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Electrochemically induced surface annealing without electrolyte immersion and its influence on pitting resistance
Surface & Coatings Technology 202 (2008) 4830–4833
Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Electrochemically induced surface annealing without electrolyte immersion and its influence on pitting resistance Zhilin Li ⁎, Jinghui Shang, Wei Liu, Bo Fei College of Materials Science and Engineering and College of Chemical Engineering, Beijing University of Chemical Technology, Beijing 100029, China
A R T I C L E
I N F O
Article history: Received 24 December 2007 Accepted in revised form 14 April 2008 Available online 22 April 2008 Keywords: Electrochemically induced surface annealing Stainless steel Electrolyte immersion Pitting resistance
A B S T R A C T The martensite in metastable-austenite-stainless-steels may decrease the corrosion resistance of the steels. Electrochemically induced surface annealing (EISA) can decrease the content of martensite in the steels, so it can promote the pitting resistance of the steels. But EISA treatment has to be operated on the condition of electrolyte immersion, which is an obstacle for its actual application. In this paper AISI304L stainless steel samples were treated with a no-immersion device. The martensite content of the steel decreased after the treatment, but the hardness of the steel almost had no change. The EISA effect was achieved with the noimmersion device. After the corrosion in a 3.5% NaCl aqueous solution, the EISA treated samples had less, smaller and shallower pits than the untreated samples had. The pitting potential of the EISA treated samples was almost equal to that of the untreated samples, and the open circuit potential of the EISA treated samples was higher than that of the untreated samples. The no-immersion EISA treatment promotes the pitting resistance of metastable-austenite-stainless-steels. © 2008 Published by Elsevier B.V.
1. Introduction Austenite stainless steels are widely used in petroleum, chemical, food and many other industries. The austenite structure of some austenite stainless steels, such as AISI304L and AISI321, are metastable at ambient temperature. Plastic deformation, cathodic hydrogen charging, and the cooling after welding can induce martensite transformation in these steels. So the steels with this character are called metastable-austenite-stainless-steels. Most stainless steel equipments in petroleum and chemical industries have to undergo plastic deformation or welding in their processes or installation and cathodic hydrogen charging during their services. So martensite often appears in the metastable-austenite-stainless-steels applied in those industries. It was noticed that the existence of martensite has much influence on the corrosion resistance of the steels [1–3]. In order to promote the corrosion resistance of the equipments, it is necessary to eliminate the martensite or decrease its content. The martensite will transform into austenite when the steel is heated at a relative high temperature [4,5]. However, it is difficult to heat the large installed equipments in petroleum and chemical industries to a high temperature. The purpose has to be achieved in other methods. In 2000, Burstein et al. reported their discovery of electrochemically induced surface annealing (EISA) [6]. A metastable-austenitestainless-steel containing martensite was treated with a series of anodic/cathodic electrochemical pulses in an 8 M aqueous solution of sodium nitrite at 80 °C. After the treatment, the martensite content of ⁎ Corresponding author. Tel.: +86 10 64421306; fax: +86 10 64437587. E-mail addresses: [email protected], [email protected] (Z. Li). 0257-8972/$ – see front matter © 2008 Published by Elsevier B.V. doi:10.1016/j.surfcoat.2008.04.063
the steel surface decreased, but the hardness of the steel surface still kept the original high value of untreated samples. They called the phenomenon electrochemically induced surface annealing [6]. EISA is of technological interest of modification of the surface properties of metals without affecting their bulk properties in demanding applications where surface stability and hardness are important [6]. It is proved experimentally that EISA can promote the pitting resistance of AISI 304 steel containing deformation induced martensite [7]. EISA is superior to heat treatment in its low temperature, so EISA is regarded as a potential method to promote the corrosion resistance of metastable-austenite-stainless-steel. However, the stainless steels need to be immersed in the electrolyte during the EISA treatment, which causes technique difficulties for the large installed equipments in petroleum and chemical industries. If the immersion can be eliminated, the actual application of EISA in steel protection will be promoted. Although the mechanism of EISA has already been noticed [6,8], it is not clear up to now. Contrary to the traditional concepts that cathodic hydrogen charging causes martensite formation, EISA makes martensite disappear. The reason was supposed as the follows. In the cathodic part of the pulse, water is reduced by electrolysis to hydrogen. Hydrogen atoms entering the metal produce considerable strain within the lattice, causing microstructural changes as well as phase transformations. In the anodic part of the potential pulse, the metal passivates by oxide film growth. Any reduced hydrogen and nitrogen generated in the previous cathodic pulse and dissolved into the metal matrix would also tend to be re-oxidized anodically and dissolve into the electrolyte. The defects generated by ingress of hydrogen (and perhaps nitrogen) would leave vacancies after re-oxidation and
Z. Li et al. / Surface & Coatings Technology 202 (2008) 4830–4833
4831
Fig. 2. XRD test result of the samples before and after the no-immersion EISA treatment.
