Composition and protective ability of rust layer formed on weathering steel exposed to various environments

Corrosion Science 48 (2006) 2799–2812 www.elsevier.com/locate/corsci

Composition and protective ability of rust layer formed on weathering steel exposed to various environments T. Kamimura

a,¤

, S. Hara b, H. Miyuki a, M. Yamashita c, H. Uchida c

a

Corporate Research and Development Laboratories, Sumitomo Metal Industries, Ltd., 1-8 Fuso-cho Amagasaki, Hyogo 660-0891, Japan b Investigation and Research Division, Sumitomo Metal Technology Inc., 1-8 Fuso-cho Amagasaki, Hyogo 660-0891, Japan c Department of Mechanical and System Engineering, Graduate School of Engineering, University of Hyogo, 2167 Shosha, Himeji, Hyogo 671-2201, Japan Received 18 May 2005; accepted 10 October 2005 Available online 29 November 2005

Abstract The compositional change of rust (corrosion products) layer formed on weathering steel exposed to atmosphere with diVerent amount of air-borne sea salt particles in Japan have been investigated by the X-ray diVraction method. The mass ratio (/) of crystalline -FeOOH to -FeOOH, in the rust layer formed on the weathering steel exposed in an industrial environment, increases with an increase in exposure duration. The / is closely related to the corrosion rate in environments when the amount of air-borne salt is less than 0.2 mg NaCl/dm2/day (2.31 £ 10¡7 g NaCl/m2/s). However this is not the case in seaside environments with a higher amount of air-borne salts. The mass ratio (/¤) of crystalline -FeOOH to the total mass of -FeOOH, -FeOOH and Fe3O4, in the rust layer formed on the weathering steel is related to the corrosion rate even in seaside environments certainly more than 0.2 mg/dm2/day (2.31 £ 10¡7 g/m2/s) of air-borne salt particles. When the /¤ is more than 1, a higher corrosion rate more than 0.01 mm/year (3.17 £ 10¡13 m/s) is not observed. The /¤ is a protective ability index of rust formed on weathering steel. © 2005 Elsevier Ltd. All rights reserved.

*

Corresponding author. Tel.: +81 6 6489 5754; fax: +81 6 6489 5757. E-mail address: [email protected] (T. Kamimura).

0010-938X/$ - see front matter © 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2005.10.004

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Keywords: Low alloy steel; Weathering steel; X-ray diVraction; Atmospheric corrosion; Rust

1. Introduction Recently, weathering steel containing small amounts of Cr, Cu, Ni and P has been widely noticed from the viewpoint of a reduction in maintenance cost of steel structures. The weathering steel forms the protective rust (corrosion products) layer, which is dense and adherent to steel, during a long-term exposure to the atmosphere. Therefore this protective rust layer reduces corrosion rate of steel. Because of this beneWcial property, the weathering steel has been applied for so many steel structures on land such as bridges. The rust layer formed on the weathering steel by atmospheric corrosion has been investigated [1–10], and it is well known that the rust is composed of mainly ferric oxyhydroxides (FeOOH) and an X-ray amorphous substance, which gives no clear peak by the X-ray diVraction technique. It is reported that diVerent structures of oxyhydroxides such as -FeOOH, -FeOOH and -FeOOH are formed, and in some cases spinel type of iron oxides such as magnetite (Fe3O4) are contained in the rust layer depending on environmental conditions; the exposure positions (vertical or horizontal), gases contained in polluted atmosphere such as SO2 gas, and air-borne salt particles originated from sea salt or de-icing salt. Recently, Mössbauer spectroscopic investigation of the rust layer formed on the weathering steel exposed at an industrial or a rural region for a long-term has been performed and the following results have been reported. The X-ray amorphous substance in the rust layer formed on the weathering steel exposed at a rural or an industrial region is mainly composed of ultra-Wne -FeOOH having the small particle size [11–16], and the diVerence in the inner rust layer between the mild steel and the weathering steel is its particle size distribution; -FeOOH particle size in the rust on weathering steel has a continuous distribution, while mild steel does not [15]. The X-ray amorphous substance in the rust layer formed on the mild steel exposed at an industrial environment comprises of ultra-Wne -FeOOH and -Fe2O3(Fe3¡O4)[17]. The alloying Cu and Cr are enriched in the rust layer, especially in the inner rust layer, and it has been pointed out that this enrichment results in the high protective ability of rust layer, which is dense and adherent to the steel surface [1,5,13]. From the above-mentioned facts, the correlation between the composition of rust layer and corrosion rate is of great importance for the evaluation methodology for the protective rust layer in the case of the maintenance and administration of the steel structures such as bridges, which have been used for many years. When the anomalous and micaceous rust is observed on the weathering steel, it is quite easy to judge whether or not the protective rust layer is formed and repairs are needed. However it is diYcult to judge whether the protective rust layer is formed or not when the rust layer is not classiWed as the above irregular and/or micaceous rust layer. The methodology for the evaluation of the protective ability of rust layer, thus, is strongly required. Our group has proposed the so-called protective ability index (PAI) focusing on the compositional change of rust formed on weathering steel. Yamashita et al. [3] performed the quantitative analysis of the composition of the rust t layers formed on the weathering steel exposed at an industrial environment by means of X-ray diVraction method, and pointed out that the mass ratio “/” ([mass fraction of -FeOOH]/[mass fraction of -FeOOH]) of crystalline -FeOOH to -FeOOH, in the rust

