Corrosion resistance of the Dhar iron pillar

The corrosion resistance of the 950-year old Dhar iron pillar has been addressed. The microstructure of a Dhar pillar iron sample exhibited characteristics typical of ancient Indian iron. Intergranular cracking indicated P segregation to the grain boundaries. The potentiodynamic polarization behaviour of the Dhar pillar iron and mild steel, evaluated in solutions of pH 1 and 7.6, indicate that the pillar iron is inferior to mild steel under complete immersion conditions. However, the excellent atmospheric corrosion resistance of the phosphoric Dhar pillar iron is due to the formation of a protective passive film on the surface. Rust analysis revealed the presence of crystalline magnetite (Fe_3−xO₄), α-Fe₂O₃ (hematite), goethite (α-FeOOH), lepidocrocite (γ-FeOOH), akaganeite (β-FeOOH) and phosphates, and amorphous δ-FeOOH phases. The rust cross-section revealed a layered structure at some locations.     

Characterization of Delhi iron pillar rust by X-ray diffraction, Fourier transform infrared spectroscopy and Mössbauer spectroscopy

Rust samples obtained from the region just below the decorative bell capital of the Delhi iron pillar (DIP) have been analyzed by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and Mössbauer spectroscopy. The identification of iron hydrogen phosphate hydrate in the crystalline form by XRD was unambiguous. Very weak diffraction from the oxyhydroxides/oxides of iron was observed indicating that these phases are most likely to be present in the amorphous form in the rust. The present XRD analysis of rust obtained from an inaccessible area of the DIP has also been compared with earlier analyses of DIP rust obtained from regions accessible to the public. FTIR indicated that the constituents of the scale were γ-, α-, δ-FeOOH, Fe_3−xO₄ and phosphate, and that the scale was hydrated. The unambiguous identification of the iron oxides/oxyhydroxides in the FTIR spectrum implied that they are present in the amorphous state, as XRD did not reveal these phases. The FTIR results have also been compared with earlier FTIR spectroscopic results of atmospheric rust formation. Mössbauer spectroscopy indicated that the rusts contained γ-FeOOH, superparamagnetic α-FeOOH, δ-FeOOH and magnetite, all in the amorphous form. The Mössbauer spectrum also confirmed that iron in the crystalline iron hydrogen phosphate hydrate, whose presence was confirmed by XRD, was in the ferric state indicating that it was a stable end corrosion product.     

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.     

低碳钢在海水中的阴极电化学行为

采用电化学技术结合XRD分析, 研究了A3碳钢在海水中的阴极电化学行为, 探讨了锈层在阴极过程中的作用. 碳钢表面生成的锈层由内锈层和外锈层组成, 内锈层主要组成相为γ-FeOOH, α-FeOOH, β-FeOOH以及Fe₃O₄与γ-Fe₂O₃的混合物. 浸泡126 d时, 外锈层主要由γ-FeOOH组成; 浸泡364 d由γ-FeOOH, α-FeOOH, Fe₃O₄和 γ-Fe₂O₃组成. 不同锈层在阴极过程中所起的作用不同. 外锈层主要作用是阻碍溶解氧到达金属表面, 内锈层除此之外还可以参与还原反应, 加速阴极反应. 提出了一个评价锈层参与还原反应程度的参数α, 在浸泡不同时期锈层参与还原反应的比例不同, 浸泡前7 d, α值上升比较明显, 随后增加比较缓慢, 浸泡168 d后基本稳定. 探讨了内、外锈层组分的变化以及锈层各组分间的相互作用.     

Morphology of rust phases formed on weathering steels in various laboratory corrosion tests

The morphology and growth characteristics of rust phases formed on ASTM A-588 weathering steel in three different types of laboratory tests—accelerated atmospheric exposure simulation tests (AAEST), salt fog test, and continuous immersion test in plain as well as salt water—are analyzed using microstructural information obtained from representative exposed specimens studied in a scanning electron microscope (SEM). The ultimate and most dominant phase in the AAEST was α-FeOOH whereas an amorphous phase designated as amorphous bulk (AB) appeared as “cotton bolls” in the adherent, sedimentary layer formed on the steel surface during continuous immersion. Crystalline phases α-, δ-, and γ-FeOOH as well as γ-Fe₂O₃.H₂O were found developed on top of the first-formed sedimentary amorphous layer, containing another amorphous phase designated as amorphous mix (AM). Magnetite was the dominant phase obtained in the salt fog test. It forms in layers and seems to transform to α-FeOOH through formation of whiskers and rods on its surface. Sandy grains of γ-Fe₂O₃.H₂O were also seen in the rusts obtained in this test.     

