Titanium alloys in diluted salt solutions under induced potential in the presence of microorganisms

The environmental behavior of Titanium alloys UNSR50250 and UNSR50400 in seawater from the Red Sea was tested under induced applied potential in the presence of microorganism. The study was carried out in a unique apparatus composed of two separated electrolytic cells, electrically connected by the tested titanium plate as a diaphragm. During the exposure of the titanium alloys to the electrolyte the anodically polarized surface potential of the titanium plates was recorded versus an Ag/AgCl reference electrode. Potentiodynamic polarization measurements curve for both UNSR50250 and UNSR50400 did not show a clear passive-transpassive transition up to a potential of about 7V vs. Ag/AgCl. However current oscillations were observed indicating metastable local sensitivity of the surface to the environment, resulting in shallow crevice and pits. To our opinion, UNSR50250 exhibited higher sensitivity to localized attack most probably due to the influence of thiobacillus bacteria. Exposing the alloys to seawater under applied potential showed that when the anodically polarized surface of the UNSR52400 exceeded a potential of 3.5V vs. Ag/AgCl a droop in the steady potential of about 0.5V vs. Ag/AgCl was observed after 5 hours. This potential drop phenomenon is related to a potential breakdown process, as confirmed by the pits observed using SEM. In contrast, polarization curve of UNSR50250 did not show any potential drop. This lack of potential drop can be explained by the higher electrical conductivity of the surface and by the almost uniform anodic dissolution of the alloy that took place when the potential was applied.