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Assessment of protective function of steel rust layers by N2 adsorption
Corrosion Science 49 (2007) 1468–1477 www.elsevier.com/locate/corsci
Assessment of protective function of steel rust layers by N2 adsorption Tatsuo Ishikawa a,*, Tomoyuki Yoshida a, Kazuhiko Kandori a, Takenori Nakayama b, Shuichi Hara c a
School of Chemistry, Osaka University of Education, 4-698-1 Asahigaoka, Kashiwara, Osaka 582-8582, Japan b Materials Research Laboratory, Kobe Steel, LTD., 5-5 Takatsukadai, 1-chome, Nishi-ku, Kobe, Hyogo 615-2271, Japan c Investigation & Research Div., Sumitomo Metal Technology Inc., 1-8 Fuso-cho, Amagasaki, Hyogo 660-0891, Japan Received 15 May 2006; accepted 8 August 2006
Abstract The specific surface area (SA) of the rusts formed by exposing various kinds of steels in different situations was determined by N2 adsorption. The SA values of the rusts increased with the increase of corrosion rate, implying that the rust layers with large SA exhibit a high resistance to corrosion. The suppression of rusting by compact rust layers was interpreted by the blockage of pores in rust layers by the adsorption and capillary condensation of water. The SA values clearly reflect the corrosion levels estimated by the external observation. It was convinced that the SA measurement is a universal quantitative technique to appraisal the protective function of rust layers. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: A. Steel; B. Alloy; B. TEM; B. XRD; C. Atmospheric corrosion; C. Rust
1. Introduction Steel rusts formed by atmospheric corrosion consists of fine particles of various iron oxyhydroxides and oxides, of which the composition depends on the exposure situation *
Corresponding author. Tel./fax: +81 729 78 3394. E-mail address: [email protected] (T. Ishikawa).
0010-938X/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2006.08.020
T. Ishikawa et al. / Corrosion Science 49 (2007) 1468–1477
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and kind of steel. The rust layer on steels functions as a protector against corrosion by resisting diffusion of ions and molecules. Weathering steels are alloyed with a small amount of metals such as Cu, Cr, and Ni to enhance the corrosion resistance of the steels. These metal ions are thought to make the rust layer more dense to improve the protective property against rusting. However, there has been no reliable method to estimate the compactness of rust layers. Since the rust layer is formed by assembly of fine rust particles and has porous structure, the compactness of rust layers is associated with the morphology of rust particles such as particle size and shape. The particle size of steel rusts is usually estimated by powder X-ray diffraction and Mo¨ssbauer spectroscopy. However, these methods do not always give geometrical particle size, because the primary rust particles are polycrystalline and contain amorphous parts. The morphology of fine particles is ordinarily observed by electron microscopy, but the rust particles always strongly agglomerate so that it is difficult to observe the morphology of primary particles. For this reason, we employed the adsorption of gas molecules for characterizing rust particles. The adsorption method is frequently used for surface characterization of porous materials and determination of specific surface area (SA) which is a measure of particle size of powder, because various gas or vapor molecules such as N2, H2O, and organic molecules used as a probe molecule can enter into micropores with a pore size larger than these molecules [1]. Masuko and Hisamatsu have reported that the SA of artificially synthesized steel rusts determined by N2 adsorption was increased by alloying with Cu [2]. In our previous studies the SA of the rusts formed by exposing different kinds of steels in various environments decreased with the increase of air-borne salinity and corrosion rate, showing that the rust particles grow with the progress of corrosion and the rust layer becomes less compact to exhibit a low protective property [3–5]. On the contrary, the SA of the rusts increases with increasing the amount of deposited SO2. This finding indicates that the presence of SO2 4 reduces the particle size of rusts and forms compact rust layers, different from the airborne salinity. Furthermore, we found that alloying with Ni reduced the particle size of the rusts formed in Cl environments and thought that the high corrosion resistance of Ni-alloying steel is ascribed to the formation of dense rust layers composed of fine particles [6]. The aim of the present study is to verify that the adsorption method is of universal application. The SA of the rusts of steels corroded to different degrees was compared to the corrosion levels classified by the external observation that is usually employed for assessing the atmospheric corrosion of steels. The rusts formed by exposing steels at different positions of bridge girder and by the cyclic corrosion test were examined. Based on the relation between SA and corrosion rate of the steels, the protective property of rust layer was discussed by the adsorption and capillary condensation of water into the pores of the rust layer. 2. Experimental 2.1. Rust samples Three series of rusts formed in different exposure situations were examined in the present study. All the rust samples were gathered by taking off from the test pieces and bridge girders by a cutter knife.
