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 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.     

Electrochemical behaviour and structure of rust formed on Si- and Al-bearing steel after atmospheric exposure

The corrosion resistance of Si- and Al-bearing steel was estimated by atmospheric exposure test, and the structure of the rust was examined by EPMA (electroprobe X-ray microanalysis) and TEM (transmission electron microscopy) analysis. Moreover, the electrochemical behaviour of rust was investigated by EIS (electrochemical impedance spectroscopy).The Si- and Al-bearing steel exhibited excellent corrosion resistance in the exposure test as compared with carbon steel (SM). EPMA and TEM analysis showed that Si and Al mainly existed in nanoscale iron complex oxides in the inner rust formed on this steel. The Al K spectrum of the rust exhibited a peak that was the same as that of Al₂O₃ in the EPMA and TEM-EELS (electron energy loss spectroscopy) analysis. This result suggests that Al was present in the complex oxides as Al³⁺. In the same way, Si was identified as being in an intermediate state in the complex oxides of the inner rust.EIS measurement of the exposure test samples revealed much higher rust resistance (R_rust) and corrosion reaction resistance (R_t) of Si- and Al-bearing steel compared to that of SM. Finally, it was found that nanoscale complex iron oxides formed in the inner rust of Si- and Al-bearing steel, resulting in increased R_rust and R_t, and corrosion suppression.     

Systematic approaches toward minimum maintenance risk management methods for weathering steel infrastructures

Since the revised Japanese Specification for Highway Bridges clearly states that degradation of bridge members to occur in a prolonged period of time must be taken into account as a risk, wide scope of discussion has been carried out in Japan. The final goal of these activities is to realize the ultra long term durability of steel infrastructures with a minimized maintenance cost. To reduce risks for anomalous rusting to occur on weathering steel structures, a computerized corrosion prediction system, visual as well as instrumental rust monitoring methods, repairing or retrofitting methods in case of contingency, etc. have been developed. Unlike organic or inorganic materials, weathering steels do not deteriorate internally. Therefore, all corrosion risks can be monitored by the inspection of steel member surfaces. The present work proposes a systematic management method for Risk Based Minimum Maintenance (RBMM) of weathering steel bridges.     

Atmospheric corrosion resistance of MnCuP weathering steel in simulated environments

In this work, atmospheric corrosion resistance of low cost MnCuP weathering steel in simulated coastal, industrial, and coastal–industrial atmospheric environments was investigated by wet/dry cyclic acceleration corrosion tests. The results indicate that MnCuP weathering steel exhibits high corrosion resistance in the three atmospheres. Besides, the alloying effect of Mn, Cu, and P elements on the anti-corrosion mechanism of MnCuP weathering steel was discussed by techniques of X-ray photoelectron spectroscopy, potential–pH diagram, and electron probe microanalysis.     

Rust formation and corrosion performance of Si- and Al-bearing ultrafine grained weathering steel

A high Si, Al type ultrafine-grained (UFG) weathering steel was produced by the multi-pass warm rolling method, and its corrosion resistance was estimated by a cyclic wet/dry corrosion test using chloride ions. The Si- and Al-bearing UFG steel exhibited excellent corrosion resistance in comparison with Si–Mn carbon steel (SM).

EPMA and TEM analyses showed that Si and Al mainly exist in nano-scale iron complex oxides in the inner rust layer formed on the developed steel. The Al K X-ray spectrum of Al₂O₃ and metallic Al had different peak positions due to chemical shifts, and that of the test sample was the same as that of Al₂O₃ in the EPMA analysis. This result suggested that Al was present in the complex oxides as Al³⁺. In the same way, Si was identified as an intermediate state such as Si²⁺ in the complex oxides of the inner rust.

EIS (electrochemical impedance spectroscopy) measurement of the corrosion test samples revealed that the rust resistance (R_rust) and corrosion reaction resistance (R_t) of the developed steel were much larger than those of SM. It was found that nano-scale complex iron oxides formed in the lower layer of iron rust in the developed steel, resulting in increased R_rust and R_t, and finally suppressing corrosion.     

The influence of chromium content on the electrochemical behavior of weathering steels

To investigate the influence of chromium content on corrosion characteristics of weathering steels, the electrochemical measurements were performed on the steels containing 0–9% Cr (wt.%) in NaHSO₃ aqueous solution. The results indicated that the open circuit potential of these steels shifted to the positive direction remarkably, because the additions of Cr improved the passivation capability of the steels. The corrosion current density of the steels containing more than 7% Cr (wt.%) decreased significantly after pre-rusted treatment, implying the corrosion resistance could be enhanced by the formation of protective goethite rust layer.     

