cycles.

Figure 22

The fatigue corrosion behavior of Ti-based alloys in 9g/L NaCl during the mechanical cycles: (a) static and (b) dynamic cycles.

Figure 23

The fatiguecorrosion behavior of Ti45Nb alloy (longitudinal and transversal sections) in 9 g/L NaCl during the mechanical cycles: (a) static and (b) dynamic.

Figure 24

The total electrical charge consumed for the fatigue corrosion tests in 9 g/L NaCl during (a) the passivation and (b) the depassivation steps (s: static and d: dynamic mechanical cycles).

Figure 25

(a) Failure of the Ti6Al4V alloy scaffold specimen before fatigue corrosion tests; (b) Ti-based alloys samples before and after test.

Figure 26

SEM images of the Ti6Al4V alloy scaffold structure taken before any machining or cleaning process.

Figure 27

Titanium alloys after the extraction test.