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The samples used for evaluation of corrosion level by the external observation were the rusts gathered from the bridge girders made of a conventional weathering steel (JIS SMA, 0.2Ni–0.3Cu–0.5Cr) of a express way built 7 years ago in a mountainous region of Hyogo Prefecture, Japan. The rusts were classified into five groups (Index 1–5) according the extent of corrosion estimated by the external observation, where the rust index decreases with the progress of corrosion [7]. To examine the influence of exposing position on corrosion, eight kinds of the rusts were gathered from the test pieces of mild steel (SM) set in different positions of girders of the Okinosuhimon Bridge located in the mouth of the Yoshino River in Tokushima Prefecture, Japan, which is close to the Inland Sea. The exposure period was 1 year. The rusts formed by the cyclic corrosion test (CCT) were also investigated. The CCT was carried out by a cycle of spraying 5 mass% NaCl solutions at 30 °C for 30 min, drying at a relative humidity (RH) of 50% and 50 °C for 6 h, wetting at RH = 98% and 30 °C for 1 h, and finally washing with water at 30 °C for 30 min. This cycle was done once a day for 60 and 120 days. The steel samples were SM, SMA, and two kinds of new weathering steels with a higher Ni content (N1, 1Ni–1Cu–0.05Ti; N2, 2.7Ni– 0.5Cu–0.04Ti). 2.2. N2 adsorption The adsorption isotherms of N2 on the rust samples were measured at the boiling temperature of liquid N2 using an automatic volumetric adsorption apparatus designed in our laboratory. Prior to the adsorption, the samples were degassed at 100 °C under 103 Torr for 2 h. The specific surface area (noted SA) was determined by fitting the BET equation to the adsorption isotherms using the cross sectional area of N2 molecule of 0.162 nm2. 3. Results 3.1. Rust index Fig. 1 displays the photographs of the steel surfaces rusted in different extent. The rusts belong to Index 1 crack and exfoliate and those of Index 2 form scale rusts. The surface of the Index 3 sample possesses small hollows of nest traces. The rusts of Indexes 4 and 5 show rather homogeneous smoothed surfaces. Although no colored photographs are shown here, the rust of Index 1 was heterogeneously colored by gray and yellowish brown while the rusts of Indexes 2 and 3 were blued. The color of the rusts of Indexes 4 and 5 was orange clearly different from the rusts of Indexes 1–3. The SA values of the rusts with Indexes 1–5 are compared in Fig. 2. The SA steadily increases with the increase of the rust index. This reveals that the particle size of rusts decreases with the increase of rust index and the rust layer built by fine rust particles exhibits a high resistance to corrosion. It should be noted that the SA of rusts well accords to the rust index. This fact definitively verifies that the SA measurement is applicable for reliable and quantitative estimation of protective nature of rusts.
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Fig. 1. Photographs of the rust appearence of weathering steels (SMA) classified in Indexes 1–5.
Fig. 2. SA of the steel rusts classified in different rust indexes. (A)–(H) are the numbers of photographs in Fig. 1.
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3.2. Exposure position Fig. 3 plots the SA of the rusts formed by setting the test pieces of the mild steel (SM) in different positions of the bridge girders against the corrosion rate. This fact indicates that the steel corrosion is strongly influenced by the exposure conditions such as wetting, sunlight, air-borne salinity, ventilation, and so forth. The SA roughly decreases with the increase of corrosion rate: the size of rust particles increases with the progress of corrosion. The same results have been obtained for the rusts formed by exposing different kinds of steels in various situations [4,6]. 3.3. CCT rusts Fig. 4 shows the SA values of the rusts formed by CCT of Ni-containing advanced weathering steels (N1 and N2), weathering steel (SMA), and mild steel (SM) as a function of the thickness loss of steels. The SA of all the steels decreases with the increase of thickness loss and is nearly constant at a large thickness loss. It should be noted that all the SA values fit a line regardless the kind of steels, meaning that the SA of rusts is affected not by the composition of steels but by the corrosion extent. The similar results have been obtained for the SA of the other steel rusts [4,6]. The SA of the CCT rusts is less than that of the rusts obtained by the atmospheric exposure of SM shown in Fig. 3, due to the acceleration of corrosion by CCT as is seen by comparing the corroded amounts of Figs. 3 and 4.