In-depth distribution of rusts on a plain carbon steel and weathering steels exposed to coastal-industrial atmosphere for 17 years

In-depth distribution of rusts on two weathering steels and a plain carbon steel exposed to atmosphere for 17 years under a bridge at a coastal + industrial region in Japan were studied. In the rust layer on all specimens, α-FeOOH, β-FeOOH, γ-FeOOH, Fe₃O₄ and so-called amorphous rust were found. Within rust layers, there were thick parts and thin parts, which were finely and complicatedly distributed on steels. Among these rust species, α-FeOOH was dominant on all specimens. α-FeOOH appeared almost homogeneously through the rust layer. Its concentration was higher on weathering steels than on plain carbon steel. β-FeOOH was found mainly at thick parts and was scarce at thin parts of rust layers. Concentration of α-FeOOH was higher and that of γ-FeOOH was lower on weathering steels than on plain carbon steel. Amorphous rust was located at the bottom of the rust layer irrespective of steel types. Concentration of magnetite was negatively correlated with concentration of β-FeOOH.     

Long-term monitoring of atmospheric corrosion at weathering steel bridges by an electrochemical impedance method

Weathering steel corrosion was monitored for one to two years under natural atmosphere by an electrochemical impedance technique. Two identical comb-shape weathering steel sheets embedded in epoxy resin were used as monitoring probe electrodes at two different bridges in Japan. Impedances at 10 kHz (Z_10kHz) and 10 mHz (Z_10mHz) were automatically measured every hour. Coupons (50 × 50 × 2 mm³) prepared from the same steel sheets were exposed together to measure the corrosion mass loss. The average (Z_10mHz)⁻¹ value for half to one year exposure correlated well with the average corrosion rate determined from the corrosion mass loss.     

Clarification of chemical state for alloying elements in iron rust using a binary-phase potential-pH diagram and physical analyses

A binary-phase potential-pH diagram has been investigated to evaluate the chemical stability of various kinds of double oxide rusts (Fe-X) to get a principle for alloy design enhancing the corrosion resistance of steels. It was found that there are the following types of alloying elements enhancing the corrosion resistance of steels in the rust: (1) iron substitution type (Ni), (2) oxide formation type (Al), (3) metallic type (Ru), and (4) oxygen-acid salt type (WO₄). X-ray photoelectron spectroscopy and transmission electron microscopy analyses have been conducted on the rust formed on the low alloy steel in a saline environment. The analytical results were discussed using potential-pH diagrams. The iron substitution type and the oxide formation type elements make spinel double oxides with iron. In the corrosion tests, steels added with Ni or Al had high corrosion resistance. Thus it is possible to obtain high corrosion resistance by the creation of spinel double oxide such as Fe₂NiO₄ and FeAl₂O₄ in an inner layer.On the other hand it was found that the metallic type and the oxygen-acid salt type elements were not contained into the iron rust. In particular the oxygen-acid salt elements were excluded from the iron rust and concentrated into the defects of the rust. It is suggested that insoluble salts like FeWO₄ are formed on the base metal in the defects to act as an anodic inhibitor. Thus, the addition of a small quantity of W gives high corrosion resistance.The penetration of Cl ions can be prevented by the spinel double oxide in an inner layer and the oxygen-acid salt in the defects. In this way, the high corrosion resistance by the addition of these elements can be understood from the potential-pH diagram and the physical analyses.     

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.     

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.     

The corrosion resistance of ultra-low carbon bainitic steel

The corrosion resistance of ultra-low carbon bainitic (ULCB) steel was compared with a weathering steel 09CuPCrNi through accelerated corrosion tests. The results indicated that the corrosion resistance was almost the same for ULCB and 09CuPCrNi based on the weight loss. It can be seen that the grain refinement did not deteriorate corrosion resistance property. The homogeneous microstructure, lower carbon content and random distributing ∑3 boundary could effectively increase the corrosion resistance of the ULCB steel. The characteristics of the rust layers indicated that the inner rust layer contained nanocrystalline Fe_3−xO₄ particles, while copper and chromium alloying additions were enriched at the rust layer and substrate interface in ULCB steel. These factors played important roles in forming a compact protective rust layer.     

Assessment of protective function of steel rust layers by N2 adsorption

The specific surface area (SA) of the rusts formed by exposing various kinds of steels in different situations was determined by N₂ adsorption. The SA values of the rusts increased with the increase of corrosion rate, implying that the rust layers with large SA exhibit a high resistance to corrosion. The suppression of rusting by compact rust layers was interpreted by the blockage of pores in rust layers by the adsorption and capillary condensation of water. The SA values clearly reflect the corrosion levels estimated by the external observation. It was convinced that the SA measurement is a universal quantitative technique to appraisal the protective function of rust layers.     

