- Email: [email protected]
The initial stage of atmospheric corrosion on interstitial free steel investigated by in situ SPM
Corrosion Science 70 (2013) 188–193
Contents lists available at SciVerse ScienceDirect
Corrosion Science journal homepage: www.elsevier.com/locate/corsci
The initial stage of atmospheric corrosion on interstitial free steel investigated by in situ SPM L.Q. Guo a, X.M. Zhao a, B.C. Wang a, Y. Bai a, B.Z. Xu b, L.J. Qiao a,⇑ a b
Corrosion and Protection Center, Key Laboratory for Environmental Fracture (MOE), University of Science and Technology Beijing, Beijing 100083, People’s Republic of China Jinzhou Petrochemical Corporation, China National Petroleum Corporation, Jinzhou, Liaoning 121001, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 9 October 2012 Accepted 20 January 2013 Available online 29 January 2013 Keywords: A. Steel B. AFM C. Atmospheric corrosion C. Intergranular corrosion
a b s t r a c t The initial stage of atmospheric corrosion on interstitial free steel was investigated by ab initio calculation and scanning probe microscopy under different operation modes. Water droplets are adsorbed preferentially on grain boundaries at 95% relative humidity due to their higher surface energy, which is confirmed by ab initio calculation. Atomic force microscopy observation indicates that corrosion preferentially occurs at grain boundaries after immersion under a thin film of NaCl electrolyte, because of the lower relative nobility of grain boundary, which is proved by scanning Kelvin probe force microscope. These results indicate that water adsorption is crucial for initializing atmospheric corrosion. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Interstitial free (IF) steel has a good ability on deep draw and is used extensively in the automotive industry [1,2]. Unfortunately, IF steel is susceptible to atmospheric corrosion, because it must work in the wet atmosphere, such as rain, moisture and dew [3]. Since slight atmospheric corrosion may cause serious damage, multiple efforts have been focused on the atmospheric corrosion of steels [4–7], but there is little research on the atmospheric corrosion behavior of IF steel, especially the initial step of atmospheric corrosion. The initial stage of atmospheric corrosion occurs on metal surface when water vapor in ambient air condenses as micro-water droplet and is absorbed on the surface [8,9]. In general, the grain boundary of steel is more susceptible to atmosphere corrosion because special crystallographic nature and atomic structure cause high interfacial energy [10–12]. Therefore, water droplets tend to be adsorbed preferentially at grain boundaries, which relates to the atmospheric corrosion process. However, it lacks direct evidence at micro-scale to characterize the adsorption behavior of micro-water droplet at grain boundaries, so the detail processes of the initiation and the early stages of atmospheric corrosion were not understood completely. Scanning probe microscopy (SPM) is an effective method to characterize both the surface topography and the surface relative nobility of steel in the atmospheric corrosion, because it can directly observe the real-time changes at nanometer scales [13– 29]. Scanning Kelvin probe force microscopy (SKPFM) is a powerful ⇑ Corresponding author. Tel.: +86 10 62334499; fax: +86 10 62332345. E-mail address: [email protected] (L.J. Qiao). 0010-938X/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.corsci.2013.01.028
technique to study the corrosion mechanism [23–29]. Based on the principle of SKPFM, Stratmann et al. [23–26] researched the Volta potential of different samples, and revealed that the Volta potential in air has a linear relationship with the corrosion potential in aqueous solution. Then the Volta potential maps can represent the relative nobility of different microstructure features relative to the matrix, which is important to determine their role in micro-galvanic interactions. Therefore, SKPFM is successfully applied in corrosion studies in recent years. In addition, with the aid of atomic force microscopy (AFM), Wang et al. [13–15] observed the condensation behavior and random adsorption of micro-water droplets on metal surfaces, such as iron and stainless steel, in ambient air. However, there is no direct observation of the selective adsorption of water droplet on some certain location, which may depend on the physical and chemical property of steel surface, such as grain boundary. The present work aims to investigate the water preferential adsorption at the grain boundary of IF steel surface and the relation between the formation of micro-droplets and the initial process of atmospheric corrosion, which gives a direct evidence to explain the intergranular corrosion.
2. Experimental 2.1. Material and sample preparation The experimental material was conventional IF steel, whose chemical composition was given in Table 1. The specimens were machined into square sheets with a dimension of 4 8 1 mm.
