Raman spectroscopy analysis of polypyrrole films as protective coatings on iron

In situ Raman spectroscopy was used to study the redox modifications of electropolymerized polypyrrole (Ppy) films used as protective coatings on iron. A preliminary study of the redox behavior of Ppy on platinum was carried out for highlighting the most sensitive bands to potential variations: the 1560–1620 cm⁻¹ as well as the 930 and 980 cm⁻¹ peaks show a strong dependence on the applied potential. In situ Raman spectra of Ppy on iron were recorded at different times after immersion in a 3% NaCl solution. They formally display the same peaks as those found for platinum, allowing to characterize the redox state of the Ppy film at the solution/film interface. Additional ex situ Raman spectra allowed to identify the corrosion products at the Ppy/metal interface. These results complement kinetics analysis early developed to understand the protection mechanism and its loss.     

Influence of an aerated/anoxic transient phase on the long-term corrosion of iron

This paper deals with the influence of an aerated/anoxic transient phase on the corrosion of ferrous matters. Actually in the context of radioactive waste disposal, metallic components could be exposed to fluctuating environmental conditions that could change the corrosion mechanisms and influence the corrosion rates. Archaeological analogues corroded in an aerated soil were exposed to an aerated/anoxic transient in an isotopic labeled solution. The corrosion mechanism in anoxic conditions does not seem to be influenced by the pre-existing thick corrosion layer formed in an aerated environment and the estimated corrosion rate is of few micrometres per year.     

690合金高温电化学性能研究

采用开路电位监测、电化学阻抗谱、动电位极化曲线测量和热力学分析等方法,研究690合金在模拟压水堆二次侧高温高压碱性水环境中(氨水调节pH_@25℃=9.5)的电化学性能。试验结果表明：在不同温度下(50,100,150,200,250或285℃)的氨水溶液中浸泡20h后,690合金的开路电位都已到达较稳定状态,表面有双层氧化膜生成,随着温度的升高,外层氧化膜阻值先减小后增加,内层氧化膜阻值逐渐减小,而总的氧化膜保护性逐渐降低。再结合试验温度下溶液pH_@T的降低,导致690合金的开路电位下降,由极化曲线获得的腐蚀电流密度和维顿电流密度增加。     

Effect of iron metal and siderite on the durability of simulated archeological glassy material

Under anoxic conditions, the presence of iron is expected to enhance the alteration rate of silicate glass. In order to understand the underlying mechanisms, experiments were performed on a synthetic glass simulating archeological artifacts to study the effect of metallic iron and siderite in a clay-type groundwater. Characterization of the altered samples at different durations reveals different impacts of iron on glass alteration. In contact with siderite, the sorption of the released silicon maintains temporarily a high alteration rate which eventually drops. With metallic iron, glass is altered near the maximum dissolution rate throughout the experiment because of Fe-silicate precipitation.     

Mechanism of corrosion of cast iron covered by aged corrosion products: Application of electrochemical impedance spectrometry

This work focuses on the corrosion of cast iron in a drinking water network. The study is performed in water and in a sandy soil saturated with the same water. In the presence of corrosion products, the inner wall surface shows a lower corrosion rate than the external wall. In the two media, impedance diagrams are described by a high-frequency loop characteristic of a protective corrosion layer and a diffusion impedance loop at low frequencies associated with oxygen diffusion. Different models involving mass transfer through a porous layer are suggested for the inner and the external corrosion layers.     

Al-coated iron particles: Synthesis, characterization and improvement of oxidation resistance

A Fluidized Bed Metal Organic Chemical Vapour Deposition (FB-MOCVD) process has been successfully applied to coat Fe particles with Al. N₂ inert gas was used to transport the organometallic Al precursor (liquid triethylaluminium) inside the fluidized bed reactor whose temperature is 350 °C. XPS analyses and TEM observations have revealed the presence of a thin alumina layer surrounding the Al coating. Oxidation treatments, performed in the temperature range 350-500 °C, demonstrate that this multi-scale coating constitutes an efficient barrier to protect iron particles against oxidation. Such a treatment may be used to perform environmental barrier coatings around magnetic powders.     

