Zum anodischen und kathodischen Verhalten des Eisens in neutralen und alkalischen Lösungen

The anodic and cathodic behaviour of iron in neutral and alkaline solutions To investigate the behaviour of iron in the p_H range from 7 to 14 in the presence of chloride ions, the authors make use of the electrochemical affinity/overpotential diagram. At p_H =7, the activation of the iron by the destruction of the FeO primary layer is possible. With increasing alkalinity, the passive zone is reduced by the direct effect of Cl^? ions, by the oxidation of FeCl₂, and by the formation of the iron complexes FeO₄^2?. The decomposition of Fe(OH)₂ by Cl^? ions is thermodynamically possible up to p_H = 10.5 only. At p_H > 10.5, there occurs the equilibrium Fe(OH)₂ = HFeO₂^? + H⁺.     

Elektrodenvorgänge am Stoffsystem Fe/H2O,Ac−

Electrode reactions at the Fe/H₂O₃ acetate system The anodic branch of the thermo-dynamic curve of the electro-chemical affinityl-excess potential diagram shows, at the Fe/H₂O, acetate system, a marked diminution of the passive zone at p_H² and the disappearance of the zone of incomplete passivity of iron due to the formation of an iron acetate complex. At p_H⁴ the influence of the latter on the disappearance of the Fe-self-passivation becomes much less significant. The log a_Fe3+/p_H-diagram also shows that a-Fe₂O₃ in the presence of acetate ions (Ac^?) at p_HO to 4 is more stable than -Fe₂O₃.     

The dynamical behavior of the electrochemical polymerization of indole on Fe in acetonitrile–water mixtures

This study reports on the stability of polyindole (PInd) films on Fe. Current oscillations emerge during the potentiostatic polymerization of indole upon its anodic oxidation on an Fe electrode in acetonitrile containing tetrabutyl-ammonium tetrafluoro-borate (N(Bu)₄BF₄). The oscillatory response of the system depends on the concentration of water, indole (Ind) and N(Bu)₄BF₄. For low concentrations of N(Bu)₄BF₄, the polymerization of Ind is possible and is enhanced in the presence of H₂O. The addition of H₂O leads also to an increase of the Fe dissolution. Increasing the concentration of Fe³⁺ ions at the Fe|polymer interface induces a mechanical instability of the polymer film. The origin of the present dynamical behavior is discussed on the basis of a breakdown and reconstruction of the polyindole film due to the accumulation of Fe³⁺ ions at the Fe|polymer interface. The kinetics of the indole polymerization coupled with the Fe electrodissolution includes the catalytic oxidation of indole by Fe³⁺ ions.     

Anodisches und kathodisches Verhalten des Chroms in salzsaurer Lösung

Anodic and cathodic behaviour of chromium in hydrochloric solution The electrochemical behaviour of chromium in aqueous solutions has been investigated with the aid of the electrochemical affinity (A) and overpotential (Δ U) diagrams in the p_H range from ?1 to 4 in the presence and absence of Cl^? ions. In the Cr/H₂O, Cl^? system, the chromium passivation is neutralized by the formation of CrCl₃. In the transpassive zone, CrCl₂ is formed. A passive layer with the composition Cr(OH)₂ was found at the Flade reference potential and under self-passivation conditions in the systems Cr/H₂O and Cr/H₂O, Cl^?.     

A Refined Scheme of the Mechanism of Iron Anodic Dissolution in Acidic Sulfate Solutions

A refined scheme of iron anodic dissolution in acidic sulfate solution, in which Fe₀ is oxidized to Fe⁺ in stages, is proposed. The first stage consists in chemisorption of H₂O molecules with participation of SO₄ ^2– anions; the degree of the participation x 0.5 per a water molecule. This stage produces adsorbed OH groups (Fe(OH)_ads), which, as a result of their interaction with sulfate anions, form an adsorption complex Fe((OH)SO₄ ^2– _ads. The schemes of oxidation of Fe⁺ to Fe²⁺ do not differ from conventional ones. The scheme put forward agrees with the literature data on the kinetics of iron dissolution under steady-state conditions.     

