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

9A, 12C/2004C001

Fe(O,OH)6 Network Nanostructure of Rusts Formed on 3 mass% Ni-added
Weathering Steel (III)
Masao KIMURA*1, Noriaki OHTA 2, Hiroshi KIHIRA3, and Masaharu NOMURA4
1
Adv. 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 effects of nickel andcopper addition on the formation of rust at early stage ofcorrosion.
Experiments
XAFS in situ measurements were performed for a
block specimen of the advanced WS (3.0Ni-0.4Cu-Fe in
mass %) [5,6]. The surface of the specimen was covered
with a thickness of ca. 20 µm of diluted artificial seawaterand then dried in air at 300 K. One wet-dry cycle wasabout 90 min. XAFS spectra of rusts were measured inthe florescence geometry using a 19-element SSD [4].
XAFS measurements were carried out at BL-9A and 12C
at PF, KEK, Tsukuba, Japan.
Results and Discussion
XAFS spectra were measured during the wet-drycycles around Ni and Cu [5,6,7]. Before corrosion begins,
Fe, Ni and Cu atoms are in a solid solution (an alloy). As
corrosion progresses, RDF around Ni shows additional
peaks, which indicating that nickel atoms substitute ironatoms in the “16d” sites in Fe3O4 to form Fe2NiO4 (Fig.1).
Cu atoms form CuO during corrosion.
The detailed analysis showed that formation of Fe2NiO4and CuO alter the evolution of “Fe(O,OH)6 network[1,2]” during corrosion (Fig. 2). This results in formationof rust composed of fine grains negatively-charged,
which prevents chloride ions penetrating from the outer
into the inner layer, and enrichment of chloride ions issuppressed [8,9]. The good corrosion resistance of theadvanced WS can be attributed to these unique structuresof the protective rusts.

140Users' Report

Fig.1 Atomic structure ofcomposedof
Fe2NiO4(Fe,Ni)O6 octahedrons andFeO4 tetrahedrons.

.
Fig. 2 Schematic diagram of the evolution of the Fe(O,OH)6
nano-network of the rust formed on the advanced WS in an
atmosphere containing relatively high air-born salinity (top),
conventional WS in milder condition (middle), and mild steel inmilder condition (bottom) [8,9].

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] M. Nomura: J. Synchrotron Rad., 5, 851 (1998).
[5] M. Kimura et al: PF Act. Rep. #20 Part B, p.200 and p.201(2003).
[6] M. Kimura et al: PF Act. Rep. #21 Part B, p.183, 184, 185(2004).
[7] M. Kimura et al: Procds. of the 13th Asian-PacificCorrosion Control Conference (Osaka, 2003).
[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