189
L.Q. Guo et al. / Corrosion Science 70 (2013) 188–193 Table 1 The chemical composition of IF steel (wt.%). Elements Content
C 0.014
Cr 0.015
Mn 0.42
Ni 0.014
Cu 0.016
The surface of specimen was wet ground with SiC paper up to 2000 grit, and polished using diamond paste to 0.5 lm. As soon as this mechanical polishing was finished, a final electrochemical polishing was performed in a mixed 5% solution of perchloric acid and ethanol for 30 s at an applied cell voltage of 28 V at 15 °C, so that grains and grain boundaries could be distinguished. Then the specimen was cleaned in absolute ethanol and dried under N2 gas flow. The specimen was kept in dry Ar for 30 min to remove surface adsorbates before water adsorption test. For the measurement of the preferential dissolution of grain boundaries, the sample was immersed under a thin film of 0.6 mol/L NaCl with a thickness of 450 nm for 48 h. To identify the scanning region, indentations were made on the surface of each specimen by digital microhardness tester (HVS-1000) before measurements. 2.2. In situ AFM continuous observation of water droplet adsorption In situ observation of the formation and adsorption of microwater droplets on IF steel surface was carried out by Agilent 5500 AFM (Agilent Technologies, USA) equipped with environmental chamber at room temperature. Relative humidity (RH) in the chamber was measured by a digital hygrometer and controlled by circulating Ar and water vapor. In situ continuous observation of the water droplet adsorption on the surface was performed using the tapping mode of AFM. The probes were Nanosensor PPP-NCH with a force constant of 2.5 N/m and a resonant frequency of 280 kHz. 2.3. AFM measurement of the intergranular corrosion behavior To investigate the preferential dissolution of grain boundaries, the surface topography change of specimens after immersion under a thin film of 0.6 mol/L NaCl with a thickness of 450 nm was monitored with a dimension AFM (Nanoscope V, Veeco Instruments Inc.) using the contact mode, which was located in a clean room and an ambient relative humidity of about 20%. The surface region (40 40 lm) was scanned at a scan frequency of 0.5 Hz. The probes used were Bruker NanoProbe™ DNP tips with force constant of 0.8 N/m. 2.4. SKPFM Volta potential mapping To evaluate the relative nobility of different microstructure features, such as grain and grain boundary, SKPFM measurements were performed to map Volta potential variations on the IF steel surface, using a dimension Nanoscope V (Veeco Instruments Inc.). All measurements were conducted in air at room temperature and an ambient relative humidity of about 25%. The probes used in the SKPFM measurements were Nanoprobe™ SCM–PIT conductive PtIr-coated silicon tips with a force constant of 2.5 N/m and a resonant frequency of 60–100 kHz. The lift mode was used to record a second signal in addition to surface topography. 2.5. Ab initio theory calculations The structural relaxation, band structure and total energy calculations were performed using project-augmented-wave (PAW) method [30] as implemented in the Vienna ab initio simulation package (VASP) codes [31]. The exchange–correlation interactions
N 0.014
Si 0.10
Al 0.056
P 0.05
Ti 0.029
S 0.0066
were treated by the spin-polarized Perdew–Burke–Ernaerhof (GGA-PBE) exchange correlation functional, as GGA-PBE can give a better predication of adsorption geometry [32]. A plane wave basis set with an energy cut-off of 400 eV and a Monkhorst–Pack [33] k-point mesh of 6 6 1 were employed. The PAW potentials with valence states 3d and 4s for Fe were used. 3. Results 3.1. The observation of selective adsorption of water droplet The continuous changes of the surface morphology are observed at 95% relative humidity, as shown in Fig. 1. In the figures grains and grain boundaries are clear. When the relative humidity increases to 95%, the surface condition changes remarkably. Obviously, water is adsorbed preferentially at the grain boundaries and nucleates as droplets. The exposure duration in humid air has a significant effect on the growth and adsorption of water droplet on the surface. With the duration of time, the water droplets grow in both diameter and height, and some new droplets successively emerge on the surface (Fig. 1b–d). After 30 min exposure, the water droplets gather along one grain boundary mainly and few droplets appear on the grains (Fig. 1b). When the exposure time further increases to 180 min, large amount of water droplets aggregate along all grain boundaries (Fig. 1c). The droplet average height increases continuously with the exposure duration, whose variation is fast in the first 180 min and then turns slow as shown in Fig. 2. The water droplet average size rises from 130 nm to 320 nm, and reaches saturation after 240 min (Fig. 1d), where the droplets are stable after 8 h. Meantime, it is noticed that water droplets are gathered along the grain boundaries, and few water droplets appear on the grains. 3.2. Characterization of the intergranular corrosion Fig. 3 shows the AFM images of the sample surface before and after immersion under a thin film of 0.6 mol/L NaCl with a thickness of 450 nm for 48 h in the same area. Compared with grains, grain boundaries are susceptible to the preferential dissolution, which is obvious in the topography profile, as shown in Fig. 3b. In addition, the corrosion in grains is much slower than that at grain boundaries (Fig. 3b). The depth of the intergranular corrosion is about 100 nm after immersion test, which is much larger than that of individual grain (about 30 nm). It indicates that the corrosion rate is larger at the grain boundaries than that of in the grains, i.e. preferential corrosion occurs at grain boundaries. 3.3. Volta potential difference variation between grain and grain boundaries The SKPFM map was performed on the IF steel surface to evaluate the relative corrosion tendency of grains and grain boundaries. Fig. 4 shows the topographic image (Fig. 4a) and the Volta potential map with Volta potential difference profile (Fig. 4b) in the same scanning area under dry condition. The Volta potential variations are clear in the Volta potential image (Fig. 4b). The grain boundaries exhibit lower Volta potential (darker) than that of grain regions, and the Volta potential is slight different in various grains. The average potential difference of at least five line analyses
190
L.Q. Guo et al. / Corrosion Science 70 (2013) 188–193
Fig. 1. In situ AFM continuous images of water droplets nucleation and growth on the surface of IF steel at 95% relative humidity with exposure duration of time, (a) 0 min, (b) 30 min, (c) 180 min and (d) 320 min.
between grain and grain boundary in dry condition is about 18 ± 3 mV. 3.4. Ab initio calculations Ab initio calculations were used to reveal the water adsorption mechanism on IF steel surface. The surface energies of (1 1 1), (1 1 2), (1 1 4), and (2 2 3) orientations in Fe lattice are calculated. Different atom layers are employed for different surfaces to maintain the surface periodicity, such as 20 atom layers for (1 1 1) sur 0 grain boundary is set up with face. The Fe bcc R3ð1 1 1Þ½1 1 three periodic layers in [112] direction and four periodic layers 0 direction. For all calculations, a 12 Å vacuum layer is used. in ½1 1 The calculated energy of different surfaces is shown in Table 2, which is in good agreement with previous reports [34]. The calculated surface energy of Fe (1 1 1) surface is 2.88 J/m2, slightly larger than that in Michelle’s result [34] (2.52 J/m2). The energies of (1 1 2), (1 1 4) and (2 2 3) surfaces are 2.46 J/m2, 2.75 J/m2, and 2.46 J/m2 respectively. Since the (1 1 1) surface has the largest energy, we calculated water adsorption on it and compared the adsorption energy with that at the grain boundary. In the calculation, one water molecular is put on (1 1) (1 1 1) 0 grain boundary with the surface and on the top of R3ð1 1 1Þ½1 1 molecular plane parallel to the surface plane. The water molecular is free to move and the atoms in the substrate are frozen without relaxation. The stable adsorption configurations are shown in Fig. 5. After relaxation, water molecular on (1 1 1) surface changes its initial configuration with one OH-group pointing to the surface and the distance between O and Fe atom is 2.35 Å. The water molecular angle changes from 104.5° to 103.9° and the bond length of the OH near surface stretches from 0.97 Å to 0.99 Å. The calculated adsorption energy is 0.53 eV.
