In this study, microstructural characteristics and corrosion behavior of the UNS S32750 super duplex stainless steel were investigated at the welding cross sections. The gas tungsten arc welding technique with direct current electrode negative polarity and AWS ER2594 filler metal was used. Noticing the importance of heat input in duplex stainless steel welding, welding process was carried out with three heat input including 801, 987 and 1298 J/mm to evaluate the effect of the heat input on the microstructure and corrosion behavior. Microstructures were examined using optical and scanning electron microscopes. In addition, EDS technique was used to understand how the alloying elements are distributed into the ferrite and austenite phases. Electrochemical tests such as potentsiodynamic and cyclic polarization were carried out to evaluate the corrosion resistance of the samples. In addition, ZRA galvanic corrosion test was done to assess the galvanic corrosion between the galvanic couple of base and weld metal. Immersion test was carried out for 100 days to investigate the corrosion at various zones such as base metal, weld metal and heat affected zone. All the corrosion tests were conducted at room temperature and in 80% v/v acetic acid containing boromide attacking ions. The results showed that the amount of austenite phase increased from 43% to 54% as the heat input increased from 801 to 1298 J/mm. According to the EDS results the partitioning ratio of the alloying elements in the weld metal to the base metal was altered. The lowest change in this ratio was for chromium element. Potentsiodynamic polarization tests showed that the base metal a had higher corrosion resistance comparing with base metal at all of the heat input conditions and the difference in their corrosion current density and potential was the lowest at 987 J/mm heat input. Cyclic polarization test exhibited that both of the base and weld metals had negative hysteresis loops and are resistant to the pitting phenomena. Considering potential difference criteria, base metal and weld metal are prone to galvanic corrosion at 801 and 1298 J/mm heat input. ZRA test results at these two cases indicated that the base metal was cathode and the weld metal was anode at a joint. ZRA test results analysis using localization index showed that the corrosion was uniform at low heat inputs and local at high heat inputs. Based on the immersion test for evaluation of corrosion at different zones it can be said that the boundary between the base metal and the weld metal had the lowest resistant to the corrosion, which means that this zone is more anodic than base metal and weld metal. Finally it can be said that, welding with 987 J/mm heat input was the best heat input for UNS S32750 super duplex stainless steel, according to the thickness and joint scheme considering the microstructure and corrosion resistant. Keywords: Super duplex stainless steel, Welding, Microstructure, Heat input, Corrosion.
