Ag and AgCu as brazing materials for Ti6Al4V-Y 3 Al 5 O 12 joints: Does ennoblement affect the galvanic behaviour in seawater?

Description

Transparent ceramic materials may be used as optical windows, sensors, gas-tight components, etc. in specific marine applications by integration into metallic structures through joining procedures. In such systems the evaluation of galvanic effects between base alloys and compounds obtained during brazing is crucial. In this work, the galvanic behaviour of Ag and AgCu brazes, used to obtain Ti6Al4V-Y3Al5O12 joints, was investigated in natural seawater for the first time. Two test sessions lasting 4 weeks each were performed. During the first, the AgCu brazed joint showed Cu selective depletion from both the interlayer and the diffusion zone, while the Ag brazed one appeared intact. Hence, the second session was devoted to a further study of the Ag brazed joint. Active-passive alloys are well known to undergo important OCP positive shift induced by the biofilm growth, commonly referred as ennoblement. The risk of Ti6Al4V to act as ennobling cathode, with acceleration of the Ag anodic dissolution in the interlayer, was simulated with Ti6Al4V and Ag freely corroding and coupled electrodes. In parallel, a 254 SMO stainless steel/Ag system was used as reference for ennoblement. OCP data were evaluated in the framework of inferential statistic in order to highlight the consistence with galvanic coupling data. The results were: 1) when coupled to Ti6Al4V, Ag showed occasional and reversible anodic inversions with galvanic current z 10-6 A x cm-2, 2) when coupled to 254 SMO, Ag inverted irreversibly as anode with galvanic current up to 10-5 A x cm-2. These differences depended on Ti6Al4V not ennobling, contrary to SS, which ennobled acting as a more powerful cathode. Finally, about dissimilar joints in natural seawater with Ti6Al4V as base alloy, AgCu is confirmed to be unsuitable as brazing material due to early Cu depletion from the interfacial zone. Differently, Ag is promising in relation to generally stable interfaces, even when there is a risk of ennoblement, thanks to Ti6Al4V not ennobling.

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