The role of rusts in corrosion and corrosion protection of iron and steel

The processes of atmospheric corrosion of iron and steel and the properties of corrosion products (rusts) are modeled based on a quantitative evaluation of the chemical reactions pertaining to corrosion to elucidate the conditions with which corrosion-protective rust films form. Based on the model, it is suggested that in the initial stage of corrosion, in the rusts, the pH of the aquatic system is maintained at 9.31 owing to an equilibrium with iron(II) hydroxide and the rate of air-oxidation at this pH is very fast, and that dense, self-repairing rust films form, protecting the underlying iron and steel. However, after corrosion stops, the rust film deteriorates due to the dissolution and shrinkage by aging, and the deteriorated rust film separates the anode and cathode reaction products (Fe²⁺ and OH⁻ ions) to cause crevice corrosion. The air-oxidation of iron(II) in anode channels without the presence of OH⁻ ions results in strongly acidic solutions (pH 1.41), causing acid-corrosion. It is proposed that good catalysts (e.g. copper(II) and phosphate ions) accelerate the air-oxidation at low pH, delaying the crevice- and acid-corrosion stages. Further, it is argued that iron compounds with negative charges due to the non-stoichiometric proportions of the lattice oxide ions and metal ions (solid oxoanions of iron) exhibit stable cation-selective permeability even with a drop in pH. Rust films including such compounds would stop the passage of aggressive anions and act to protect iron and steel.     

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.     

Nano-scale chemical analysis of rust on a 2% Si-bearing low alloy steel exposed in a coastal environment

Corrosion products of the rust layer formed on a 2% Si-bearing low alloy steel during atmospheric exposure at a coastal area in Japan for three years were characterized. The inner layer of the rust, i.e., near oxide/metal interface, has a layer structure and the average grain size of corrosion products is finer than that in the upper region that is few micro meter above the interface. A series of nano probe energy disperse X-ray spectroscopy (EDS) and selected area electron diffraction (SAED) analyses performed using a field emission gun (FEG) transmission electron microscope (TEM) revealed that the inner layer consists of an iron rich, coarse grained β-FeOOH matrix and the silicon enriched narrow band composed a mixture of α-FeOOH and nano-meter size silicon oxide. On the other hand, the α-FeOOH single phase area exists where the layer structure did not develop. The result of detailed microstructural analyses indicated that the silicon plays an important role in modifying the rust layer structure.     

Influence of carbon content and microstructure on corrosion behaviour of low alloy steels in a Cl containing environment

Corrosion behaviour of low alloy steels (A and B) with different carbon content was studied by a salt fog test and an outdoor test. A commercial weathering steel 09CuPCrNi was used for comparison. The corrosion resistance of steels A and B with homogeneous microstructures was better than that of the commercial weathering steel 09CuPCrNi in the salt fog test. Steel A with an ultra-low-carbon content had far less weathering resistance than the other steels in the outdoor test. Selective corrosion of large pearlite produces stress in initial corrosion product films. Uniform corrosion product films with few cracks tend to form on homogeneous microstructures such as ferrite and bainite, and this is advantageous for the formation of a compact rust layer in the initial stage of atmospheric corrosion. However, uniform microstructures will result in over even interfaces between rust layers and bases, which will lead to frequent peeling of rust layers from bases because stress is induced by large temperature fluctuations and wet-dry alternations. Protection of the rust layer on a low alloy steel is dependent on the rust density and the bonding performance of the rust-base rather than the proportion of the rust phase in the initial stage of atmospheric corrosion. These results indicate that homogenous microstructures, proper amounts of carbon content and fine carbon-rich phases that are produced by appropriate processes are beneficial for the corrosion resistance of steels.     

Enhancement of electric conductivity of the rust layer by adsorption of water

The rust layers on the weathering steels exposed for 17 or 18 y at seven different sites in Japan were studied by electrochemical impedance under wet condition and measurement of amount of water vapor adsorbed. The transient of amount of adsorbed water vapor into the rust layer during humidity jump from 0% to 80% RH indicated that saturation of adsorbed water vapor took place within 1 h. From the semi-infinite model of diffusion, apparent diffusion coefficients of H₂O in the rust were calculated. It was found the apparent diffusion coefficient was higher when the amount of air-borne salt of exposure sites was higher. The impedance diagram of the rusted steels was analyzed with an equivalent circuit consisting of a series connection between an ohmic resistance and a parallel circuit of charge transfer resistance-redox capacitance. The double layer capacitance on the gold electrodes pressed onto the rust layer was further added on the circuit. The impedance showed that the rust layer behaved as a dielectric layer under dry condition, while the conductance was greatly enhanced with wet condition. The charge transfer resistance evaluated from the simulation under wet condition was largely dependent on the amount of air-borne salt in environment. It was concluded that the rust layer formed in the site with relatively large amount of air-borne salt revealed high conductivity under wet condition. For the rust formation followed by the Evans model during wet-dry cycles, the higher conductivity induced the more facilitative reduction of the rust layer under wet condition and the larger growth of the rust layer in a wet-dry cycle.