Fig. 1. Schematic diagram of the no-immersion EISA treatment device.
provide for surface relaxation of the metal atoms over very short range, giving the observed annealing. The defects that remain would then be available for penetration of further reduced atoms in the subsequent cathodic pulse. In this way a deeply affected layer could be induced [6]. If the immersion is eliminated, both the cathodic and anodic processes of EISA may change. So the possibility of the elimination has to be proved first. Our experiment in this paper proved that the elimination can be achieved and the no-immersion EISA treatment can also promote the pitting resistance of metastableaustenite-stainless-steels. 2. Experimental The samples were AISI 304L type stainless steel with the size of 10 × 10 × 4 (mm). Sealed with epoxy resin, the samples had a working area of 10 × 10 (mm). The working areas of the sealed samples were ground with six successive grades of abrasive papers, in order to form martensite. The total thickness of the sample and the epoxy resin was about 10 mm after the grinding. The content of martensite was tested with a Rigaku D/max 2500 type X-ray diffractometer (XRD). The following conditions were adopted: Cu Kα radiation, X-ray generator power at 40 kV × 200 mA, a scanning speed of 5°/min, and a sampling width of 0.01°. The content of martensite was estimated through the relative diffraction strength of its {110} crystal plane. The working part of the samples was treated with a no-immersion EISA treatment device which is shown in Fig. 1. The working electrode was the sample. The auxiliary electrode was a 3 mm thick graphite electrode. A piece of polyurethane sponge was placed between the two electrodes to absorb the solution. To keep the device tight, the sponge in the device was compressed from its initial thickness of 10 mm to about 6 mm. The whole system was open to the atmosphere. The temperature of the sample can be controlled at 80 ± 3 °C by adjusting the interval of the heating cord. An 8 M aqueous solution of sodium nitrite was dripped into the sponge with the solution dripper. The dripping speed was controlled at about 120 drops/min by adjusting the valve and the height of the solution pot. When the solution was dripping, the two electrodes were in conducting state and the electrochemical processes for EISA might happen. A series of anodic/cathodic electrochemical pulses were carried out with the pulse generator, the output voltages of which were −4.1 V for 280 s and +1.2 V for 60 s, alternately. This was equivalent to applied electrode potential pulses between about −1.8 V (w.r.t. SCE) and about +0.28 V (w.r.t. SCE)—here w.r.t. SCE indicates with respect to the standard calomel electrode [6]. The pulse treatment was held for 8 h.
