Structure of titanium-doped goethite rust

doi:10.1016/j.corsci.2004.11.030

Corrosion Science

Volume 47, Issue 10, October 2005, Pages 2521-2530

https://doi.org/10.1016/j.corsci.2004.11.030 Get rights and content

Abstract

To investigate the influence of titanium addition on the formation and structure of goethite (α-FeOOH) rust which is one of main corrosion products of weathering steel, the artificially synthesized α-FeOOH rusts were prepared by hydrolysis of aqueous solutions of Fe(III) containing Ti(IV) at different atomic ratios (Ti/Fe) in the range 0–0.1. The obtained rusts particles were observed by TEM. Characterization by XRD, N₂ absorption, Mössbauer spectroscopy was also done. TEM observation revealed that the α-FeOOH rust particle size increased with the increase of Ti/Fe, and that Ti-enriched poorly crystalline particles were formed around the rust particles. XRD confirmed that the crystallite size increased with the increase of Ti/Fe, while the XRD peaks decreased in intensity. Specific surface area obtained by N₂ absorption increased with the increase of Ti/Fe. It is deduced from the obtained results that the addition of Ti(IV) increases the crystallite size of α-FeOOH, and produces double domain particles consisting of the particle core and a porous poorly crystalline shell. It is thought that such unique rust structure produced by titanium addition contributes to the protective properties of rust layer of the weathering steel.

Introduction

Recently, much attention has been devoted to steels with better atmospheric corrosion resistance of the various industries. For example, new weathering steels have been researched and developed, according to the increase of requirements of reduced initial construction and maintenance costs for steel bridges [1]. So far, un-coated conventional weathering steels for bridges have not been adopted in the chlorides environment such as marine and coastal areas. This is because the steels encountered the formation of flaky rust with poor adherence, resulting into deterioration of corrosion resistance at the same level of mild steel [2]. Automobile corrosion has also a serious problem in regions where a large amount of de-icing salt is used in the winter. β-FeOOH(akaganeite), one of the polymorphs of ferric oxy-hydroxides is known as a component of corrosion products of steel in such chloride environments [3]. The authors have recently found that the corrosion resistance in chloride environments was well correlated to the formation of β-FeOOH rust, and that the corrosion resistance was increased with the decrease of the fraction [4], [5], [6]. Shiotani, et al. [7] and Yamashita et al. [8] have also confirmed the good correlation between fraction of β-FeOOH rust and corrosion thickness loss of weathering steels conducted as on-site exposure tests at 41 bridges for the periods of 17–18 years by Public Works Research Institute, Kozai Club, and Japan Association of Steel Bridge Construction [2]. On the other hand, the authors have found that Ti was effective as an alloying element to improve corrosion resistance of steel in chloride environments to decrease the formation of β-FeOOH [4], [5], [6], [9].

As well as β-FeOOH, α-FeOOH(goethite), γ-FeOOH(lepidocrocite), Fe₃O₄(magnetite) and poorly crystallized iron oxide (X-ray amorphous species) are also known as a major component of corrosion products of steel formed by the atmospheric corrosion [1]. Among them, α-FeOOH(goethite) which is a polymorph of iron(III) oxy-hydroxides, is known as a most stable corrosion product. Inoue et al. found a marked inhibitory effect of Cu(II) on the formation of α-FeOOH and interpreted its anti-corrosion character by a mechanism in which distortion of octahedral in α-FeOOH crystals induced by the Jahn–Teller effect of Cu(II) led to a dense rust layer on steel [10]. It has been reported that Cr(III) can be substituted for Fe(III) in α-FeOOH by up to 0.10 in terms of Cr/(Cr + Fe) atomic ratio [11]. Yamashita et al. proposed that Cr-substituted α-FeOOH particles are related to the formation of a stable rust layer on a weathering steel exposed for 26 years [12]. Ni(II) also substitutes for Fe(III) of α-FeOOH particles up to an atomic ratio of ca. 0.1 without morphological change despite the different valency of Ni(II) [13]. On the other hand, the authors have found that Ti exerts a significant anti-corrosive action in steel for used in an environment containing chloride ions, as mentioned above [4], [5], [6]. However, the mechanism of the formation of iron(III) oxy-hydroxides, including α-FeOOH, has been much less explored than the action of Cu(II), Ni(II) or Cr(III). This paper aims to investigate the influence of Ti on the crystallinity and particle sizes of α-FeOOH by artificial rust synthesis experiments. Ti-doped α-FeOOH particles were characterized by transmission electron microscopy and other analytical tools. The results here will serve to provide a basic understanding of the mechanism involved in the anti-corrosion action of not only Ti but also other alloying elements.