Fig. 2. Growth of water droplet height with time on the grain boundaries of IF steel at 95% RH. Inserts show surface morphology with the same area outlined.
The stable site of water adsorption on grain boundary is on the top of one iron atom near the center of grain boundary with two OH-groups pointing out of surface plane. The distance between O and Fe atoms is 2.15 Å. The molecular angle changes to 105.8° and the OH bond length becomes 0.98 Å. The calculated adsorption energy is 0.98 eV. Compared with water on (1 1 1) surface, the adsorption energy of water at grain boundary is larger, which suggests that water is easier to be adsorbed at grain boundaries. The calculated result is consistent with the experiment observations.
191
L.Q. Guo et al. / Corrosion Science 70 (2013) 188–193
Fig. 3. AFM observations of intergranular corrosion of the sample (a) before and (b) after immersion under a thin film of 0.6 mol/L NaCl with a thickness of 450 nm for 48 h on the same scanning area.
Fig. 4. (a) Topography and (b) SKPFM images of IF steel surface in dry condition with corresponding potential profiles in mV superimposed.
4. Discussion
Table 2 The calculated energy of different surfaces of IF steel (J/m2).
In situ AFM continuous observation shows that the preferential adsorption of micro-water droplet occurs at grain boundaries at 95% relative humidity. Moreover, AFM measurement displays that the grain boundary is susceptible to the preferential dissolution after immersion under a thin film of electrolyte. These results indicate that the formation and adsorption of micro-water droplet on the surface are related to the initial atmospheric corrosion process of IF steel. The intergranular corrosion and its relationship with the micro-water droplet adsorption on the surface can be discussed from the differences of relative nobility and surface energy between grain and grain boundary. Mantel and Wightman [35] showed that the ability of stainless steel surface to attract water after cleaning treatments depends on the surface composition, because the heterogeneity of composition distribution on the surface results in different surface energy [36]. It has been widely accepted that material with higher surface energy is susceptible to the preferential water adsorption. In general, the grain boundary has higher surface energy due to the special
Surface Surface energy (J/m2)
(1 1 1) 2.88
(1 1 2) 2.46
(1 1 4) 2.75
(2 2 3) 2.46
crystallographic nature and irregular arrange of atom [37]. It is demonstrated by ab initio calculations that the adsorption energy of water at grain boundary is larger than that in grain in IF steel (Table 2 and Fig. 5). It suggests that the energy decrement by water adsorption is larger at grain boundary than that in grain, namely the interaction between water molecule and grain boundary is stronger, so water prefers to be adsorbed at grain boundaries. This is consistent with the in situ AFM continuous observation that water droplets preferentially gather at grain boundaries, as shown in Fig. 1. With the duration of time, water droplets grow larger and become more, most of which spread along grain boundaries and few in grains. The preferential adsorption of water droplets is caused by different surface energy between grain boundary and grain [38].
192
L.Q. Guo et al. / Corrosion Science 70 (2013) 188–193
0 grain boundary. (a and c) Side and top view of water on top of grain boundary. Fig. 5. Stable adsorption configuration of water on surface (1 1 1) and on top of R3ð1 1 1Þ½1 1 (b and d) Side and top of view water on surface (1 1 1).
The micro-water droplets are formed and absorbed preferentially at the grain boundaries of metal surfaces, where the atmospheric corrosion occurs preferentially (Fig. 3), because of different relative nobility between grain and grain boundary. Although the SKPFM measurement was performed in air, it has a strong correlation with the corrosion potential in solution [39–41] and indicates the tendency of corrosion, namely low Volta potential indicates low relative nobility. The SKPFM map (Fig. 4) shows the corrosion tendency and the difference of relative nobility between grain and grain boundary. A higher Volta potential of grain reveals that it is nobler than grain boundary and has a higher corrosion potential. Therefore, the lower Volta potential at grain boundaries make them preferred sites for the onset of corrosion. Moreover, our previous work displayed that the Volta potential decreases with the water adsorption on the metal surfaces [42]. Therefore, the preferential adsorption of micro-water droplets at grain boundaries can further lower the Volta potential, which results in a worse corrosion resistance in humid condition. In addition, the SKPFM map results (Fig. 4) and previous reports [42,43] indicate that the Volta potential is different in the different grains, so the corrosion rate of these
grains is different (see Fig. 3). However, the grains always have higher Volta potential than the grain boundaries, which results in higher corrosion resistance. Therefore, the initiation of grain corrosion is later than that at grain boundaries. 5. Conclusions The ab initio calculations and SPM direct measurements were used to characterize the water droplet preferential adsorption, which depends on the nanoscale surface structure and nature of IF steel. The in situ AFM continuous observation reveals that the water droplets are absorbed preferentially at grain boundaries at 95% relative humidity due to higher surface energy, which is confirmed by ab initio calculations. The AFM observation proves that corrosion preferentially occurs at grain boundaries after immersion under a thin film of NaCl electrolyte because of their lower Volta potential measured by SKPFM. The micro-water droplet formation and its adsorption on the surface are related to the initial atmospheric corrosion process of IF steel, namely water adsorption is crucial for initializing atmospheric corrosion.
