Estudo eletroquímico da influência da energia de soldagem na resistência à corrosão de soldas do aço inoxidável superduplex UNS S32760

Abstract

The oil industry requires materials with special properties such as high mechanical and corrosion resistance, so it has invested in research of new materials.These researches permit the use of super duplex stainless steels to meet such expectations.However, the effect of welding thermal cycles can be harmful to the properties of these alloys by modifying ferrite/austenite microstructure and intermetallic phases precipitation. The aim of this work is therefore to evaluate the influence of heat input on corrosion resistance and microstructural transformations of super duplex stainless steel UNS S32760 in multipass welding process MIG/MAG. The samples were produced and welded joints were prepared by modifying the heat input at levels of 0.5, 1.0 and 2.0 kJ/mm.Two regions of the welded joint were obtained for study: moltenzone (ZF) and the heat affected zone (ZAC). A sample unwelding was used as reference. The corrosion resistance was evaluated by means of electrochemical techniques: open circuit potentialmonitoring,potentiodynamic polarization, electrochemical impedance spectroscopy and determination of critical pitting temperature (CPT).The electrochemical tests showed that ZAC energy 1.0 kJ/mm was more susceptible to corrosion. The CPT test showed no significant difference in the results., The nucleation of pits occurred in the ferritic phase in both the regions ZAC and ZF and did not show a relationship with the regions where precipitation occurred intermetallic phases. The results indicated that the heat input was the factor that had the greatest influence on the average content of ferrite in the root zone of the joints. There was precipitation of chromium nitrides in all samples and sigmaphase in the samples of 1.0 kJ/mm and 2.0 kJ/mm. It was observed that the chemical compositions between the ferrite and austenite phases are fairly similar, but the ferrite phase has presented a pitting resistance equivalente (PREN) lower than the austenite.