An ESCA study of the Fe2+/Fe3+ ratio in passive films on iron-chromium alloys

X-ray photoelectron spectra of Fe $\text{2p}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ electrons are measured for passive films on iron and FeCr alloys passivated for 1 h in 1M H₂SO₄ at + 100 and + 500 mV (s.c.e.). When chromium content of FeCr alloys increased to ca. 12 at.%, binding energy of Fe $\text{2p}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ electrons from oxidized state shifted abruptly from ca. 710·3 to ca. 709·6 eV and no more change in binding energy was observed at high chromium content. This abrupt shift of binding energy is in accord with the fact that the Fe²⁺/Fe³⁺ ratio showed sharp increase at ca. 12 at.% chromium. This phenomenon can be attributed to the inhibition of oxidation of iron due to the change in film structure and properties.     

Hydrometallurgical recovery of zinc from industrial hot dipping top ash

Top ash from hot-dip galvanizing plant was investigated as a source of secondary zinc to be returned to galvanizing bath. The waste material contained 63% Zn as metallic, oxide and hydroxychloride phases. It was leached in H₂SO₄ solutions (20% and 25%) at various bath loadings (100−300 g/L). Leaching behaviors of zinc, manganese, iron and chloride ions were investigated. A few strategies of iron elimination from leaching liquors were examined. Flocculant addition was harmful for subsequent filtration of iron precipitates due to increased viscosity of solution, while a combination of zinc oxide and calcium carbonate for rising pH resulted in the formation of dense suspension unenforceable to separate from zinc sulphate solution. Zinc electrowinning was carried out at different pH (from −0.5 to 2.8) using a range of current densities (3−10 A/dm²). Optimal conditions for pure metal recovery were: leaching in 20% H₂SO₄ solution at zinc ash content 100−150 g/L, Fe₂O₃·xH₂O precipitation using H₂O₂ and CaCO₃, zinc electrowinning at pH of 0.1−1.0 at 3−6 A/dm². Correlations between pH and free H₂SO₄ concentration in electrolyte solutions were also discussed. pH−acid concentration dependence for zinc electrolyte was between experimental and calculated curves for pure H₂SO₄ solutions, while the curve was shifted towards lower pH if ferric ions were in the solution.     

Water Vapour Effect on Ferritic 4509 Steel Oxidation Between 800 and 1000?��C

The 4509 alloy (Fe?C18Cr?CNb?CTi) was oxidised in dry and wet air in the 800?C1000 °C temperature range. Results showed that the formation of a chromia layer acts as a good diffusion barrier under isothermal conditions at 800 and 900 °C, under 7.5 vol.% water vapour and dry air. Nevertheless, a breakaway is generally observed at 1000 °C, under wet air 7.5 vol.% H₂O. It is proposed that the oxidant H⁺/OH^? species react at the internal interface with iron in the chromium-depleted alloy zone. Wüstite reacts with Cr₂O₃ to form FeCr₂O₄. Outward iron diffusion leads to Fe₃O₄ and Fe₂O₃ formation. The chromia scale was consumed by reaction with wüstite, but chromia also internally forms owing to a chromium oxidation process with the inner chromium-rich alloy area.     

Determination of the corrosion products of iron in sea water with and without sulphides

A method was developed to characterize and quantify iron corrosion products in clean and sulphide polluted sea water. This method is based upon a selective dissolution with suitable reagents (methanol, glycine, bromine-methanol, hydrochloric acid etc.) of the various compounds and the subsequent chemical analysis of the various dissolved elements. The information thus obtained is integrated by a diffractometric X-ray analysis. The following corrosion products were found:

• Fe(OH)₂ = Fe₃O₄ ? FeO(OH) ? Fe₂O₃ + unidentified compound Cr (probably oxychloride) in sea water with pH = 8.1 having a 7 ppm D.O. content.

