HCl solutions

Corrosion Science 48 (2006) 696–708 www.elsevier.com/locate/corsci

Effect of electrolyte composition on the active-to-passive transition behavior of 2205 duplex stainless steel in H2SO4/HCl solutions I-Hsuang Lo a, Yan Fu b, Chang-Jian Lin b, Wen-Ta Tsai a

a,*

Department of Materials Science and Engineering, National Cheng Kung University, Tainan 701, Taiwan b Department of Chemistry, State Key Laboratory for Physical Chemistry of Solid Interface, Xiamen University, Xiamen 361005, China Received 6 September 2004; accepted 3 February 2005 Available online 28 March 2005

Abstract Selective dissolution could occur in duplex stainless steels (DSSs) due to the difference in chemical composition between the two constituent phases. In this study, the effect of H2SO4/HCl composition on the selective dissolution behavior was investigated. The results indicated that there were two distinct peaks appeared in the active-to-passive transition region in the polarization curve. The peak appeared at a lower potential region was associated with the preferential dissolution of ferrite phase while that for austenite at a higher potential. In the concentration ranges of 0.25–2 M of H2SO4 and 0.25–2 M of HCl, the magnitude of the peak anodic current density and the resolution between these two peaks greatly depended on the composition of H2SO4/HCl. However, the anodic peaks corresponding to the respective dissolutions of ferrite and austenite became less distinguishable when the concentrations of HCl exceeded 1.2 M. Image analysis using scanning electron microscopy (SEM) was performed to confirm the selective dissolution of each constituent phase after potentiostatic polarization at the respective anodic peak potential. Ó 2005 Elsevier Ltd. All rights reserved.

*

Corresponding author. Tel.: +886 6 2757575x62927; fax: +886 6 2754395. E-mail address: [email protected] (W.-T. Tsai).

0010-938X/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2005.02.004

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Keywords: (A) Duplex stainless steel; (C) Selective dissolution; (C) Active-to-passive transition

1. Introduction Selective dissolution in a duplex stainless steel (DSS) is expected to occur due to the difference in chemical composition of the constituent phases. Yau and Streicher [1] indicated that selective corrosion of ferritic (a) phase did occur for Fe–Cr– 10%Ni DSS in reducing acid. Similar observations have been reported by others [2–5]. However, selective dissolution of austenitic (c) phase has also been reported by Sridhar and Kolts [6]. The occurrence of selective dissolution has also been found in the progress of environmentally-assisted cracking [7]. Each phase in a DSS may exhibit an electrochemical potential different from the other, which may facilitate the occurrence of galvanic corrosion. As pointed out by Symniotis [2], the weight loss of SAF 2205 DSS in 2 M H2SO4 + 0.1 M HCl solution was composed of two contributions from a and c phases, respectively, which were potential dependent. The local difference in potentials between a and c phases has been measured and confirmed by using a scanning vibrating electrode technique [8]. The existence of two separate peaks in the active-to-passive anodic transition peak has further been elucidated by Tsai et al. [9] using electrochemical potentiokinetic reactivation (EPR) method. The higher anodic peak corresponded to the preferential attack of c phase in 2 M H2SO4 + 0.5 M HCl solution while that at a lower potential was for a phase. In situ studies using scanning tunneling microscopy (STM) [10,11] and atomic force microscopy (AFM) [12] gave the images of the preferential dissolution behavior in DSSs. Though the metallurgical factors affecting the selective dissolution behavior in DSSs, including composition, heat treatment, a/c area ratio, alloying partitioning etc., have been investigated in the past [1–6,12–14], the environmental effect was less explored. In this investigation, the effect of composition of the mixed H2SO4/HCl solution on the selective dissolution in 2205 DSS was studied. Microscopical examination to reveal the morphological difference after potentiostatic etching was emphasized.

2. Experimental The chemical composition of 2205 DSS used in this investigation is listed in Table 1. The steel rod with a diameter of 15 mm was solution heat treated at

Table 1 Chemical composition (wt.%) of 2205 DSS used Element

Fe

C

Cr

Ni

Mo

Mn

Si

Cu

P

S

N

wt.%

Bal.

