Effect of Vacuum Brazing on Corrosion Resistance of UNS S31803 and UNS S31200 Duplex Stainless Steels

Duplex stainless steel (DSS) has a combination of good mechanical strength and pitting corrosion resistance, but the precipitated brittle sigma (σ) phase formed during cooling from a welding procedure will rapidly degrade its properties. To avoid the precipitation of secondary phases in the welds, the vacuum brazing effects on UNS S31803 and UNS S31200 DSS weld properties were investigated. Three fillers (copper foil and Ni-based alloys, BNi-3 and BNi-7) were used to evaluate the chemical composition influence on the corrosion resistance. In a nitrogen gas quench, a cooling rate of 0.85°C/s and σ phase precipitation could be inhibited for the studied DSS. Under the quenched state, the weight loss was low for the UNS S31803 DSS dipped in 6% ferric chloride (FeCl₃) solution at 47.5°C for 24 h. However, the σ phase precipitation in UNS S31803 increased as the cooling rate decreased. When the cooling rate decreased to 0.067°C/s, the UNS S31803 weight loss increased. Owing to the compositional difference between the filler and DSS, selective corrosion occurred at the interface of brazed joints soaked in 6% FeCl₃ solution for 24 h.