Polyaniline-coated iron: studies on the dissolution and electrochemical activity as a function of pH

Polyaniline coatings were electrodeposited from an oxalic acid solution onto iron and their electrochemical activity and corrosion protection properties studied as a function of pH. It was found that the coating (emeraldine salt) had a limited effect on the corrosion protection of iron in acidic solutions. However, in an alkaline borate solution, where the conducting polyaniline was converted to the emeraldine base, the coating had a clear beneficial effect on the local breakdown of iron by chloride anions; much higher pitting potentials were recorded following a 2 h immersion period for the polyaniline-coated substrate relative to the uncoated electrode. Relatively small anions, such as acetates, nitrates and borates, were transported readily across the polymer interface. However, the emeraldine base inhibited the transport of the much larger ethylenediamine tetraacetate (EDTA) species to the iron interface, preventing complexation of the iron by EDTA.     

Buried iron archaeological artefacts: Corrosion mechanisms related to the presence of Cl-containing phases

Deterioration after excavation of archaeological iron artefacts buried in soil is often associated with the presence of Cl-containing phases in corrosion products, leading to serious problems for conservation of metallic objects of cultural heritage. Thus, in order to better understand the corrosion process related to the presence of chlorine, some high-resolution techniques of materials characterisation are implemented. Particularly this paper shows the great utility of the combination of micro-X-ray diffraction, micro-Raman spectroscopy and micro-X-ray absorption spectroscopy. The analyses are realised on the cross sections of iron corroded objects excavated on archaeological sites dated from the 12th to the 16th century AD. In addition to the common oxyhydroxide containing chloride, akaganeite (β-FeOOH) often mentioned in the literature, a ferrous hydroxychloride β-Fe₂(OH)₃Cl was also found in the corrosion layers. In order to explain the corrosion system formed during burial, a corrosion mechanism including the presence of chlorine, is proposed.     

Effect of long term, high temperature aging on luminescence from Eu-doped YSZ thermal barrier coatings

The use of the luminescence from europium doped yttria-stabilized zirconia thermal barrier coatings for non-contact thermometry has been previously studied up to 850 °C [Choy, K.L. et al., Surface Engineering, 16 [6] (2000) 496] and 1150 °C [M.M. Gentleman and D.R. Clarke, Surface and Coatings Technology, 188-189 (2004) 93]. In this contribution we examine the effect of long-term, high temperature aging and martensitic phase transformation on both the details of the luminescence spectrum and the temperature-dependent lifetime of the luminescence. It is found that even after prolonged aging, the wavelength of the ⁵D₀ → ⁷F₂ emission peak shifts only very slightly with increasing percentage of transformation to the monoclinic phase. Preliminary data suggests that the effect on the decay lifetime up to 1150 °C is very slight, expressed in a slight shift of the onset of thermal quenching to lower temperatures; the slope of the characteristic decay time-constant versus temperature appears relatively unaffected.     

Electrochemical studies of packed iron powder electrodes: Effects of common constituents of natural waters on corrosion potential

Using a powder disk electrode (PDE) made with micron-sized, high purity iron metal we investigated how the corrosion of this material is affected by solution conditions that are relevant to the degradation of contaminants in environmental remediation applications. Changes in corrosion potential (E_corr) with time showed that low pH, high concentrations of chloride, and natural organic matter led to breakdown of the passive film. Bicarbonate caused E_corr to decline rapidly into the active potential region, but then E_corr rose back into the passive region over 10s of hours. The rate of decline in E_corr was greatest at higher pHs, suggesting a specific effect of rather than a general effect of pH.     