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formed increases with an increase in exposure duration. When the / is more than a cetain value, the higher corrosion rate is not observed and therefore this / can be an index for the evaluation of the protectiveness of the rust layer formed on the weathering steel [18]. Moreover, the mass ratio “/¤” [where ¤ D (mass fraction of -FeOOH) + (mass fraction of -FeOOH) + (mass fraction of Fe3O4)] of crystalline -FeOOH to the sum of -FeOOH, -FeOOH and Fe3O4 was proposed in the case of seaside environments with high amount of air-borne salts [19,20]. -FeOOH has a hollandite structure, and Cl is necessary for forming this tunnel structure by locating at the tunnel. -FeOOH is therefore the representative rust composition observed in the rust layer formed on the steel exposed in an environment containing much chloride. When the above mass ratio is calculated, the spinel type iron oxides observed by X-ray diVraction technique was dealt with as magnetite (Fe3O4). Since Fe3O4 and -Fe2O3 have a similar structure, which cannot be easily distinguished only by X-ray diVraction technique [17,21], these spinel-type iron oxides both Fe3O4 and -Fe2O3 were expressed as Fe3O4 in this study. Quite recently, Dillmann et al. [22] focused on the relationship between the /, which was analyzed by XRD, and protectiveness of the rust, and reported the compositional change of rust formed on steel exposed for a distinctly long period of over 1600 years (5 £ 1010 s) to predict the corrosion behavior of the nuclear waste containers. They veriWed that the / increased with the age of the samples. In this study, the correlation between the mass ratio of the crystalline rust and corrosion rate has been investigated for rust layer formed on the weathering steel, which had been exposed at various places, with diVerent amounts of salt particles, and for various exposure durations. Finally, we veriWed the relationship between the corrosion rate and the PAI /¤. 2. Experimental 2.1. Samples and exposure test The chemical composition of steel used for the exposure test is listed in Table 1. Sample steel plates (60 £ 100 £ 4.0 mm3) were exposed at an angle of 30° facing south in ten sites with diVerent amounts of air-borne sea salt particles for 20 years. Fig. 1 shows the map of exposure locations under study. They were Kitahiroshima-city in Hokkaido pref. classiWed as a rural condition (amount of air-borne salt particles: 0.01 mdd (mg/dm2/day [0.12 £ 10¡7 g/m2/s])), Amagasaki-city in Hyogo pref. as an industrial region (0.01 mdd [0.12 £ 10¡7 g/m2/s]), Choshi-city in Chiba pref., Wakayama-city in Wakayama pref., Kitakyusyu-city in Fukuoka pref. and Kashima-city in Ibaraki pref. as a seashore region (0.2 mdd [2.31 £ 10¡7 g/m2/s]), and Joetsu-city in Niigata pref., Naha-city, Itoman-city, the inland at Miyako island in Okinawa pref. as a severe seashore region (>0.5 mdd [5.79 £ 10¡7 g/m2/s]). The measurement for the amount of air-bone salt particles was performed by the dry gauze method according to JIS Z 2382 [23]. The Japanese Archipelago ranges a considerable distance from north to south, and therefore there are varied climatic zones throughout the archipelago: the subarctic zone (Hokkaido/annual mean temperature: approximately 6–8 °C), the temperate zone, which occupies the largest area of the Table 1 Chemical compositions of weathering steel tested (mass%)

