Zur Elektrodenkinetik von Reinst – Eisen im Phosphatelektrolyten bei Feststoffeinschlag (Simulation von Erosionskorrosion)

Electrode kinetics of high purity iron in phosphate electrolytes under solid particle impact (Simulation of erosion corrosion) Erosion-corrosion of high purity iron was simulated by means of a vibration cell in a neutral phosphate electrolyte. The investigations were carried out to explain the initiation processes of this type of corrosion. The results confirm the general conception of the central role of reaction layer destruction in erosion-corrosion processes. Under these conditions a qualitative change in reaction layer formation occur. By solid impact the layers are destroyed locally and so conditions of pit nucleation exist. The destruction of the layers depends on the action of shear forces resulting from tensile or compressive strains.     

Der Oxidationsmechanismus von Szomolnokit im intermittierend befeuerten Warmwasserkessel

The oxidation mechanism of szomolnokite in intermittently fired domestic boilers Using thermogravimetry and exposure experiments on initially formed iron(II)-sulfate hydrates the consecutive reactions of the primary corrosion product szomolnokite under the typical operating conditions of domestic boilers were investigated. The reaction mechanism thus deduced is in accordance with thermodynamic equilibrium considerations in the system Fe₂O₃? H₂SO₄? H₂O/crystalline phase. The reaction mechanism consists of the following steps: With this reaction mechanism the most frequently occurring crystalline phases in corrosion samples from oil fired domestic boilers can be explained.     

Die aktive und passive Molybdän-Elektrode in sauren Lösungen

The active and passive molybdenum electrode in acid solution It follows from the anodic and cathodic thermodynamic polarization curves in the system molybdenum/aqueous solution (p_H 0 to 10) that the favourable behaviour of molybdenum in acid solutions is determined by the extension of the immunity region becomes smaller, but without simultaneous passivation of molybdenum; this fact explains the active behaviour of molybdenum in alcaline solution. The passivation of molybdenum starts as soon as the half cell reaction sets in, where The p_H-dependence of the cathodic passivation potential in acid solutions follows the equation      

Investigations on the passivity of iron in borate and phosphate buffers, pH 8.4

In the present work surface analytical experiments (XPS and AES) on the passive film on iron formed in borate and phosphate buffers (pH 8.4) have been carried out. In the passive film formed in phosphate buffer a significant amount of phosphates is found in the outer part of the film. Boron species are not significantly incorporated in the passive film formed in borate buffer. The mechanism of the reduction of the passive film depends strongly on the electrolyte composition. In borate buffer, cathodic polarization leads to reductive dissolution of the passive film whereas in phosphate buffer the passive film is converted into metallic iron without dissolution but via laterally inhomogeneously formation of an intermediate Fe(II) phosphate layer.     

Untersuchungen zum Aktiv/Passiv-Übergang von nichtrostenden Chromnickelstählen in organisch wäßrigen Medien. Teil 3. Resultate der Impedanzmessungen und Passivierungsmodell einer Legierung

Investigations into the active/passive transition of 304 stainless steel in organic media containing water and hydrogen chloride Part 3. Results of the impedance measurements and passivation model of an alloy Steady state polarization curves and electrode impedances were measured during the active/passive transition of type 304 stainless steel in dearated ethanolic solution containing hydrogen chloride and different amounts of water. The passivation potential and the critical current density for passivation strongly depend on the water content of the solution. The impedance measurements in the active/passive transition show the same sequence of diagrams independent of the water content of the solution. They indicate the onset of passivation before the maximum current density and show two time constants related to two different passivating species on the alloy surface. The experimental results were interpreted on the basis of a reaction model with parallel dissolution and passivation mechanism of the iron and the chromium compound of the alloy. The resulting total surface composition (related to the steady-state polarization curves) can be described with a reaction model of iron–the alloy behaviour is that of pure metal. The fundamental passivation reaction is described as a potential dependent equilibrium between adsorbed Me(II)- and passivating Me(III)-hydroxide, water molecules being directly involved in the formation of this primary passivating film. In the case of stainless steel this primary passivating film mainly consists of chromium (III) adsorbates. Finally, a general model for the passivation is proposed: The passivation of a pure metal or of an alloy can be understood as the coupling of the stepwise deprotonation of the water molecules at the interface metal/solution and the formation of a high cation charge density in this adsorbed hydroxide/oxide film to build up the passive layer. The effect of water content, pH, adding of passivating species to the solution or the alloying with chromium on the passivation potential and the critical current density thus can be explained.     