Influence of manganese on iron oxyhydroxides and oxides formed in aqueous solution

Structural analysis techniques such as X-ray diffraction and anomalous X-ray scattering were used for characterizing the influence of manganese on iron oxyhydroxides and oxides formed from green rust (GR) in an aqueous solution. The results showed that the formation of Fe₃O₄ was enhanced by the addition of manganese ions during the conversion of GR2 to α-FeOOH and Fe₃O₄. The results obtained from anomalous X-ray scattering showed that manganese was present both in α-FeOOH and Fe₃O₄ particles. The incorporation of manganese in α-FeOOH appears to induce the distortion of the atomic-scale structure of α-FeOOH particles formed in an aqueous solution.     

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.     

海水飞溅区Ni-Cu-P钢的锈层和耐点蚀性能研究

在海水飞溅区对实验室冶炼的Ni-Cu-P钢、含Cu低合金钢和碳钢进行660 d的挂片实验,评价Ni-Cu-P钢的耐蚀性能;采用Fourier变换红外(FTIR)光谱、电感耦合等离子体原子发射光谱(ICP-AES)、电子探针(EPMA)、SEM和EDAX等技术,分析3种钢表面的锈层特征.结果表明,Ni-Cu-P钢表现出比...     

Influence of silicate ions on the formation of goethite from green rust in aqueous solution

We investigated the influence of silicate ions on the formation of goethite converted from hydroxysulphate green rust, which was synthesized by neutralizing mixted solution of Fe₂(SO₄)₃ and FeSO₄ with NaOH solution, by O₂ in an aqueous solution. The pH and oxidation-reduction potential of the suspension and the Fe and Si concentrations in supernatant solutions were analyzed. X-ray diffraction results for the solid particles formed during the conversion were consistent with the results of the solution analyses. The results indicated that silicate ions suppressed the conversion from green rust to α-FeOOH and distorted the linkages of FeO₆ octahedral units in the α-FeOOH structure.     

低碳贝氏体钢在青岛曝晒一年的锈层力学性能和抗热震性能研究

研究了低碳贝氏体钢在青岛曝晒一年后表面锈层的相组成、形貌、力学性能以及抗热震性能.结果表明:锈层由α-FeOOH、β-FeOOH、γ-FeOOH、Fe3O4以及非晶相组成;锈层的表面和截面形貌显示锈层较为致密,合金元素Cr和Cu在锈层中接近钢基体处有明显的富集现象;锈层的弹性模量和硬度随距锈层/钢基体界面的距离增大而降低;锈层/钢基体的结合强度和抗热震能力均高于锈层本身.     

Cu、Mn的协同作用对低合金钢在模拟海洋大气环境中腐蚀的影响

    用增重法、扫描电镜、X射线衍射、电子探针和X射线光电子能谱等手段研究了在模拟海洋大气干湿交替环境下16Mn钢和Cu-Mn耐候钢的腐蚀行为及Cu、Mn共添加对低合金钢腐蚀行为的影响.结果表明：Cu-Mn耐候钢的腐蚀速率低于16Mn钢,其锈层更致密;两种钢的铁锈均由Fe₃O₄,α-FeOOH,β-FeOOH,γ-FeOOH和大量无定形相组成;添加Cu使Fe₃O₄含量增加,添加Mn使γ-FeOOH含量减少;Cu在Cu-Mn耐候钢锈层中以CuFeO₂存在;Mn在两种钢锈蚀初期以MnO存在,后期为Mn₃O₄.Cu、Mn的协同作用使Cu-Mn耐候钢抗大气腐蚀性能优于16Mn钢.     

The reductive dissolution of γ-FeOOH in corrosion scales formed on cast iron in near-neutral waters

Corrosion scales which form on cast iron in oxygenated, near-neutral waters at 50°C contain green rust, Fe₂O₄ and γ-FeOOH. Upon de-oxygenation of the water, γ-FeOOH → Fe₃O₄ occurs by dissolution-precipitation as a cathodic reaction in the corrosion cell. Mechanisms of corrosion reactions are postulated in which it is proposed that the amount of γ-FeOOH is controlled by a balance between its rate of precipitation and its rate of reductive dissolution. The mechanism explains the eventual predominance of Fe₃O₄ in scales formed in oxygenated waters and the absence of γ-FeOOH from scales formed in waters with [O₂] < ～ 1 ppm.     