Weathering steel (JIS SMA 490AW)

C

Si

Mn

P

S

Cu

Ni

Cr

0.11

0.24

0.75

0.014

0.005

0.33

0.12

0.49

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Fig. 1. Map of exposure locations under study.

archipelago, and the subtropical zone (Okinawa/annual mean temperature: approximately 22–24 °C). The samples after the exposure test were covered with relatively adherent rust layer, which were not easily delaminated and their thicknesses were less than 200 m. The rust layer was scraped oV from the steel surface with keeping the steel substrate intact by a cutter, and residual rust on steel was removed by immersion in a solution of 10% diammonium hydrogen citrate +0.3% inhibitor (Asahi Kagaku Kougyou: IBIT No. 30AR). The weight loss was measured after the exposure, and then converted to thickness loss (corrosion loss). 2.2. Quantitative analysis of composition for the powdered rust by X-ray diVraction method The skyward face of the rust layer formed on the steel was scraped oV from the steel surface by a cutter. In this case the steel substrate was also partly scrapped oV to contain

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the rust in the vicinity of the interface between the rust layer and the steel substrate, which was diVerent from the above procedure measuring corrosion loss. The scraped rust was ground into powder. After drying the powdered rust in desiccators for more than 1 week, the powdered samples were characterized by means of X-ray diVraction technique (XRD). The measurements were carried out by using a Rigaku Model RU200 diVractometer with a Co target under the condition of 30 kV-100 mA. The quantitative determination of the rust constituents comprising crystalline -FeOOH, -FeOOH, -FeOOH and Fe3O4 was carried out by measuring and analyzing the diVraction intensities, and the quantitative determination of -Fe in the rust, which had been mixed while scraping the rust layer, was also carried out. The amount of the X-ray amorphous substance was calculated by subtracting the total amount of the above crystalline substances from the total amount of the powdered rust sample. The standard samples are -FeOOH and -FeOOH manufactured by Rare Metallic Co, Ltd, magnetite manufactured by Kojundo Chemical Laboratory, Co., Ltd. and -FeOOH, which was synthesized by the dehydration of 0.1 M FeCl3 at 100 °C [24,25]. The diVraction intensities of (0 1 1) reXection of -FeOOH, (1 1 0) reXection of -FeOOH, (0 2 0) reXection of -FeOOH, (2 2 0) reXection of Fe3O4 and (1 1 0) reXection of -Fe were measured and referred to (1 0 0) reXection of ZnO powder (Wako Pure Chemical Industries, Ltd.: particle size »5 m), which had been well mixed with rust samples as an internal standard [26]. The ZnO were mixed with the same ratio of 30% to the corrosion products. The ratio of -, -, -FeOOH, Fe3O4 and -Fe to ZnO peak intensities were 0.191, 0.307, 0.179, 0.287 and 0.887, respectively. The quantitative analysis by XRD must be paid attention to, because the calibration curve for the quantitative analysis of rust depends on the properties such as crystalline and particle size of the standard materials. Moreover, there is a case that the corrosion products containing large amounts of chloride salt have moisture-absorption characteristics, and therefore, if the water-soluble material such as KCl is used as an internal standard, this leads to wrong results. 3. Results 3.1. Exposure duration dependence of corrosion loss and crystalline rust composition Fig. 2 shows a typical corrosion loss as a function of the exposure duration at sites containing diVerent amounts of air-borne salt particles. The increase in corrosion loss is suppressed as the exposure proceeds in the industrial region (amount of air-bone salt: 0.01 mdd [0.12 £ 10¡7 g/m2/s]). This can be caused by a formation of the protective rust layer formed on weathering steel. The higher corrosion loss in a seashore region (0.2 mdd [2.31 £ 10¡7 g/m2/s]) was observed compared to the corrosion loss in an industrial region. The corrosion loss with high amounts of air-borne salt particles of more than 0.5 mdd (5.79 £ 10¡7 g/m2/s) was twice as much as that in an industrial region (0.01 mdd [0.12 £ 10¡7 g/m2/s]). The corrosion loss (>0.5 mdd [5.79 £ 10¡7 g/m2/s]) increased linearly with the exposure duration, whereas the increase tendency of the corrosion loss (0.01 mdd [0.12 £ 10¡7 g/m2/s]) decreased after several years of exposure. Therefore, it should be therefore concluded that the corrosion resistance of the weathering steel is greatly aVected by the amount of air-borne salt particles. Fig. 3 shows the typical X-ray diVraction patterns of rust formed on the weathering steels exposed in an industrial region and a seashore region containing more than 0.5 mdd (5.79 £ 10¡7 g/m2/s) of air-borne salt particles. In the case of an exposure site of 0.01 mdd

