Ex-situ and in-situ X-ray diffractions of corrosion products freshly formed on the surface of an iron-silicon alloy

Ex-situ X-ray diffraction measurements of a small amount of samples extracted from wet corrosion products freshly formed on a pure iron and iron-2 mass% silicon surfaces have been conducted using synchrotron radiation for clarifying the formation process of corrosion products. The results showed that γ-FeOOH was formed on the outer side of wet corrosion products formed on the surface of the pure iron by sodium chloride solution, while γ-FeOOH, α-FeOOH, Fe₃O₄, and green rusts were formed on the inner side. On the other hand, in comparison to the case of the pure iron, a significant formation of β-FeOOH was observed in the iron-silicon alloy. Influences of silicon alloying on corrosion products formed by aqueous solution containing sulfate ions were also observed. Furthermore, in-situ diffraction measurements by a conventional X-ray source were conducted for analyzing corrosion products formed on the pure iron and iron-silicon alloy surfaces by cyclic exposure to wet and dry atmospheres. The results obtained by the in-situ diffraction and ex-situ diffraction measurements on the corrosion products were consistent.     

Corrosion behaviors of carbon steel induced by life activities of Phaeodactylum tricornutum in a marine environment

Microbiologically influenced corrosion induced by bacteria has been studied for many years. Corrosion is known to be sensitive to the presence of microalgae, such as Phaeodactylum tricornutum. However, the life activity of P. tricornutum that influences the general and localized corrosion of carbon steel is not fully understood. The current study uses a combination of immersion tests and electrochemical experiments with a detailed surface characterization to reveal the naturally formed corrosion products with/without the presence of P. tricornutum. The results show that samples suffer from pitting corrosion and the averaged pit depths are approximately 15 μm under a light–dark cycle condition or a 24-h constant light condition. Meanwhile, the corrosion products are mainly comprised of γ-FeOOH and Fe₃O₄ in a constant light condition. However, γ-FeOOH, Fe₃O₄, and FeCO₃ are found in a light–dark cycle. This study proposes the fundamental mechanisms of the effect of P. tricornutum life activities on the corrosion performance of Q235 carbon steel, to fulfill the knowledge gaps of the presence of microalgae inducing the general and pitting corrosion of carbon steel.     

Corrosion behaviour investigation of 460 low alloy steels exposed in the natural deep-sea environment

ABSTRACT

The corrosion behaviours of 460 low alloy steels were investigated by the field exposure corrosion experiments with various long periods and 1200–3000?m depths in the natural deep-sea environment. XRD analysis results indicate that corrosion products of β-FeOOH, γ-FeOOH and Fe₃O₄ increase and the corrosion product of α-FeOOH decreases with the exposure sea depth extending. Electrochemical experiment results reveal the corrosion status is aggravated with the exposure sea depth extending: corrosion potentials shift negatively to a minimum self-corrosion potential of ?0.590?V (vs. SCE) and charge transfer resistances decrease to a minimum of 278?Ω?cm². Corrosion behaviours analysis shows that the average corrosion rate decreases with the time increasing and reaches a maximum value of 101?μm?a^?1. The maximum pitting depth varies between 63.0?μm and 138?μm. Gray relational analysis shows that the corrosion rate of 460 steel is most influenced by the pressure and dissolved oxygen concentration of deep sea, exhibiting a positive correlation relationship.     

The mechanism of formation of iron oxide and oxyhydroxides in aqueous solutions at room temperature

For a better understanding of the atmospheric rusting of iron and steels, the present work is aimed to explore the mechanism of formation of green rusts, Fe₃O₄, α-FeOOH, β-FeOOH, γ-FeOOH, δ-FeOOH and amorphous ferric oxyhydroxide in aqueous solution at room temperature. The formation processes on which end products are determined are strongly affected by the oxidation rate, pH and the structure and composition of initial and intermediate species of iron. The systematic diagram of formation processes of iron oxide and oxyhydroxides has been presented, in which both dissolved and solid species of iron are included.     

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.     

Microstructure and corrosion performance of X100 steel in the oilfield produced water

ABSTRACT

The corrosion performances of X100 steels in the oilfield produced water with different temperature (30°C, 40°C, 60°C) under the simulated working conditions were studied by potentiodynamic polarization (PDP) and electrochemical impedance spectroscopy (EIS) test. Energy dispersive spectroscopy (EDS) was used to measure the element composition of the sample after corrosion. The phase and chemical compositions of the specimens were analyzed by X-ray diffraction (XRD) and Raman spectrometry. The electrochemical results showed that the corrosion resistance of the specimen exposed for 24 hours increased with the increase of the testing temperature, indicating the better protective effect due to the corrosion product film formed under high temperature despite the impact of turbulence. The corrosion products consist of Fe₃O₄, Fe₂O₃, α-FeOOH, and γ-FeOOH. The corrosion mechanism model of X100 steel was established and the corrosion behavior was analyzed under simulated working conditions.     

