Structure of the rust layer of weathering steel in A high chloride environment: A detailed characterization via HRTEM, STEM-EDS, and FIB-SEM
Introduction
Conventional weathering steel (WS), which contains small amounts of Cu (0.25∼0.40 %), P (≤0.15 %), Cr (0.40∼1.00 %), and Ni (0.02∼0.65 %) and possesses high corrosion resistance, has been widely applied as a structural material in urban, industrial, and rural atmospheric environments [[1], [2], [3]]. The higher corrosion resistance of WSs compared with plain carbon steels can be attributed to the formation of a compact and adherent rust layer which can prevent the penetration of water and corrosive ions from the atmosphere.
The corrosion resistance of steels is related to the rust layer structure, which mainly depends on the type and content of alloying elements and the atmospheric environment. Yamashita et al. [[4], [5], [6], [7]] proposed that Cr can enrich the inner rust layer of steel. Cr3+ coordinated with (7 ± 1) O2− is distributed in the vacant sites in the FeO3(OH)3 octahedra network in goethite (α-FeOOH), resulting in the formation of a dense, packed aggregation of nanophase Cr-substituted α-FeOOH (α-(Fe1-x,Crx)OOH) with cation selectivity in the rust layer. Misawa et al. [8,9] indicated that Cu and P can catalyze Fe2+ complex transformation to amorphous ferric oxyhydroxide, which will be transformed into densely packed α-FeOOH nano-particles in the inner rust layer. Nishimura et al. [10] and Kimura et al. [11,12] found that Ni can enrich the rust layer in its bivalent state and partly replace Fe2+ to form Fe2NiO4 inverse spinel oxide, which can provide sites for the nucleation of the Fe(O,OH)6 nano-network, leading to the formation of a rust layer comprised of compact grains. In addition, the presence of Fe2NiO4 reverses the ion-exchanging properties of the rust layer from anion to cation selectivity. The rust layer is constituted by various types of oxides, namely, hydrated oxides, oxy-hydroxides, and amorphous substances [13]. Among these, α-FeOOH, akaganeite (β-FeOOH), lepidocrocite (γ-FeOOH), and magnetite (Fe3O4) are the most common. The composition of the rust layer depends on environmental factors. For example, α-FeOOH and γ-FeOOH are the main corrosion products in low chloride deposition environments such as urban, rural, and industrial, while β-FeOOH is formed in high chloride environments, such as areas where MgCl2 and CaCl2 are applied as anti-freezing and snow-melting agents, and marine atmospheres.
Many researchers have explored the structure of the rust layer formed on conventional WS in low to moderate aggressive environments. The rust layer formed in urban, rural, and industrial environments consists of two layers, namely, the outer layer and the inner layer. The outer layer is loose and mainly comprised of γ-FeOOH, while the inner layer is compact and mainly composed of densely packed α-FeOOH nano-particles [4,[14], [15], [16], [17]]. However, as noted by Alcántara et al. [18], only a few studies have investigated the rust layer structure formed on WS in high chloride environments. Some controversies remain to be clarified, such as the location of β-FeOOH and the existence of α-FeOOH nanograins. de la Fuente et al. [19] and Morcillo et al. [20] characterized the rust layer formed on mild steel exposed to marine atmospheres using scanning electron microscopy (SEM), Micro-Raman, and X-ray diffraction (XRD) techniques, and found that β-FeOOH mainly formed in the inner part of the rust layer, close to the base steel. Asami and Kikuchi [21] studied the rust layer on conventional WS exposed for 17 years in coastal-industrial atmospheres using transmission electron microscopy (TEM) and selected area electron diffraction (SAED), and found that β-FeOOH was mainly distributed in the external section of the thick rust layer. Diaz et al. [1] investigated the atmospheric corrosion of a conventional WS and three Ni-advanced WSs after exposure in a moderately aggressive marine atmosphere for 1 year. Mössbauer spectroscopy analysis of the rust showed that the fraction of superparamagnetic α-FeOOH with particle size lower than 15 nm increased with Ni content in the advanced WS. In contrast, α-FeOOH present in the rust of conventional WS has a particle size greater than 15 nm. The superparamagnetic α-FeOOH can hardly be formed on conventional WS in marine atmospheres, even after long exposure times. Nishimura [22] investigated the formation mechanism of the rust layer on Ni-bearing low alloy steels in a high SOx and Cl− environment, with exposure tested in Hainan Island for 3 years. It was concluded that nano-size α-FeOOH and Fe3O4 formed in the inner rust layer and increased the corrosion resistance of Ni-bearing low alloy steels. However, these nanophases were not clearly seen in TEM micrographs.
In view of the above, it is necessary to conduct more detailed research on the nano-scale structure of the rust layer. A combination of high-resolution TEM (HRTEM) and scanning transmission electron microscopy equipped with energy dispersive X-ray spectroscopy (STEM-EDS) is a powerful method for the analysis of the structure and element distribution. However, TEM/STEM study of the rust layer structure on WS has been rarely reported so far, likely because of the great difficulty in sample preparation due to the loose rust layer.