Influence of lactate ions on the formation of rust

The formation of rust can be simulated by oxidation of aqueous suspensions of Fe(OH)₂ obtained by mixing solutions of NaOH and a Fe(II) salt. The aim of this study was to investigate the influence of organic species associated with microbially influenced corrosion. The lactate anion, often used as a carbon and electrons source for the development of microorganisms, was chosen as an example. Then, in the first part of the study, Fe(OH)₂ was precipitated using iron(II) lactate and NaOH. Its oxidation process involved two stages, as usually observed. The first stage led to a Fe(II-III) intermediate compound, the lactate green rust, . This compound has never been reported yet. Its existence demonstrates that the GR structure is able to incorporate a very wide range of anions, whatever the size and geometry. The second stage corresponded to the oxidation of . It led to ferrihydrite, the most poorly ordered form of iron(III) oxides and oxyhydroxides. In the second part of the study, the formation of rust in seawater was simulated by oxidation of Fe(OH)₂ in an aqueous media containing both Cl⁻ and anions. The first stage led to the sulphate green rust, , the second stage to lepidocrocite γ-FeOOH. Small amounts of iron(II) lactate were added to the reactants. Lactate ions did not modify the first stage but drastically perturbed the second stage, as ferrihydrite was obtained instead of γ-FeOOH.     

The effects of anions on the behaviour of scratched iron electrodes in aqueous solutions

Two distinct types of electrochemical behaviour are observed when iron electrodes held at constant potential are rapidly scratched in the presence of reactive anions in aqueous solutions. The results presented here deal with the effects of bicarbonate, chloride and phosphate ions in alkaline solutions. Chloride and phosphate each accelerate the first oxidative step in the reaction by direct formation of adsorbed surface complexes with Fe(I). Bicarbonate does not accelerate the first oxidative step and does not complex with Fe(I) at a significant rate. It does, however, enter the second oxidative step by reacting at a significant rate with adsorbed FeOH to give an Fe(II) complex, thereby maintaining a bare reaction surface at higher potentials. The results are compared with those obtained in non-aggressive electrolytes of similar pH. In all cases the Fe(I) intermediate forms to complete monolayer coverage at potentials below the Fe/Fe(II) reversible potentials.     

Sauerstoffübertragung aus CO2?CO?und H2O?H2?Gemischen auf Metalle und Oxide

Oxygen transfer from CO₂-CO- and H₂O-H,-mixtures to metals and oxides In flowing CO₂-CO or H₂O-H,-mixtures oxygen is transferred from the gas phase to the solid, and equilibria are established according to the reactions These oxygen transfer reactions are rate determining for the linear oxidation of metals or for the linear reduction of oxides in these gases. With the aid of isotope exchange reactions the dependence of the oxygen transfer on the oxygen activity was measured on metals and oxides. The dependence on the oxygen activity is similar for reaction ( 1 ) and (2) on the same solid andit can be explained by oxygen adsorption according to the Freundlich-isotherm. From the measurement of the isotope exchange reactions during oxidation or reduction processes the stationary oxygen activity at the surface the solid can be determined. such measurements during the linear oxidation of iron to Wustite, during the linear reduction of magnetite to wustite and during the reduction of wustite to iron are described.     

亚铵法制浆过程中碳钢蒸球的腐蚀行为与防腐蚀措施

在亚铵法制纸浆蒸煮过程中升温到120℃附近时,碳钢蒸球内壁将由活化转为钝化。钝化膜表层为亚硫酸亚铁铵结晶层,里层为铁的氧化物层。活化碳钢的表面大部份是疏松多孔的Fes层。对影响碳钢钝化的因素也进行了研究。 根据碳钢在亚铵法蒸煮时的腐蚀特性,在纸厂现场试验了控制蒸煮液pH、设置防滑钉和快速升温等三种简易的防腐蚀措施,使用这些方法可使蒸球腐蚀率降到0.2mm/y以下。     

