Microstructure by Thermal Attack under Vacuum of a Superduplex Stainless Steels and Electrochemical Behavior in H2S/CO2-Saturated Synthetic Seawater

Abstract

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 °C, 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 H 2 S/CO 2 -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 H 2 S/CO 2 (40/60 ppm) at room temperature (25 °C).

Dilatometric Cycles in the Study of Precipitation of Intermetallic Phases in Duplex Stainless Steels

“…Beraha's chemical reagent revealed the DSS microstructure, austenite and ferrite, and sigma phases 3,7,8 , and then quantitative metallography was performed in eight fields for each sample with LEICA light microscope, equipped with a Motic Advanced 3.2 image analyzer system.…”

“…The good properties of this steel class can be attributed to the low C content and a two-phase microstructure composed of elements of stabilizing ferrite and austenite alloys 5,6 . Consequently, duplex stainless steels can be used under most severe temperature conditions and in chloride environments, where austenitic steels are usually susceptible to pitting, cracking and stress corrosion [2][3][4]7,8 .…”