L.Q. Guo et al. / Corrosion Science 70 (2013) 188–193
Acknowledgements The authors acknowledge support from the National Natural Science Foundation of China under Grant Nos. 51271026, 511611 60563 and Beijing Municipal Commission of Education under Grant No. YB20091000801.
References [1] I.G. Rodionova, E.T. Shapovalov, M.E. Kovalevskaya, O.N. Baklanova, Metallurgist 49 (2005) 314–318. [2] S.Q. Cao, J.X. Zhang, J.S. Wu, J.G. Chen, Scripta Mater. 52 (2005) 25–28. [3] Branislav Hadzima, Milos Janecek, Yuri Estrin, Hyoung Seop Kim, Mater. Sci. Eng. A 462 (2007) 243–246. [4] S. Andersson, C. Nyberg, C.G. Tengstl, Chem. Phys. Lett. 104 (1984) 305–309. [5] T. Handa, Y. Miyata, Zairyo-to-Kankyo, Corros. Eng. 49 (2000) 649–654. [6] P. Qiu, C. Leygraf, J. Electrochem. Soc. 158 (2011) C172–C178. [7] F. Heslot, N. Fraysse, A.M. Cazabat, Nature 640 (1989) 338–341. [8] P.A. Thiel, T.E. Madey, Surf. Sci. Rep. 7 (1987) 211–216. [9] S. Lee, R.W. Staehle, Mater. Corros. 48 (1997) 86–91. [10] P. Lin, G. Palumbo, U. Erb, Scripta Metall. Mater. 33 (1995) 1387–1391. [11] H.Y. Bi, H. Kokawa, Z.J. Wang, Scripta Mater. 49 (2003) 219–223. [12] M. Shimada, H. Kokawa, Z.J. Wang, Y.S. Sato, I. Karibe, Acta Mater. 50 (2002) 2331–2335. [13] R. Wang, M. Takeda, K. Mukai, M. Kido, Scripta Mater. 46 (2002) 83–87. [14] R. Wang, M. Takeda, K. Mukai, M. Kido, Mater. Lett. 54 (2002) 140–144. [15] R. Wang, L. Cong, K. Mukai, M. Kido, Appl. Surf. Sci. 191 (2002) 74–78. [16] S. Herminhaus, A. Fery, D. Reim, Ultramicroscopy 211 (1997) 69–75. [17] T. Pompe, A. Fery, S. Herminghaus, Langmuir 2585 (1998) 14–19. [18] L.Q. Guo, X.M. Zhao, M. Li, Y. Bai, L.J. Qiao, Appl. Surf. Sci. 258 (2012) 213–218.
193
[19] L.Q. Guo, M. Li, X.L. Shi, Y. Yan, X.Y. Li, L.J. Qiao, Corros. Sci. 53 (2011) 3733– 3743. [20] M. Jonsson, D. Thierry, N. LeBozec, Corros. Sci. 48 (2006) 1193–1208. [21] B.S. Tanem, G. Svenningsen, J. Mardalen, Corros. Sci. 47 (2005) 1506–1519. [22] S.M. Gheno, F.S. Santos, S.E. Kuri, J. Appl. Phys. 103 (2008) 053906–0539010. [23] M. Stratmann, H. Steckel, Corros. Sci. 30 (1990) 681–686. [24] S. Yee, R.A. Oriani, M. Stratmann, J. Electrochem. Soc. 55 (1991) 138–144. [25] M. Stratmann, Corros. Sci. 27 (1987) 869–872. [26] M. Stratmann, K.T. Kim, H. Steckel, Z. Metallkd, Corros. Sci. 30 (1990) 715–719. [27] N. Sathirachinda, R. Pettersson, S. Wessman, J. Pan, Corros. Sci. 52 (2010) 179– 186. [28] N. Sathirachinda, R. Gubner, J. Pan, U. Kivisäkk, Solid State Lett. 11 (2008) C41– C45. [29] N. Sathirachinda, R. Pettersson, S. Wessman, U. Kivisakk, J.S. Pan, Electrochim. Acta 56 (2011) 1792–1798. [30] P.E. Blöchl, Phys. Rev. B 50 (1994) 17953–17956. [31] G. Kresse, D. Joubert, Phys. Rev. B 59 (1999) 1758–1762. [32] J.P. Perdew, J.A. Chevary, S.H. Vosko, K.A. Jackson, M.R. Pederson, D.J. Singh, C. Fiolhais, Phys. Rev. B 46 (1992) 6671–6674. [33] H.J. Monkhorst, J.D. Pack, Phys. Rev. B 13 (1972) 5188–5191. [34] Spencer, Andrew Hung, Ian K. Snook, Irene Yarovsky, Surf. Sci. 513 (2002) 389–395. [35] M. Mantel, J.P. Wightman, Surf. Interf. Anal. 21 (1994) 595–601. [36] M.K. Bernett, W.A. Zisman, J. Phys. Chem. 74 (1970) 2309–2314. [37] T.R. Bieler, P. Eisenlohr, F. Roters, D. Kumar, D.E. Mason, M.A. Crimp, D. Raabe, Int. J. Plast. 25 (2009) 1655–1659. [38] D.Y. He, L.J. Qiao, Alex A. Volinsky, Y. Bai, M. Wu, W.Y. Chu, Appl. Phys. Lett. 98 (2011) 062905-1–062905-3. [39] P. Schmutz, G.S. Frankel, J. Electrochem. Soc. 145 (1998) 2285–2295. [40] P. Schmutz, G.S. Frankel, J. Electrochem. Soc. 145 (1998) 2295–2306. [41] M. Li, L.Q. Guo, L.J. Qiao, Y. Bai, Corros. Sci. 60 (2012) 76–81. [42] L.Q. Guo, X.M. Zhao, Y. Bai, L.J. Qiao, Appl. Surf. Sci. 258 (2012) 9087–9091. [43] N. Sathirachinda, R. Pettersson, J.S. Pan, Corros. Sci. 8 (2009) 1850–1860.
