Magnetic property based characterization of rust on weathering steels

The characterization of rusts on weathering steels is important in understanding the origin of their corrosion resistance. Rust consists of several phases, e.g. α-, β- and γ-FeOOH, which are anti-ferromagnetic with different Neél temperatures. Rust on so-called advanced weathering steel containing 3 wt.% Ni [H. Kihira, A. Usami, K. Tanabe, M. Ito, G. Shigesato, Y. Tomita, T. Kusunoki, T. Tsuzuki, S. Ito, T. Murata, in: Proc. Symp. on Corrosion and Corrosion Control in Saltwater Environments, Honolulu, 1999, The Electrochemical Soc., pp. 127-136] contains in addition a ferrimagnetic spinel phase [M. Kimura, H. Kihira, Y. Ishii, T. Mizoguchi, in: Proc. 13th Asian-Pacific Corrosion Control Conference, Osaka, 2003; M. Kimura, H. Kihira, N. Ohta, M. Hashimoto, T. Senuma, Corros. Sci., this volume; M. Kimura, N. Ohta, H. Kihira, Mater. Trans. JIM, in press]. The nanostructure of real rust cannot be elucidated satisfactorily only with conventional analytical methods such as X-ray diffraction, because of the complex mixture of phases with fine and imperfect crystallites. Because of the short range of the super-exchange coupling between Fe ions in a solid, the magnetic properties can give information on local configurations even in the absence of perfect crystalline coherence. Therefore, the magnetic properties of rust samples were investigated in detail using a Superconducting Quantum Interference Device (SQUID) magnetometer and Mössbauer spectroscopy. SQUID magnetometry is effective to determine the quantity of the ferrimagnetic phase. The temperature dependence of the Mössbauer spectrum gives information about not only the fractions of the phases but also the distribution of grain volume, V, in each phase according to the super-paramagnetic relaxation effect. This approach has been applied to rust of conventional [T. Okada, Y. Ishii, T. Mizoguchi, I. Tamura, Y. Kobayashi, Y. Takagi, S. Suzuki, H. Kihira, M. Ito, A. Usami, K. Tanabe, K. Masuda, Jpn. J. Appl. Phys. 39 (2003) 3382] and advanced weathering [H. Kihira, A. Usami, K. Tanabe, M. Ito, G. Shigesato, Y. Tomita, T. Kusunoki, T. Tsuzuki, S. Ito, T. Murata, in: Proc. Symp. on Corrosion and Corrosion Control in Saltwater Environments, Honolulu, 1999, The Electrochemical Soc., pp. 127-136] steels.The grains of the rust formed on advanced weathering steel have clearly bimodal Gaussian distributions of volume with peaks at V ≈ 5 × 10⁻²⁴ m³ and V ≈ 16 × 10⁻²⁴ m³ in α-FeOOH and β-FeOOH phases. The outer layer has grains of γ-FeOOH which are an order of magnitude smaller. The inner layer, in contrast, has a continuous distribution of grain volume, consistent with the formation of a continuous densely packed thin protective rust layer which prevents further corrosion.     

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

The compositional change of rust (corrosion products) layer formed on weathering steel exposed to atmosphere with different amount of air-borne sea salt particles in Japan have been investigated by the X-ray diffraction method. The mass ratio (α/γ) of crystalline α-FeOOH to γ-FeOOH, in the rust layer formed on the weathering steel exposed in an industrial environment, increases with an increase in exposure duration. The α/γ 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 (α/γ^∗) of crystalline α-FeOOH to the total mass of γ-FeOOH, β-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 α/γ^∗ is more than 1, a higher corrosion rate more than 0.01 mm/year (3.17 × 10⁻¹³ m/s) is not observed. The α/γ^∗ is a protective ability index of rust formed on weathering steel.     

Characterization of the passive film formed on weathering steels

Raman spectroscopy, infrared spectroscopy and electrochemical reduction techniques were used to characterize the passive film formed on weathering steels. Test pieces of weathering steel were exposed to an industrial environment for 4.5 and 8 yrs. In both cases the passive film was composed predominantly of 8-FeOOH with 10–20% γ-FeOOH and possibly some α-FeOOH. Raman spectra from different depths of the surface were identical which implied that the film composition was homogeneous.     

