Selective Dissolution Characteristics of 26Cr-7Ni-2.5Mo-3W Duplex Stainless Steel in H 2 SO 4 /HCl Mixed Solution

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1 Materials Transactions, Vol. 50, No. 5 (2009) pp to 1218 #2009 The Japan Institute of Metals EXPRESS REGULAR ARTICLE Selective Dissolution Characteristics of 26Cr-7Ni-2.5Mo-3W Duplex Stainless Steel in H 2 SO 4 /HCl Mixed Solution Heejoon Hwang, Gwanyong Lee, Soonhyeok Jeon and Yongsoo Park Department of Metallurgical Engineering, Yonsei University, 134 Shinchon-dong, Seodaemun-gu, Seoul , Korea Selective dissolution of hyper duplex stainless steel was studied by potentiodynamic and potentiostatic test in various concentrations of H 2 SO 4 /HCl solutions at various temperatures. There were two peaks in the active-to-passive transition region in potentiodynamic test in 2 M H 2 SO M HCl solution at 60 C. In potentiostatic tests, the curve at 340 mv showed stable current density. As the potential increased, the current density increased and at above 310 mv potential, there was a much longer initial period of nonsteady current value. As the potential reached at 280 mv, the current density started to be stabilized and the current density was completely stabilized at 250 mv. It was found that a preferential dissolution of ferrite phase occurred at 330 mv and with the increase of potential, austenite phase was corroded at a high rate. On the other hand, both two phases were passivated at the potential above 270 mv, so that selective dissolution was absent. [doi: /matertrans.mer ] (Received June 5, 2008; Accepted March 3, 2009; Published April 15, 2009) Keywords: hyper duplex stainless steel, selective dissolution, potentiostatic test, active-to-passive transition 1. Introduction Duplex stainless steel (DSS) is the stainless steel (SS) that has microstructure where both ferrite and austenite phases are present in approximately equal volume fraction. DSS has a high mechanical strength, excellent corrosion resistance, and a better cost performance than austenitic SS because of its lower Ni content. Highly alloyed DSS with Cr, Mo and N has excellent resistance to localized corrosion and SCC, so that it is suitable for chemical industries or an marine equipment that requires excellent corrosion resistance. 1,2) The corrosion behavior of DSS is greatly affected by the difference in chemical composition between ferrite and austenite phases. In general, Cr, Mo and W contents are higher in ferrite while Ni and N are much higher in austenite. For this reason, there is a difference in dissolution rate between ferrite and austenite phases of DSS. 3 6) Selective dissolution of the two phases of DSS in acid solutions has been widely studied. 7 14) If lower potential than passivation potential is applied to the specimen, dissolution of ferrite phase takes place preferentially, whereas if higher potential is done, usually austenite phase is dissolved and ferrite phase dissolves very slowly or is passivated. It is well known that if DSS is dipped in acidic solution, galvanic coupling between the two phases affects preferential dissolution. Symniotis measured weight loss as a function of potential for the two phases of SAF 2205 in 2 M H 2 SO M HCl solution. 9) According to Symniotis, the ferrite and austenite phases had similar dissolution rates at different potentials, and the potential of the maximum dissolution rate of austenite was 100 mv more noble than ferrite. These results mean that austenite phase facilitates the dissolution of ferrite phase by galvanic coupling. By Tsai et al., 15) it is revealed that two separate peaks exist in anodic active-to-passive transition region of polarization curve of SAF2205 in H 2 SO 4 /HCl mixed solution. At higher anodic peak, the preferential dissolution of austenite phase occured, while the lower peak corresponded to the ferrite phase. 16,17) The existing researches were progressed for DSS and super DSS of PREW 40 grades. However, to date, few studies have been focused on hyper DSS of PREW 50 grades, having higher Cr, Mo and W than DSS and super DSS, to substitute the 6% Mo austenite stainless steels in more severe environment. The purposes of this study are to observe the active-topassive transition of 26.2Cr-6.99Ni-2.37Mo-2.88W-0.36N hyper DSS having PREW 49.5 in H 2 SO 4 /HCl mixed solution through potentiodynamic polarization test, and to verify the selective dissolution of ferrite and austenite phases through potentiostatic polarization test at different potentials close to passivation potential. 2. Experimental Procedure The DSS was vacuum induction melted and then hot rolled to 6 mm-thick plate. Chemical composition of the DSS used in this study is shown in Table 1. The specimen was cut and solution heat treated at 1090 C for 30 minutes. This was ground with SiC papers, followed by polishing with 1 mm diamond paste, and then electrochemically etched at 2.5 V in 10 N KOH solution at room temperature. Microstructure of DSS was observed with optical microscope. In order to investigate the electrochemical behavior of DSS, the specimen was mounted in epoxy-resin and ground with SiC paper down to #2000, the specimen was conducted to potentiodynamic anodic polarization test (APT) in 2 M H 2 SO 4,2MH 2 SO M HCl and 2 M H 2 SO 4 +1M HCl mixed solutions at 40 and 60 C. After potentiostatic test was conducted at potential close to passivation potential for 2 h, the surface of the specimen was examined with SEM. Besides, to the specimen after the potentiostatic test, XRD was conducted to identify the phases existing on the surface. 3. Results and Discussion Figure 1 shows the microstructure of DSS which was solution heat treated at 1090 C for 30 minutes. The austenite phase can be found as island phase on the background of

