The corrosion behaviour of duplex and superduplex stainless steels depends on both composition and microstructure. Although the corrosion resistance of the base materials can be carefully controlled, after welding this resistance may be impaired due to precipitation of intermetallic phases and/or the development of an unbalanced microstructure.

In this work, the effect of multipass welding on both standard duplex (UNS S31803) and super-duplex (UNS 32550) alloys were evaluated. A Gleeble® thermomechanical simulator was used to simulate single and three-pass HAZ in small laboratory samples. Three different thermal cycles with different interpasses temperatures (natural heating, 200 and 300^oC) were used. The effect of these thermal cycles on the corrosion performance of the duplex and super-duplex HAZs was analysed by intergranular corrosion (ASTM A262 Practice A) and pitting corrosion tests (ASTM G48 and cyclic polarisation in 3.5% NaCl solution). Microstructure of simulated samples was analysed using both optical and scanning electron microscopy. Composition of the ferrite and austenite phases was determined using an electron dispersive spectrometer attached to the SEM.

Intergranular corrosion testing presented a better result for 300C interpass temperature for both duplex grades. These results are confirmed by metallographic examinations that showed a lower amount of inter and intragranular intermetallic precipitated phases and chromium nitrides (Cr₂N) with a 300^oC interpass temperature. This is due to lower cooling rates that generate a cleaner microstructure. It means less to inter and intragranular intermetallic phases and more to austenite precipitated. The results of FeCl₃ pitting corrosion tests showed no difference between critical pitting temperature for single and multipass thermal cycles. The calculated critical pitting temperature by EDS analysis of the ferrite and austenite showed a good agreement with the experimental results for UNS S31803. This technique should be improved to give better results for all duplex stainless steel grades. Cyclic polarisation curves presented a decreasing trend in pitting potential with increasing interpass temperature for UNS S31803 in both tested temperatures. On the other hand, UNS S32550 exhibited little difference in pitting potential for testing temperature in all simulated thermal cycles. Comparing single and multipass samples, one can notice a lower pitting potential for duplex stainless steels single pass conditions.