Contents lists available at SciVerse ScienceDirect
Corrosion Science journal homepage: www.elsevier.com/locate/corsci
The initial stage of atmospheric corrosion on interstitial free steel investigated by in situ SPM L.Q. Guo a, X.M. Zhao a, B.C. Wang a, Y. Bai a, B.Z. Xu b, L.J. Qiao a,⇑ a b
Corrosion and Protection Center, Key Laboratory for Environmental Fracture (MOE), University of Science and Technology Beijing, Beijing 100083, People’s Republic of China Jinzhou Petrochemical Corporation, China National Petroleum Corporation, Jinzhou, Liaoning 121001, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 9 October 2012 Accepted 20 January 2013 Available online 29 January 2013 Keywords: A. Steel B. AFM C. Atmospheric corrosion C. Intergranular corrosion
a b s t r a c t The initial stage of atmospheric corrosion on interstitial free steel was investigated by ab initio calculation and scanning probe microscopy under different operation modes. Water droplets are adsorbed preferentially on grain boundaries at 95% relative humidity due to their higher surface energy, which is confirmed by ab initio calculation. Atomic force microscopy observation indicates that corrosion preferentially occurs at grain boundaries after immersion under a thin film of NaCl electrolyte, because of the lower relative nobility of grain boundary, which is proved by scanning Kelvin probe force microscope. These results indicate that water adsorption is crucial for initializing atmospheric corrosion. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Interstitial free (IF) steel has a good ability on deep draw and is used extensively in the automotive industry [1,2]. Unfortunately, IF steel is susceptible to atmospheric corrosion, because it must work in the wet atmosphere, such as rain, moisture and dew [3]. Since slight atmospheric corrosion may cause serious damage, multiple efforts have been focused on the atmospheric corrosion of steels [4–7], but there is little research on the atmospheric corrosion behavior of IF steel, especially the initial step of atmospheric corrosion. The initial stage of atmospheric corrosion occurs on metal surface when water vapor in ambient air condenses as micro-water droplet and is absorbed on the surface [8,9]. In general, the grain boundary of steel is more susceptible to atmosphere corrosion because special crystallographic nature and atomic structure cause high interfacial energy [10–12]. Therefore, water droplets tend to be adsorbed preferentially at grain boundaries, which relates to the atmospheric corrosion process. However, it lacks direct evidence at micro-scale to characterize the adsorption behavior of micro-water droplet at grain boundaries, so the detail processes of the initiation and the early stages of atmospheric corrosion were not understood completely. Scanning probe microscopy (SPM) is an effective method to characterize both the surface topography and the surface relative nobility of steel in the atmospheric corrosion, because it can directly observe the real-time changes at nanometer scales [13– 29]. Scanning Kelvin probe force microscopy (SKPFM) is a powerful ⇑ Corresponding author. Tel.: +86 10 62334499; fax: +86 10 62332345. E-mail address: [email protected] (L.J. Qiao). 0010-938X/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.corsci.2013.01.028
technique to study the corrosion mechanism [23–29]. Based on the principle of SKPFM, Stratmann et al. [23–26] researched the Volta potential of different samples, and revealed that the Volta potential in air has a linear relationship with the corrosion potential in aqueous solution. Then the Volta potential maps can represent the relative nobility of different microstructure features relative to the matrix, which is important to determine their role in micro-galvanic interactions. Therefore, SKPFM is successfully applied in corrosion studies in recent years. In addition, with the aid of atomic force microscopy (AFM), Wang et al. [13–15] observed the condensation behavior and random adsorption of micro-water droplets on metal surfaces, such as iron and stainless steel, in ambient air. However, there is no direct observation of the selective adsorption of water droplet on some certain location, which may depend on the physical and chemical property of steel surface, such as grain boundary. The present work aims to investigate the water preferential adsorption at the grain boundary of IF steel surface and the relation between the formation of micro-droplets and the initial process of atmospheric corrosion, which gives a direct evidence to explain the intergranular corrosion.
2. Experimental 2.1. Material and sample preparation The experimental material was conventional IF steel, whose chemical composition was given in Table 1. The specimens were machined into square sheets with a dimension of 4 8 1 mm.
