Localized Corrosion of a super-austenitic stainless steel (Fe-24Ni-20Cr-6.3Mo-0.22N) brazed with Ni-based braze (Ni-22Cr-6.3Si-3.8P): Effect of Braze Clearance on Corrosion Resistance

摘要

The microstructural and local corrosion characteristics of a super-austenitic 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) are investigated. The isolated SASS base material is intrinsically resistant to pitting and crevice corrosion in 0.6 M NaCl solution at room temperature. Isolated braze alloy, 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 in 0.6 M NaCl well below that of the SASS. This is due to both the low pitting resistance equivalency number (PREN) 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 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/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 after isothermal brazing. 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. A maximum braze clearance of 30 μm was determined for this specific SASS brazed with the commercially available Ni-based alloy at 1150 °C for 60 minutes.