Section snippets

Materials

α-FeOOH rust particles were prepared by co-precipitation with Ti(IV) at a variety of atomic ratios (Ti/Fe) in the range of 0–0.1 as follows. A determined quantity of a 2.0 mol Ti(SO₄)₂ solution was added to 25 cm³ of a 2.0 mol Fe(NO₃)₃ solution and then the total volume of the solutions was adjusted to 50 cm³ by adding water. The solution pH was set to 12 with 1.0 mol NaOH solution under stirring. The resulting precipitates were aged in a polypropylene vessel at 50 °C for 5 days. After the aging, the

X-ray diffraction

Fig. 1 shows X-ray diffraction patterns of the artificial synthesized α-FeOOH rusts at different Ti/Fe ratio. All diffraction peaks of the samples are characteristics of α-FeOOH (JCPDS 13–157). The diffraction peaks of the materials increase with increasing Ti/Fe ratio at Ti/Fe ⩽ 0.02, while above this value they decrease and show narrowed peak width. None of the peaks shifts with an increase of Ti/Fe which indicates that the unit cell parameters of α-FeOOH do not vary by adding Ti(IV). Fig. 2

Conclusions

The results presented above allow us to reach the following conclusions. Titanium-doped goethite (α-FeOOH) rust has a double domain structure consisting of the particle core and a porous poorly crystalline shell. The core is crystalline α-FeOOH, while their outer shells are composed of agglomerates of ultra-fine α-FeOOH particles. Such unique rust structure produced by titanium addition is thought to contribute to the protective properties of rust layer of the weathering steel.

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The increase in Cr notably shifted the open circuit potential of the steel in the positive direction and improved the corrosion resistance of WS, which was greater the higher the Cr content, by promoting the formation of a protective goethite rust layer and enhancing its passivation capability. Nakayama et al. artificially synthesised goethite rusts by the addition of Ti with atomic ratios (Ti/Fe) in the 0–0.1 wt% range [153]. Ti-enriched UFG particles plugged the pores in the rust film and significantly increased the passivation ability of the rust; the rust structure that was produced contributed to the protective properties of the rust layer.
The design of the first weathering steels was purely empirical and focused on a small number of conventional alloying elements such as manganese, silicon, chromium, nickel, copper and phosphorus, mainly. The environmental conditions that promote the formation of protective rust layers: existence of wet/dry cycling, absence of very long wetness times, atmospheres without a marine component, etc., were identified by trial and error. This paper makes a bibliographic review of the abundant literature that has been published on the atmospheric corrosion of weathering steels, setting out in chronological order the advances made in the scientific knowledge of important matters such as: atmospheric corrosion mechanisms of weathering steel, formation of protective rust layers, and the role played by alloying elements. The work ends with an overview of the scientific design of new weathering steels, placing special emphasis on the new compositions developed for application in marine atmospheres.
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The length and width of the crystals calculated were 0.917 ± 0.18 and 0.190 ± 0.014 μm for pure and 1.719 ± 0.576 and 0.3439 ± 0.105 μm for Pb-doped goethite, which suggests that the doping of Pb2+ ions results in an increase in the particle size of goethite. Effect on the size of goethite particles by metal ions incorporation has been reported by many researchers [4,26,30]. Traces of an amorphous phase along with the crystals of goethite can also be noted in the TEM image (B) of Pb-doped goethite.
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Structure of titanium-doped goethite rust

Abstract

Introduction

Section snippets

Materials

X-ray diffraction

Conclusions

Corros. Sci.

Geochim. Cosmochim. Acta

J. Soc. Mater. Sci., Japan

J. Miner. Perol. Econ. Geal.

CAMP-ISIJ

R&D Kobe Steel Engineering Reports