189
L.Q. Guo et al. / Corrosion Science 70 (2013) 188–193 Table 1 The chemical composition of IF steel (wt.%). Elements Content
C 0.014
Cr 0.015
Mn 0.42
Ni 0.014
Cu 0.016
The surface of specimen was wet ground with SiC paper up to 2000 grit, and polished using diamond paste to 0.5 lm. As soon as this mechanical polishing was finished, a final electrochemical polishing was performed in a mixed 5% solution of perchloric acid and ethanol for 30 s at an applied cell voltage of 28 V at 15 °C, so that grains and grain boundaries could be distinguished. Then the specimen was cleaned in absolute ethanol and dried under N2 gas flow. The specimen was kept in dry Ar for 30 min to remove surface adsorbates before water adsorption test. For the measurement of the preferential dissolution of grain boundaries, the sample was immersed under a thin film of 0.6 mol/L NaCl with a thickness of 450 nm for 48 h. To identify the scanning region, indentations were made on the surface of each specimen by digital microhardness tester (HVS-1000) before measurements. 2.2. In situ AFM continuous observation of water droplet adsorption In situ observation of the formation and adsorption of microwater droplets on IF steel surface was carried out by Agilent 5500 AFM (Agilent Technologies, USA) equipped with environmental chamber at room temperature. Relative humidity (RH) in the chamber was measured by a digital hygrometer and controlled by circulating Ar and water vapor. In situ continuous observation of the water droplet adsorption on the surface was performed using the tapping mode of AFM. The probes were Nanosensor PPP-NCH with a force constant of 2.5 N/m and a resonant frequency of 280 kHz. 2.3. AFM measurement of the intergranular corrosion behavior To investigate the preferential dissolution of grain boundaries, the surface topography change of specimens after immersion under a thin film of 0.6 mol/L NaCl with a thickness of 450 nm was monitored with a dimension AFM (Nanoscope V, Veeco Instruments Inc.) using the contact mode, which was located in a clean room and an ambient relative humidity of about 20%. The surface region (40 40 lm) was scanned at a scan frequency of 0.5 Hz. The probes used were Bruker NanoProbe™ DNP tips with force constant of 0.8 N/m. 2.4. SKPFM Volta potential mapping To evaluate the relative nobility of different microstructure features, such as grain and grain boundary, SKPFM measurements were performed to map Volta potential variations on the IF steel surface, using a dimension Nanoscope V (Veeco Instruments Inc.). All measurements were conducted in air at room temperature and an ambient relative humidity of about 25%. The probes used in the SKPFM measurements were Nanoprobe™ SCM–PIT conductive PtIr-coated silicon tips with a force constant of 2.5 N/m and a resonant frequency of 60–100 kHz. The lift mode was used to record a second signal in addition to surface topography. 2.5. Ab initio theory calculations The structural relaxation, band structure and total energy calculations were performed using project-augmented-wave (PAW) method [30] as implemented in the Vienna ab initio simulation package (VASP) codes [31]. The exchange–correlation interactions
N 0.014
Si 0.10
Al 0.056
P 0.05
Ti 0.029
S 0.0066
were treated by the spin-polarized Perdew–Burke–Ernaerhof (GGA-PBE) exchange correlation functional, as GGA-PBE can give a better predication of adsorption geometry [32]. A plane wave basis set with an energy cut-off of 400 eV and a Monkhorst–Pack [33] k-point mesh of 6 6 1 were employed. The PAW potentials with valence states 3d and 4s for Fe were used. 3. Results 3.1. The observation of selective adsorption of water droplet The continuous changes of the surface morphology are observed at 95% relative humidity, as shown in Fig. 1. In the figures grains and grain boundaries are clear. When the relative humidity increases to 95%, the surface condition changes remarkably. Obviously, water is adsorbed preferentially at the grain boundaries and nucleates as droplets. The exposure duration in humid air has a significant effect on the growth and adsorption of water droplet on the surface. With the duration of time, the water droplets grow in both diameter and height, and some new droplets successively emerge on the surface (Fig. 1b–d). After 30 min exposure, the water droplets gather along one grain boundary mainly and few droplets appear on the grains (Fig. 1b). When the exposure time further increases to 180 min, large amount of water droplets aggregate along all grain boundaries (Fig. 1c). The droplet average height increases continuously with the exposure duration, whose variation is fast in the first 180 min and then turns slow as shown in Fig. 2. The water droplet average size rises from 130 nm to 320 nm, and reaches saturation after 240 min (Fig. 1d), where the droplets are stable after 8 h. Meantime, it is noticed that water droplets are gathered along the grain boundaries, and few water droplets appear on the grains. 3.2. Characterization of the intergranular corrosion Fig. 3 shows the AFM images of the sample surface before and after immersion under a thin film of 0.6 mol/L NaCl with a thickness of 450 nm for 48 h in the same area. Compared with grains, grain boundaries are susceptible to the preferential dissolution, which is obvious in the topography profile, as shown in Fig. 3b. In addition, the corrosion in grains is much slower than that at grain boundaries (Fig. 3b). The depth of the intergranular corrosion is about 100 nm after immersion test, which is much larger than that of individual grain (about 30 nm). It indicates that the corrosion rate is larger at the grain boundaries than that of in the grains, i.e. preferential corrosion occurs at grain boundaries. 3.3. Volta potential difference variation between grain and grain boundaries The SKPFM map was performed on the IF steel surface to evaluate the relative corrosion tendency of grains and grain boundaries. Fig. 4 shows the topographic image (Fig. 4a) and the Volta potential map with Volta potential difference profile (Fig. 4b) in the same scanning area under dry condition. The Volta potential variations are clear in the Volta potential image (Fig. 4b). The grain boundaries exhibit lower Volta potential (darker) than that of grain regions, and the Volta potential is slight different in various grains. The average potential difference of at least five line analyses
190
L.Q. Guo et al. / Corrosion Science 70 (2013) 188–193
Fig. 1. In situ AFM continuous images of water droplets nucleation and growth on the surface of IF steel at 95% relative humidity with exposure duration of time, (a) 0 min, (b) 30 min, (c) 180 min and (d) 320 min.
between grain and grain boundary in dry condition is about 18 ± 3 mV. 3.4. Ab initio calculations Ab initio calculations were used to reveal the water adsorption mechanism on IF steel surface. The surface energies of (1 1 1), (1 1 2), (1 1 4), and (2 2 3) orientations in Fe lattice are calculated. Different atom layers are employed for different surfaces to maintain the surface periodicity, such as 20 atom layers for (1 1 1) sur 0 grain boundary is set up with face. The Fe bcc R3ð1 1 1Þ½1 1 three periodic layers in [112] direction and four periodic layers 0 direction. For all calculations, a 12 Å vacuum layer is used. in ½1 1 The calculated energy of different surfaces is shown in Table 2, which is in good agreement with previous reports [34]. The calculated surface energy of Fe (1 1 1) surface is 2.88 J/m2, slightly larger than that in Michelle’s result [34] (2.52 J/m2). The energies of (1 1 2), (1 1 4) and (2 2 3) surfaces are 2.46 J/m2, 2.75 J/m2, and 2.46 J/m2 respectively. Since the (1 1 1) surface has the largest energy, we calculated water adsorption on it and compared the adsorption energy with that at the grain boundary. In the calculation, one water molecular is put on (1 1) (1 1 1) 0 grain boundary with the surface and on the top of R3ð1 1 1Þ½1 1 molecular plane parallel to the surface plane. The water molecular is free to move and the atoms in the substrate are frozen without relaxation. The stable adsorption configurations are shown in Fig. 5. After relaxation, water molecular on (1 1 1) surface changes its initial configuration with one OH-group pointing to the surface and the distance between O and Fe atom is 2.35 Å. The water molecular angle changes from 104.5° to 103.9° and the bond length of the OH near surface stretches from 0.97 Å to 0.99 Å. The calculated adsorption energy is 0.53 eV.