On the corrosion resistance of the Delhi iron pillar

The nature of the protective passive layer on the corrosion resistant Delhi iron pillar (DIP) has been addressed based on a detailed characterization of its rust. Rust characterization clearly established that the major constituents of the scale were crystalline iron hydrogen phosphate hydrate (FePO₄·H₃PO₄·4H₂O), α-, γ-, δ-FeOOH and magnetite. The iron oxide/oxyhydroxides were present in the amorphous form. The role of slag particles in the matrix of the DIP iron in enhancing the passive film formation is briefly addressed initially. The process of protective rust formation on DIP iron is outlined based on the rust analysis. Initially, the corrosion rate of iron is high due to the presence of slag particles. This results in enhancement of surface P content. In the presence of P, the formation of a protective amorphous compact layer of δ-FeOOH, next to the metal surface, is catalyzed and this confers the initial corrosion resistance. The critical factor contributing to the superior corrosion resistance of the DIP, however, is the formation of iron hydrogen phosphate hydrate, as a thin layer next to the metal–metaloxide interface. The formation of the crystalline modification of this phosphate from the amorphous form is aided by alternate wetting and drying cycles (i.e. the environmental factor). The rate of corrosion is further lowered due to the low porosity content of the crystalline phosphate phase. The passive film formation on the DIP has been contrasted with the rusting of normal and weathering steels.     

A study of the evolution of rust on Mo–Cu-bearing fire-resistant steel submitted to simulated atmospheric corrosion

The corrosion evolution of a Mo–Cu-bearing fire-resistant steel in a simulated industrial atmosphere was investigated by corrosion weight gain, XRD, EPMA, XPS, and polarization curves. The results indicate that the corrosion kinetics is closely related to the rust composition and electrochemical properties. As the corrosion proceeds, the relative content of γ-FeOOH and Fe₃O₄ decreases and α-FeOOH increases, and the rust layer becomes compact and adherent to steel substrate. Molybdenum and copper enrich in the inner rust layer, especially at the bottom of the corrosion nest, forming non-soluble molybdate and Cu(I)-bearing compounds responsible for enhanced corrosion resistance of the rust layer.     

Phasenumwandlungen im Rost

Phase conversions in rust The principal components of rust are lepidocrocite (γ-FeOOH), göthite (α-FeOOH) and magnetite (Fe₃O₄). The first crystal phase, possibly via (FeOH)₂, is lepidocrocite. This may later be transformed into göthite and magnetite. The paper describes pseudo-morphoses of gothite and magnetite after lepidocrocite. Magnetite, which at higher temperatures is formed directly on the iron, grows epitactically and forms a protective layer which prevents further corrosion. In contrast, magnetite transformed from lepidocrocite cannot be expected to have such an effect. The necessary adhesion and freedom from pores is conditioned by the epitaxy of a protective layer.     

Dynamic equilibria of phases in the processes of the mechanosynthesis of an alloy with composition Fe72.6C24.5O1.1N1.8

X-ray diffraction, Mössbauer spectroscopy, and measurements of the dynamic magnetic susceptibility have been used to investigate phase states of the Fe_72.6C_24.5O_1.1N_1.8 alloy at different stages of the mechanosynthesis (MS) in a planetary ball mill. The introduction of impurities of O and N into an Fe₇₅C₂₅-based alloy changes the sequence of the formation of phases during MS: instead of Fe₃C, the Fe₇C₃ carbide is first to be formed. The processes of phase formation in the alloy preliminarily subjected to MS have unidirectional nature upon the continuation of the MS and upon annealings and are determined by the interaction of the alloy components with one another under the effect of the accumulated excess energy. The phase compositions of the MS alloys depend on the conditions of the dynamic equilibrium between the crystalline and amorphous phases.     