0.014

22.40

5.42

3.24

1.43

0.41

0.21

0.025

0.004

0.198

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Table 2 Compositions of testing solutions used for electrochemical tests Constituent

H2SO4

HCl

Concentration (M)

2.0 2.0 2.0 2.0 2.0 2.0 2.0

0.25 0.5 0.7 1.0 1.2 1.5 2.0

0.25 0.5 0.7 1.0 1.2 1.5 2.0

0.5 0.5 0.5 0.5 0.5 0.5 0.5

1100 °C for 30 min. This rod was sliced into several discs each with a thickness of 2 mm. The disc was mounted in epoxy resin, ground with SiC papers to a grit finish of #2000, followed by polishing with 0.1 lm Al2O3 powders. Finally, the mounted specimen was cleaned with distilled water before electrochemical testing. The mixed H2SO4/HCl solutions with the compositions listed in Table 2 were used for electrochemical tests. Potentiodynamic polarization curve measurements were conducted in the H2SO4/ HCl solutions with various compositions at ambient temperature. The potential was scanned from the cathodic towards anodic direction at a rate of 1 mV/s. All the potentials were measured with respect to a saturated calomel electrode (SCE). A platinum sheet was used as the counter electrode. Potentiostatic etching in each of the above electrolyte at the peak potential in the active-to-passive transition region was also performed for 3 h. After potentiostatic etching, the specimen was examined with a scanning electron microscope (SEM). The major alloying elements in the constituent phases were analyzed using energy dispersive spectroscopy (EDS).

3. Results and discussion Fig. 1 presents the metallograph of 2205 DSS after solution heat treatment at 1100 °C for 30 min. The etchant used was a boiling Murukami reagent. The white region in Fig. 1 corresponds to the austenitic phase (c) while the gray region is the ferritic phase (a). As shown in this figure, the island-like c phase was embedded in the continuous a matrix. Table 3 lists the average compositions of the individual c and a phases analyzed by EDS. Image analysis showed that the a/c phase area ratio was 0.53/0.47.

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Fig. 1. Optical micrograph of 2205 DSS after heat treating at 1100 °C for 30 min.

Table 3 EDS results of the major alloying elements (wt.%) of c and a phase in 2205 DSS Element

Fe

Cr

Ni

Mo

Mn

c phase a phase

69.5 68.7

20.4 23.5

7.2 4.9

1.2 2.1

1.7 0.8

1500 2205 DSS scan rate = 1 mV/sec

A: 2 M H2SO4 B: 0.5 M HCl C: 2 M H2SO4 + 0.5 M HCl

Potential (mV vs. SCE)

1000

500

0 A

B

C

-500 1E-008 1E-007 1E-006 1E-005 1E-004 1E-003 1E-002 1E-001 Current density (A/cm2)

Fig. 2. Potentiodynamic polarization curves of 2205 DSS in 2 M H2SO4, 0.5 M HCl and 2 M H2SO4 + 0.5 M HCl mixed solution, respectively.

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Fig. 2 shows the potentiodynamic polarization curves of 2205 DSS in 2 M H2SO4, 0.5 M HCl, and 2 M H2SO4 + 0.5 M HCl solutions, respectively. As shown in this figure, no distinctive active-to-passive transition region was observed in 2 M H2SO4

Fig. 3. SEM micrographs of 2205 DSS after potentiostatic etching at (a) Ec max = Ea max = 320 mV for 3 h in 2 M H2SO4 + 0.5 M HCl solution.

255 mV, and (b)

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solution, but that region was detected in 0.5 M HCl solution. In the mixed solution with a composition of 2 M H2SO4 + 0.5 M HCl, an enhanced anodic current density in the active-to-passive transition region was observed. It is worth of noting that two distinguishable peaks existed in the active-to-passive transition region in 2 M H2SO4 + 0.5 M HCl solution. The results were consistent with those reported in the literature [8,9]. The SEM micrographs of 2205 DSS after potentiostatic etching for 3 h in 2 M H2SO4 + 0.5 M HCl solution at the two respective potentials are shown in Fig. 3. As depicted in Fig. 3(a), preferential dissolution of c phase occurred at the higher peak potential ( 255 mV). On the contrary, a phase corroded at a fast rate at the lower peak potential ( 320 mV), as shown in Fig. 3(b). Based on these observations, the peak potential at which either c or a phase dissolved preferentially was thus defined as Ec max or Ea max. The corresponding anodic current density was noted as Ic max or Ia max. It has been pointed out previously that the active-to-passive transition region could be deconvoluted into two anodic peaks associated with the respective activeto-passive transitions for a and c phases [9]. Fig. 4 gives the schematic diagram showing the deconvolution for the active-to-passive transition region in the anodic polarization curve of 2205 DSS in mixed H2SO4/HCl solutions. At potentials below Ea max, the dissolution rate of c phase is lower than that of a phase. At potentials above Ec max, on the other hand, both phases are in passive states, and preferential