2 Selective Dissolution Characteristics of 26Cr-7Ni-2.5Mo-3W Duplex Stainless Steel in H 2 SO 4 /HCl Mixed Solution 1215 Table 1 Chemical composition of duplex stainless steel (DSS) in the present study. Element Cr Ni Mo W Mn N P S Ce Ba Fe PREW mass% Bal PRE (Pitting Corrosion Equivalent) ¼ %Cr þ 3:3 ð%mo þ 1=2 %WÞþ30 %N Fig. 1 Optical micrograph of DSS after solution heat treatment at 1090 C for 30 min. ferrite phase which looks relatively dark. Besides, affected by hot rolling, the specimen has elongated texture parallel to the rolling direction. Potentiodynamic anodic polarization test was conducted in 2 M H 2 SO 4,2MH 2 SO M HCl and 2 M H 2 SO M HCl mixed solutions at different temperatures to see anodic polarization behavior. Results of the APT in H 2 SO 4 /HCl mixed solution at 60 C are shown in Fig. 2. Among the solutions of different concentrations, two peaks were detected in the active-passive transition region at Fig. 3 Potentiodynamic polarization curves of 2205 DSS and the respective constituent phases in 2 M H2SO M HCl solution by Tsai et al. 15) from 400 mv to 200 mv on the polarization curve only in 2 M H 2 SO M HCl solution at 60 C. This result is very similar to the polarization behavior of SAF 2205 (DSS of PRE 38) in 2 M H 2 SO M HCl solution at room temperature reported by Tsai et al. 17) (Fig. 3). However, Fig. 2 Potentiodynamic anodic polarization curves of DSS in (a) 2 M H 2 SO 4 at 60 C, (b) 2 M H 2 SO M HCl at 60 C, (c) 2 M H 2 SO M HCl at 60 C and (d) 2 M H 2 SO M HCl solution at 40 C.

3 1216 H. Hwang, G. Lee, S. Jeon and Y. Park Fig. 4 The enlarged transition region of Fig. 2(b) of DSS in deaerated 2 M H 2 SO M HCl solution at 60 C. (a) (b) (c) (d) (e) (f) (g) (h) (i) (j) Fig. 5 Potentiostatic measurements in 2 M H 2 SO M HCl solution at 60 C for 2 h at different anodic potential.