Fig. 3. Plots of SA against corrosion rate for the rusts formed by exposing mild steels (SM) at different positions of bridge girder.
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Fig. 4. SA of the rusts formed on different steels of N1, N2, SMA, and SM by CCT for 60 and 120 days. s; 60 days, d; 120 days.
4. Discussion The foregoing reveals that the SA well corresponds to the extent of corrosion. The SA of rusts decreases with the increase of corroded amount, demonstrating that the compact rust layer with a high SA is protective against corrosion. Now, we discuss in more detail the relation between SA and protective nature of rusts. Since the rust layer build by assembly of fine rust particles is porous, the capillary condensation of water in the pores takes place in the atmosphere. The pore size of rust layer is related to the particle size of rust which is estimated from the SA value. Now, assuming that the rust particles are spherical, the mean particle diameter (D) is represented as the following equation: D ¼ 6=ðSAÞd
ð1Þ
where d is the density of rust and regarded as 4 g/cm3. The radii of pores where the capillary condensation of water takes place can be related to the relative pressure of water by the Kelvin equation r ¼ 2cV m =½RT lnðP =P 0 Þ
ð2Þ
where r is the radius of pores, c is the surface tension of water, Vm is the molar volume of water, R is the gas constant, T is temperature, and P/P0 is the relative pressure of water or relative humidity (RH). Here, assuming that the rust particles are spherical and the pores in rust layer are formed by contact of three spherical rust particles, the radii (r) of the pores in rust layer can be related to the diameter of rust particles (D) by the equation
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of 2r/D = 0.15. This relation and T = 25 °C are substituted for Eqs. (1) and (2) to give the following equation showing the relation between SA and P/P0, SA ¼ 0:106 103 lnðP =P 0 Þ ½m2 =g:
ð3Þ
Eqs. (2) and (3) are shown in Fig. 5. From observing this figure, the rust layer with higher SA causes the capillary condensation of water at lower RH. For instance, in the rust layer with SA = 38 m2/g or r = 3 nm the capillary condensation occurs at RH P 70% (P/P0 P 0.7). On the other hand, at a low RH of 30% (P/P0 = 0.3) the pores in the rust layers with SA P 128 m2/g or r 6 1.8 nm are filled with water by the capillary condensation. The filling of the micropores with r 6 1 nm by water is considered to be caused not by the capillary condensation but by the strong adsorption of H2O molecules owing to overlapping of the attractive potential from the pore walls close to each other [8]. Since the H2O molecules under the strong potential in the micropores must exhibit a clustering structure different from the bulk water [9], the diffusion of ions through the micropores filled with water is rather restricted. Therefore, the rust layer with higher SA or smaller pore size is more protective against corrosion by interfering with the diffusion of ions and molecules. To confirm the effect of the pore filling with water in other rusts, the results published in our previous papers are shown in Figs. 6 and 7 which are the plots of SA against the corrosion rate for the rusts formed by exposing various kinds of steels in different situations [4,6]. The SA values of all the rusts decrease with the increase of corrosion rate and then become almost invariable, quite similar to the CCT result in Fig. 3. It is noteworthy that the resemble relation between SA and corrosion rate is obtained independently of the kind of steels and the rusting conditions. The constant values of SA at
Fig. 5. Plots of SA and pore radius (r) against P/P0 according to Eqs. (2) and (3).
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Fig. 6. Plots of SA against corrosion rate for the rusts formed by exposing various kinds of steels at different bridges in Japan for 17 years. The details of the steels and exposure environments were shown elsewhere [4]. s, mild steel (SM); n, weathering steel B; h, weathering steel C; d, weathering steel D; m, weathering steel E.
