Pitting corrosion of UNS S41000 and UNS S42000 stainless steels

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Rodrigo Magnabosco
European Corrosion Congress 2009, EUROCORR 2009
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BORGES, A. O.; ALONSO-FALLEIROS, N.; MAGNABOSCO, R. Pitting corrosion of UNS S41000 and UNS S42000 stainless steels. European Corrosion Congress 2009, EUROCORR 2009, v. 1, p. 440-452, sept. 2009.
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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.