-100

Experimental

2205 DSS scan rate 1 mV/sec 2M H2SO4 + 0.5M HCl

γ α

Potential (mV vs SCE)

-200

γ

-300

α

-400

1E-005

1E-004

1E-003

Current density (A/ cm2)

Fig. 4. Schematic diagram showing the contributions of ferrite (a) and austenite (c) phase on the anodic peak current density of 2205 DSS in 2 M H2SO4 + 0.5 M HCl solution and at potentials in the active-topassive transition region.

702

I-H. Lo et al. / Corrosion Science 48 (2006) 696–708 400 2205 DSS scan rate = 1 mV/sec 2 M H2SO4 + x M HCl

Potential (mV vs SCE)

200

A: x = 0.25 M B: x = 0.5 M C: x = 0.7 M

D: x = 1 M E: x = 1.2 M F: x = 1.5 M G: x = 2 M

G 0

F D

E

C

B -200 A -400

-600 1E-007

1E-006

1E-005

1E-004

1E-003

1E-002

1E-001

Current density (A/cm2)

Fig. 5. Effect of HCl concentration on the potentiodynamic polarization curves of 2205 DSS in 2 M H2SO4 base solution.

dissolution is absent. However, if the potential is controlled in the range between Ea max and Ec max, preferential dissolution and its reversion can be observed. The polarization curves for 2205 DSS determined in 2 M H2SO4 solutions containing various concentrations of HCl are demonstrated in Fig. 5. The active-to-passive transition region was found to shift towards the anodic direction when HCl concentration was increased. As revealed in this figure, Ic max was increased with a faster rate than that of Ia max by increasing the concentration of HCl in 2 M H2SO4 solution. Fig. 6 gives the SEM micrographs showing the selective dissolutions of c and a after etching at the respective peak potentials, namely 250 and 315 mV, respectively in 2 M H2SO4 + 1 M HCl solution for 3 h. The extent of dissolution of each phase was enhanced as the concentration of HCl was increased from 0.5 to 1.0 M. As revealed in Fig. 5, it was noted that the peak potentials, Ec max and Ea max, were also affected by the change of HCl concentration. Ec max and Ea max became less distinguishable as the concentration of HCl was increased above 1.2 M. In such high HCl concentration and at the peak potential, extensive dissolution of both a and c phases took place, leaving severely corroded surface morphology as demonstrated in Fig. 7 for 2205 DSS exposed in 2 M H2SO4 + 2 M HCl solution at 190 mV for 3 h. The change in the characteristic feature of the active-to-passive transition region was also seen in 0.5 M HCl solutions with various H2SO4 additions. As illustrated in Fig. 8, the anodic current density increased with increasing H2SO4 concentration. Unlike that found in H2SO4 base solutions, the two anodic peaks corresponding

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Fig. 6. SEM micrograph showing the surface morphology of 2205 DSS after potentiostatic etching at: (a) Ec max = 250 mV, and (b) Ea max = 315 mV for 3 h, in 2 M H2SO4 + 1 M HCl solution.

to different phase dissolution of 2205 DSS were still distinguishable in 0.5 M HCl solution with the addition of H2SO4 up to 2 M. The SEM micrographs showing the surface morphologies of 2205 DSS after potentiostatic etching at Ec max

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Fig. 7. SEM micrograph showing the surface morphology of 2205 DSS after potentiostatic etching at the peak potential of 190 mV for 3 h, in 2 M H2SO4 + 2 M HCl solution.

200 2205 DSS scan rate = 1 mV/sec 0.5 M HCl + y M H2 SO4 A: y = 0.25 M B: y = 0.5 M C: y = 0.7 M

Potential (mV vs SCE)

0

C -200

B

D: y = 1 M E: y = 1.2 M F: y = 1.5 M G: y = 2 M

G F D A

E

-400

-600 1E-007

1E-006

1E-005

1E-004

1E-003

1E-002

Current density (A/cm2)

Fig. 8. Effect of H2SO4 concentration on the potentiodynamic polarization curves of 2205 DSS in 0.5 M HCl base solution.