Hydrogen permeation and corrosion behavior of high strength steel MCM 430 in cyclic wet-dry SO2 environment

Hydrogen permeation caused by corrosion under a cyclic wet (2 h)-dry (10 h) SO₂ condition was investigated for a high strength steel of MCM 430 by using an electrochemical technique in addition to the corrosion behavior obtained from weight loss measurement and the determination of corrosion products by using X-ray diffraction method. The hydrogen content converted from hydrogen permeation current density was observed in both wet and dry periods. The origin of proton was estimated to be from (1) the hydrolysis of ferrous ions, (2) the oxidation of ferrous ions and ferrous hydroxide, and (3) hydrolysis of SO₂ and formation of FeSO₄, but not from the dissociation of H₂O. With respect to the determination of the corrosion products consisting of inner (adherent) and outer (not adherent) layers, the outer layer is composed of α-FeOOH, amorphous phase and γ-FeOOH, where α-FeOOH increases with the increase in the wet-dry cycle, and amorphous phase shows the reverse trend. The corrosion product in the inner layer is mainly Fe₃O₄ with them. On the basis of the results obtained, the role of the dry or wet period, the effect of SO₂ and the corrosion process during the cyclic wet-dry periods were discussed.     

Corrosion resistance and mechanical properties of low-alloy steels under atmospheric conditions

The corrosion resistance and mechanical strength of four newly developed low-alloy steels (LAS) were compared with a carbon steel (SS400) and a weathering steel (Acr-Ten A) using a laboratory-accelerated test that involved cyclic wet/dry conditions in a chloride environment (5 wt.% NaCl). The new LAS were designated 1605A, 1605B, 1604A, and 1604B. After 72 cycles of cyclic corrosion tests, the susceptibility of the steels to corrosion could be listed in the following order based on their weight loss (from high to low): SS400 > Acr-Ten A > 1604B ? 1604A > 1605B ? 1605A. The change in mechanical properties by corrosion was the least for SS400, Acr-Ten A was second, and effects of corrosion on the mechanical properties of the other four low-alloy steels were similar. Finally, the characteristics of the rust layers on each LAS sample were observed by SEM, and analyzed by FTIR and EPMA. The results indicated that most of the rust layers on the test steels were composed of a loose outer rust layer and a dense inner rust layer. The outer rust layer of each steel was composed of α-FeOOH, γ-FeOOH, magnetite (Fe₃O₄), H₂O, and amorphous ferric oxyhydroxide (FeO_x(OH)_3−2x, x=0-1), while the inner rust layer was composed mainly of Fe₃O₄ with a little α-FeOOH. In addition, it was apparent that the copper and chromium alloying additions were enriched, respectively, at the rust-layer/substrate interface and in the rust layers. Finally, combining the results of the accelerated tests and the rust layer analysis showed that low-alloy steels, such as 1605A and 1605B, have better weathering steel properties than Acr-Ten A for use in the humid and salty weather.     

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.     

Thermally induced crystallization of Fe73.5Cu1Nb3Si15.5B7 amorphous alloy

Thermally induced crystallization of Fe_73.5Cu₁Nb₃Si_15.5B₇ amorphous alloy occurs in two well-separated stages: the first, around 475 °C, corresponds to formation of α-Fe(Si)/Fe₃Si and Fe₂B phases from the amorphous matrix, while the second, around 625 °C, corresponds to formation of Fe₁₆Nb₆Si₇ and Fe₂Si phases out of the already formed α-Fe(Si)/Fe₃Si phase. Mössbauer spectroscopy suggests that the initial crystallization occurs through formation of several intermediate phases leading to the formation of stable α-Fe(Si)/Fe₃Si and Fe₂B phases, as well as formation of smaller amounts of Fe₁₆Nb₆Si₇ phase. X-ray diffraction (XRD) and electron microscopy suggest that the presence of Cu and Nb, as well as relatively high Si content in the as-prepared alloy causes inhibition of crystal growth at annealing temperatures below 625 °C, meaning that coalescence of smaller crystalline grains is the principal mechanism of crystal growth at higher annealing temperatures. The second stage of crystallization, at higher temperatures, is characterized by appearance of Fe₂Si phase and a significant increase in phase content of Fe₁₆Nb₆Si₇ phase. Kinetic and thermodynamic parameters for individual steps of crystallization suggest that the steps which occur in the same temperature region share some similarities in mechanism. This is further supported by investigation of dimensionality of crystal growth of individual phases, using both Matusita–Sakka method of analysis of DSC data and texture analysis using XRD data.