A corrosion prediction method for weathering steels

Japanese Specification for Highway Bridges [Japan Road Association: Specification for Highway Bridges—I & II, 2002.3] has been revised in 2002, which clearly states that degradation of bridge members, including those of weathering steels, to occur in prolonged period of time must be taken into account at the design stage to realize long term structural durability. To cope with the revised paradigm, SABI chemistry committee in Japan Society of Corrosion Engineers has proposed durable state concept [SABI Chemistry Committee of Japan Society of Corrosion Engineers, in: Proc. 132nd Symposium for Corrosion and Protection, 2001.6.25, p. 3], which is defined as a condition in which corrosion rate of steel is slow enough not to form thick rust [H. Kihira, K. Shiotani, H. Miyuki, T. Nakayama, M. Takemura, Y. Watanabe, Doboku Gakkai Ronbun-shu (J. Japan Soc. Civil Engineers) No. 745/I-65, 2003.10, pp.77-87]. Since corrosiveness of atmosphere differs by location, a corrosion prediction method for both conventional and advanced weathering steels [H. Kihira, A. Usami, K. Tanabe, M. Ito, G. Shigesato, Y. Tomita, T. Kusunoki, T. Tsuzuki, S. Ito, T. Murata, in: Electrochemical Society Proceedings, vol. 99-26, 2000, pp. 127-136] is needed to ensure structural durability for future. Thus, versatile computational scheme for it has been derived through mathematical modeling based upon the durable state concept.     

Abnormal relationship between rust particles size and rust layer compactness of weathering steels

In order to reveal evolution of the rust layer during atmospheric corrosion, commercial weathering steel (WS) 09CuPCrNi and a recently developed bainite WS were subjected to a salt fog test. The protection and compactness of the rust layer were evaluated by electrochemical analysis, absorption-desorption test, etc. The experimental results indicate that more compact rust layer could be derived from the rust particles with larger size. The well-established notion that the rust particle growth can induce the decrement of corrosion resistance of WS. However, the present investigation shows that the notion might not be universally accurate. The unusual result means that the rust particles might grow anisotropically during corrosion.     

Effects of rare earth elements on corrosion behaviors of low-carbon steels and weathering steels

The effects of rare earth elements on the corrosion properties of low-carbon steel and weathering steel were investigated. To elucidate the roles of rare earth elements (Ce and La) and the corrosion behavior of steels, salt spray tests, electrochemical techniques, X-ray diffractometer, scanning electron microscopy, Electron Probe Micro Analysis (EPMA), and Auger Electron Spectroscopy (AES) were conducted. The results showed that the addition of rare earth elements enhances the corrosion resistance of both low-carbon steels and weathering steels, indicated by lower corrosion current density and salt spray corrosion rate after rare earth alloying. On the one hand, rare earth elements modify the morphology of inclusions and thus slow down the micro-area electrochemical corrosion, which improves the electrochemical corrosion resistance of these two steels. On the other hand, rare earth atoms tend to segregate toward the interface between the rust layer and the matrix. Hence, salt spray corrosion resistance is improved due to the enhancement of adhesion and compactness of the rust by the addition of rare earth elements.     

Effect of nickel on ion-selective property of rust formed on low-alloying weathering steel

The effect of nickel on the ion-selective property of the rust layers formed on the low alloy weathering steels under marine atmosphere was investigated by an alternate wet-dry accelerated corrosion test.Four types of low-alloying steels with different Ni contents were prepared for the corrosion test.The rust formed at various stages of corrosion was characterized by electrochemical impedance spectra(EIS),electron microprobe analysis(EMPA) and scanning electron microscopy(SEM).Afterwards,the corrosion resistance property of the rust was studied in the light of the experimental results.It is shown that increasing Ni content has significantly improved the corrosion resistance of the low-alloying steels under marine atmosphere.Furthermore,it is noted that an evident cation-selective property emerges in the rust layers with the Ni content of the rust layers up to 1.82%.However,when the Ni content of steel the matrix is below 1.5%,it has little effect on the cation-selective property.The ratio of Ni content in between the rust layer and the steel substrate decreases with the increase of Ni content in the steel substrate and varies little as the corrosion proceeds.     

Study of the corrosion behavior of weathering steels in atmospheric environments

The effect of chemical composition of steel and environmental conditions on the atmospheric corrosion behavior of weathering steel and carbon steel exposed in the different environments has been investigated. The results showed that the corrosion rate of weathering steel was lower compared with carbon steel due to the alloying additions of Cr, Cu, P, and Ni. The different environmental factor affected the corrosion behavior of steel in the different stages. The chemical composition of steel and environmental conditions influenced the corrosion rate, as well as the morphology and composition of the rust layer.     

