Influence of microstructure on the corrosion resistance of the duplex stainless steel UNS S31803 V.S. Moura, L.D. Lima, J.M. Pardal, A.Y. Kina, R.R.A. Corte, S.S.M. Tavares ⁎ Universidade Federal Fluminense, Departamento de Engenharia Mecânica/PGMEC, Rua Passo da Pátria, 156, CEP 24210-240, Niterói/RJ, Brazil ARTICLE DATA ABSTRACT Article history: Received 16 August 2007 Accepted 6 September 2007 Duplex stainless steels (DSS) are high strength corrosion resistant materials used in the chemical and petrochemical industries. When compared to conventional austenitic grades (AISI 304L, 316L, 317L), DSS are more expensive, but present higher strength levels and are more resistant to Cl - environments. In the present work it was investigated how microstructural variations can affect the corrosion resistance of a DSS UNS S31803 in artificial sea water media. Pitting corrosion resistance in 3.5%NaCl solution was evaluated by cyclic polarization tests at room temperature and 60 °C. The double loop electrochemical potentiodynamic reactivation test (DL-EPR) was also used to evaluate the degree of sensitization due to tertiary phases, such as σ and Cr 2 N. Structures containing sigma phase were found to be the most susceptible to pitting and localized corrosion. Cr 2 N, present in the as-melted sample, decreases the pitting potential but does not cause sensitization. © 2007 Elsevier Inc. All rights reserved. Keywords: Duplex stainless steel Microstructure Pitting corrosion 1. Introduction The high corrosion resistance of duplex stainless steels (DSS) is due to alloying elements such as Cr, Mo and N, but also depends on the microstructure. The best general properties are obtained with approximately equal amounts of austenite and ferrite and the absence of third phases such as σ and χ [1,2]. These phases can be formed from ferrite when the DSS is welded or hot worked. Welding specifications are designed to obtain phase proportions near 1:1 and to avoid σ and χ precipitation by limiting the heat input to 1.5 or 2.0 kJ/mm [3]. However, as the austenite phase forms by diffusion on cooling below 1300 °C, too low welding energy can lead to high ferrite contents (N 70%), which is also undesirable. In this case, chromium nitride (Cr 2 N) precipitation is prone to occur in the ferrite domains, due to low solubility of nitrogen in this phase. In recent work [4] we have reported on the effect of microstructure on the toughness and hardness of DSS UNS S31803. Samples rapidly cooled from 1300 °C presented higher hardness and lower toughness than samples that were slow cooled and solution treated. An increase of the ferrite phase can be related to intense chromium nitride (Cr 2 N) precipita- tion [4,5]. The high ferrite content and the Cr 2 N precipitation decrease the low temperature toughness. In addition, Cr 2 N particles are also believed to be nucleation sites for pitting [6]. In the present work, various microstructures were pro- duced in order to evaluate the influence of austenite/ferrite proportions, Cr 2 N and σ on the pitting corrosion resistance. The effect of cold deformation on the pitting corrosion resistance was also studied. 2. Experimental Sheets of UNS S31803 duplex steel with the chemical composition shown in Table 1 were purchased in the solution treated condition. The microstructure of the material was varied by using a range of thermal and mechanical processes as indicated in Table 2. MATERIALS CHARACTERIZATION 59 (2008) 1127 – 1132 ⁎ Corresponding author. Tel.: +55 21 2629 5584; fax: +55 21 2629 5368. E-mail address: ssmtavares@terra.com.br (S.S.M. Tavares). 1044-5803/$ – see front matter © 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.matchar.2007.09.002