high corrosion range are 40–50 m2/g for the atmospheric exposure rusts (Figs. 6 and 7) and about 20 m2/g for the CCT rusts (Fig. 3). These constant SA values suggest the existence of a minimum SA or a maximum pore size of rust layers showing the corrosion resistance, which is related to the pore filling by capillary condensation at the exposure RH. Since the pores larger than the maximum size are not filled by water at the exposure RH, they are accessible to molecules such as O2 and SO2 and the air-bone salinity, resulted in losing the protective character of the rust layer. To put it concretely using a schema shown in Fig.8, the rust layers with the minimum SA of 50 m2/g cause the capillary condensation at RH = 62% according to the Eq. (3). Therefore, the pores in the rust layer with SA = 50 m2/g filled by wetting with dew or rain are opened by drying at RH 6 62% (P/P0 6 0.62), that is, a repeat of opening and closing of large pores by drying and wetting facilitates the progress of rusting. Furthermore, various ions are able to transfer through the water phase in the large pores as well as the bulk water, because the nature of water in large pores is close to that of the bulk water. On the contrary, the rust layer with small pores or large SA is anytime filled with water by the adsorption and capillary condensation even at low RH and prevents the transfer of ions and molecules into the pores, leading to a high protective nature to the atmospheric corrosion of steels. 5. Conclusions (1) The compact rust layers with high SA or small particle size exhibit a high resistance to corrosion.
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Fig. 7. Plots of SA against corrosion rate for the rusts on binary phase alloys of Fe–Ni (s), Fe–Cr (n), and Fe– Cu (h) exposed in Miyakojima, Okinawa Prefecture, Japan for 2 years. The details of the alloys and exposure condition were shown in the previous paper [6].
Fig. 8. Schematic representation of pore filling by repeating drying and wetting.
(2) The corrosion resistance of rust layers is caused by pore filling by the adsorption and capillary condensation of water. (3) The SA measurement is a qualitative method to evaluate the protective function of rust layers. References [1] F. Rouquerol, J. Rouquerol, K. Sing, Adsorption by Powders & Porous Solids, Academic Press, London, 1999. [2] N. Masuko, N. Hisamatsu, Corros. Eng. (Jpn.) 17 (1969) 539–542. [3] M. Yamashita, K. Asami, T. Ishikawa, T. Ohtsuka, H. Tamura, T. Misawa, Zairyo-to-Kankyo 50 (2001) 521– 530.
T. Ishikawa et al. / Corrosion Science 49 (2007) 1468–1477 [4] [5] [6] [7]
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T. Ishikawa, M. Kumagai, A. Yasukawa, K. Kandori, Corrosion (NACE) 57 (2001) 346–352. T. Ishikawa, Zairyo-to-Kankyo 54 (2005) 361–367. T. Ishikawa, A. Maeda, K. Kandori, T. Tahara, Corrosion (NACE) 62 (2006) 559–567. H. Kihira, K. Shiotani, H. Miyuki, T. Nakayama, M. Takemura, Y. Watanabe, Doboku Gakkai Ronbun-shu (J. Jpn. Soc. Civil Eng.) No. 745/I-65 10 (2003) 77–87. [8] D.H. Everett, J.C. Powl, J. Chem. Soc., Faraday Trans. 172 (1976) 619–636. [9] T. Iiyama, K. Nishikawa, T. Suzuki, K. Kaneko, Chem. Phys. Lett. 274 (1997) 152–154.
Assessment of protective function of steel rust layers by N2 adsorption Tatsuo Ishikawa a,*, Tomoyuki Yoshida a, Kazuhiko Kandori a, Takenori Nakayama b, Shuichi Hara c a
School of Chemistry, Osaka University of Education, 4-698-1 Asahigaoka, Kashiwara, Osaka 582-8582, Japan b Materials Research Laboratory, Kobe Steel, LTD., 5-5 Takatsukadai, 1-chome, Nishi-ku, Kobe, Hyogo 615-2271, Japan c Investigation & Research Div., Sumitomo Metal Technology Inc., 1-8 Fuso-cho, Amagasaki, Hyogo 660-0891, Japan Received 15 May 2006; accepted 8 August 2006
Abstract The specific surface area (SA) of the rusts formed by exposing various kinds of steels in different situations was determined by N2 adsorption. The SA values of the rusts increased with the increase of corrosion rate, implying that the rust layers with large SA exhibit a high resistance to corrosion. The suppression of rusting by compact rust layers was interpreted by the blockage of pores in rust layers by the adsorption and capillary condensation of water. The SA values clearly reflect the corrosion levels estimated by the external observation. It was convinced that the SA measurement is a universal quantitative technique to appraisal the protective function of rust layers. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: A. Steel; B. Alloy; B. TEM; B. XRD; C. Atmospheric corrosion; C. Rust
1. Introduction Steel rusts formed by atmospheric corrosion consists of fine particles of various iron oxyhydroxides and oxides, of which the composition depends on the exposure situation *
Corresponding author. Tel./fax: +81 729 78 3394. E-mail address: [email protected] (T. Ishikawa).