Characterization of the rust formed on weathering steel exposed to Qinghai salt lake atmosphere

The product formed on weathering steel exposed to salt lake atmosphere for 12 months was investigated by X-ray diffraction (XRD), infrared transmission spectroscopy (IRS), scanning electron microscopy (SEM), electron probe micro analyzer (EPMA) and electrochemical techniques. The rust was mainly composed of β-FeOOH, Fe₈(O,OH)₁₆Cl_1.3 and a little γ-FeOOH. Amorphous δ-FeOOH was only on skyward surface. The rust layer suppressed anodic reaction and facilitated the cathodic reaction. The very small value of rust resistance R_r in this work indicated that the rust had poor protective ability. Cl element was rich in the whole rust layer and played an important role in accelerating the corrosion of weathering steel in salt lake atmosphere.     

Atmospheric corrosion of Ni-advanced weathering steels in marine atmospheres of moderate salinity

One conventional and three Ni-advanced weathering steels have been exposed for one year in two marine atmospheres of moderate aggressivity (30 and 75 mg Cl⁻/m² d). The rusts generated have been analysed by polarised light optical microscopy, SEM, XRD, Raman spectroscopy and Mössbauer spectroscopy.The presence of high nickel (1–3% weight) contents in the steel composition leads to higher corrosion resistance in moderate marine atmospheres. The presence of nickel in the weathering steel also raises the proportion of nanophasic (superparamagnetic) goethite in the inner rust layer.     

Taxonomy for protective ability of rust layer using its composition formed on weathering steel bridge

For a quantitative evaluation of the protectiveness of a rust layer formed on a weathering steel bridge, the relationship between the corrosion rate of the bridge and the composition of the rust layers formed on the girders was first investigated. These corrosion rates were clearly classified by the protective ability index (PAI) of α/γ^∗ and (β + s)/γ^∗, where α, γ^∗, β and s are the mass ratio of crystalline α-FeOOH, the total of γ-FeOOH, β-FeOOH and the spinel-type iron oxide (mainly Fe₃O₄), β-FeOOH and spinel-type iron oxide, analyzed by XRD, respectively. The inequality of the former index α/γ^∗ > 1 expressed the protectiveness criterion of the rust layer, while that of the latter index, (β + s)/γ^∗< 0.5 or > 0.5, classified the corrosion rate of the non-protective rust layer. The PAI is useful for a quantitative evaluation of the protectiveness of a rust layer formed on a weathering steel bridge and is an important item for the corrosion assessment of the bridge.     

A new interpretation of the corrosion loss processes for weathering steels in marine atmospheres

Most available data sets for the long-term corrosion loss of various grades of weathering steel exposed to marine atmospheric environments are demonstrated to be consistent with the multi-phase corrosion model previously proposed for steels exposed to marine environments. This means that the early corrosion of weathering steels by oxidation is gradually inhibited by the build-up of corrosion products. These produce anoxic and sub-oxic conditions that may permit microbiological activity to govern the longer-term corrosion loss process. This new interpretation for the long-term corrosion of weathering steels may have implications for the design of such steels.     

Cu和Cr对耐候钢的力学性能及耐蚀性能的影响

研究了不同Cu和Cr含量对低合金高强耐候钢的力学性能及耐腐蚀性能的影响规律。结果表明,Cu元素含量对合金的强度影响较为明显,随Cu含量增加,合金的屈服强度逐渐提高;随Cr含量增加,合金屈服强度也略有上升,但强化效果不明显。Cu和Cr的加入均会提高合金的韧脆转变温度,但冲击功的绝对值仍保持在较高的水平。Cu和Cr元素的加入,增加了锈层的致密度,使得合金的耐腐蚀性能得到明显提升。     

低合金钢在海水中局部腐蚀锈层结构的研究

使用光学显微镜、电子探针、电子显微镜、微电化学测试等方法对两种低合金钢在海洋条件下的局部腐蚀锈层结构进行了研究。研究工作表明,低合金钢虽然具有较碳钢严重的局部腐蚀,但当该种合金钢的局部腐蚀锈层具有合金元素富集、致密等特征时,局部腐蚀将受到抑制,其原因是蚀坑内含富集合金元素的腐蚀产物成了难溶性盐,复盖了蚀坑底部基体,阻滞了腐蚀过程,降低了局部腐蚀速度。而在蚀坑外的平坦区域则没有发现合金元素的明显富集——局部腐蚀由向深度方向转而朝着扩大蚀孔直径的方向发展。这种变化的规律可由微电化学的蚀坑内外ΔmV-d曲线明显地看到所研究的钢种中,AlCuP钢具有较为典型的上述特征。NlCuAs钢的局部腐蚀行为与碳钢相近。     