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Fig. 9. SEM micrograph showing the surface morphology of 2205 DSS after potentiostatic etching at: (a) Ec max = 290 mV, and (b) Ea max = 355 mV for 3 h, in 0.25 M H2SO4 + 0.5 M HCl solution.

( 290 mV) and Ea max ( 355 mV) in 0.25 M H2SO4 + 0.5 M HCl solution are shown in Fig. 9. As revealed in Fig. 8, the anodic current density in the active-to-passive transition region was very low in the mixed solution of 0.25 M H2SO4 + 0.5 M HCl as compared with those containing high concentrations of H2SO4. The less

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Fig. 10. SEM micrograph showing the surface morphology of 2205 DSS after potentiostatic etching at the saddle point potential of 275 mV for 3 h, in 2 M H2SO4 + 0.5 M HCl solution.

extent of selective dissolution revealed in Fig. 9, comparing with those shown in Fig. 3, was due to the low concentration of H2SO4 in the mixed acid. At a potential inbetween the two peak potentials, say 275 mV in 2 M H2SO4 + 0.5 M HCl solution, potentiostatic etching for 3 h gave a feature showing simultaneous dissolution of a and c phases as depicted in Fig. 10. At such saddle point potential, a phase began to be passivated while c phase was in the active state. Close examination of Fig. 10 indicated that c phase corroded slightly faster than a phase, but the difference in etching step was less than those shown in Fig. 3. The effect of electrolyte composition on the respective anodic peak densities for ferrite and austenite dissolution of 2205 DSS is demonstrated in Fig. 11. In 2 M H2SO4 base solution, the addition of HCl caused an increase in the Ic max, which became more pronounced as the concentration of HCl was increased. Though the addition of HCl also caused an increase in the peak anodic current density for ferrite phase, the concentration dependence was less significant. As shown in Fig. 11(a), however, the difference between Ic max and Ia max was magnified as the concentration of HCl was increased up to 2 M. In contrast, the addition of H2SO4 into 0.5 M HCl base solution resulted in a steeper increase of Ia max than that of Ic max with increasing concentration of H2SO4 (Fig. 11(b)). These results indicated that selective dissolution of either ferrite or austenite in 2205 DSS at potentials in the active-to-passive region strongly depended on the composition of the mixed acid encountered. Clearly, preferential dissolution of austenite could be promoted by increasing HCl concentration in

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30 x M HCl + 2 M H2SO4

Peak Current density (mA/cm2)

Iα max Iγ max

20

10

0 0

0.4

0.8

1.2

1.6

2

Concentration of HCl (M)

(a) 1.2

0.5 M HCl + x M H2SO4

Peak Current density (mA/cm2)

Iαmax Iγmax

0.8

0.4

0 0

(b)

0.4

0.8

1.2

1.6

2

Concentration of H2 SO4 (M)

Fig. 11. Effects of (a) HCl concentration and (b) H2SO4 concentration on the peak current densities, in the active-to-passive transition region, of 2205 DSS in 2 M H2SO4 and 0.5 M HCl solutions, respectively.

H2SO4 solution, while ferrite dissolution could be enhanced by addition of H2SO4 in HCl base solution. The specific explanation to the above observation was not clear and remained to be explored. Perhaps, the competitive adsorption between chloride and hydroxyl ions, as postulated by Sato [15], on the respective a and c phases at the peak potentials might explain the unique roles of HCl and H2SO4 found in this investigation.

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4. Conclusions In the mixed H2SO4/HCl solutions, there existed two anodic peaks in the activeto-passive transition region of the potentiodynamic polarization curves of 2205 DSS. The higher anodic peak corresponded to the selective dissolution of austenite phase while the lower anodic peak to that of ferrite phase. The magnitudes of the anodic current densities and the peak potentials for the selective dissolution of these two phases varied with the concentration of H2SO4 and/or HCl. In 2 M H2SO4 base solution, the peak current density for austenite phase dissolution was enhanced with increasing concentration of HCl. In 0.5 M HCl base solution, on the other hand, the anodic current density for preferential dissolution of ferrite increased with increasing concentration of H2SO4. The distinguishable surface morphologies showing the characteristic feature of selective dissolution in the active-to-passive transition region were clearly demonstrated by SEM micrographs.

Acknowledgment The authors of I-Hsuang Lo and Wen-Ta Tsai would like to thank the National Science Council of the Republic of China for the partial support of this investigation through the contract of NSC89-2216-E-006-068.

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