Corrosion protection of Zn-phosphate containing water borne dispersion coatings on steel: Part 1: Design and analysis of model water based latex films on iron substrates

Model thin film coated substrates were developed to investigate the functionality of Zn-phosphates, the polymeric binder and emulsifiers during the interface formation on iron substrates. By means of grazing incidence FTIR-spectroscopy, X-ray photoelectron spectroscopy and atomic force microscopy, it was observed that ultra-thin films of zinc phosphates and hydroxides, with a thickness of less than 2 nm, adsorb on the iron surface during the formation of the latex coating/metal interface. Moreover, film formation conditions were developed for the deposition of hexagonal closed packed defect-free-monolayers of latex particles over large areas. This allowed the application of time of flight SIMS for the analysis of the monolayer-metal interface and the study of its morphology before and after film annealing. After annealing, the monolayer film showed an enrichment of surfactants at the particle boundaries and at the interface to the phosphate and hydroxide covered metal substrate.     

Synthesis, structure, and high temperature Mössbauer and Raman spectroscopy studies of Ba1.6Sr1.4Fe2WO9 double perovskite

Ba_1.6Sr_1.4Fe₂WO₉ has been prepared in polycrystalline form by solid-state reaction method in air, and has been studied by X-ray powder diffraction method (XRPD), and high temperature Mössbauer and Raman spectroscopies. The crystal structure was resolved at room temperature by the Rietveld refinement method, and revealed that Ba_1.6Sr_1.4Fe₂WO₉ crystallizes in a tetragonal system, space group I4/m, with a = b = 5.6489(10)Å, c = 7.9833(2)Å and adopts a double perovskite-type A₃B′₂B″O₉ (A = Ba, Sr; B′ = Fe/W, and B″ = Fe/W) structure described by the crystallographic formula (Ba_1.07Sr_0.93)_4d(Fe_0.744W_0.256)_2a(Fe_0.585W_0.415)_2bO₆. The structure contains alternating [(Fe/W)_2aO₆] and [(Fe/W)_2bO₆] octahedra. Mössbauer studies reveal the presence of iron in the 3+ oxidation state. The high temperature Mössbauer measurements showed a magnetic to paramagnetic transition around 405 ± 10 K. The transition is gradual over the temperature interval. The decrease in isomer shift is in line with the general temperature dependence. While the isomer shift is rather linear over the whole temperature range, the quadratic dipolar ΔE temperature dependence shows an abrupt change at 405 K. The latter results allow concluding that a temperature-induced phase transition had occurred. The high temperature Raman study confirms the Mössbauer results on the magnetic to paramagnetic transition.     

Electrochemical behaviour of cermet coatings with a bond coat on Al7075: Pseudopassivity, localized corrosion and galvanic effect considerations in a saline environment

The behaviour of an HVOF WC-17Co/Ni-5Al coating on Al7075 in aqueous NaCl is investigated. The coating susceptibility to localized corrosion depended on the potential of polarization reversal. A two-stage pseudopassivity was observed for WC-17Co: At low overpotentials, oxidation occurred in the binder leading to surface films composed of anhydrous Co- and W-oxides. At high overpotentials, oxidation extended to the carbide phase leading to the formation of hydrated WO₃ films. Ni-5Al notably reduced the galvanic effect between WC-17Co and Al7075, whereas it hindered corrosion propagation into the substrate. The coating showed a high corrosion resistance during salt spraying for 49 days.     

Influence of the borohydride concentration on the composition of the amorphous Fe–B alloy produced by chemical reduction of synthetic, nano-sized iron oxide particles: Part II. Goethite

A systematic study of the effect of the NaBH₄ concentration upon the composites produced by chemical reduction of nano-sized goethite (-FeOOH) particles suspended in water is presented. Amorphous Fe–B alloys with boron contents between 2 and 8 at.% have been produced. The obtained samples were characterized by X-ray diffraction, wet chemical analyses, thermal analyses, scanning electron microscopy, transmission Mössbauer spectroscopy (TMS) and integral low-energy electron Mössbauer spectroscopy (ILEEMS). It was observed that the boron content of this amorphous composite sensitively depends on the borohydride concentration. The TMS analyses have shown that the contribution of the Fe_1−xB_x phase, which is present as a superposition of a broad sextet and a doublet, accounts for about 38% of the total spectral area. ILEEMS has further revealed that the reduction process mainly affects the surface layers of the goethite grains, without causing any changes in both particle shape and sizes.