Accelerated Degradation of Ag-Cu-Ti Alloy in Molten Sodium Via Dissolution of Sliver
Authors: Jung Keeyoung , Park Yoon-Cheol , Heo Hoe-Jun , Kang Chung-Yun 
Published: February 2020

Ag-Cu-Ti alloys are widely used in active metal brazing (AMB) to bond between heterogeneous metal-ceramic joints. This is mainly because brazed joints with the alloys show acceptable bonding strength and good oxidation resistance at high temperatures of 400~600 oC through a simple but low cost bonding process. For example, a brazed joint between 304 stainless steel and α-Al2O3 with a commercial Ag-35.2Cu-5.4Ti (in wt.%) alloy showed excellent shear bonding strength of 100~200 MPa, and the oxidation kinetics of the alloy in air obeys the parabolic rate law with the rate constant of about 10-10~10-12 g2 cm−4 s−1 in the temperature range of 400~600oC. Previous studies showed that the alloy formed slowly growing protective Cu- and/or Ti- oxide layers. However, when the brazing alloys are applied to applications associated with molten sodium (Na) environments, such as cooling pipe joints in nuclear power plants and heterogeneous joints of high temperature sodium beta-alumina batteries, the joints show poor bonding performance resulting in failure in less than 100 hours at 350oC.
In order to elucidate the phenomena different from a general expectation, there is a need to investigate the effect of molten sodium on the degradation kinetics of Ag-Cu-Ti alloys. In this study, disk shaped coupon specimens (10 mm in diameter, 5 mm in thickness) made of 430 stainless steel and α-Al2O3 were prepared and the heterogeneous specimens were brazed with a Ag-35.2Cu-5.4Ti alloy. The bonded assemblies were exposed to molten sodium at 350oC. Their degradation behaviors were characterized via (1) cross-sectional observation through optical microscopy (OM), secondary electron microscopy (SEM), transmission electron microscopy (TEM), and field-emission electron probe micro analyzer (FE-EPMA), and (2) kinetics analysis with Na penetration depth measurements at different exposure times. As the exposure proceeds, it was characterized that Ag in the Ag-Cu-Ti alloy continuously dissolves into the Na melt at the alloy-Na interface leaving the unreacted Cu and Ti to be a porous structure. The kinetics of the Na penetration depth into the alloy is governed by the linear rate law. It turns out that (1) no continuous and protective layer at the reaction front between the Na melt and alloy is formed and (2) Ag concentration in the melt in the vicinity of the reaction front is maintained below its maximum solubility in molten Na by sufficiently rapid homogenization of the Na melt (with a small amount of dissolved Ag) at this temperature.
It was concluded that rapid degrdation of Ag-Cu-Ti in molten Na is not via corrosion but via dissolution of alloying element(s) in the melt although its resulting microstructure is hardly distinguishable from that formed by corrosion process. Since the reaction front is continuously refreshed, the velocity of the reaction front, i.e., the penetration depth of Na into the alloy as a function of time, is governed by the linear rate law which is analogous to gaseous corrosion forming porous or discontinuous corrosion products.