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x 108 (s) 0

1

2

3

4

5

6

7

Corrosion loss (mm)

>0.5 >0.5mdd mdd 0.2 0.2mdd m

0.15

0.01 0.01mdd m

0.10

0.05

0.00 0

5 10 15 Exposure duration (years)

20

Fig. 2. Typical change in corrosion loss as a function of exposure duration for weathering steel exposed at places with diVerent amounts of air-borne salts (0.01 mdd [0.12 £ 10¡7 g/m2/s]: Amagasaki-city, Hyogo pref., 0.2 mdd [2.31 £ 10¡7 g/m2/s]: Wakayama-city, Wakayama pref., >0.5 mdd [5.79 £ 10¡7 g/m2/s]: Itoman-city, Okinawa pref.).

(0.12 £ 10¡7 g/m2/s), the peaks of -FeOOH and -FeOOH were clearly observed as crystalline rust constituents. The peak intensity of -FeOOH increased with an increase in the exposure duration, and the peak intensity of -FeOOH, on the other hand, decreased. The rust layer formed on the weathering steel exposed in a seashore region (>0.5 mdd [5.79 £ 10¡7 g/m2/s]) was composed of -FeOOH and Fe3O4 as well, which however was not observed for the rust layer formed on the weathering steel exposed in an industrial region, with the exception of -FeOOH and -FeOOH. 3.2. Correlation between mass ratio of crystalline rust and corrosion rate The correlation of the / and the exposure duration are given in Fig. 4. The X-ray amorphous substance was contained 50–60 mass% in the rust formed on the weathering steels independent of the exposure conditions. Only crystalline -FeOOH and -FeOOH were obtained by XRD in the rust formed on the weathering steel exposed in an industrial region (0.01 mdd [0.12 £ 10¡7 g/m2/s]), as mentioned above. The / has a tendency to increase with an increase of the exposure duration in an industrial region, which agrees well with the data reported by Yamashita et al. [2,3], as shown in Fig. 4(a). On the other hand, the increase tendency of the / with an increase of exposure duration was likely observed in an environment with high amounts of air-borne salt particles (0.2 [2.31 £ 10¡7], >0.5 mdd [5.79 £ 10¡7 g/m2/s]), but the correlation between them is quite low, as shown in Fig. 4(b), compared to the results at the industrial region. Especially, the / after more than 10 year exposure is widely distributed with the exposure duration. In the case of the

T. Kamimura et al. / Corrosion Science 48 (2006) 2799–2812

Intensity (aribitrary unit)

ZnO (002) ZnO (101)

(220)

ZnO (100)

β-FeOOH Fe3O4 (011)

(020)

(110)

α-FeOOH γ-FeOOH

2805

(a)

(b)

(c)

(d)

(e)

10˚

20˚

30˚ 2θ

40˚

50˚

Fig. 3. X-ray diVraction patterns of rust formed on weathering steels. (a) 1-year exposure, (b) 6-year exposure, (c) 15-year exposure in an industrial environment (0.01 mdd [0.12 £ 10¡7 g/m2/s]: Amagasaki-city, Hyogo pref.), (d) 15-year exposure in a seaside environment (>0.5 mdd [5.79 £ 10¡7 g/m2/s], Itoman-city, Okinawa pref.) and (e) 3year exposure in a seaside environment (>0.5 mdd [5.79 £ 10¡7 g/m2/s]: Miyako island, Okinawa pref.).