A Mössbauer spectroscopic study of rust formed during simulated atmospheric corrosion

Steel coupons were subjected to 100% relative humidity and were inoculated every day with 100 μl of 0.01 N solutions of NaCl, Na₂SO₄, LiCl or CsCl. The first solid rust constituent that formed contained significant amounts of both γ-FeOOH and ferrihydrite. In contrast, only γ-FeOOH was observed in the rust formed during atmospheric corrosion and during wet-dry cycling with distilled water in the laboratory. The ferrihydrite in time was converted to γ-FeOOH, α-FeOOH, and γ-Fe₂O₃. The fractions of ferrihydrite + γ-FeOOH in the rust formed as a function of time during atmospheric exposure and during rusting in the laboratory environment were the same in the two cases.     

Lost iron and iron converted into rust in steels submitted to dry-wet corrosion process

During the electrochemical corrosion of iron and steel not all the iron removed from the steel forms later part of the iron phases that remain stick to the surface. Here we report numerical data that demonstrates that the conversion of iron into adherent rust is not always equal to one, at least for steels submitted to chloride ions in dry-wet cyclical processes. For that purpose, the adherent (AR) and non-adherent rusts (NAR) formed on weathering and carbon steels exposed to these tests were weighted and fully characterized using different techniques. We have found that 2-38% of the corroded iron is lost, while 3-18% is converted into NAR. The NAR for both steels are composed of lepidocrocite, goethite, hematite, and traces of akaganeite, independent of the investigated exposure times and chloride concentrations. The AR was composed of lepidocrocite, spinel phase, goethite and akaganeite. Comparison of these results with those obtained in total immersion tests is also addressed.     

某耐候钢桥锈层稳定化处理技术的探索

文中对闪光焊接在铁塔用耐候钢上的应用展开研究,闪光焊接是通过焊接面的短路进行加热,加载顶锻变形过程界面上动态再结晶形成焊缝。焊接件经退火后,焊缝与热影响区均得到与基体相近的珠光体+铁素体组织,焊接试样与基体力学性能趋于一致。耐候钢闪光焊接无需引入其他焊料从而保证了焊缝与基体化学成分相同,并可通过热处理消除焊缝与基体之间组织差异,获得焊缝与基体力学性能的一致性。这对于无涂装输变电铁塔应用,闪光焊接一方面保证了焊接构件机械性能稳定性,另一方面避免了焊缝与基体之间因组织成分差异所带来的较大电位差,在自然环境下导致电化学腐蚀,从而满足耐候钢焊接构件免涂装服役性能要求。

     

Morphology of corrosion products formed on cobalt and nickel in argon-oxygen-chlorine mixtures at 1000 K

The morphology of corrosion products formed on cobalt and nickel in argonoxygen-chlorine mixtures at 1000 K have been examined using optical microscopy, scanning electron microscopy, and X-ray diffraction. The corrosion products formed under conditions where the oxides are thermodynamically stable consist of porous oxides containing little or no chlorine. This morphology is consistent with a corrosion mechanism involving vapor phase transport of volatile metal chlorides.     

Weathering structural steels corrosion in atmospheres of various degrees of pollution in Romania

The atmospheric corrosion behaviour of CORTEN weathering steel grade in various atmospheric environments in Romania is presented. The researches were accomplished on two types of weathering structural steels exposed 20 years in urban‐industrial and rural environments (characterised according to ISO 8565 and ISO 9223 as corrosivity classes C5 and C1). The experimental results emphasise that corrosion rate stabilised at 0.08–0.10 mm/year after about 10 exposure years, and electronic microscope examination showed that corrosion products layer enriches in alloying elements (Cr, Ni, P, Cu) and in stable crystalline products like Fe₂O₃ and α‐FeOOH that passivate steel. Passive layer compactness, studied by its moistening capacity, increases with time. Corrosion products layer composition and elements distribution was studied using an electronic microprobe; crystalline phases composition was determined by X‐ray diffraction.