Fig. 2. Growth of water droplet height with time on the grain boundaries of IF steel at 95% RH. Inserts show surface morphology with the same area outlined.
The stable site of water adsorption on grain boundary is on the top of one iron atom near the center of grain boundary with two OH-groups pointing out of surface plane. The distance between O and Fe atoms is 2.15 Å. The molecular angle changes to 105.8° and the OH bond length becomes 0.98 Å. The calculated adsorption energy is 0.98 eV. Compared with water on (1 1 1) surface, the adsorption energy of water at grain boundary is larger, which suggests that water is easier to be adsorbed at grain boundaries. The calculated result is consistent with the experiment observations.
191
L.Q. Guo et al. / Corrosion Science 70 (2013) 188–193
Fig. 3. AFM observations of intergranular corrosion of the sample (a) before and (b) after immersion under a thin film of 0.6 mol/L NaCl with a thickness of 450 nm for 48 h on the same scanning area.
Fig. 4. (a) Topography and (b) SKPFM images of IF steel surface in dry condition with corresponding potential profiles in mV superimposed.
4. Discussion
Table 2 The calculated energy of different surfaces of IF steel (J/m2).
In situ AFM continuous observation shows that the preferential adsorption of micro-water droplet occurs at grain boundaries at 95% relative humidity. Moreover, AFM measurement displays that the grain boundary is susceptible to the preferential dissolution after immersion under a thin film of electrolyte. These results indicate that the formation and adsorption of micro-water droplet on the surface are related to the initial atmospheric corrosion process of IF steel. The intergranular corrosion and its relationship with the micro-water droplet adsorption on the surface can be discussed from the differences of relative nobility and surface energy between grain and grain boundary. Mantel and Wightman [35] showed that the ability of stainless steel surface to attract water after cleaning treatments depends on the surface composition, because the heterogeneity of composition distribution on the surface results in different surface energy [36]. It has been widely accepted that material with higher surface energy is susceptible to the preferential water adsorption. In general, the grain boundary has higher surface energy due to the special
Surface Surface energy (J/m2)
(1 1 1) 2.88
(1 1 2) 2.46
(1 1 4) 2.75
(2 2 3) 2.46
crystallographic nature and irregular arrange of atom [37]. It is demonstrated by ab initio calculations that the adsorption energy of water at grain boundary is larger than that in grain in IF steel (Table 2 and Fig. 5). It suggests that the energy decrement by water adsorption is larger at grain boundary than that in grain, namely the interaction between water molecule and grain boundary is stronger, so water prefers to be adsorbed at grain boundaries. This is consistent with the in situ AFM continuous observation that water droplets preferentially gather at grain boundaries, as shown in Fig. 1. With the duration of time, water droplets grow larger and become more, most of which spread along grain boundaries and few in grains. The preferential adsorption of water droplets is caused by different surface energy between grain boundary and grain [38].
192
L.Q. Guo et al. / Corrosion Science 70 (2013) 188–193
0 grain boundary. (a and c) Side and top view of water on top of grain boundary. Fig. 5. Stable adsorption configuration of water on surface (1 1 1) and on top of R3ð1 1 1Þ½1 1 (b and d) Side and top of view water on surface (1 1 1).
The micro-water droplets are formed and absorbed preferentially at the grain boundaries of metal surfaces, where the atmospheric corrosion occurs preferentially (Fig. 3), because of different relative nobility between grain and grain boundary. Although the SKPFM measurement was performed in air, it has a strong correlation with the corrosion potential in solution [39–41] and indicates the tendency of corrosion, namely low Volta potential indicates low relative nobility. The SKPFM map (Fig. 4) shows the corrosion tendency and the difference of relative nobility between grain and grain boundary. A higher Volta potential of grain reveals that it is nobler than grain boundary and has a higher corrosion potential. Therefore, the lower Volta potential at grain boundaries make them preferred sites for the onset of corrosion. Moreover, our previous work displayed that the Volta potential decreases with the water adsorption on the metal surfaces [42]. Therefore, the preferential adsorption of micro-water droplets at grain boundaries can further lower the Volta potential, which results in a worse corrosion resistance in humid condition. In addition, the SKPFM map results (Fig. 4) and previous reports [42,43] indicate that the Volta potential is different in the different grains, so the corrosion rate of these
grains is different (see Fig. 3). However, the grains always have higher Volta potential than the grain boundaries, which results in higher corrosion resistance. Therefore, the initiation of grain corrosion is later than that at grain boundaries. 5. Conclusions The ab initio calculations and SPM direct measurements were used to characterize the water droplet preferential adsorption, which depends on the nanoscale surface structure and nature of IF steel. The in situ AFM continuous observation reveals that the water droplets are absorbed preferentially at grain boundaries at 95% relative humidity due to higher surface energy, which is confirmed by ab initio calculations. The AFM observation proves that corrosion preferentially occurs at grain boundaries after immersion under a thin film of NaCl electrolyte because of their lower Volta potential measured by SKPFM. The micro-water droplet formation and its adsorption on the surface are related to the initial atmospheric corrosion process of IF steel, namely water adsorption is crucial for initializing atmospheric corrosion.
L.Q. Guo et al. / Corrosion Science 70 (2013) 188–193
Acknowledgements The authors acknowledge support from the National Natural Science Foundation of China under Grant Nos. 51271026, 511611 60563 and Beijing Municipal Commission of Education under Grant No. YB20091000801.