Zum Rosten des Eisens

Rust formation on iron - A model A model is presented on the basic of which the phenomenological variety of rusting can be reduced to a few basic process. The first step of the formation of rust results in the transfer reaction of iron (II)-ions from the metal surface into the adherent boundary layer, the ions being surrounded there by water molecules or hydroxid ions. Here or after their diffusion into the adhearent electrolyte solution they are oxidized to Fe(III)-ions by oxygen or other oxidants, eventually also electrochemically. The nuclei of the structures of FeOOH modifications develop from hydrogen bonded iron hydroxo- and/or-aquocomplexes by transposition of the iron ions and water loss. If only Fe(III)-ions participate in the nucleation process, then the stabile oxides or oxyhydroxides in relation to the solution are formed. The metastable structures result from simultaneous participation of Fe(II)- and Fe(III)-ions.     

Das anodische und kathodische Verhalten des Eisens in sulfathaltigen Elektrolyten. Chaire d'Electrochimie,Faculté des Sciences,Strasbourg, France

The anodic and cathodic behaviour of iron in sulphate containing electrolytes The formation of Fe₂(SO₄)₃ on passive iron at p_H = 1 appears probable from a thermodynamical point of view. At high SO₄^2? concentrations the equilibrium system contains but low concentrations of Fe³⁺, and no Fe²⁺ ions, a fact showing the relatively elevated stability of the Fe₂(SO₄)₃ layer on passive iron. In slightly acid solution (p_H = 4) the passivity of the iron is determined by iron oxide layers. The formation of FeSO₄ from metallic iron and sulphate ions is restricted to the transpassive zone (p_H 4 to 7), in alkaline solutions even to the active zone. In the p_H region 2 to 14 the passive layer on iron has about the same composition in the systems Fe|H₂O + SO₄^2? and Fe|H₂O.     

Kinetics of Pit Initiation at the Alloy Fe5Cr

Pit initiation at the passive alloy Fe5 Cr was studied and compared to the results obtained with pure iron. At constant electrode potential the addition of chloride to the solution increases the dissolution rate of Fe(III). Above the critical pitting potential the first pits develop during a time of incubation after addition of chloride. The total current density and the dissolution rate of Fe(II) rise simultaneously and suddenly. The logarithm of the times of incubation increase linearly with the reciprocal of the difference between the actual electrode potential and the pitting potential. Galvanostatic experiments show that, with Fe 5 Cr as with iron, chloride catalyzes the transfer reaction of oxygen ions at the interface between oxide and electrolyte more strongly than the dissolution of Fe(III). Addition of chloride. The total current density and the dissolution rate of Fe(II) rise simultaneously and suddenly. The logarithm of the times of incubation increase linearly with the actual electrode potential and the pitting potential. Galvanostatic experiments show that, with Fe5Cr as with iron, chloride catalyzers the transfer reaction of oxygen ions at the interface between oxide and electrolyte more strongly than the dissolution of Fe(III). Addition of Chromium lowers the current efficiency of Fe(III) dissolution and favors oxide growth. The ionic conductivity of the passivating oxide is not significantly changed by chloride but is lowered by chromium. Fluctuations connected to the onset of pitting are slower with Fe 5 Cr than with iron.     

Elektronenoptische und röntgenographische Untersuchung einer Eisenphosphat-Schutzschicht auf Eisen

Electron-optical and radiographical examination of a protective iron phosphate layer on iron The protective effect of a novel anticorrosive containing phosphoric acid has been examined for its dependence on mineralogical conditions. The base material contains, intro alia, phosphoric acid, potash mica, organic wetting agents and dyeing agents. Vivianite formation has been precluded by radiographic comparison with natural vivianite and with data quited in technical literature. The scraped protective layer was found to contain, apart from muscovite and α iron, dendritic formations which were observed under the electron microscope and identified, from their diffraction interferences, as cubic form of the potassium iron (III) phosphate leucophosphite \documentclass{article}\pagestyle{empty}\begin{document}$ KFe_2 [OH(PO_4)_2] \cdot 2\ H_2 O. $\end{document} KFe₂[OH(PO₄)₂] · 2 H₂O. Precision zone diffraction diagrams of individual dendrites showed a_o = 7.75 ± 0.05 Å and the transmission directions [100]^X [010]^X [202]^X, identical to those observed with natural pharmacosiderite, an iron arsenate corresponding to leucophosphite. Natural and synthetic leucophosphite are monoclinic; if Fe³⁺ is replaced by Al³⁺, and (PO₄)^3? by (AsO₄)^3?, a cubic alumopharmacosiderite is formed. The formation is thereofore interpreted as a cubic modification of leucophosphite, corresponding to pharmacosideriote. A structure comparison shows the epitactic growth of the leucophosphite with [110] and [333] on α iron [100] and [111]. The presence of leucophosphite directly on the iron surface is proved by electron diffraction with grazing incidence of the electron ray.     

