Photon Factory Activity Report 2004 #22 Part B (2005)
Materials Science

11B/2004C002

State of Chlorine in Rusts Formed on 3 mass% Ni-added Weathering Steel (III)
Masao KIMURA*1, Noriaki OHTA 2, Hiroshi KIHIRA3 and Yoshinori KITAJIMA4
1Adv. Tech. Res. Lab., Nippon Steel Corp., Futtsu, Chiba, 293-8511, Japan
2
Nippon Steel Technoresearch, Chiba 293-8500, Japan
3
Steel Res. Lab., Nippon Steel Corp., Futtsu, Chiba, 293-8511, Japan
4
Photon Factory, KEK, Tsukuba, Ibaraki, 305-0801, Japan
Introduction
Improvement of the corrosion resistance of steel
without coating is very important from the viewpoint oflife-cycle assessments of materials. Weathering steel(WS) has been developed for the application to bridges
without coating. WS, containing a 0.3-0.7 mass% Cr andCu, forms a protective rust layer after a few years ofcorrosion in atmosphere and shows a good corrosion
resistance [1,2].
However, a conventional WS shows a good corrosion
resistance only when it is exposed to atmosphere
containing little air born salt. A new type of WS (“theadvanced WS”) was developed; the addition of 3 mass %Ni increases the corrosion resistance by a factor morethan 10 [3].
In this study, the mechanism has been
investigated using X-ray absorption fine structure(XAFS) with a special attention to chloride ions which
are thought to enhance corrosion.
Experiments
The advanced WS was prepared and has been exposedin costal atmosphere (Kimitsu, Chiba, Japan) for 9 years.
Microstructures of cross sections near the surface were
investigated by TEM and EPMA. Atomic structures ofrusts were analyzed by XRD, TEM and XAFS. XAFS
spectra of rusts at Cl-K edge were measured by total
electron yield and X-ray fluorescence[4].
XAFS
measurements were carried out at BL-11B at PF, KEK,
Tsukuba, Japan.
Results and Discussion
After the corrosion for 9 years, enrichment of nickel
and cupper was found in the inner part of the protective
rust formed on the surface of the advanced WS.
Concentration of chlorine in the inner layer was much
lower than that of the outer layer, whereas concentrationof nickel and cupper is lower than that of the inner layer.
The “protective” inner layer is composed of fine grains as
small as a few nm. Protective inner rusts formed on theadvanced WS “breathe out” chloride ions [5,6,7].
XAFS measurements were carried out in order toinvestigate states of chloride ions that were breathed out
from the interface between the inner rust and the ironsubstrate[6,7]. Figure 1 shows XAFS spectra around ClK edge of the rust and reference specimens : synthesized
β-FeOOH, FeCl3 and NaCl. The spectra of specimen issimilar to that of β-FeOOH, whose structure is shown inFig.2. It is expected that chloride ions in the rust form
networking with Fe(O,OH)6 octahedrons [8.9].

Detailed analysis has shown that energies of Cl-K edge
differ slightly among specimens [8]. That of the innerrust is about 0.9eV higher than those of the outer rust andNaCl. It is expected that chloride ions in the inner ismore stable in the “Fe(O,OH)6 network [1]” than those in
the outer rust.
This can be attributed to the highperformance of t the advanced WS [9].

Fig.1 XANES spectra around the Cl-K edge: (a) the specimen
after 9 years exposure , (b) β-FeOOH, (c) FeCl3 and (d) NaCl.

Fig.2 Atomic structure of β-FeOOH. A chloride ion(green) is located in the center of Fe(O,OH)6 octahedrons.
References[1] M. Kimura et al: J. Japan Inst. Metals, 66, 166 (2002).
[2] M. Kimura et al: ISIJ International, 42, 1534 (2002).
[3] H. Kihira et al: Zairyo-to-Kankyo, 49, 30 (2000).
[4] Y. Kitajima: J. Elec. Spec. Relat. Phenom., 80, 405 (1999).
[5] M. Kimura et al: Procds. of the 13th Asian-PacificCorrosion Control Conference (Osaka, 2003).
[6] M. Kimura et al: PF Act. Rep. #20 Part B, p.200 and p.201(2003).
[7] M. Kimura et al: PF Act. Rep. #21 Part B, p.183, 184, 185(2004).
[8] M. Kimura et al: : Procds. of ECS (Honolulu, 2004), ECS
PV 2005-14, p133.
[9] M. Kimura et al: Corrosion Sci. (in print,2005)* kimura@re.nsc.co.jp

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