In the present work, HRTEM and STEM-EDS analysis were performed on the rust layer structure of WS by the wet-dry cyclic corrosion test (CCT) [10,23] in high chloride environments, which simulates a severe marine atmosphere. The TEM samples were prepared by focused ion beam-SEM (FIB-SEM), which can section a specific portion of the sample and preserve the original rust layer structure. In addition, corrosion kinetics and the effect of the rust layer structure on corrosion resistance were discussed.
Section snippets
Sample preparation
WS, with a composition of C 0.051, Mn 1.21, Si 0.31, Cr 0.68, Cu 0.32, Ni 0.34, Mo 0.04, Nb 0.024, Ti 0.016, P 0.008, S 0.002 (wt.%) and Fe balance, was used in this study. The material was machined into samples with dimensions 20 mm × 20 mm × 4 mm. Then, the samples were inset in resin, and only the surfaces (20 mm × 20 mm) were periodically exposed to a corrosive electrolyte. The exposed surface of each sample was ground using silicon carbide paper to 1500 grit, cleaned ultrasonically in
Corrosion kinetics
The corrosion kinetics of steel in atmosphere has been found to follow an empirical equation, according to previous results [[28], [29], [30]]:ΔW = ANnwhere ΔW (mg/cm2) refers to weight gain, N refers to CCT number, and, A and n are constants. Eq. (1) often takes the logarithmic form, which is called the bilograrithmic law, to estimate the atmospheric corrosion behavior of the steel, expressed as:log ΔW=log A+n log N
The corrosion kinetics feature of the steel can be reflected by the n value,
Phase formation and distribution in the rust layer
The formation of the α-FeOOH, γ-FeOOH, and β-FeOOH phases in the rust layer is largely dependent on the local chemical conditions [33]. Refait group [34] designed an experiment to reveal the formation mechanism of β-FeOOH and α-FeOOH in chloride-containing environments. The experiment consisted of changing the experimental ratio R = [Cl−]/[OH−] through mixing of FeCl2-4H2O and NaOH aqueous solutions. The results revealed that for R ≥ 8, only β-FeOOH was produced via the formation of the
Conclusions
- (1)
The rust layer formed on WS in high chloride environments after a wet/dry corrosion test was studied and showed a three-layer structure: an inner layer, an outer layer, and an outermost layer. The inner layer was composed of densely packed α-FeOOH nanograins and the outer layer was a mixture of spindle- and rod-like α-FeOOH, and rod- and fiber-like β-FeOOH. In addition, the outermost layer was composed of laminar γ-FeOOH.
- (2)
Cu and Ni were relatively evenly distributed in the rust layer. In
Data availability statement
The original/processed data required to produce these findings in this paper cannot be shared at this moment as the data is reserved for the further ongoing study.
CRediT authorship contribution statement
Yuefeng Wang: Conceptualization, Investigation, Methodology, Data curation, Writing - original draft, Writing - review & editing. Jianguang Li: Investigation, Data curation. Lie Zhang: Investigation. Linfeng Zhang: Investigation. Qingfeng Wang: Data curation. Tiansheng Wang: Conceptualization, Supervision, Resources, Writing - review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by the National Key Research and Development Program of China (Grant No. 2017YFB0304800 and Grant No. 2017YFB0304802 for the second sub project).
References (53)
- et al.
Atmospheric corrosion of Ni-advanced weathering steels in marine atmospheres of moderate salinity
Corros. Sci.
(2013) - et al.
Atmospheric corrosion data of weathering steels
A review, Corros. Sci.
(2013) - et al.
Optimizing the nickel content in weathering steels to enhance their corrosion resistance in acidic atmospheres
Corros. Sci.
(2017) - et al.
The long term growth of the protective rust layer formed on weathering steel by atmospheric corrosion during a quarter of a century
Corros. Sci.
(1994) - et al.
Structure and protective performance of atmospheric corrosion product of Fe-Cr alloy film analyzed by Mössbauer spectroscopy and with synchrotron radiation X-rays
Corros. Sci.
(2003) - et al.
The mechanism of atmospheric rusting and the protective amorphous rust on low alloy steel
Corros. Sci.
(1974) - et al.
The mechanism of atmospheric rusting and the effect of Cu and P on the rust formation of low alloy steels
Corros. Sci.
(1971) - et al.
Effect of Co and Ni on the corrosion behavior of low alloy steels in wet/dry environments
Corros. Sci.
(2000) - et al.
Control of Fe(O,OH)6 nano-network structures of rust for high atmospheric-corrosion resistance
Corros. Sci.
(2005) - et al.
Weathering steels: from empirical development to scientific design
A review, Corros. Sci.
(2014)