FeOC1 ? Fe(OH)₂ ? Fe₃O₄ ? FeO.OH in partially deoxygenated sea water (D.O. = 3 ppm) at pH 8.1; Fe_0.95S ? Fe(OH)₂ in sea water at pH = 7 with 10 ppm of sulphides; Fe_0.95S ? FeS ? Fe_3.6 · Fe_0.9(O · OH · C1) and FeOC1 ? iron oxisulphide ? Fe₃O₄ ? FeO(OH) ? Fe₂O₃ · H₂O in sea water at pH = 7 and 10 ppm initial sulphides left to oxidate. Since the method of chemical analysis thus developed supplies quantitative information in iron distribution among the various anions and on the various oxidation forms, it is deemed a useful tool for investigation of the corrosion kinetics and mechanism.     

Thermodynamic analysis of Na-S-Fe-H2O system for Bayer process

Thermodynamic diagrams of Na-S-Fe-H₂O system were constructed to analyze the behavior of sulfur and iron in the Bayer process. After digestion, iron mainly exists as Fe₃O₄ and Fe₂O₃ in red mud, and partial iron transfers into solution as Fe(OH)₃^-, HFeO₂^-, Fe(OH)₄^- and Fe(OH)₄^2-. The dominant species of sulfur is S^2-, followed by SO₄^2-, and then SO₃^2- and S₂O₃^2-. The thermodynamic analysis is consistent with the iron and sulfur species distribution in the solution obtained by experiments. When the temperature decreases, sulfur and iron can combine and precipitate. Controlling low potential and reducing temperature are beneficial to removing them from the solution. XRD patterns show that NaFeS₂·2H₂O, FeS and FeS₂ widely appear in red mud and precipitates of pyrite and high-sulfur bauxite digestion solution. Thermodynamic analysis can be utilized to guide the simultaneous removal of sulfur and iron in the Bayer process.     

Evaluation of corrosion resistance of electrodeposited nanocrystalline Ni–Fe alloy coatings

Nickel–iron alloys with a compositional range of 24–80?wt-% iron were electrodeposited on a copper substrate from a sulphate-based bath and using a stirring rate of 100?rev?min^?1. The effect of applied current density and Ni²⁺/Fe²⁺ metal ion ratio of plating bath on the properties of alloy coatings was examined. Crystal structure and grain size of Ni–Fe alloy coatings were investigated using X-ray diffraction technique. Field emission scanning electron microscopy and energy dispersive X-ray spectroscopy were used to analyse the surface morphology and chemical composition of coatings. Microhardness test was applied to evaluate the hardness of the coatings. Finally, the electrochemical behaviour of the Ni–Fe alloy coatings was studied by a polarisation test in 10?wt-% H₂SO₄ solution. Results revealed that current density and plating bath composition had a strong effect on the characteristics of coatings. As the iron content of alloys produced increased, their corrosion resistance improved with the best corrosion resistivity being achieved at a metal ion ratio of 0.5 and applied current density of 2.5?A?dm^?2.     

Formation of the Fe(II)-Fe(III) hydroxysulphate green rust during marine corrosion of steel

Rust layers formed on steel sheet piles immersed 1 m above the mud line for 25 years were analysed by Raman spectroscopy, scanning electron microscopy and elemental X-ray mappings (Fe, S, O). They consist of three main strata, the inner one mainly composed of magnetite, the intermediate one of iron(III) oxyhydroxides and the outer one of hydroxysulphate green rust GR(SO₄²⁻). Simulations of GRs formation in solutions having large [Cl⁻]/[SO₄²⁻] ratios revealed that the hydroxysulphate GR(SO₄²⁻) was obtained instead of the hydroxychloride GR(Cl⁻), as demonstrated by X-ray diffraction and transmission Mössbauer spectroscopy analyses. Measurements of the [S], [Fe] and [Cl] concentrations allowed us to establish that GR(SO₄²⁻) formed along with a drastic impoverishment of the solution in sulphate ions; the [Cl⁻]/[SO₄²⁻] ratio increased from 12 to 240. The GR, acting like a “sulphate pump”, may favour the colonisation of the rust layers by sulphate reducing bacteria.     