exposure site of more than 0.5 mdd, there was no correlation between the / and the exposure duration. Fig. 5 shows the relationship between the corrosion rate and the / in an industrial region (0.01 mdd [0.12 £ 10¡7 g/m2/s]). The corrosion rate was calculated from the slope of the corrosion loss curve vs. the exposure duration as was shown in Fig. 2. The correlation between the corrosion rate and the / was clearly observed in an industrial region, and the corrosion rate became less than 0.01 mm/year (3.17 £ 10¡13 m/s) when the / becomes more than approximately 1. Fig. 6 shows the relationship between the corrosion rate and the / in a seashore region of 0.2 (2.31 £ 10¡7) and more than (0.5 mdd [5.79 £ 10¡7 g/m2/s]). When the / is more than 1, the high corrosion rate was observed in some cases in the seashore region. It should be noted that the / is not always related to the corrosion rate when large amounts of air-borne salt particles exists. The rust samples, which did not show the correlation between the / and the corrosion rate, contained a considerable amount of -FeOOH and Fe3O4. For example, the rust samples formed on the weathering steel exposed at Miyako-island (>0.5 mdd [5.79 £ 10¡7 g/m2/s]) and Joetsu-city in Niigata (>0.5 mdd [5.79 £ 10¡7 g/m2/s]) contained approximately 20% of -FeOOH, and approximately 10% of Fe3O4, respectively. For the rust layer formed on the weathering steels exposed in environments with high amounts of air-borne salt particles (0.2 [2.31 £ 10¡7], >0.5 mdd [5.79 £ 10¡7 g/m2/s]),

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4

0

2

a

4

8

Amount of air-borne salt

0.01 0.01mdd mdd

3 α/γ

6

x108 (s) 10

2 1 0 4

b

Amount of air-borne salt

0.2 0.2 mdd mdd >0.5 >0.5mdd mdd

α/γ

3 2 1 0

0

5

10 15 20 25 30 Exposure duration (years)

35

Corrosion rate (mm/y)

0.01 mdd 0.03

12 10

-13

Amount of air-borne salt

x10

0.04

(m /s )

Fig. 4. Change in / value as a function of exposure duration ((a) air-borne salt 0.01 mdd [0.12 £ 10¡7 g/m2/s], (b) air-borne salt 0.2 [2.31 £ 10¡7] and >0.5 mdd [5.79 £ 10¡7 g/m2/s]).  and  are the mass fraction of -FeOOH and -FeOOH, respectively.

8 0.02

6 4

0.01 2 0.00 0

1

2

3

0

α/γ Fig. 5. Relationship between corrosion rate and / in an industrial environment (0.01 mdd [0.12 £ 10¡7 g/m2/s]).  and  are the mass fraction of -FeOOH and -FeOOH, respectively.

the /¤ of crystalline -FeOOH to the total mass of -FeOOH, -FeOOH and Fe3O4, in the rust was plotted vs. the corrosion rate in Fig. 7. The correlation between the corrosion rate and the /¤ was clearly observed: The corrosion rate decreased with an increase of the

0.08

25 Amount of air-borne salt

Corrosion rate (mm/y)

0.07

0.2 0.2mdd mdd >0.5 >0.5mdd mdd

0.06 0.05

20

2807

x10-13 (m/s)

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15

0.04

Formation of β-FeOOH and/or spinel

0.03

10

0.02 5 0.01 0.00

0

0

1

2

3

α/γ

0.08

25 Amount of air-borne salt

Corrosion rate (mm/y)

0.07

0.01 0.01mdd mdd 0.2 0.2mdd mdd >0.5 >0.5mdd mdd

0.06 0.05

20

x10-13 (m/s)

Fig. 6. Relationship between corrosion rate and / at places with air-borne salt (0.2 [2.31 £ 10¡7] and >0.5 mdd [5.79 £ 10¡7 g/m2/s]).  and  are the mass fraction of -FeOOH and -FeOOH, respectively.

