Engenharia de Materiais

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Pit morphology and its microstructure relation in 850°C aged UNS S31803 (SAP 2205) duplex stainless steel
(2003-07-24) Rodrigo Magnabosco; ALONSO-FALLEIROS, N.
This work described the relationship between microstructure of UNS S 31803 (SAP 2205) aged at 850°C and pitting morphology after cyclic polarization in chloride aqueous solution. The initial material, solution treated for 30 minutes at 1120°C and water quenched, was aged at 850°C for periods up to 100 hours. Cyclic polarization in 3,5% NaCl aqueous solution was conducted on #600 grounded sample surfaces. After polarization, the samples were sectioned transversal and perpendicular to polarized surface to allow the inner view of pitting corrosion using scanning electron microscope. It was found that sigma phase formation after 850°C aging treatment reduced pitting potential, and pitting corrosion occurred as selective corrosion of chromium and molybdenum impoverished regions, like interfaces between sigma phase and metallic matrix.
Pitting corrosion of UNS S41000 and UNS S42000 stainless steels
(2009-09-06) BORGES, A. O.; ALONSO-FALLEIROS, N.; Rodrigo Magnabosco
The aim of this work is to evaluate the pitting corrosion resistance, through potentiodynamic polarization tests, in two commercial martensitic stainless steels, named UNS S41000 and UNS S42000, by using four electrolytes of 0.10M Na2SO4, with different concentration of chloride: 0.01M NaCl; 0.05M NaCl; 0.10M NaCl; 0.60M NaCl (3.5%). The received samples showed a ferritic microstructure with spheroidized carbides (annealed), whereas it was observed a microstructure of tempered martensite after heat treatment. Considering the same condition - annealed or quenched and tempered - the hardness of steel UNS S42000 was always higher than steel UNS S41000. In potentiodynamic tests, the potential scan rate was defined as 1mV/s after exploratory tests which showed this parameter influence on the curve shape. From then on, it was possible to obtain polarization curves with scanning rate of 1mV/s, starting at open-circuit potential after a five minute immersion and 600 grit surface finish. The results showed that the higher chloride ion concentration, the lower the pitting potential. For a given electrolyte, pitting potential measured for annealed steel UNS S41000 is slightly higher than the same steel on quenched and tempered condition. The intense chromium carbide precipitation at the grain boundaries of former prior austenite, a phenomenon that can lead to sensitization, may be a reason for it. Through immersion tests, it was found out that pitting potential of UNS S41000, after heat treatment, is lower than corrosion potential in the electrolyte composed of (3.5% NaCl + 0.10M Na2SO4). Otherwise, steel UNS S42000 presented higher pitting potential after heat treatment. In this case, intergranular carbide precipitation was not observed and the quench and temper treatment, at first, generates a higher content of chromium in solid solution if compared with the annealing treatment, leading to a greater corrosion resistance. Pitting density and their sizes are chloride concentration dependent: the higher ion chloride concentration, the lower is pit density and the larger is pit width. According to the studied conditions, the most suitable electrolyte for pitting potential determination is composed of (0.10M NaCl + 0.10M Na2SO4). The shape of the obtained potentiodynamic polarization curves with this electrolyte allowed the identification and accurate determination of the pitting potential.
Influenzce of the sigma phase formation in pitting potential of UNS s31803 duplex stainless steel in 0,6M NACL AGED at 850 E 900°C
(2010-07-26) Daniella Caluscio dos Santos; Rodrigo Magnabosco
The main purpose of this work is to analyze the mechanisms and kinetics of sigma phase formation of UNS S31803 stainless steel after isothermal aging between 850 e 900°C, evaluating the influence of sigma phase formation on pitting potential in 0.6M NaCl solution. Solution treated samples of the UNS S31803 duplex stainless steel were isotermically aged at 850°C or 900°C up to 360 h. The microstructural characterization of the samples was made through quantitative metallography, ferritscope analysis and Vickers microhardness. The cyclic polarization tests used to the determination of the pitting potential were conducted in polished samples. The electrochemical behavior of aged UNS S31803 duplex stainless steel in 0.6M NaCl solution is strongly dependent on the microstructure, since both changes in mechanisms and kinetics of sigma phase formation influenced the pitting potential of the material. Increasing aging time lead to the decrease of pitting potential, due to the formation of chromium impoverished regions. Oscillations in pitting potential values were noted due to the chromium redistribution enabled by easy diffusion of this element in high temperatures.
Comparison between stainless steels and Nickel alloys through pitting corrosion resistance electrochemical tests
(2021-04-27) FRAZÃO, I.; Rodrigo Magnabosco; DELBLANC, A.
© 2021 Universidade Federal de Sao Carlos. All rights reserved.Critical pitting temperature (CPT) is one of the most accepted criteria for alloy classification concerning pitting corrosion. The techniques used nowadays to determine CPT have different parameters for alloys of different material classes, preventing the comparison between stainless steels and nickel based alloys. This study aims to compare the corrosion resistance of nickel based alloys and stainless steels with high corrosion resistance through potentiostatic tests for CPT determination, using a 3M MgCl2 aqueous solution. CPT values were effectively determined for the stainless steels studied but the technique did not have the same efficiency for nickel based alloys due to the occurrence of crevice corrosion, even considering the higher PRE of nickel based alloys.

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