References [1] I.G. Rodionova, E.T. Shapovalov, M.E. Kovalevskaya, O.N. Baklanova, Metallurgist 49 (2005) 314–318. [2] S.Q. Cao, J.X. Zhang, J.S. Wu, J.G. Chen, Scripta Mater. 52 (2005) 25–28. [3] Branislav Hadzima, Milos Janecek, Yuri Estrin, Hyoung Seop Kim, Mater. Sci. Eng. A 462 (2007) 243–246. [4] S. Andersson, C. Nyberg, C.G. Tengstl, Chem. Phys. Lett. 104 (1984) 305–309. [5] T. Handa, Y. Miyata, Zairyo-to-Kankyo, Corros. Eng. 49 (2000) 649–654. [6] P. Qiu, C. Leygraf, J. Electrochem. Soc. 158 (2011) C172–C178. [7] F. Heslot, N. Fraysse, A.M. Cazabat, Nature 640 (1989) 338–341. [8] P.A. Thiel, T.E. Madey, Surf. Sci. Rep. 7 (1987) 211–216. [9] S. Lee, R.W. Staehle, Mater. Corros. 48 (1997) 86–91. [10] P. Lin, G. Palumbo, U. Erb, Scripta Metall. Mater. 33 (1995) 1387–1391. [11] H.Y. Bi, H. Kokawa, Z.J. Wang, Scripta Mater. 49 (2003) 219–223. [12] M. Shimada, H. Kokawa, Z.J. Wang, Y.S. Sato, I. Karibe, Acta Mater. 50 (2002) 2331–2335. [13] R. Wang, M. Takeda, K. Mukai, M. Kido, Scripta Mater. 46 (2002) 83–87. [14] R. Wang, M. Takeda, K. Mukai, M. Kido, Mater. Lett. 54 (2002) 140–144. [15] R. Wang, L. Cong, K. Mukai, M. Kido, Appl. Surf. Sci. 191 (2002) 74–78. [16] S. Herminhaus, A. Fery, D. Reim, Ultramicroscopy 211 (1997) 69–75. [17] T. Pompe, A. Fery, S. Herminghaus, Langmuir 2585 (1998) 14–19. [18] L.Q. Guo, X.M. Zhao, M. Li, Y. Bai, L.J. Qiao, Appl. Surf. Sci. 258 (2012) 213–218.
193
[19] L.Q. Guo, M. Li, X.L. Shi, Y. Yan, X.Y. Li, L.J. Qiao, Corros. Sci. 53 (2011) 3733– 3743. [20] M. Jonsson, D. Thierry, N. LeBozec, Corros. Sci. 48 (2006) 1193–1208. [21] B.S. Tanem, G. Svenningsen, J. Mardalen, Corros. Sci. 47 (2005) 1506–1519. [22] S.M. Gheno, F.S. Santos, S.E. Kuri, J. Appl. Phys. 103 (2008) 053906–0539010. [23] M. Stratmann, H. Steckel, Corros. Sci. 30 (1990) 681–686. [24] S. Yee, R.A. Oriani, M. Stratmann, J. Electrochem. Soc. 55 (1991) 138–144. [25] M. Stratmann, Corros. Sci. 27 (1987) 869–872. [26] M. Stratmann, K.T. Kim, H. Steckel, Z. Metallkd, Corros. Sci. 30 (1990) 715–719. [27] N. Sathirachinda, R. Pettersson, S. Wessman, J. Pan, Corros. Sci. 52 (2010) 179– 186. [28] N. Sathirachinda, R. Gubner, J. Pan, U. Kivisäkk, Solid State Lett. 11 (2008) C41– C45. [29] N. Sathirachinda, R. Pettersson, S. Wessman, U. Kivisakk, J.S. Pan, Electrochim. Acta 56 (2011) 1792–1798. [30] P.E. Blöchl, Phys. Rev. B 50 (1994) 17953–17956. [31] G. Kresse, D. Joubert, Phys. Rev. B 59 (1999) 1758–1762. [32] J.P. Perdew, J.A. Chevary, S.H. Vosko, K.A. Jackson, M.R. Pederson, D.J. Singh, C. Fiolhais, Phys. Rev. B 46 (1992) 6671–6674. [33] H.J. Monkhorst, J.D. Pack, Phys. Rev. B 13 (1972) 5188–5191. [34] Spencer, Andrew Hung, Ian K. Snook, Irene Yarovsky, Surf. Sci. 513 (2002) 389–395. [35] M. Mantel, J.P. Wightman, Surf. Interf. Anal. 21 (1994) 595–601. [36] M.K. Bernett, W.A. Zisman, J. Phys. Chem. 74 (1970) 2309–2314. [37] T.R. Bieler, P. Eisenlohr, F. Roters, D. Kumar, D.E. Mason, M.A. Crimp, D. Raabe, Int. J. Plast. 25 (2009) 1655–1659. [38] D.Y. He, L.J. Qiao, Alex A. Volinsky, Y. Bai, M. Wu, W.Y. Chu, Appl. Phys. Lett. 98 (2011) 062905-1–062905-3. [39] P. Schmutz, G.S. Frankel, J. Electrochem. Soc. 145 (1998) 2285–2295. [40] P. Schmutz, G.S. Frankel, J. Electrochem. Soc. 145 (1998) 2295–2306. [41] M. Li, L.Q. Guo, L.J. Qiao, Y. Bai, Corros. Sci. 60 (2012) 76–81. [42] L.Q. Guo, X.M. Zhao, Y. Bai, L.J. Qiao, Appl. Surf. Sci. 258 (2012) 9087–9091. [43] N. Sathirachinda, R. Pettersson, J.S. Pan, Corros. Sci. 8 (2009) 1850–1860.