The Vickers hardness of the samples was tested with a digital microhardness tester of the HXZ-1000 type. The load was 100 g and the loading time was 12 s. The hardness of a sample was the average of three testing positions. A 1.5 V voltage was applied to epoxy resin sealed samples in a 3.5% NaCl aqueous solution at 50 °C for 25 min, to compare their pitting corrosion. The corrosion morphology was observed with a 4XC type metallurgical microscope at 400 times of magnification. Potentiodynamic scanning was carried out with a Model 283 Potentiostat/Galvanostat at ambient temperature in a 3.5% NaCl aqueous solution. The film on the samples was removed by means of cathode polarization at −0.2 V (based on open circuit potential with a saturated calomel electrode used as the reference electrode) for 3 min. When the potential reached stable, dynamic potential scanning was carried out at a speed of 0.4 mV/s, from −200 mV based on the open circuit potential. When the anodic polarization current density reached 1 mA cm− 2, or the potential reached 200 mV based on the open circuit potential, the potential was decreased until it reached the start value, or until the potential–current curve was finally closed. 3. Results and discussion 3.1. No-immersion EISA treatment result Fig. 2 shows the XRD test results of the samples before and after the no-immersion EISA treatment. It can be seen the relative diffraction strength of martensite {110} crystal plane decreased after the treatment, which indicates that the martensite content decreased after the treatment. Table 1 shows the hardness of the samples before and after the no-immersion EISA treatment. It shows that the hardness after the treatment almost had no change. It is the character of EISA phase transformation that the hardness keeps the initial high value after the decrease of martensite content. In a bath the aqueous solution of sodium nitrite is stable, so the processes of anode oxidating, polarization and the cathode hydrogen reducing, charging may occur on the basis of ion diffusion. During the no-immersion treatment, the anode processes and cathode processes may also occur on the basis of ion diffusion if the sponge was simply wetted with the solution. But the sponge and the sample had to be
Table 1 Hardness test results before and after the no-immersion EISA treatment Hardness (Hv)
Before treatment After treatment
Sample no. 1
2
3
226.7 227
225.1 218.5
204.3 211.5
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Z. Li et al. / Surface & Coatings Technology 202 (2008) 4830–4833
Fig. 3. Micrograph of the corroded samples: (a) no-immersion EISA treated; (b) untreated.
heated in order to keep the temperature of 80 ± 3 °C. The heating would dry the sponge. In order to keep the sponge wet and keep the solution concentration in the sponge stable, the solution was designed dripping continuously into the sponge, as shown in Fig. 1. Because the solution in the sponge keeps flow during the dripping, the anode processes and cathode processes may include ion flow, which may cause the anode processes and cathode processes to be different from those based on ion diffusion. Although the detailed mechanism need further research, our results indicate that the no-immersion EISA phase transformation occurred with our device. Based on this device, large no-immersion EISA treatment device can be designed with water absorbing materials and dripping solution. Such devices can provide much convenience to the actual application of EISA to the corrosion prevention. 3.2. Pitting resistance after the no-immersion EISA treatment Before and after the no-immersion EISA treatment, a 1.5 V voltage was applied to epoxy resin sealed samples in a 3.5% NaCl aqueous solution at 50 °C for 25 min. Then the corrosion morphology was observed using a metallurgical microscope set at 400 times magnification, which are shown in Fig. 3. It indicates that the largest pit of the EISA treated sample was much smaller than that of the untreated one. The numbers of the pit larger than 1.2 μm were counted. The observed pit number of the 3 untreated samples was 256–340 cm− 2, but that of the 3 EISA treated samples was only 166–216 cm− 2. The corroded samples were polished, and the pit numbers were counted at various sample thicknesses, until no pits could be observed in the samples. The various pit depths can be deduced from the pits observed at corresponding sample thicknesses. Fig. 4 shows the pit
Fig. 4. Pit numbers at various distances to the surface of the samples.
numbers at various pit depths, in EISA treated samples and untreated samples. It can be seen that the deepest pit in the 3 EISA treated samples was only 30–54 μm, whereas that in the 3 untreated sample was about 158–187 μm. Although most pits in the untreated samples were shallow, there were at least 50 pits deeper than the largest pit depth of 54 μm in the EISA treated samples. Therefore, the pitting resistance of the steel within the environment of the Cl− ion increases after the no-immersion EISA treatment. Fig. 5 shows an example of the dynamic potential scanning results of the EISA treated and untreated samples. The pitting potential (the potential of the point at which the scan current starts to increase sharply) of the EISA treated sample was almost equal to that of the untreated sample. Therefore, the EISA treatment would not increase the pitting tendency. The open circuit potentials of the EISA treated sample and the untreated one were −0.10 V and −0.22 V, respectively. The higher open circuit potential of the EISA treated sample can also prove that the no-immersion EISA treatment has an anti-corrosion effect. Similar phenomenon was observed in the repeated test on three pairs of samples. Take all pitting number, pitting depth, pitting size, pitting potential and open circuit potential into account, it can be deduced that the noimmersion EISA treatment promotes the pitting resistance of metastable-austenite-stainless-steels. 4. Conclusions 1. The martensite content of the samples treated with the noimmersion device decreased, but the hardness of the samples almost had no change. Therefore, the EISA effect was achieved with the no-immersion device.
