Experimental Investigation of long-term ageing effect on the structural and electrochemical behaviour of self-organized TiO2 array nanotubes on Ti-6Al-4V alloy

The correlation between anodization conditions and the ageing effect on TiO2 nanotubes (TNT) surface has been widely studied in different media and conditions (physiological solutions, mechanical stresses in water, etc.) for the prediction of their behaviour over a long period of time. In the present study, the synthesized TiO2 nanotubes (TNT) from Ti-6Al-4V alloy, which were left unattended and exposed to environmental conditions (i.e., humidity and ambient temperature) for more than 4 years, were investigated to underline any important alteration/changes and ageing effects, on the surface morphology, the surface composition, and the electrochemical behaviour. The nanotubes were made in 2018 by anodization in different potentials (20V, 40V, 50V, and 60V) for different times (30 min, 60 min, 90 min, 150 min and 180 min) in an Ethylene Glycol solution for other purposes. For the surface morphology characterisation, electronic microscopy (SEM) was performed to depict any tendency with anodization conditions: potential and time. The comparison study between the obtained results and the SEM pictures taken on similar samples made and characterized under the same conditions in 2018, reveals a noticeable alteration in the morphology and a change in the TNT’s external diameter. Surface composition was checked using energy dispersive spectrometry (EDXS). The EDXS spectra analysis was realised to investigate the storage time impact on structure surface stability. A drastic decrease in the amount of oxygen was noticed on all of the surfaces where wettability measurements by contact angle were performed to confirm the latter. The verification of the hydrophobicity of TNT surfaces attested that all aged samples are hydrophobic in concordance with EDXS analysis and X-ray photoelectron spectroscopy (XPS). To affirm the surface modification during the storage duration and its impact on the electrical behaviour: cyclic voltammetry (CV), open circuit potential (OCP) measurements, and Tafel plots are undergone on the aged samples and compared with the freshly synthesised samples [1],[2]. The plotted CV curved as a function of the scan rate and the composition of the electrolyte showed a correlation between the different samples electrochemical behaviour and their surface morphologies as well as the existence of surface states for all samples. From the previous characterisation, it was obvious that the sample prepared at 40V over 3 hours showed a remarkable electrochemical behaviour. The ageing effect is closely related to the anodization conditions. It was also noticed that the amount of water in the electrolyte solution EG played a contributing factor in the onset of ageing. High water content causes the formation of nanograss [3] which have a non-negligible influence on the morphology [4],[5]. Exposing nanotube surfaces to ambient conditions without taking any precautionary measures and without knowing their historical anodization conditions can cause drastic changes in the electrochemical behaviour of TNT. These changes affect considerably their function for different applications. These results can open a new way for the optimization of the storage conditions according to anodization conditions (electrolyte, voltage, time, and temperature annealing) of this material as well as for the study of the life cycle of products made from TiO2 nanotubes.