Corrosion of a super-austenitic stainless steel (Fe-24Ni-20Cr-6.3Mo-0.22N) brazed with Nicrobraze 31 (Ni-22Cr-6.3Si-3.8P): Electrochemical Corrosion Behavior of Isolated and Combined Materials

摘要

The corrosion of a super-austenitic stainless steel (SASS) A1-6XN (Fe-24Ni-20Cr-6.3Mo-0.22N in wt.%, UNS N08367) when brazed with a commercial Nicrobraze alloy (Nicrobraze 31: Ni-22Cr-6.3Si-3.8P) in 0.6 M NaCl at 25°C is reported. The SASS sheet material was intrinsically resistant to pitting and crevice corrosion in marine environments at room temperature, as indicated by its performance in both multiple crevice assembly exposures and electrochemical polarization testing in 0.6 M NaCl. The open circuit potential of isolated AL-6XN fell roughly 900 mV below the critical potential for crevice corrosion repassivation (E_(crev,r)). Isolated beads of complex multiphase Nicrobraze 31 exhibited an open circuit potential equal to or greater than the measured E_(crev,r) of -0.200 V_(SCE) when prepared at 1150°C for 60 minutes in a vacuum furnace. This was related to both the low PRE number of the solid solution phase and the segregation of beneficial chromium due to the complex microstructure of the alloy in a bead form, representative of a brazed joint of infinite clearance. The complex microstructure is a result of the formation of intermetallic compounds involving the melting point depressants (Si and P) in the braze alloy. Brazing AL-6XN with Nicrobraze 31(1150°C/60 minutes) produced an extrinsic mode of crevice corrosion which initiated in the braze surface but propagated into the SASS. Multiple crevice assembly tests in 6 wt% FeCl_3 and also 0.6 M NaCl with neutral pH revealed that the brazed SASS alloy was attacked heavily on the brazed surface and not at all on the bare AL-6XN surface. Crevice corrosion attack greatly exceeded the thickness of the Nicrobraze 31 layer, indicating corrosion penetration into the SASS. This crevice corrosion of the brazed SASS could be explained by the dissimilar metal crevice corrosion mechanism and was not due to intrinsic sensitization by the braze thermal treatment.