Fig. 5. Dynamic potential scanning results of the no-immersion EISA treated and untreated samples.
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Z. Li et al. / Surface & Coatings Technology 202 (2008) 4830–4833
2. After the corrosion in a 3.5% NaCl aqueous solution, the EISA treated samples had less, smaller and shallower pits than the untreated samples had. The pitting potential of the EISA treated samples was almost equal to that of the untreated samples, and the open circuit potential of the EISA treated samples was higher than that of the untreated samples. The no-immersion EISA treatment promotes the pitting resistance of metastable-austenite-stainless-steels. Acknowledgement The authors thank Dr. Jinping Xiong and Dr. Xuhui Zhao for their helpful discussion. This work was supported by the Beijing Natural Science Foundation (Grant No. 2072014).
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References [1] A. Cigada, B. Mazza, P. Pedeferri, G. Salvago, D. Sinigaglia, G. Zanini, Corros. Sci. 22 (1982) 558. [2] K. Hidehiko, J. Jpn. Inst. Met. 58 (1994) 414. [3] C.T. Kwok, H.C. Man, F.T. Cheng, Surf. Coat. Technol. 126 (2000) 238. [4] A.D. Schino, I. Salvatori, J.M. Kenny, J. Mater. Sci. 37 (2002) 4561. [5] Y. Zhang, X.T. Jing, B.Z. Lou, F.S. Shen, F.Z. Cui, J. Mater. Sci. 34 (1999) 3291. [6] G.T. Burstein, I.M. Hutchings, K. Sasaki, Nature 407 (2000) 885. [7] Z. Li, W. Liu, J. Qi, Metall. Mater. Trans. 37A (2006) 435. [8] G.T. Burstein, K. Sasaki, I.M. Hutchings, Electrochem. Solid-State Lett. 6 (2003) 13.
Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Electrochemically induced surface annealing without electrolyte immersion and its influence on pitting resistance Zhilin Li ⁎, Jinghui Shang, Wei Liu, Bo Fei College of Materials Science and Engineering and College of Chemical Engineering, Beijing University of Chemical Technology, Beijing 100029, China
A R T I C L E
I N F O
Article history: Received 24 December 2007 Accepted in revised form 14 April 2008 Available online 22 April 2008 Keywords: Electrochemically induced surface annealing Stainless steel Electrolyte immersion Pitting resistance
A B S T R A C T The martensite in metastable-austenite-stainless-steels may decrease the corrosion resistance of the steels. Electrochemically induced surface annealing (EISA) can decrease the content of martensite in the steels, so it can promote the pitting resistance of the steels. But EISA treatment has to be operated on the condition of electrolyte immersion, which is an obstacle for its actual application. In this paper AISI304L stainless steel samples were treated with a no-immersion device. The martensite content of the steel decreased after the treatment, but the hardness of the steel almost had no change. The EISA effect was achieved with the noimmersion device. After the corrosion in a 3.5% NaCl aqueous solution, the EISA treated samples had less, smaller and shallower pits than the untreated samples had. The pitting potential of the EISA treated samples was almost equal to that of the untreated samples, and the open circuit potential of the EISA treated samples was higher than that of the untreated samples. The no-immersion EISA treatment promotes the pitting resistance of metastable-austenite-stainless-steels. © 2008 Published by Elsevier B.V.
