Abstract
Sigma phase leads to deterioration of corrosion resistance of super duplex stainless steels (SDSS) and its volume fraction increases as the time which the materials is exposed in determined temperature increases. In the present study, the effect of the short aging time on the formation of the sigma phase and on the corrosion behavior of UNS S32520 SDSS at 3.5% sodium chloride solution was evaluated using microstructural characterization, linear potentiodynamic polarization curve and electrochemical impedance spectroscopy. The results indicated that the SDSS sample solution-treated at 1150oC and aged at 850oC for 5 min may improve the corrosion resistance in comparison with the as-received and solution-treated conditions, due to the better partition of the elements Cr, Mo and dissolution of certain amount of sigma phase in the microstructure. However, results also showed that when SDSS solution-treated at 1150oC is aged at 850oC with the aging time slightly superior (10 min), the precipitation of sigma phase increases leading to a rapid deterioration of the corrosion resistance of the material.
Highlights
Micrograph of the UNS S32520 super duplex stainless steels (SDSS) solution-treated at a temperature of 1150°C for 30 min and cooled to water, are shown in Figure 1b, note that the austenite is in the form of lamella islands, distributed in the ferritic matrix
Small oscillations in current density resulting from the nucleation and repassivation of metastable pits, which grow and repassivate in a few seconds, caused variations in current density, which accumulated during the corrosion process
The small variation in the potentiodynamic polarization and in the electrochemical impedance results is due to the non-uniform distribution of the chemical elements, which favors the formation of a heterogeneous film with different properties at different points
Summary
Super duplex stainless steels (SDSS) are basically Fe-Cr-Ni-Mo-N system alloys presenting a microstructure of approximately 50 vol% ferrite (α) and 50 vol% austenite (γ), and their high Cr and Mo content allow the combination of excellent mechanical properties and high corrosion resistance, mainly in environments containing halogen family ions, specially the chloride ion[1,2,3,4].The combination of these properties make SDSS extremely interesting for use in the oil, chemical, offshore petroleum, and electric power industries, which require materials with high strength and toughness combined with high corrosion and stress corrosion resistance[5,6].Super duplex duplex stainless steels are useful as wrought alloys, castings and weld, and have replaced successfully austenitic stainless steels in many severe operational applications where higher pitting, crevice and stress corrosion resistance in high temperatures are mandatory requirements, such as seamless tubes, valves and wellheads.despite their superior mechanical and corrosion resistance properties, they are susceptible to precipitation of intermetallic phases as a consequence of the thermodynamic instability of the ferrite in the temperature range from 300 to 1050oC7.The formation of these intermetallic phases, such as the sigma (σ) phase in SDSS is totally undesirable since it causes the depletion of Cr and Mo, in the regions adjacent to the matrix due to their consumption in solid solution, during the alloying process, welding process, heat treatment or during material aging, which results in the reduction of the corrosion resistance and mechanical properties[7,8,9].This phase is basically Fe-Cr-Mo intermetallic compound and it is formed in the austenite/ferrite interface by nucleation and grown processes. Super duplex duplex stainless steels are useful as wrought alloys, castings and weld, and have replaced successfully austenitic stainless steels in many severe operational applications where higher pitting, crevice and stress corrosion resistance in high temperatures are mandatory requirements, such as seamless tubes, valves and wellheads Despite their superior mechanical and corrosion resistance properties, they are susceptible to precipitation of intermetallic phases as a consequence of the thermodynamic instability of the ferrite in the temperature range from 300 to 1050oC7. Depending on its amount in the SDSS, the corrosion resistance decreases considerably whereas the hardness and mechanical strength increase[10,11,12,13]
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