Ag as brazing metal in Ti6Al4V/Ag/YAG joints: galvanic effects in seawater

The electrochemical behavior of Ag and AgCu, used as brazing material, was compared in Ti6Al4V/YAG (Yttrium-Aluminium garnet) joints. The concern was about galvanic effects in seawater, which can cause possible sources of weakness of the interlayer. Ag and AgCu brazed Ti6Al4V/YAG joints have been already tested in terms of wettability and interfacial reactivity elsewhere. In the present work, Ti6Al4V/Ag/YAG and Ti6Al4V/AgCu/YAG samples were initially immersed for four weeks in a seawater mesocosm. SEM observations showed that the interlayer resulting from AgCu brazing was locally corroded due to Cu depletion. On the other hand, Ag-based interlayer appeared integer. Hence, Ag behavior, as brazing material, was further investigated. Although Ag and Ti6Al4V can be regarded as noble-behaving materials, in case of Ag behaving as an anode, possible worsening effects due to coupling it with Ti6Al4V can be related to the geometry of the system, since Ti6Al4V has a more extensive area than the interlayer, and the environment of application, since Ti6Al4V can enhance cathodic effects due to its ennoblement induced by biofilm growing. Therefore, the study of the Ag interlayer was performed using Ti6Al4V and Ag electrodes, coupled and not coupled, immersed in seawater. Biofilm induced ennoblement was investigated by comparison with 254 SMO/Ag systems. After 4 weeks, the couples were disconnected, open circuit potential was monitored for 1 hour, potentiodynamic polarization curves were performed. Freely corroding electrodes were polarized as well. Results show that Ag behaved initially as a cathode in 254 SMO/Ag and Ti6Al4V/Ag couples; after few days, reversal of polarization was observed, making Ag behaving as an anode. This event was a stable feature only for the 254 SMO/Ag couples, and this trend was consistent with freely corroding electrode behaviors: the corrosion potential of 254 SMO electrodes shifted to potentials larger than that of Ag, with sigmoidal kinetic, typical of biofilm ennoblement; on the other hand, Ti6Al4V potential shifted to potentials comparable to Ag ones, with a saturation kinetic typical of passivation layer strengthening. Important evidences about Ti6Al4V/Ag couples were: a) Ti6Al4V did not undergo biofilm induced ennoblement, b) galvanic current densities on Ag were about 10-6 A cm-2, whatever the sign of the current. Then, Ag, better than AgCu, can be used as brazing metal for Ti6Al4V/interlayer/YAG joints in seawater, since no appreciable galvanic effects occur.