15

0.04 10

0.03 0.02

5

0.01 0.00

0

1

α/γ∗

2

3

0

Fig. 7. Relationship between corrosion rate and /¤ in places with diVerent amounts of air-borne salt (0.01 mdd [0.12 £ 10¡7 g/m2/s], 0.2 mdd [2.31 £ 10¡7 g/m2/s], >0.5 mdd [5.79 £ 10¡7 g/m2/s]).  and ¤ are the mass fraction of -FeOOH and total mass fraction of -FeOOH, -FeOOH and Fe3O4, respectively.

/¤. It should be emphasized that the high corrosion rate of more than 0.01 mm/year (3.17 £ 10¡13 m/s) is never observed if the /¤ is over 1. This means that the steel does not show the high corrosion rate if the /¤ is over 1, while the corrosion rate cannot be determined easily if the /¤ is less than 1. These results show the possibility that there is a correlation between the corrosion rate and the /¤, and the /¤ will be of great importance for

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the evaluation of the protectiveness of the rust layer formed in a wider range of environmental conditions including higher salt environment, and to understand the steps of the atmospheric corrosion of a weathering steel. 4. Discussion 4.1. Correlation between corrosion rate and the mass ratio change of crystalline rust composition with exposure duration A lot of literature has been reported on the relation between the reactivity of rust and corrosion behavior under the wet/dry cyclic conditions, which is a typical atmospheric condition [21,27–37]. Stratmann et al. [31–36] have studied the corrosion behavior of pure iron under the wet/dry transition and the behavior of the reduction and re-oxidation of rust formed on pure iron, and they have elucidated the following inXuence of rust on corrosion behavior of pure iron in the initial wet/dry cyclic condition. During the wet/dry transition, the cathodic reaction, which is the counter reaction of Fe dissolution reaction, is the reduction of -FeOOH, which is formed at the previous wet/dry transition, just after wetting of the dry surface. -FeOOH is thermodynamically stable and not easily reduced, although it is partly reduced at a very negative potential below ¡0.5 VSHE. On the other hand, FeOOH is easily reduced. -FeOOH is reduced to Fe · OH · OH, which is formed probably on the surface of -FeOOH as an intermediate [34]. This intermediate containing Fe2+ is reoxidized to -FeOOH again during the drying process, and then re-oxidized -FeOOH works again as a cathode site at the subsequent wet/dry process. Furthermore, Fe2+ species are created within the lattice of the -FeOOH, therefore increasing the conductivity of the n-type semiconducting oxide. During subsequent drying, the thickness of the electrolyte layer decreases and oxygen is reduced with a high rate on the reduced -FeOOH in the rust, which behaves like a large area cathode. At the negative potential, such as ¡0.4 VSHE, which is nearly the corrosion potential of Fe [33] and, that is, the potential near the steel/ rust interface, -FeOOH is reduced to Fe3O4. Fe3O4 shows quite high conductivity, and therefore works as a large cathode area and will accelerate the corrosion. They also demonstrated that the Fe3O4 formed by the reduction of -FeOOH is oxidized back not to FeOOH but to -Fe2O3. The alloying elements added are expected to aVect the reduction behavior of -FeOOH. It can therefore be concluded that -FeOOH does not aVect the corrosion in the wet/dry cyclic condition, and -FeOOH and Fe3O4 accelerate corrosion from an electrochemical point of view. Although the formation condition, which decides whether -FeOOH is a main constituent in the rust layer on the steel or -FeOOH, is still unclear, it can be understandable from an electrochemical point of view that the corrosion rate decreases if -FeOOH is a main constituent in the rust with an increase of the exposure duration or by the eVect of the coexistence of ions [38]. Misawa et al. [7,39] have proposed the following atmospheric rusting process based on their results for the formation process of iron oxyhydroxides in the solution. In the neutral solution where FeOH+ is stable, -FeOOH is formed by a relatively fast oxidation of FeOH+ by dissolved oxygen. In the slightly acidic solution, -FeOOH is formed via amorphous ferric oxyhydroxide. If the rust layer becomes thick enough and the oxidation rate decreases, the green rust is formed and then Fe 3O4 is formed under the rust layer, where the oxygen supply is very slow. They also pointed out that the formed -FeOOH is re-dissolved in conditions where pH decreases, and -FeOOH is formed by re-precipitation. In the initial

