Joshua James and John R. Scully, University of Virginia; Jim Fitz-Gerald, Center for Electrochemical Science and Engineering
This investigation focuses on the microstructural and local corrosion characteristics of a super-austenitic stainless steel, AL-6XN (Fe-24Ni-20.5Cr-6.3Mo-0.22N, UNS N08367), when brazed with a commercially available Ni-based filler alloy, (Nicrobraz 31: Ni-22Cr-6.5Si-3.5P). The isolated SASS base material is intrinsically resistant to pitting and crevice corrosion in marine environments at room temperature. Isolated Nicrobraz 31, tested as a bead prepared at 1150°C for 60 minutes in a vacuum furnace, without the diffusive egress of melting point depressants (MPDs) exhibited critical potentials for localized corrosion well below that of the SASS. This is due to both the low PRE number of the Ni-Cr solid solution and the loss of beneficial Cr due to formation of silicide and phosphide phases, enabled by the presence of these MPDs. Further characterization was performed on a novel tapered brazed SASS joint sample that linearly increases the braze gap from 0 µm to 200 µm. Such a variation in braze gap may exist from node to node of a brazed cellular metal structure as the gap between core and face sheet varies. This test configuration enabled the simultaneous collection of both microstructural details and local corrosion results over a wide range of braze gaps/ diffusion lengths and, hence, levels of residual MPD concentrations. Scanning electron microscopy, energy dispersive spectroscopy, and X-ray diffraction were used to assess the fate of melting point depressants, Si and P, and the phases formed over a range of braze gaps/diffusion lengths. This information was correlated with the subsequent corrosion resistance and the minimum braze clearance was determined. Insight on corrosion mitigation strategies via either braze alloy design or post-braze heat treatment was subsequently developed.
