Abstract Duplex stainless steels (DSS) are used in off-shore platforms due to their good properties resulting from the combination of two phases, namely austenite and ferrite. Secondary intermetallic phases, such as alpha prime (α´) and sigma (σ), which are harmful, can be formed at temperatures above 400 °C, or by a casting process. This study investigates the formation of sigma phase by thermal cycles running in a dilatometer (800 °C, 850 °C, 900 °C and 1,000 °C) for 1h, and at 850 °C for 3h and 7h. The optical microscopy of the DSS microstructure subjected to 800, 850 and 900 °C for one hour revealed a small fraction of σ phase with nucleation in the contour of ferrite/austenite phases, and tests by ferritoscope indicated a decrease (~4%) in a magnetic fraction (ferrite). However, samples subjected to dilatometric cycles at 850 °C for 3 hours showed an 18% decrease in the magnetic phase and quantitative metallography revealed a 33% fraction of σ phase precipitated in the ferrite phase. Tests conducted at 850 ºC for 7 hours indicated a high fraction of σ phase precipitated (approximately 50%), starting in the grain boundary, austenite/ferrite, and advancing in ferrite grain.

Superduplex stainless steels (SSS) composed in both equal fraction ferritic-austenite phases, classically defined by ASTM A995M Gr.5A, in order to obtain a better relation of mechanical properties and corrosion properties, are applied as components for use in petrochemical industries in off-shore platforms. The corrosion resistance of stainless steels is due to the oxides formed on the surface, which is dependent on the temperature and environment in which they are exposed. This paper investigates a microstructure duplex during steps of thermal attack under vacuum at high temperatures through observation of the coloration and growth of oxide films. Some coloration was observed in the phases, austenite and ferrite, with the increase of the temperature to 1100 ºC, which is indicative of the presence of chromium and iron oxides. In the cooling occurs the stability of oxides present in the films. Electrochemical tests were conducted in H2S/CO2-saturated in two different concentrations of chloride sodium electrolyte, simulating a seawater environment. Potentiodynamic polarization curves indicate that pitting corrosion resistance and repassivation process in media of 3.5 and 9.0% (wt.%) NaCl are not affected by the presence of H2S/CO2 (40/60 ppm) at room temperature (25 ºC).