1. Introduction Austenite stainless steels are widely used in petroleum, chemical, food and many other industries. The austenite structure of some austenite stainless steels, such as AISI304L and AISI321, are metastable at ambient temperature. Plastic deformation, cathodic hydrogen charging, and the cooling after welding can induce martensite transformation in these steels. So the steels with this character are called metastable-austenite-stainless-steels. Most stainless steel equipments in petroleum and chemical industries have to undergo plastic deformation or welding in their processes or installation and cathodic hydrogen charging during their services. So martensite often appears in the metastable-austenite-stainless-steels applied in those industries. It was noticed that the existence of martensite has much influence on the corrosion resistance of the steels [1–3]. In order to promote the corrosion resistance of the equipments, it is necessary to eliminate the martensite or decrease its content. The martensite will transform into austenite when the steel is heated at a relative high temperature [4,5]. However, it is difficult to heat the large installed equipments in petroleum and chemical industries to a high temperature. The purpose has to be achieved in other methods. In 2000, Burstein et al. reported their discovery of electrochemically induced surface annealing (EISA) [6]. A metastable-austenitestainless-steel containing martensite was treated with a series of anodic/cathodic electrochemical pulses in an 8 M aqueous solution of sodium nitrite at 80 °C. After the treatment, the martensite content of ⁎ Corresponding author. Tel.: +86 10 64421306; fax: +86 10 64437587. E-mail addresses: [email protected], [email protected] (Z. Li). 0257-8972/$ – see front matter © 2008 Published by Elsevier B.V. doi:10.1016/j.surfcoat.2008.04.063
the steel surface decreased, but the hardness of the steel surface still kept the original high value of untreated samples. They called the phenomenon electrochemically induced surface annealing [6]. EISA is of technological interest of modification of the surface properties of metals without affecting their bulk properties in demanding applications where surface stability and hardness are important [6]. It is proved experimentally that EISA can promote the pitting resistance of AISI 304 steel containing deformation induced martensite [7]. EISA is superior to heat treatment in its low temperature, so EISA is regarded as a potential method to promote the corrosion resistance of metastable-austenite-stainless-steel. However, the stainless steels need to be immersed in the electrolyte during the EISA treatment, which causes technique difficulties for the large installed equipments in petroleum and chemical industries. If the immersion can be eliminated, the actual application of EISA in steel protection will be promoted. Although the mechanism of EISA has already been noticed [6,8], it is not clear up to now. Contrary to the traditional concepts that cathodic hydrogen charging causes martensite formation, EISA makes martensite disappear. The reason was supposed as the follows. In the cathodic part of the pulse, water is reduced by electrolysis to hydrogen. Hydrogen atoms entering the metal produce considerable strain within the lattice, causing microstructural changes as well as phase transformations. In the anodic part of the potential pulse, the metal passivates by oxide film growth. Any reduced hydrogen and nitrogen generated in the previous cathodic pulse and dissolved into the metal matrix would also tend to be re-oxidized anodically and dissolve into the electrolyte. The defects generated by ingress of hydrogen (and perhaps nitrogen) would leave vacancies after re-oxidation and
Z. Li et al. / Surface & Coatings Technology 202 (2008) 4830–4833
4831
Fig. 2. XRD test result of the samples before and after the no-immersion EISA treatment.
Fig. 1. Schematic diagram of the no-immersion EISA treatment device.
provide for surface relaxation of the metal atoms over very short range, giving the observed annealing. The defects that remain would then be available for penetration of further reduced atoms in the subsequent cathodic pulse. In this way a deeply affected layer could be induced [6]. If the immersion is eliminated, both the cathodic and anodic processes of EISA may change. So the possibility of the elimination has to be proved first. Our experiment in this paper proved that the elimination can be achieved and the no-immersion EISA treatment can also promote the pitting resistance of metastableaustenite-stainless-steels. 2. Experimental The samples were AISI 304L type stainless steel with the size of 10 × 10 × 4 (mm). Sealed with epoxy resin, the samples had a working area of 10 × 10 (mm). The working areas of the sealed samples were ground with six successive grades of abrasive papers, in order to form martensite. The total thickness of the sample and the epoxy resin was about 10 mm after the grinding. The content of martensite was tested with a Rigaku D/max 2500 type X-ray diffractometer (XRD). The following conditions were adopted: Cu Kα radiation, X-ray generator power at 40 kV × 200 mA, a scanning speed of 5°/min, and a sampling width of 0.01°. The content of martensite was estimated through the relative diffraction strength of its {110} crystal plane. The working part of the samples was treated with a no-immersion EISA treatment device which is shown in Fig. 1. The working electrode was the sample. The auxiliary electrode was a 3 mm thick graphite electrode. A piece of polyurethane sponge was placed between the two electrodes to absorb the solution. To keep the device tight, the sponge in the device was compressed from its initial thickness of 10 mm to about 6 mm. The whole system was open to the atmosphere. The temperature of the sample can be controlled at 80 ± 3 °C by adjusting the interval of the heating cord. An 8 M aqueous solution of sodium nitrite was dripped into the sponge with the solution dripper. The dripping speed was controlled at about 120 drops/min by adjusting the valve and the height of the solution pot. When the solution was dripping, the two electrodes were in conducting state and the electrochemical processes for EISA might happen. A series of anodic/cathodic electrochemical pulses were carried out with the pulse generator, the output voltages of which were −4.1 V for 280 s and +1.2 V for 60 s, alternately. This was equivalent to applied electrode potential pulses between about −1.8 V (w.r.t. SCE) and about +0.28 V (w.r.t. SCE)—here w.r.t. SCE indicates with respect to the standard calomel electrode [6]. The pulse treatment was held for 8 h.