4 Selective Dissolution Characteristics of 26Cr-7Ni-2.5Mo-3W Duplex Stainless Steel in H 2 SO 4 /HCl Mixed Solution 1217 (a) (b) (c) Fig. 6 SEM micrographs of DSS after potentiostatic test at (a) 330 mv, (b) 300 mv and (c) 270 mv for 2 h in 2 M H 2 SO M HCl solution at 60 C. Fig. 7 XRD of DSS after potentiostatic test in 2 M H 2 SO M HCl solution at 60 C for 2 h. when the Potentiodynamic test was conducted in 2 M H 2 SO M HCl solution at 40 C, only one peak was detected. Femenia et al. 18) reported that the higher alloyed duplex steels exhibited a more equal corrosion resistance between the two phases. High contents of the alloying elements Cr, Mo, W and N in the DSS improved dissolution resistance of the ferrite and austenite phases as well as of phase boundaries, and subsequently this type of DSS showed more homogeneous dissolution behavior. The hyper DSS of PRE 49.5 used in this study contained aforementioned elements highly, so that the dissolution tendency of ferrite and austenite phases in 2 M H 2 SO M HCl solution at 40 C was balanced, but it was thought that the balance was broken as the temperature increased. So, the two peaks of hyper DSS in 2 M H 2 SO M HCl solution were detected at 60 C although the two peaks of SAF 2205, 1st generated DSS were found at room temperature. Figure 4 is the enlarged transition region of Fig. 2(b). The lower peak is located at around 330 mv and the upper at around 270 mv. According to previous reports the peak at lower potential corresponded to the dissolution of ferrite phase, whereas the peak at upper potential to that of austenite phase. Figure 5 showed tendencies of anodic current as a function of time through potentiostatic test in 2 M H 2 SO M HCl at 60 C for 2 h at different potentials. Figure 5(a) representing the test at 340 mv for 2 h showed stable current density. As the potential increased, the current density increased and at above 310 mv potential, the curve denoted that at the beginning the current density decreased but it increased later as time passed. As the potential reached at 280 mv, the current density started to be stabilized, at the same time, time required for the stabilization of the current density was shortened gradually and current density was completely stabilized at 250 mv. Namely, the current density increased as the selective dissolution of ferrite phase started with increasing potential at 340 mv and at the potential of the transition range from 330 mv to 280 mv, ferrite phase was passivated gradually whereas austenite phase was activated. And both two phases were passivated at above 280 mv, so that they could have a constant current density. Figure 6 is microstructures of the specimens conducted to potentiostatic test by SEM. It was found that a selective dissolution between ferrite and austenite phases of the specimen occurred. It could be ascertained that ferrite phase is dissoluted preferentially when potentiostatic tested at 330 mv for 2 h. in SEM image of Fig. 6(a), but in Fig. 6(c), austenite phase was dissolved selectively with ferrite phase remained after tested at 270 mv. Figure 7 shows the results of X-ray diffraction (XRD) of the specimen before and after the potentiostatic test. In XRD result (a) before the potentiostatic test, both peaks of ferrite and austenite phases were present. But after the potentiostatic test at 330 mv (b), some of the peaks of ferrite phase didn t show up and after the test at 300 mv (c) the peak of austenite phase disappeared. It was found that a preferential dissolution of ferrite phase occurred at 330 mv and with the increase of potential, austenite phase was corroded at a fast rate. On the other hand, both two phases were passivated at the potential above 270 mv, so that normal XRD peaks could be detected. 4. Conclusions (1) When the potentiodynamic tests were conducted in H 2 SO 4 /HCl mixed solutions, two separate peaks related to ferrite and austenite phases were detected in active-to-passive transition region of polarization curve only in 2 M H 2 SO M HCl solution at 60 C.

5 1218 H. Hwang, G. Lee, S. Jeon and Y. Park (2) In the potentiostatic test at from 340 mv to 250 mv in 2 M H 2 SO M HCl solution at 60 C for 2 h, the curve showed stable current density at first, but as the potential increased, the current density increased and at above 310 mv potential, the curve denoted that at the beginning the current density decreased but it increased later as time passed. The current density started to be stabilized from 280 mv, at the same time, time required for the stabilization of the current density reduced gradually and the current density was completely stabilized at 250 mv. (3) In XRD result on the surface of the specimen after potentiostatic test, both peaks of ferrite and austenite phases were present before the potentiostatic test. But after the potentiostatic test at 330 mv, some of the peaks of ferrite phase didn t show up and after the test at 300 mv the peak of austenite phase disappeared. It was found that a preferential dissolution of ferrite phase occurred at 330 mv and with the increase of potential, austenite phase was corroded at a fast rate. On the other hand, both two phases were passivated at the potential above 270 mv, so that selective dissolution was stopped. REFERENCES 1) J. O. Nilsson: Mater. Sci. Tech. 8 (1992) ) Y. S. Park, S. T. Kim, I. S. Lee and C. B. Song: Met. Mater. Int. 8 (2002) ) S. Gutiérrez de Sáinz-Solabarría, I. Gutiérrez Urrutia and J. M. San Juan: Nuñez Química e Industria (Quibal) 90 (1992). 4) M. Martins and L. C. Casteletti: Mater. Charact. 55 (2005) ) Y. Maehara, Y. Ohmori, J. Murayama, N. Fujino and T. Kunitake: Met. Sci. 17 (1983) ) Y. Park: Corros. Sci. Tech. 31 (2002) 1. 7) M. Stern: Corrosion 14 (1958) ) Y. H. Yau and M. A. Streicher: Corrosion 43 (1987) ) F. Mansfeld: Corrosion 27 (1971) ) E. Symniotis: Corrosion 46 (1990) 2. 11) E. Symniotis: Corrosion 51 (1995) ) A. Laitinen and H. Hänninen: Corrosion 52 (1996) ) E. Schmidt-Rieder, X. Q. Tung, J. P. G. Farr and M. Aindow: Brit. Corros. J. 31 (1996) ) N. Sridhar and J. Kolts: Corrosion 43 (1987) ) W.-T. Tsai, K.-M. Tsai and C.-J. Lin: Corrosion/2003 NACE, paper #03398 (2003). 16) I-Hsuang Lo, Fu Yan, Chang-Jian Lin and W.-T. Tsai: Corros. Sci. 48 (2006) ) W.-T. Tsai and L.-R. Chen: Corros. Sci. 49 (2007) ) M. Femenia, J. Pan and C. Leygraf: J. Electrochem. Soc. 149 (2002) 194.