Coprecipitation thermodynamics of iron(II-III) hydroxysulphate green rust from Fe(II) and Fe(III) salts

Iron(II-III) hydroxysulphate GR(SO₄²⁻) was prepared by precipitating a mixture of Fe(II) and Fe(III) sulphate solutions with NaOH, accompanied in most cases by iron(II) hydroxide, spinel iron oxide(s) or goethite. Its [Fe(II)]/[Fe(III)] ratio determined by transmission Mössbauer spectroscopy was 2±0.2, whatever the initial [Fe(II)]/[Fe(III)] ratio in solution. Proportion of Fe(OH)₂ increased when the initial [Fe(II)]/[Fe(III)] ratio increased, whereas proportion of α-FeOOH or spinel oxide(s) increased when this ratio decreased. GR(SO₄²⁻) is metastable vs. Fe₃O₄ except in a limited domain around neutral pH. Precipitation from solutions containing both Fe(II) and Fe(III) dissolved species seems to favour GRs formation with respect to stable systems involving iron (oxyhydr)oxides.     

Inhibitory effects of manganeous,cadmium and zinc ions on hydrogen evolution reaction and corrosion of iron in sulphuric acid solutions

The presence of metal ions (Cd²⁺, Mn²⁺, Zn²⁺), more electronegative than the cathodic potential for the hydrogen evolution reaction on iron in a 0.25M H₂SO₄ solution, inhibits the hydrogen evolution reaction and corrosion of iron. This effect has been explained as the under-potential deposition of the adatoms of these metals on iron.     

Studies of galvanised steel atmospheric corrosion in Saudi Arabia

Abstract

This paper summarises the results obtained for galvanised steel specimens exposed in Saudi Arabia region for four years at four pure marine and five mixed marine (SO₂ polluted) sites. The atmospheres at these sites were characterised climatologically and in terms of their pollution level so that their corrosivity could be expressed in accordance with ISO standards. Chemical characterisation of the galvanised steel corrosion product layers was performed using X-ray diffraction. The main phases determined were zincite (ZnO), simonkolleite [Zn₅(OH)₈Cl₂.H₂O], smithsonite (ZnCO₃), magnetite (Fe₃O₄), gordaite [NaZn₄(SO₄)Cl(OH)₆Cl.6(H₂O)], hematite (Fe₂O₃), zinkosite (ZnSO₄), zinc chloride (ZnCl₂), zinc hydroxide sulphate hydrate [(Zn(OH)₂)₃(ZnSO₄)(H₂O)₃] and zinc sulphate hydroxide hydrate [ZnSO₄(OH)₂.5H₂O] was found on the specimens. The results obeyed well with the empirical kinetics equation of the form C?=?Ktⁿ, where K and C are the corrosion losses in mg cm^?2 after 1 and ‘t’ years of the exposure respectively, and ‘n’ is constant. Based on ‘n’ values, the corrosion mechanism of galvanised steel is predicted. The results obtained show that the corrosion rate of galvanised steel is a function of both the chloride, SO₂ pollution level and the humidity. Corrosion rate of galvanised steel specimens have been obtained by loss of weight after each year of exposure.     

Influence of tensile stress on corrosion behaviour of high‐strength galvanized steel bridge wires in simulated acid rain

Corrosion behaviour of the high‐strength galvanized steel wires under tensile stress was researched by electrochemical polarization and salt spray test (SST) using simulated acid rain as electrolyte. Electrochemical polarization and SST results showed corrosion rate rose significantly with increasing tensile stress; white grains were observed by SEM after polarization, while cellular and dendritic crystals appeared on the rust layer after SST. XRD and TG‐DTA results revealed (Zn(OH)₂)₃ · ZnSO₄ · 5H₂O was the main corrosion product, and traces of Fe₂(SO₄)₂O · 7H₂O, Fe₂(SO₄)₃, Fe₂O₃ · H₂O were also detected. A three‐stage corrosion process for the galvanized steel wires during SST was proposed.     