Characteristics of iron corrosion scales established under blending of ground, surface, and saline waters and their impacts on iron release in the pipe distribution system

Interior scales on PVC, lined ductile iron (LDI), unlined cast iron (UCI) and galvanized steel (G) were analyzed by XRD, RMS, and XPS after contact with varying water quality for 1 year. FeCO₃, α-FeOOH, β-FeOOH, γ-Fe₂O₃, Fe₃O₄ were identified as primary UCI corrosion products. No FeCO₃ was found on G. The order of Fe release was UCI > G ? LDI > PVC. For UCI, Fe release decreased as % Fe₃O₄ increased and as % Fe₂O₃ decreased in scale. Soluble Fe and FeCO₃ transformation indicated FeCO₃ solid was controlling Fe release. FeCO₃ model and pilot data showed Fe increased as alkalinity and pH decreased.     

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.     

Mössbauer and x-ray diffraction phase analysis of rusts from atmospheric test sites with different environments in Sweden

Rust samples from mild steel panels exposed at eight different atmospheric test sites in Sweden have been studied by transmission ⁵⁷Fe-Mössbauer spectroscopy and X-ray powder diffraction. All samples exhibited superparamagnetic effects in Mössbauer spectra recorded at room temperature, due to fine particles. The corrosion products were mainly identified in low temperature (∼10 K) Mössbauer spectra and with X-ray powder diffraction. In all samples, α-FeOOH and γ-FeOOH were the main constituents (>70%). Only in the marine samples were β-FeOOH and Fe_3-σO₄ found. The α-FeOOH/γ-FeOOH ratio obtained was found to increase with the SO₂ concentration at the test sites.     

Investigations on the oxygen corrosion behaviors of P110 steel in a dynamic experiment simulating nitrogen injection

Nitrogen injection is widely applied to enhance oil recovery, but the corrosion failure risk of tubing and casing is increased by residual oxygen in nitrogen. In this study, the corrosion behaviors of P110 steel under dynamic conditions with different oxygen partial pressures (2.5–0.003 MPa) simulating high-pressure (30 MPa) nitrogen injection were studied. The morphology and composition of corrosion products were characterized by scanning electron microscopy, energy-dispersive X-ray spectroscopy and energy-dispersive X-ray analysis methods; furthermore, the localized corrosion rate and three-dimensional surface morphologies were analyzed. The results show that the general corrosion rate decreased significantly with decreasing O₂ partial pressure, especially when O₂ partial pressure was below 0.5 MPa, and there is a corrosion morphology change from general corrosion to localized corrosion (pitting). Besides, the localized corrosion rates are about 20 times the general corrosion rates. The corrosion products are mainly FeOOH and Fe₃O₄. In addition, the scales in the intact area are relatively flat and dense, whereas the damaged scales are porous and brittle. Fluid flows not only changed the morphologies and microstructures, resulting in a more protective product layer but also promoted the transfer of the corrosive mediators and affected the development of localized corrosion.     

An electrochemical study of phase-transitions in rust layers

Isolated rust layers have been investigated by electrochemical methods to find out whether their reduction and re-oxidation can affect the atmospheric corrosion of iron. At potentials below 0 mV, first a thin Fe²⁺-containing surface layer is formed on top of the γ-FeOOH. This reduced surface layer can dissolve into the cell electrolyte at acid pH, or at lower potentials the Fe²⁺-ions can react with γ-FeOOH to Fe₃O₄. The formation of magnetite could be followed by in-situ magnetic measurements. The reduced surface layer can easily be oxidized back to γ-FeOOH, magnetite can partly be oxidized to γ-Fe₂O₃.     

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.     

西南地区某输油管道外腐蚀分析

目的研究西南地区某输油管道外腐蚀行为。方法现场检测,对土壤理化性质及腐蚀产物成分进行室内分析。结果开挖点A的土壤呈弱碱性,土壤腐蚀性弱,管道发生轻微均匀腐蚀,腐蚀产物主要为FeO(OH),Fe(OH)3和Fe3O4等铁的氧化物,对应的阴极反应为吸氧反应;开挖点B的土壤酸性强,硫酸根离子浓度高,土壤腐蚀性强,管道发生严重坑蚀,腐蚀产物主要成分为FeSO4·7H2O及少量碱式硫酸铁,对应的阴极反应以析氢反应为主。结论开挖点土壤的理化性质差异,导致管道的腐蚀形态、腐蚀产物和腐蚀机理显著不同。     

Morphology and phase composition of corrosion products formed at the zinc-iron interface of a galvanized steel