0010-938X/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2006.08.020
T. Ishikawa et al. / Corrosion Science 49 (2007) 1468–1477
1469
and kind of steel. The rust layer on steels functions as a protector against corrosion by resisting diffusion of ions and molecules. Weathering steels are alloyed with a small amount of metals such as Cu, Cr, and Ni to enhance the corrosion resistance of the steels. These metal ions are thought to make the rust layer more dense to improve the protective property against rusting. However, there has been no reliable method to estimate the compactness of rust layers. Since the rust layer is formed by assembly of fine rust particles and has porous structure, the compactness of rust layers is associated with the morphology of rust particles such as particle size and shape. The particle size of steel rusts is usually estimated by powder X-ray diffraction and Mo¨ssbauer spectroscopy. However, these methods do not always give geometrical particle size, because the primary rust particles are polycrystalline and contain amorphous parts. The morphology of fine particles is ordinarily observed by electron microscopy, but the rust particles always strongly agglomerate so that it is difficult to observe the morphology of primary particles. For this reason, we employed the adsorption of gas molecules for characterizing rust particles. The adsorption method is frequently used for surface characterization of porous materials and determination of specific surface area (SA) which is a measure of particle size of powder, because various gas or vapor molecules such as N2, H2O, and organic molecules used as a probe molecule can enter into micropores with a pore size larger than these molecules [1]. Masuko and Hisamatsu have reported that the SA of artificially synthesized steel rusts determined by N2 adsorption was increased by alloying with Cu [2]. In our previous studies the SA of the rusts formed by exposing different kinds of steels in various environments decreased with the increase of air-borne salinity and corrosion rate, showing that the rust particles grow with the progress of corrosion and the rust layer becomes less compact to exhibit a low protective property [3–5]. On the contrary, the SA of the rusts increases with increasing the amount of deposited SO2. This finding indicates that the presence of SO2 4 reduces the particle size of rusts and forms compact rust layers, different from the airborne salinity. Furthermore, we found that alloying with Ni reduced the particle size of the rusts formed in Cl environments and thought that the high corrosion resistance of Ni-alloying steel is ascribed to the formation of dense rust layers composed of fine particles [6]. The aim of the present study is to verify that the adsorption method is of universal application. The SA of the rusts of steels corroded to different degrees was compared to the corrosion levels classified by the external observation that is usually employed for assessing the atmospheric corrosion of steels. The rusts formed by exposing steels at different positions of bridge girder and by the cyclic corrosion test were examined. Based on the relation between SA and corrosion rate of the steels, the protective property of rust layer was discussed by the adsorption and capillary condensation of water into the pores of the rust layer. 2. Experimental 2.1. Rust samples Three series of rusts formed in different exposure situations were examined in the present study. All the rust samples were gathered by taking off from the test pieces and bridge girders by a cutter knife.
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The samples used for evaluation of corrosion level by the external observation were the rusts gathered from the bridge girders made of a conventional weathering steel (JIS SMA, 0.2Ni–0.3Cu–0.5Cr) of a express way built 7 years ago in a mountainous region of Hyogo Prefecture, Japan. The rusts were classified into five groups (Index 1–5) according the extent of corrosion estimated by the external observation, where the rust index decreases with the progress of corrosion [7]. To examine the influence of exposing position on corrosion, eight kinds of the rusts were gathered from the test pieces of mild steel (SM) set in different positions of girders of the Okinosuhimon Bridge located in the mouth of the Yoshino River in Tokushima Prefecture, Japan, which is close to the Inland Sea. The exposure period was 1 year. The rusts formed by the cyclic corrosion test (CCT) were also investigated. The CCT was carried out by a cycle of spraying 5 mass% NaCl solutions at 30 °C for 30 min, drying at a relative humidity (RH) of 50% and 50 °C for 6 h, wetting at RH = 98% and 30 °C for 1 h, and finally washing with water at 30 °C for 30 min. This cycle was done once a day for 60 and 120 days. The steel samples were SM, SMA, and two kinds of new weathering steels with a higher Ni content (N1, 1Ni–1Cu–0.05Ti; N2, 2.7Ni– 0.5Cu–0.04Ti). 2.2. N2 adsorption The adsorption isotherms of N2 on the rust samples were measured at the boiling temperature of liquid N2 using an automatic volumetric adsorption apparatus designed in our laboratory. Prior to the adsorption, the samples were degassed at 100 °C under 103 Torr for 2 h. The specific surface area (noted SA) was determined by fitting the BET equation to the adsorption isotherms using the cross sectional area of N2 molecule of 0.162 nm2. 3. Results 3.1. Rust index Fig. 1 displays the photographs of the steel surfaces rusted in different extent. The rusts belong to Index 1 crack and exfoliate and those of Index 2 form scale rusts. The surface of the Index 3 sample possesses small hollows of nest traces. The rusts of Indexes 4 and 5 show rather homogeneous smoothed surfaces. Although no colored photographs are shown here, the rust of Index 1 was heterogeneously colored by gray and yellowish brown while the rusts of Indexes 2 and 3 were blued. The color of the rusts of Indexes 4 and 5 was orange clearly different from the rusts of Indexes 1–3. The SA values of the rusts with Indexes 1–5 are compared in Fig. 2. The SA steadily increases with the increase of the rust index. This reveals that the particle size of rusts decreases with the increase of rust index and the rust layer built by fine rust particles exhibits a high resistance to corrosion. It should be noted that the SA of rusts well accords to the rust index. This fact definitively verifies that the SA measurement is applicable for reliable and quantitative estimation of protective nature of rusts.