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stage of atmospheric exposure, the rust layer is mainly composed of -FeOOH. As the exposure duration increases, the corrosion rate slows down because of the thick and dense rust formation. At this stage the formation rate of the rust is slow, the above change from FeOOH to -FeOOH via amorphous ferric oxyhydroxide will be the main reaction, and therefore the mass fraction of -FeOOH may increase. This supports the fact that the rust layer formed on the steel exposed in polluted conditions, where the atmosphere contains much SO2 gas, contains a considerable amount of -FeOOH, and weathering steel shows superior performance to mild steel especially in industrial environments [40]. Yamashita et al. [3] have discussed the change in compositions of crystalline rust with exposure duration in an industrial environment: The main rust composition changes from -FeOOH in the initial stage to the X-ray amorphous substance with an increase of the exposure duration, and Wnally to -FeOOH after a long-term exposure. In the initial stage, the corrosion rate is high because -FeOOH is the main constituent, which is electrochemically active and works as a cathode site. As the corrosion proceeds according to the above process, the corrosion rate slows down because the rust changes from -FeOOH to FeOOH, which is thermodynamically stable and electrochemically inactive. Furthermore, in the stage that -FeOOH is the main constituent in the rust layer, and newly formed Fe2+ is inhibited, that is, the corrosion rate is low, the rust composition changes from -FeOOH to -FeOOH by dissolution and the re-precipitation process mainly proceeds. Therefore, there is a high correlation between the corrosion rate and the mass ratio, and the corrosion rate decreases with an increase of the / as shown in Fig. 5, which can be clearly observed for the rust layer formed on the weathering steel exposed in an industrial environment. The corrosion rate can be correlated to the macro-structures such as crack and porosity which dominates the denseness of the rust layer. The fact that there is a correlation between the corrosion rate and rust composition, probably suggests that the macro-structure of the rust layer is related to the rust composition. In this study, we did not take into account the so-called X-ray amorphous substance. From the recent Mössbauer spectroscopic studies [12–17], it is found that the X-ray amorphous substance in the rust layer formed on the weathering steel exposed in a rural or an industrial region is mainly ultra-Wne -FeOOH. Fig. 8 shows the comparison between the / obtained by XRD and by Mössbauer spectroscopy for the rust formed on weathering steel exposed in an industrial region (Amagasaki-city, Hyogo pref.) [41]. It is clearly observed that the / obtained by Mössbauer spectroscopy was higher than that by XRD. Moreover, there is a linear relationship between the / obtained by XRD and by Mössbauer spectroscopy. This indicates that the / obtained by XRD is reXected in the above ultra-Wne -FeOOH. The above discussion, however, cannot explain the composition change of rust formed on the weathering steels exposed to the atmosphere containing much air-borne salt particles because of the formation of -FeOOH and Fe3O4. In the case of the exposure to the atmosphere containing large amounts of air-borne salt particles, as shown in Fig. 4, the tendency to increase the / is not observed with an increase in the exposure duration, and there is no correlation between the corrosion rate and the /, especially in the case of exposure to the atmosphere containing over 0.5 mdd of air-borne salt particles. Nishimura et al. [42] demonstrated the possibility that -FeOOH is easily reduced by the electrochemical measurement for the steel with rust containing -FeOOH, which is formed by the wet/ dry cyclic condition containing chloride. This result suggests that -FeOOH can also accelerate the corrosion in the initial wet/dry transition as it works as a cathode. Although a

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4

32

α/γMS

3

2

15 6 1

0.5

1

0.25 0.08 0

0

1

2 α/γXRD

3

4

Fig. 8. Comparison of / value obtained by X-ray diVraction (/XRD) and by Mössbauer spectroscopy (/MS). The numbers near the plots indicate the exposure duration (years) at Amagasaki-city, Hyogo pref.