The Vickers hardness of the samples was tested with a digital microhardness tester of the HXZ-1000 type. The load was 100 g and the loading time was 12 s. The hardness of a sample was the average of three testing positions. A 1.5 V voltage was applied to epoxy resin sealed samples in a 3.5% NaCl aqueous solution at 50 °C for 25 min, to compare their pitting corrosion. The corrosion morphology was observed with a 4XC type metallurgical microscope at 400 times of magnification. Potentiodynamic scanning was carried out with a Model 283 Potentiostat/Galvanostat at ambient temperature in a 3.5% NaCl aqueous solution. The film on the samples was removed by means of cathode polarization at −0.2 V (based on open circuit potential with a saturated calomel electrode used as the reference electrode) for 3 min. When the potential reached stable, dynamic potential scanning was carried out at a speed of 0.4 mV/s, from −200 mV based on the open circuit potential. When the anodic polarization current density reached 1 mA cm− 2, or the potential reached 200 mV based on the open circuit potential, the potential was decreased until it reached the start value, or until the potential–current curve was finally closed. 3. Results and discussion 3.1. No-immersion EISA treatment result Fig. 2 shows the XRD test results of the samples before and after the no-immersion EISA treatment. It can be seen the relative diffraction strength of martensite {110} crystal plane decreased after the treatment, which indicates that the martensite content decreased after the treatment. Table 1 shows the hardness of the samples before and after the no-immersion EISA treatment. It shows that the hardness after the treatment almost had no change. It is the character of EISA phase transformation that the hardness keeps the initial high value after the decrease of martensite content. In a bath the aqueous solution of sodium nitrite is stable, so the processes of anode oxidating, polarization and the cathode hydrogen reducing, charging may occur on the basis of ion diffusion. During the no-immersion treatment, the anode processes and cathode processes may also occur on the basis of ion diffusion if the sponge was simply wetted with the solution. But the sponge and the sample had to be
Table 1 Hardness test results before and after the no-immersion EISA treatment Hardness (Hv)
Before treatment After treatment
Sample no. 1
2
3
226.7 227
225.1 218.5
204.3 211.5
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Z. Li et al. / Surface & Coatings Technology 202 (2008) 4830–4833
Fig. 3. Micrograph of the corroded samples: (a) no-immersion EISA treated; (b) untreated.
