Microstructure Influence on Corrosion Resistance of Brazed AISI 304L/NiCrSiB Joints

The characterization of corrosion resistance, which is essential to estimate the lifetime of brazed joints in corrosive environments, is of central importance for many industrial applications and a basic requirement for the reliable and economic operation of brazed components. High temperature vacuum brazing with thin amorphous-crystalline foils is used for numerous applications such as exhaust gas heat exchangers. In this study one industrial BNi-5a® and two experimental rapidly solidified filler metal foils of Ni7Cr7.5Si4Fe1.5B and Ni20Cr7.5Si4Fe4Mo1.5B wt% were used to braze joints of AISI 304L. In addition, two holding times at 1160 °C were chosen to investigate the effect of the resulting microstructural differences on corrosion resistance. Especially the amount and distribution of borides and silicides within the brazing seam could be changed by the time-dependent diffusion processes, as could be shown by metallographic cross sections. Accelerated intercrystalline corrosion tests were carried out to evaluate the influence of the microstructure on the corrosion depth and damage mechanisms. Additionally, potentiodynamic polarization measurements in synthetic exhaust gas condensate as an application-oriented corrosion medium were performed for a comparative evaluation of corrosion properties and rate. The combination of high chromium-containing filler metal and increased holding time, which led to a more homogeneous microstructure, resulted in a more than five times improved corrosion resistance within both investigations.