The corrosion products formed on the inner wall of pipes made of galvanized low carbon steel, exposed for ∼2 years to water flowing in a large household heating system, were analysed using X-ray diffraction, Mössbauer and Raman spectroscopic techniques, as well as metallographic techniques. Products grew in the form of large-sized tubercles that gradually developed causing base metal losses up to perforation of the steel pipe. Considerable differences in the phase composition were found between the products formed in contact with the steel and those constituting the outer part of tubercles. The former were mainly made of FeCO₃ (siderite), with small amounts of Zn₅(CO₃)₂(OH)₆ (hydrozincite), ZnCO₃ (smithsonite), (Fe,Zn)CO₃ mixed carbonate and CaCO₃ (calcite), the latter mainly by Fe(III) oxyhydroxide goethite. Both parts of the tubercles also contained small amounts of other ferric oxyhydroxides, γ-FeOOH (lepidocrocite) and β-FeOOH (akaganeite), and very small amounts of hematite. The procedures used proved effective for an adequate identification of both the iron-containing and iron-free compounds in the corrosion products as well as for suggesting a corrosion mechanism.     

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.     

Effects of temperature and applied strain on corrosion of X80 pipeline steel in chloride solutions

In this work, corrosion of X80 pipeline steel was investigated in 3.5?wt-% NaCl solutions with two different dissolved oxygen (DO) concentrations by electrochemical measurements and surface analysis techniques. The effects of temperature and applied strain on the steel corrosion were determined. Results show that an elevated temperature increases the corrosion activity of the steel and its corrosion reaction kinetics, which is mainly realised by the anodic reaction, rather than cathodic reaction. At a high DO concentration (i.e. 7?mg?L^?1) in the solution, the corrosion is diffusion-controlled. When the DO concentration is reduced to 3?mg?L^?1, it becomes mixed controlled. The applied strain, although increasing the thermodynamic activity of the steel, does not obviously affect the corrosion of the steel due to the generation of corrosion scales, primarily γ-FeOOH, which provides somewhat protection to the steel and offsets the increased corrosion activity.     

High-temperature corrosion of Y-doped Fe3Al in environments containing chlorine and oxygen

The high-temperature corrosion performance of Y-doped Fe₃Al was studied in various environments containing chlorine, oxygen, and argon. Results were compared with Y-free Fe₃Al in terms of corrosion rate and microstructure of corrosion products. The kinetic pattern shown in the two types of alloy features two consecutive stages: initial slow mass loss followed by fast linear mass loss. However, addition of Y decreased the rate but increased the duration of the first stage of the corrosion. Microstructure examination indicates that the beneficial effect of Y may be due to the formation of pegs and the development of dense oxide layers.     

Carbon steel corrosion under anaerobic–aerobic cycling conditions in near-neutral pH saline solutions – Part 1: Long term corrosion behaviour

A corrosion mechanism has been developed to describe tubercle formation along pipeline steels during successive anaerobic–aerobic cycles. Small concentrations of O₂ under nominally anaerobic conditions can lead to the separation of anodes and cathodes. Under subsequent aerobic conditions localized corrosion is then promoted by O₂ reduction on the general magnetite-covered surface. Subsequently, the conversion of magnetite to maghemite passivates the general surface, and focuses corrosion within one major tubercle-covered pit. On switching from aerobic to anaerobic conditions, corrosion is temporarily supported by the galvanic coupling of lepidocrocite (γ-FeOOH) reduction (to γ-Fe-OH·OH) to steel dissolution primarily within the tubercle-covered pit.     

The influence of dissolved oxygen concentration on the corrosion of grey cast iron in water at 50°C

The formation of corrosion scales has been studied on grey cast iron in flowing water at 50°C as a function of O₂ concentrations from 0.1 to 3.95 ppm O₂. Below 1.0 ppm O₂, nodular scales form containing Fe₃O₄ and a green rust, GR. At higher O₂ concentrations, a continuous scale eventually forms, consisting of a porous subscale of Fe₂O₄ + GR overlaid with a compact crust of Fe₃O₄ + GR and a thin surface layer of γ-FeOOH. ‘Chimneys’ oriented in the water flow direction grow out of the crust. γ-FeOOH is reduced to Fe₃O₄ which becomes the principal constituent of the scale. Scales on cast iron components from central heating systems closely resemble those found in the present work.     

Mechanisms of localized corrosion of carbon steel associated with magnetite/mackinawite layers in a cement grout

Carbon steel electrodes covered with a specific low-pH cement grout (pH ~10.7 at 20°C), designed for nuclear waste management applications, were immersed for 30 days in a 0.01-M NaCl + 0.01-M NaHCO₃ solution (pH 7 measured at 20°C), in aerated conditions, at 80°C. The corrosion processes were studied by voltammetry and linear polarization resistance measurements while the corrosion product layers were analyzed by µ-Raman spectroscopy and X-ray diffraction. Most of the electrodes (75%) suffered from localized corrosion, a phenomenon associated with the formation of a heterogeneous Fe₃O₄/FeS layer. It is proposed that the mechanisms of the particular corrosion process observed here are associated with galvanic effects, the large magnetite-covered zone acting as cathode and the locally mackinawite-covered zones being anodic regions.