Formation of the Fe(II-III) hydroxysulphate green rust during marine corrosion of steel associated to molecular detection of dissimilatory sulphite-reductase

Accelerated corrosion phenomena of carbon steel constantly immersed in seawater could be simulated in situ via a galvanic coupling of the samples with steel port structures. Three harbours located on different seas and various conditions of immersion were considered so as to study the eventual correlation between dissimilatory sulphite-reductase genes and sulphate-containing corrosion products. In each case, after 6 or 12 months, the rust layers proved to be made of an inner black layer, close to steel surface, and an orange outer layer. Scanning electron microscopy, chemical analyses by inductively coupled plasma/atomic emission spectroscopy, X-ray diffraction and micro-Raman spectroscopy were used to obtain a detailed characterisation of these layers. The inner one proved to be mainly composed of iron sulphides FeS and Fe(II-III) hydroxysulphate green rust , the outer one of Fe(III) oxyhydroxides, with lepidocrocite γ-FeOOH as a major component. The molecular detection of dissimilatory sulphite-reductase, the key enzyme in dissimilatory sulphate reduction by micro-organisms, was applied for the first time to rust layers. This detection was positive in most cases, especially for the inner part of the rust layers. This demonstrates that sulphate reducing bacteria are associated to inside the rust layers, more likely playing the role of a source of sulphate. The systematic presence of iron sulphides also testifies the activity of sulphate reducing bacteria and/or thiosulphate reducing bacteria.     

Das anodische Verhalten des Zinns bei Selbstpassivierung

The anodic behaviour of tin in the case of auto-passivation The diagrams: electrochemical affinity(A): overvoltage (U) characterizing the behaviour of tin in the p_H range 4–14 occupy an intermediate position between the groups Ni, Co, Cu, and Ti, Zr, Nb, U. On passive tin the formation of a double layer electrode of the type sn|SnO₂|Sn(OH)₄ appears thermodynamically probable. The anodic Flade reference potential of tin, depending from the p_H value obeys the equation .     

Localised corrosion of carbon steel in NaHCO3/NaCl electrolytes: role of Fe(II)-containing compounds

Pitting corrosion processes of carbon steels in 0.1 M NaHCO₃ + NaCl aqueous solutions induce rapidly the precipitation of Fe(II)-containing phases. The mechanisms leading to those transient compounds were studied by coupling classical electrochemical methods (cyclic voltammetry, chronoamperometry) with electrochemical microprobes and analyses by micro-Raman spectroscopy. Anodic polarisation above the breakdown potential induces the formation of a white corrosion product, identified as iron carbonate by Raman spectroscopy. The activity of a given pit, followed by Scanning Vibrating Electrode Technique, reaches a maximum before to decrease, in agreement with the evolution of the overall current density that stabilises whereas the number of pits increases. Iron carbonate is oxidised by dissolved O₂ into the hydroxycarbonate Green Rust. The study of the oxidation of aqueous suspensions of Fe(II) compounds precipitated from FeCl₂ · 4H₂O, NaOH and NaHCO₃ solutions allowed to clarify the mechanisms, that are governed by the concentration ratios and . The results were confronted to a thermodynamic approach.     

