The electrochemical polishing behavior of duplex

Int J Adv Manuf Technol (2007) 34:904–910 DOI 10.1007/s00170-006-0654-8

ORIGINAL ARTICLE

The electrochemical polishing behavior of duplex stainless steel(SAF 2205) in phosphoric-sulfuric mixed acids C. A. Huang & C. C. Hsu

Received: 6 February 2006 / Accepted: 5 May 2006 / Published online: 24 October 2006 # Springer-Verlag London Limited 2006

Abstract In this paper the electrochemical polishing behavior of duplex stainless steel (DSS) in phosphoricsulfuric mixed acids with volume ratios of 1:1, 2:1, and 3:1 was studied. The electrochemical polishing was conducted at 70°C by using arotating disc electrode. Before the polishing, the steel specimens were heated at 1080°C for 10 min and cooled with different rates to obtain dissimilar microstructures. Experimental results show that a brightening surface of each DSS specimen can be obtained by polishing in the mixed acids at 70°C. However, the dissolution rate between α and γ phases in a DSS specimen is different during potentiostatic polishing in the mixed acids and the rate of α phase is obviously higher than that of γ phase. However, the difference in the dissolution rate can be reduced as the DSS specimen was polished in a high H3PO4-content mixed acid. Some small round σ-phases were found to precipitate along the α/γ interface in a DSS specimen, which can be obtained by heating at 1080°C and then cooling in the furnace. The presentation of σ phase increased the hardness and microstructure fraction of the γ phase in the DSS specimen. Moreover, the σ phase can be leveled together with α and γ phases as polishing in the 3:1 v/v mixed acid. Keywords Duplex stainless steel . Electrochemical polishing

C. A. Huang (*) : C. C. Hsu Department Mechanical Engineering, Chang Gung University, Taoyuan, Gweishan, Taiwan, Republic of China e-mail: [email protected]

1 Introduction The duplex stainless steel (DSS) has become more important in recent years owing to its excellent mechanical properties and pitting resistance in chloride-contained environment [1–3]. It has been reported that the DSS, unlike the austenitic stainless steel, is suitable for welding without losing itscorrosion resistance [4, 5]. Therefore, the DSS is preferably used in some applications, such as transport pipes for chemical reactive species. The microstructures of DSS comprise mainly two phases, the ferric alpha (α) and the austenitic gamma (γ), in general applications. Normally the volume fraction of both phases in a DSS is roughly equal. However, the amount of the phases or even presentation of other phases is strongly dependent on chemical composition and heat treating of the DSS [6–8]. The electropolishing was performed in the limitingcurrent plateau of the anodic polarization curve, in which anodic dissolution was under mass-transfer controlled limitation. However, the chemical species governing the mass-transfer controlled mechanism is significantly dependent on both the metallic electrode and electrolyte [9, 10]. It can be therefore expected that the electrochemical behavior is affected by the galvanic effect due to different phases in the DSS. For the different orientations of the grains, dissimilar phases and crystalline imperfections (grain boundary, twin boundary, dislocation and inclusions) of the metallic workpiece, a leveling and brightening surface can be achieved through anodic dissolution in a suitable electrolyte by electrochemical polishing [11–13]. Several researchers have pointed out that many types of Cr, and Cr-Ni stainless steels can be electrochemically polished in phosphor-sulfuric mixed acids at a temperature higher than 65°C [9]. The mixed acids with different

Int J Adv Manuf Technol (2007) 34:904–910

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Table 1 Chemical composition of the duplex stainless steel (SAF2205) Element

Fe

C

Si

Mn

Cr

Ni

Mo

N

Cu

Mass %

64.5

0.04

0.5

1.65

22.7

5.4

3.45

0.18

0.06

volume ratios have been proposed for polishing dissimilar types of stainless steels [14, 15]. The volume-ratio of H3PO4-H2SO4 mixed acid commonly lies between 3:1 and 1:1 for electrochemical polishing [16, 17]. It is reported that depending on the types of stainless steel the electrochemical polishing processes, leveling and brightening, could be some or less affected by the volume ratio ofthe mixed acid [16–18]. So far as we know that scare reports have mentioned the electrochemical polishing behavior of DSS in the mixed acids with different volume ratios. In this study we polished four different pretreated conditions of DSS in the mixed acids with volume ratios of 1:1, 2:1, and 3:1. The effect of the volume ratio on the polishing behavior of four pretreated DSS specimens was discussed in detail.