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Fig. 1. Photographs of the rust appearence of weathering steels (SMA) classified in Indexes 1–5.
Fig. 2. SA of the steel rusts classified in different rust indexes. (A)–(H) are the numbers of photographs in Fig. 1.
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3.2. Exposure position Fig. 3 plots the SA of the rusts formed by setting the test pieces of the mild steel (SM) in different positions of the bridge girders against the corrosion rate. This fact indicates that the steel corrosion is strongly influenced by the exposure conditions such as wetting, sunlight, air-borne salinity, ventilation, and so forth. The SA roughly decreases with the increase of corrosion rate: the size of rust particles increases with the progress of corrosion. The same results have been obtained for the rusts formed by exposing different kinds of steels in various situations [4,6]. 3.3. CCT rusts Fig. 4 shows the SA values of the rusts formed by CCT of Ni-containing advanced weathering steels (N1 and N2), weathering steel (SMA), and mild steel (SM) as a function of the thickness loss of steels. The SA of all the steels decreases with the increase of thickness loss and is nearly constant at a large thickness loss. It should be noted that all the SA values fit a line regardless the kind of steels, meaning that the SA of rusts is affected not by the composition of steels but by the corrosion extent. The similar results have been obtained for the SA of the other steel rusts [4,6]. The SA of the CCT rusts is less than that of the rusts obtained by the atmospheric exposure of SM shown in Fig. 3, due to the acceleration of corrosion by CCT as is seen by comparing the corroded amounts of Figs. 3 and 4.
Fig. 3. Plots of SA against corrosion rate for the rusts formed by exposing mild steels (SM) at different positions of bridge girder.
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Fig. 4. SA of the rusts formed on different steels of N1, N2, SMA, and SM by CCT for 60 and 120 days. s; 60 days, d; 120 days.
4. Discussion The foregoing reveals that the SA well corresponds to the extent of corrosion. The SA of rusts decreases with the increase of corroded amount, demonstrating that the compact rust layer with a high SA is protective against corrosion. Now, we discuss in more detail the relation between SA and protective nature of rusts. Since the rust layer build by assembly of fine rust particles is porous, the capillary condensation of water in the pores takes place in the atmosphere. The pore size of rust layer is related to the particle size of rust which is estimated from the SA value. Now, assuming that the rust particles are spherical, the mean particle diameter (D) is represented as the following equation: D ¼ 6=ðSAÞd
ð1Þ
where d is the density of rust and regarded as 4 g/cm3. The radii of pores where the capillary condensation of water takes place can be related to the relative pressure of water by the Kelvin equation r ¼ 2cV m =½RT lnðP =P 0 Þ
ð2Þ
where r is the radius of pores, c is the surface tension of water, Vm is the molar volume of water, R is the gas constant, T is temperature, and P/P0 is the relative pressure of water or relative humidity (RH). Here, assuming that the rust particles are spherical and the pores in rust layer are formed by contact of three spherical rust particles, the radii (r) of the pores in rust layer can be related to the diameter of rust particles (D) by the equation
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of 2r/D = 0.15. This relation and T = 25 °C are substituted for Eqs. (1) and (2) to give the following equation showing the relation between SA and P/P0, SA ¼ 0:106 103 lnðP =P 0 Þ ½m2 =g:
ð3Þ