Extraction Separation of Sc(III) and Fe(III) from a Strongly Acidic and Highly Concentrated Ferric Solution by D2EHPA/TBP

The solvent extraction and separation process of Sc(III) and Fe(III) from a strongly acidic and highly concentrated ferric solution using mixtures of di(2-ethylhexyl) phosphate (D2EHPA) and tri-n-butyl phosphate (TBP) in sulfonated kerosene was studied. The effects of various parameters, including D2EHPA concentration, dosage of TBP, and phase ratio, were investigated for the extraction process. The results indicated that the extraction rate of Sc(III) was 99.72% with 1.09% Fe(III) co-extracted after two stages of counter-current extraction under optimal conditions. Moreover, saturation capacity and slope analysis were used to determine the reaction mechanism. Sc(III) is extracted in the form of HSc(SO₄)₂·4HL. Further separation of Sc(III) and Fe(III) was realized in a scrubbing and stripping process. First, 98.67% of the co-extracted iron in the loaded organic phase was scrubbed with a dilute HCl solution by a three-stage counter-current scrubbing. Then, 85.00% of Sc(III) can be stripped efficiently with a 2-mol/L NaOH solution saturated in 1-mol/L NaCl by three-stage cross-current stripping.     

XPS investigations of electrochemically formed passive layers on Fe/Cr-alloys in 0.5 M H2SO4

Passive layers on Fe20Cr and Fe15Cr were prepared by anodic oxidation in 0.5 M H₂SO₄ and were investigated by X-ray photoelectron spectroscopy. The preparation of the sputter-cleaned samples and their transfer to the ultrahigh vacuum was performed under protection of purified argon without any contact to the laboratory atmosphere. The prepassive layer consists of a homogeneous film with Cr(III)hydroxide and Cr(III)-sulphate with a low content of Fe(II). In the passive potential range a bilayer structure with enrichment of Cr(III)-oxides with an outer hydroxide and an inner oxide part is formed. Iron exists only in a bivalent oxidation state. In the transpassive potential range a pronounced change of the layer composition was observed: The outer part of the transpassive layer is formed predominantly by Fe(III) species whereas the inner part still contains a strong enrichment of Cr₂O₃.     

Zur anodischen auflösung von reineisen im bereich zwischen aktivem und passivem verhalten

The kinetics of anodic iron dissolution in sulphate ions containing solutions, which are free of oxygen, are investigated in a over potential range between active and passive states of the metal. The measurements were carried out on a rotating disk electrode in the pH-range between 3·5 and 5·5 using the potentiostatic triangular voltage sweep method under quasi-steady-state conditions. The measured current density-potential curves show two current maxima. At relatively low anodic overvoltages, i.e. before the maxima I is reached, the used recrystallized iron dissolves in the active state corresponding to the consecutive (Bockris-) mechanism. At high anodic overvoltages the passive state arises after passing the maxima II. In the transition range, i.e. in the potential range of the maximum I, a second mechanism of iron dissolution is discussed. This mechanism takes place parallel to the consecutive one. An intermediate [Fe (OH)₂]_ads will be formed, which dissolves in a following rate-determining chemical step or is changed in an oxide phase respectively.     

Anodische Korrosionsprozesse auf Stählen in inerten und in oxydierenden Atmosphären

Anodic corrosion processes on steels in inert and oxidising atmospheres The purpose of the investigation here described was to find out to what extent electrochemical techniques lend themselves to the examination of steel corrosion in the presence of molten sulphates and combustion gases. The measured equilibrium potentials of an inert metal electrode are intended to serve for the determination of the redox potential in the salt melt/combustion gases system. If no current is flowing, the stationary potential of a platinum electrode in a sulphate melt at 600° C can be expressed by the equation where E signifies the potential related to the silver/silver sulphate electrode, and p_SO2 p_O2 the partial pressures (in atmospheres) of the two gases in an inert gas (nitrogen + carbon dioxide). The formula permits the conclusion that the electrode reaction can be expressed by the equilibrium condition . The anode currents set up if the potential of a mild steel or pure iron electrode is kept at the above-mentioned temperature viz. ?0.3 V with N₂ + CO₂ + 5% O₂, or + 0.4 V with N₂ + CO₂ + 0.2% SO₂, show that under the test conditions, these metals would be greatly exposed to corrosion. Stainless steels become passive after a few hours although a residual corrosion current at + 0.4 V remains. These observations give rise to the expectation that electro-chemical examinations may well represent a useful means of examining corrosion phenomena caused by molten salts in the presence of combustion gases, so that they merit more detailed investigation.