Electrochimica Acta 51 (2006) 1842–1846 Evaluation of selective corrosion in UNS S31803 duplex stainless steel with electrochemical impedance spectroscopy R. Chaves a , I. Costa b , H.G. de Melo c , S. Wolynec a,∗ a Department of Metallurgical and Materials Engineering of the Polytechnic School of the University of S˜ ao Paulo, Brazil b Energy and Nuclear Research Institute – IPEN/CNEN-SP, S˜ ao Paulo, Brazil c Chemical Engineering Department of the Polytechnic School of the University of S˜ ao Paulo, Brazil Received 23 June 2004; received in revised form 25 November 2004; accepted 25 February 2005 Available online 19 August 2005 Abstract The precipitation of certain phases, in particular of sigma phase, in duplex stainless steels makes them susceptible to selective corrosion. In the present work, the effect of heat treatments upon selective corrosion of UNS S31803 duplex stainless steel was investigated in 2M H 2 SO 4 + 0.5 M NaCl + 0.01 M KSCN solution at 30 ◦ C using electrochemical impedance spectroscopy (EIS). For this purpose, samples were solution annealed at 1050 ◦ C for 30 min and, subsequently, heat-treated at 750 ◦ C or 850 ◦ C for times varying from 1 h to 30 h in order to precipitate the sigma phase and other phases. After these heat-treatments the specimens were immersed in the test solution and submitted to EIS tests. These tests, performed at the open circuit potential after 84 h of immersion in the test solution, showed higher impedances for samples tested at 850 ◦ C than at 750 ◦ C, implying lower selective corrosion rates for the former samples. Moreover, for samples tested for longer times at 850 ◦ C, there are evidences of an increase of the high frequency phase angle, which seems to be associated with the recovery of Cr and Mo content in the depleted zone near the precipitated sigma phase. These results are in good agreement with those obtained in tests performed with the electrochemical potentiodynamic reactivation double loop method. © 2005 Elsevier Ltd. All rights reserved. Keywords: Duplex stainless steel; Electrochemical impedance spectroscopy; Selective corrosion; Sigma phase precipitation 1. Introduction Due to their microstructure, composed of approximately equal volumetric fractions of ferrite (α) and austenite (γ ), duplex stainless steels display adequate mechanical proper- ties and resistance to corrosion in an ample range of appli- cations, thus justifying their increasing use in several fields such as chemical processing, electrical energy generation and others. Notwithstanding these advantages, in some applica- tions involving welding and high temperature, these steels are susceptible to the precipitation of additional phases that affect both the corrosion and the mechanical properties. Depending on temperature, different phases can precipi- tate, such as sigma (), chi (), nitrides (Cr 2 N) and carbides ∗ Corresponding author. Tel.: +55 11 3091 5236; fax: +55 11 3091 5243. E-mail address: swolynec@usp.br (S. Wolynec). (M 23 C 6 and M 7 C 3 ) [1–3]. In a duplex stainless steel such as UNS S31803, heat-treated at 850 ◦ C for 5 h, the main compo- nents of sigma phase are Fe, Cr and Mo in approximate wt% amounts of 57, 31 and 7, respectively. The remaining 5 wt% is Ni, Mn and Si [4]. The chi phase, which is formed in lesser amounts, has also the same main components in approximate amounts of 56 wt% Fe, 17 wt% Cr and 27 wt% Mo [5]. Dur- ing their precipitation these phases remove from their vicinity elements responsible for the corrosion resistance of the steel. In this way, regions depleted in these elements are formed, making the material susceptible to selective corrosion in a manner similar to sensitization of austenitic stainless steels, which renders them susceptible to intergranular corrosion. The latter is a peculiar type of selective corrosion, in which the chromium depleted regions form along the grain boundaries. In duplex stainless steels the regions susceptible to selective corrosion are chromium and molybdenum depleted and they 0013-4686/$ – see front matter © 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2005.02.105