Eqs. (2) and (3) are shown in Fig. 5. From observing this figure, the rust layer with higher SA causes the capillary condensation of water at lower RH. For instance, in the rust layer with SA = 38 m2/g or r = 3 nm the capillary condensation occurs at RH P 70% (P/P0 P 0.7). On the other hand, at a low RH of 30% (P/P0 = 0.3) the pores in the rust layers with SA P 128 m2/g or r 6 1.8 nm are filled with water by the capillary condensation. The filling of the micropores with r 6 1 nm by water is considered to be caused not by the capillary condensation but by the strong adsorption of H2O molecules owing to overlapping of the attractive potential from the pore walls close to each other [8]. Since the H2O molecules under the strong potential in the micropores must exhibit a clustering structure different from the bulk water [9], the diffusion of ions through the micropores filled with water is rather restricted. Therefore, the rust layer with higher SA or smaller pore size is more protective against corrosion by interfering with the diffusion of ions and molecules. To confirm the effect of the pore filling with water in other rusts, the results published in our previous papers are shown in Figs. 6 and 7 which are the plots of SA against the corrosion rate for the rusts formed by exposing various kinds of steels in different situations [4,6]. The SA values of all the rusts decrease with the increase of corrosion rate and then become almost invariable, quite similar to the CCT result in Fig. 3. It is noteworthy that the resemble relation between SA and corrosion rate is obtained independently of the kind of steels and the rusting conditions. The constant values of SA at
Fig. 5. Plots of SA and pore radius (r) against P/P0 according to Eqs. (2) and (3).
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Fig. 6. Plots of SA against corrosion rate for the rusts formed by exposing various kinds of steels at different bridges in Japan for 17 years. The details of the steels and exposure environments were shown elsewhere [4]. s, mild steel (SM); n, weathering steel B; h, weathering steel C; d, weathering steel D; m, weathering steel E.
high corrosion range are 40–50 m2/g for the atmospheric exposure rusts (Figs. 6 and 7) and about 20 m2/g for the CCT rusts (Fig. 3). These constant SA values suggest the existence of a minimum SA or a maximum pore size of rust layers showing the corrosion resistance, which is related to the pore filling by capillary condensation at the exposure RH. Since the pores larger than the maximum size are not filled by water at the exposure RH, they are accessible to molecules such as O2 and SO2 and the air-bone salinity, resulted in losing the protective character of the rust layer. To put it concretely using a schema shown in Fig.8, the rust layers with the minimum SA of 50 m2/g cause the capillary condensation at RH = 62% according to the Eq. (3). Therefore, the pores in the rust layer with SA = 50 m2/g filled by wetting with dew or rain are opened by drying at RH 6 62% (P/P0 6 0.62), that is, a repeat of opening and closing of large pores by drying and wetting facilitates the progress of rusting. Furthermore, various ions are able to transfer through the water phase in the large pores as well as the bulk water, because the nature of water in large pores is close to that of the bulk water. On the contrary, the rust layer with small pores or large SA is anytime filled with water by the adsorption and capillary condensation even at low RH and prevents the transfer of ions and molecules into the pores, leading to a high protective nature to the atmospheric corrosion of steels. 5. Conclusions (1) The compact rust layers with high SA or small particle size exhibit a high resistance to corrosion.
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Fig. 7. Plots of SA against corrosion rate for the rusts on binary phase alloys of Fe–Ni (s), Fe–Cr (n), and Fe– Cu (h) exposed in Miyakojima, Okinawa Prefecture, Japan for 2 years. The details of the alloys and exposure condition were shown in the previous paper [6].
Fig. 8. Schematic representation of pore filling by repeating drying and wetting.
(2) The corrosion resistance of rust layers is caused by pore filling by the adsorption and capillary condensation of water. (3) The SA measurement is a qualitative method to evaluate the protective function of rust layers. References [1] F. Rouquerol, J. Rouquerol, K. Sing, Adsorption by Powders & Porous Solids, Academic Press, London, 1999. [2] N. Masuko, N. Hisamatsu, Corros. Eng. (Jpn.) 17 (1969) 539–542. [3] M. Yamashita, K. Asami, T. Ishikawa, T. Ohtsuka, H. Tamura, T. Misawa, Zairyo-to-Kankyo 50 (2001) 521– 530.
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