2 Experimental procedures The commercial duplex stainless steel, SAF 2205, in bar type with a diameter of 27 mm was used in this study and its chemical composition is given in Table 1. To obtain different microstructures of SAF 2205, as-received steel bars were heated at 1080°C for 10 min then cooled in water, air and furnace respectively. Four different pretreated steel specimens, namely as-received, water-, air- and furnace-cooled DSS specimens, were then prepared for electrochemical testing and polishing. After the heat treatment, the specimen was cut in a cylindrical form, 8 mm in diameter and 6 mm in length, with a turning machine for preparing the rotating disc electrode (RDE). The schematic construction of RDE is shown in Fig. 1, in which the cylindrical specimen was bound tightly with G 1 glue (Gatan Company; USA) in a Teflon-sleeve insulator with an exposed area of 1.0 cm2, and inside the electrode a flexible spring was used to connect the spindle of the RDEcell kid (EG&GModel 616). The schematic construction of three-electrode cell for electrochemical test and polishing is presented in Fig. 2. A platinized Ti-mesh and the Ag/AgClsat. were used as counter and reference electrodes,

Fig. 1 Schematic presentation of the RDE used in this study

respectively. Before electrochemical testing and polishing, the exposed surface of the RDE was mechanically ground with 600 grit emery paper, cleaned with distilled water, dried with cool air blaster. The electrochemical test and polishing were carried out at 70°C in the mixed acids, which were composed of analytical grade of concentrated phosphoric- and sulfuricmixed acids with volume-ratios of 3:1, 2:1, and 1:1, respectively. The anodic polarization behavior of the RDE was detected with a scan rate of 3 mV/s starting from -250 mV (vs. Ecorr) to the limiting-current potential by using potentiostat/galvanostat (EG&G Model273A). According to the anodic polarization curve of the RDE, the potential range corresponding to limiting-current plateau can be determined, and then potentiostatic polishing was conducted in the middle potential of the plateau. After the electrochemical polishing, the RDE was cleaned with distilled water, acoustically cleaned in an acetone bath for 10 min, dried with cold air blaster, and then prepared for surface profiler (Hommel T 4000) measurement, Rockwell hardness test, and scanning electron microscope (SEM, Hitachi Model S-4700) examination.

3 Results and discussions 3.1 Metallographic examination The specimens were mechanically polished with 0.3 μm diamond paste and then immersed in an etchant composed of 100 ml HCl+20 g picric acid [19] for a few seconds to reveal their microstructures. Metallographic micrographs of the as-received, water-, air-and furnace-cooled specimens were shown in Fig. 3. As indicated in Fig. 3, an obvious grain growth can be found in water-,air-,and furnace-cooled specimens. That is, grain growth is obvious as the asreceived specimen was heated at 1080°C for 10 min. As shown in Fig. 3, the metallographic micrographsof waterand air-cooled specimens are almost the same; on the other