detailed formation process of -FeOOH has not been clariWed up to now, as compared to other oxyhydroxides, it is reported that it forms under the low pH [43]. At the anodic site under conditions containing chloride, chloride can move by electrophoresis to keep electroneutrality, and therefore -FeOOH may form in the vicinity of the steel surface. As shown in Fig. 7, the correlation between the corrosion rate and the /¤ is clearly observed; there is no high corrosion rate over 0.01 mm/year (3.17 £ 10¡13 m/s) when the /¤ is over 1. This can be also explained by the idea discussed above for the /, and this result, in other words, implies that -FeOOH is electrochemically active and accelerates corrosion. The samples used in this study were exposed at an angle of 30° facing south, and rain directly washes the steel surface, and therefore the deposited chloride cannot be accumulated on the steel surfaces under this condition. This correlation between the /¤ and the corrosion rate have been veriWed even under the sheltered conditions by using the data for the rust layer formed on the weathering steel exposed for 17 and/or 18 years in 41 nationwide atmospheres having diVerent amounts of air-borne salt particles [44]. It should be emphasized that there is no high corrosion rate over 0.01 mm/year (3.17 £ 10¡13 m/s) when the /¤ is over 1, whereas the corrosion rate is distributed or scattered when the /¤ is less than 1. It is noted that the threshold value of the /¤ is aVected by the analytical curve (see Section 2.2). The above fact that at least the high corrosion rate is not observed when the /¤ is over 1 exhibits that the evaluation by the /¤ is applicable for the evaluation of the protectiveness of rust layer formed on the weathering steel even if it is exposed to the atmosphere containing large amounts of air-borne salt particles. 4.2. Evaluation for corrosion resistance of weathering steel by rust protective ability /¤ As discussed in Section 4.1, the results that there were no high corrosion rates when the /¤ is over 1 is of great importance for evaluating the protectiveness of the rust layer

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formed on the weathering steel. The /¤ corresponds to the mass ratio of the active rust and inactive rust, and the strong correlation between the /¤ and the corrosion rate, as a result, can be observed. When the rust is taken from the weathering steel bridges, and the /¤ is less than 1, it is not easy to evaluate its protectiveness at the given condition. In such a case, it is necessary to evaluate its protectiveness in the conjunction with other measurements [45–47]. When the /¤ is, on the other hand, over 1, it is possible to determine that the rust layer is protective and the corrosion rate is slow enough at the given condition. Therefore, the /¤ is the PAI of the rust layer formed on the weathering steel. 5. Conclusions The correlation between corrosion rate and compositions of crystalline corrosion products formed on weathering steel exposed to atmosphere for 20 years with diVerent amounts of air-borne sea salt particles ranging from less than 0.01 (0.12 £ 10¡7) to more than 0.5 mg NaCl/dm2/day (5.79 £ 10¡7 g NaCl/m2/s) has been investigated by the X-ray diVraction method. The conclusions obtained are summarized as follows: (1) The mass ratio / of crystalline -FeOOH to -FeOOH, in the rust layer formed on the weathering steel exposed at industrial and rural environments increases with an increase in exposure duration. This tendency is not observed in cases that the weathering steel is exposed to the seashore environments where -FeOOH and Fe3O4 are formed. (2) The / is closely related to the corrosion rate in the industrial and rural environments where the amounts of air-borne salt is less than 0.2 mg NaCl/dm2/day (2.31 £ 10¡7 g NaCl/m2/s). When the / is more than a certain value, the low corrosion rate less than 0.01 mm/year (3.17 £ 10¡13 m/s) is observed. (3) There is less correlation between the / and the corrosion rate in seashore environments with higher amount of air-borne salts, where -FeOOH and Fe3O4 are formed. (4) The /¤ of crystalline -FeOOH to the total mass of -FeOOH, -FeOOH and Fe3O4, in the rust layer formed on the weathering steel is related to the corrosion rate even in seaside environments with more than 0.2 mg/dm2/day (2.31 £ 10¡7 g/m2/s) of air-borne sea salt particles. (5) When the /¤ is more than a certain threshold value, the higher corrosion rate more than 0.01 mm/year (3.17 £ 10¡13 m/s) is not observed. (6) The /¤ is a protective ability index of rust layer formed on weathering steel. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]

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