Effect of Braze Clearance on Localized Corrosion of a Superaustenitic Stainless Steel Brazed with a Ni-Based Alloy (Ni-22Cr-6.3Si-3.8P)

The microstructural and local corrosion characteristics of a superaustenitic stainless steel (SASS) (Fe-24Ni-20.5Cr-6.3Mo-0.22N, UNS N08367), brazed with a commercially available Ni-based filler alloy (Ni-22Cr-6.5Si-3.5P), were investigated. The isolated SASS base material was intrinsically resistant to pitting and crevice corrosion in 0.6 M sodium chloride (NaCl) solution at room temperature. The isolated braze alloy, tested as a bead prepared at 1,150°C for 60 min in a vacuum furnace without the diffusive egress of melting point depressants (MPD), exhibited critical potentials for crevice corrosion in 0.6 M NaCl solution at ambient temperature well below that of the SASS. This is due to both the low localized corrosion resistance of the Ni-Cr solid solution and the further loss of beneficial Cr due to formation of silicide and phosphide phases, enabled by the presence of Si and P MPD. 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 in brazed cellular metal structures 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 and, hence, levels of residual MPD concentrations. Scanning electron microscopy, energy-dispersive spectroscopy, and x-ray diffraction were used to assess the fate of MPD, Si and P, and the phases formed over a range of braze gaps and thus diffusion lengths. This information was correlated with the subsequent corrosion resistance as a function of braze gap. The minimum braze clearance was determined for good seawater corrosion resistance. A maximum braze clearance of 30 μm was determined for UNS N08367 brazed with Ni-based alloy NB 31 at 1,150°C for 60 min. Insight into corrosion mitigation strategies via either braze alloy design or post-braze heat treatment was subsequently discussed.