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Fig. 2 Schematic presentation of the construction of the electrochemical polishing cell

hand, a dissimilar metallographic micrograph of the furnace-cooled specimen can be observed. There are much more γ-grains, which demonstrate bright image in the micrograph, in the furnace-cooled specimen than those of the other specimens. Furthermore, in the furnace-cooled specimen some small round σ-phases can be found along the γ/α interface. Lai et al. [8] have reported that σ- and γphases were developed in the DSS through eutectoidic decomposition of α-phase as DSS was slowly cooled from high temperatures. Thus, the more σ-phase precipitates, the more γ-phase presents and the lesser α-phase exists in the DSS. From the results of the hardness measurements shown in the right bottom of each micrograph, the highest hardness of furnace-cooled specimen and almost the same hardness of the other specimens were detected. From the

results of metallographic examination, it can be realized that the relatively high hardness of the furnace-cooled specimen was attributed to the presentation of some hard and brittle σ-particles in the substrate [4–8]. In this paper we focus our study on the electrochemical polishing behavior of the DSS treated with the afore-mentioned four different pretreated RCE specimens in sulfuric-phosphoric mixed acids. 3.2 Electrochemical behavior Figure 4 presents the anodicpolarization curves of the asreceived specimen in the 1:1 v/v mixed acid at the temperatures of 30,50, and 70°C. Obvious limiting current plateaus can be found in these polarizationcurves as the

Fig. 3 The metallographic microstructures of (a) as-received, (b) water-cooled, (c) air-cooled and (d) furnace-cooled DSS (SAF2205) specimens


500 2

potential is higher than 2.2 V. Although electrochemicalpolishing of a DSS specimen would be performed in the potential corresponding to the limiting current plateau in its anodic polarization curve, a bright surface of the DSS specimen can be visually observed only at 70°C, but a matt and dull surface at 30 and 50°C after anodic polarization measurement. It implies that the electrochemical polishing for DSS can not be achieved in the 1:1 v/v mixed acid at temperatures of 30°C and 50°C. This is the same results of electrochemical polishing for Cr- and Cr-Ni stainless steels reported by Matlosz [9], who confirmed that aleveling and brightening surface of stainless steel was achieved when polishedin the mixed acid at a temperature above 65°C. The anodic polarization curves of as-received, water-, air-, and furnace-cooled specimens at 70°C in the mixed acids with volume-ratios of 1:1, 2:1 and 3:1 are presented in Fig. 5. Their anodic polarization behaviors demonstrate active dissolution in lower overpotential region, then limiting current plateau in the potential region between

Fig. 6 The relation between current density and polishing time of four pretreated DSS specimens. (Potentiostatic polishing in 2.5 V in the 3:1 v/v mixed acid at 70°C)

I/area ( mA/cm )

Fig. 4 The anodicpolarization curves of as-received DSS specimen in 1:1 v/vH3PO4-H2SO4 mixed acid at 30, 50, and 70°C. (RDE with 1000 rpm)

907

400

as-received

300

water-cooled

200

air-cooled furnace-cooled

100 0 1:1

2:1

3:1

H3PO4(vol%) : H2SO4(vol%)

Fig. 7 The current densities of four-pretreated DSS specimens potentiostatically polished in 2.5 Vat 70°C in the 1:1, 2:1 and 3:1 v/v mixed acids

2.2 V and 2.7 V, and oxygen evolution together with anodic dissolution of specimen for further increasing the overpotential. Besides, apparent different limiting current densities for specimens can be observed in the mixed acids with different volume ratios. However, a shinning surface of DSS can bevisually seen after each anodic polarization experiment. It implies that the DSS would be possibly polished in the mixed acids at 70°C. To evaluate the polishing behavior of DSS, potentiostatic polishing was performed in the middle potential of limiting current

Table 2 The Ra-values in micrometer of polished surface after potentionstatic polishing invarious concentration of H3PO4-H2SO4 Mixed acids in 2.5 V at 70°C. (RED with a rotating speed of 1000 rpm)

Fig. 5 The anodicpolarization curves of as-received DSS specimen in the H2SO4-H3PO4 mixed acids with volume ratios of 1:1, 2:1 and 3:1 at 70°C. (RDE with arotating speed of 1000 rpm)