heated in order to keep the temperature of 80 ± 3 °C. The heating would dry the sponge. In order to keep the sponge wet and keep the solution concentration in the sponge stable, the solution was designed dripping continuously into the sponge, as shown in Fig. 1. Because the solution in the sponge keeps flow during the dripping, the anode processes and cathode processes may include ion flow, which may cause the anode processes and cathode processes to be different from those based on ion diffusion. Although the detailed mechanism need further research, our results indicate that the no-immersion EISA phase transformation occurred with our device. Based on this device, large no-immersion EISA treatment device can be designed with water absorbing materials and dripping solution. Such devices can provide much convenience to the actual application of EISA to the corrosion prevention. 3.2. Pitting resistance after the no-immersion EISA treatment Before and after the no-immersion EISA treatment, a 1.5 V voltage was applied to epoxy resin sealed samples in a 3.5% NaCl aqueous solution at 50 °C for 25 min. Then the corrosion morphology was observed using a metallurgical microscope set at 400 times magnification, which are shown in Fig. 3. It indicates that the largest pit of the EISA treated sample was much smaller than that of the untreated one. The numbers of the pit larger than 1.2 μm were counted. The observed pit number of the 3 untreated samples was 256–340 cm− 2, but that of the 3 EISA treated samples was only 166–216 cm− 2. The corroded samples were polished, and the pit numbers were counted at various sample thicknesses, until no pits could be observed in the samples. The various pit depths can be deduced from the pits observed at corresponding sample thicknesses. Fig. 4 shows the pit
Fig. 4. Pit numbers at various distances to the surface of the samples.
numbers at various pit depths, in EISA treated samples and untreated samples. It can be seen that the deepest pit in the 3 EISA treated samples was only 30–54 μm, whereas that in the 3 untreated sample was about 158–187 μm. Although most pits in the untreated samples were shallow, there were at least 50 pits deeper than the largest pit depth of 54 μm in the EISA treated samples. Therefore, the pitting resistance of the steel within the environment of the Cl− ion increases after the no-immersion EISA treatment. Fig. 5 shows an example of the dynamic potential scanning results of the EISA treated and untreated samples. The pitting potential (the potential of the point at which the scan current starts to increase sharply) of the EISA treated sample was almost equal to that of the untreated sample. Therefore, the EISA treatment would not increase the pitting tendency. The open circuit potentials of the EISA treated sample and the untreated one were −0.10 V and −0.22 V, respectively. The higher open circuit potential of the EISA treated sample can also prove that the no-immersion EISA treatment has an anti-corrosion effect. Similar phenomenon was observed in the repeated test on three pairs of samples. Take all pitting number, pitting depth, pitting size, pitting potential and open circuit potential into account, it can be deduced that the noimmersion EISA treatment promotes the pitting resistance of metastable-austenite-stainless-steels. 4. Conclusions 1. The martensite content of the samples treated with the noimmersion device decreased, but the hardness of the samples almost had no change. Therefore, the EISA effect was achieved with the no-immersion device.
Fig. 5. Dynamic potential scanning results of the no-immersion EISA treated and untreated samples.
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Z. Li et al. / Surface & Coatings Technology 202 (2008) 4830–4833
2. After the corrosion in a 3.5% NaCl aqueous solution, the EISA treated samples had less, smaller and shallower pits than the untreated samples had. The pitting potential of the EISA treated samples was almost equal to that of the untreated samples, and the open circuit potential of the EISA treated samples was higher than that of the untreated samples. The no-immersion EISA treatment promotes the pitting resistance of metastable-austenite-stainless-steels. Acknowledgement The authors thank Dr. Jinping Xiong and Dr. Xuhui Zhao for their helpful discussion. This work was supported by the Beijing Natural Science Foundation (Grant No. 2072014).
4833
References [1] A. Cigada, B. Mazza, P. Pedeferri, G. Salvago, D. Sinigaglia, G. Zanini, Corros. Sci. 22 (1982) 558. [2] K. Hidehiko, J. Jpn. Inst. Met. 58 (1994) 414. [3] C.T. Kwok, H.C. Man, F.T. Cheng, Surf. Coat. Technol. 126 (2000) 238. [4] A.D. Schino, I. Salvatori, J.M. Kenny, J. Mater. Sci. 37 (2002) 4561. [5] Y. Zhang, X.T. Jing, B.Z. Lou, F.S. Shen, F.Z. Cui, J. Mater. Sci. 34 (1999) 3291. [6] G.T. Burstein, I.M. Hutchings, K. Sasaki, Nature 407 (2000) 885. [7] Z. Li, W. Liu, J. Qi, Metall. Mater. Trans. 37A (2006) 435. [8] G.T. Burstein, K. Sasaki, I.M. Hutchings, Electrochem. Solid-State Lett. 6 (2003) 13.