H3PO4:H2SO4 Heat treatment

1:1

2:1

3:1

as-received water-cooled air-cooled furnace-cooled

0.162 0.058 0.096 0.058

0.072 0.058 0.070 0.064

0.074 0.064 0.066 0.066

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Fig. 8 The micrographs of polished surfaces of as-received specimens in (a) 1:1, (b) 2:1 and (c) 3:1 v/v mixed acids after potentiostatic polishing in 2.5 V at 70°C. (FESEM)

Fig. 9 The micrographs of polished surfaces of water-cooled specimens in (a) 1:1, (b) 2:1 and (c) 3:1 v/v mixed acids after potentiostatic polishing in 2.5 V at 70°C. (FESEM)

plateau in the mixed acids at 70°C. After polishing, the roughness values of polished surfaces were measured with surface profiler and surface morphologies were examined with SEM. 3.3 Potentiostatic polishing As already mentioned, the potentiostatic polishing of asreceived, water-, air-, and furnace- cooled specimens in the middle potential of limiting current plateau was performed in the mixed acids at 70°C. Figure 6 shows the relation between current density and polishing time of four pretreated DSS specimens in the 2:1 v/v mixed acid. As clearly seen in Fig. 6, the current densities could reach constant values within 200 s; that is, anodic dissolution of four pretreated DSS specimens in 2:1 v/v mixed acid is stable. The same trend of stable dissolution can also be found in 1:1 and 3:1 v/v mixed acids.The dissolution current densities of four pretreated DSS specimens in the mixed acids are presented in Fig. 7, in which the current density lies between 320 and 420 mA/cm2and tends to increase with an increasing H3PO4-content in the mixed acids.The relation between dissolution current density and four pretreated conditions of DSS is not clear. However the lowest current densities of the as-received specimen compared with those of the other pretreated specimens polished in the same mixed acid can be found.

Fig. 10 The micrographs of polished surfaces of furnacecooled specimens in (a) 1:1, (b) 2:1 and (c) 3:1 v/v mixed acids after potentiostatic polishing in 2.5 V at 70°C. (FESEM)

Although a relatively high volume fraction of γ phase and presentation of σ phase is found in the furnace-cooled specimen, the dissolution current density of furnace-cooled specimen in limiting current plateau shows no obvious difference from those of the other DSS specimens in the mixed acids. 3.4 Surface morphologies of DSS after potentiostatic polishing The values of the center-line average surface roughness, Ra, of the polished surfaces are measured with surface profiler after the potentiostatic polishing, and the results were summarized in Table 2, in which all the Ra values are lower than 0.2 μm. The highest value was recorded when the asreceived specimen was polished in the 1:1 v/v mixed acid. Owing tothe wave length of visual light and light refraction, it is well known, ashinning surface can be observed by the naked eye as the Ra value of the surface is lower than 0.2 μm. That is, four pretreated DSS specimens have shinning surfaces after potentiostatic polishing in the mixed acids with volume ratios of 1:1, 2:1, and 3:1. From the results of Ra values presented in Table 2, it can be seen that a relatively lower surface roughness could be obtained when the specimens were polished in the 3:1 v/v mixed acid. The surface morphologies of all DSS specimens after potentiostatic polishing in three selected mixed acids were observed with SEM and shown in Figs. 8, 9, 10, 11. From


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Fig. 11 The micrographs of polished surfaces of furnacecooled specimens in (a) 1:1, (b) 2:1 and (c) 3:1 v/v mixed acids after potentiostatic polishing in 2.5 V at 70°C. (FESEM)

these surface morphologies, it can be seen that different dissolution rates between α and γ phases can befound as the DSS specimen polished in the mixed acid. Moreover, the dissolutionrate of α phase is higher than that of γ phase. This different dissolution rate becomes relatively small as DSS specimens were polished in the 3:1 mixed acid.That is, the difference in dissolution rate between α- and γ phases in a DSSspecimen can be reduced by increasing the H3PO4 content in the mixed acid. The largest different dissolution rate between α and γ phasescan be observed in as-received specimen polished in the 1:1 v/v mixed acid [see Fig. 8a]. Meanwhile, some small round pits (