Comparative studies of the electrochemical behaviour of aluminium in 1 M H2SO4/water and in 1 M H2SO4/dimethylformamide (anhydrous)

Anodic current potential curves of Al (99.999%) in H₃O/1 M H₂SO₄ are quite different from that in anhydrous DMF/1 M H₂SO₄. Al is active in DMF/H₂SO₄, and at a potential of 0.4 V (SCE) gives a constant current-time curve, where in H₂O/H₂SO₄ the current decreases in the same time to a low value. In DMF, it decreases also at a later step. This happens when the solution is saturated with anhydrous Al₂(SO₄)₃ (0.18 g l^?1). Comparison of cut-off curves after polarization to 0.4 and 3 V (SCE) shows that the curves are similar, but the potential decreases more slowly in DMF/H₂SO₄ than in H₂O/H₂SO₄. This proves that the space charge of Al with a Al₂(SO₄)₃ layer in anhydrous DMF has a much higher half-time of potential decay than that of Al with an oxide layer in water. This can be explained on the basis that the oxide layer in water has incorporated water molecules and perhaps H⁺ or OH^?, whereas the anhydrous Al₂(SO₄)₃ layer is free from solvent molecules, and the ionic conductivity of Al₂(SO₄)₃ itself is very low. The high space charge in DMF/H₂SO₄ could also be shown by potentiostatic measurements using an interrupter. The different behaviour of Al, when passivated by salt layers on the one hand or by oxide layers on the other, is discussed.     

Inhibitoren der Korrosion 21 (1) – Autoxidationsstudien an Fe2+-Salzen der Tetramethylen-1,4-bis-phosphonsäure,der Hexamethylen-1,6-bis-phosphonsäure,der Benzol-1,4-bis-phosphonsäure,der Phosphorsäure und Pyrophosphorsäure als Modell einer die Korrosion des Eisens hemmenden Deckschicht

Corrosion Inhibitors 21 (1) – Investigations into the autoxidation of Fe²⁺ salts of tetramethylene-1,4-bis-, hexamethylene-1,6-bis-, benzene-1,4-bis-phosphonic acids, phosphoric acid and pyrophosphoric acid as a model of a surface layer inhibiting the corrosion of iron The efficiency of organic bisphosphonic acids as inhibitors of the corrosion of iron is due to the fact that Fe²⁺ ions leaving the iron surface react with the acid immediately and give rise to the formation of threedimensional surface layers. The more dense the structure of these layers, the more resistant these network is against oxygen and the more resistant to hydrolysis are the Fe? OP bonds, the better the protective effect. Oxygen as well as chloride ions are then prevented excess to the metal surface. The distribution of phosphorus in the precipitates obtained during the model experiments yields information concerning the degree of crosslinking.     

Effects of iron-containing phases on transformation of sulfur-bearing ions in sodium aluminate solution

Sulfides in the high-sulfur bauxite lead to serious steel equipment corrosion and alumina product degradation via the Bayer process, owing to the reactions of sulfur and iron-containing phases in the sodium aluminate solution. The effects of iron-containing phases on the transformation of sulfur-bearing ions (S^2–, S₂O₃^2?, SO₃^2? and SO₄^2?) in sodium aluminate solution were investigated. Fe, Fe₂O₃ and Fe₃O₄ barely react with SO₃^2? and SO₄^2?, but all of them, particularly Fe, can promote the conversion of S₂O₃^2? to SO₃^2? and S^2– in sodium aluminate solution. Fe can convert to Fe(OH)₃^? in solution at elevated temperatures, and further react with S^2– to form FeS₂, but Fe₂O₃ and Fe₃O₄ have little influence on the reaction behavior of S^2– in sodium aluminate solution. Increasing temperature, duration, dosage of Fe, mole ratio of Na₂O_k to Al₂O₃ and caustic soda concentration are beneficial to the transformation of S₂O₃^2? to SO₃^2? and S^2–. The results may contribute to the development of technologies for alleviating the equipment corrosion and reducing caustic consumption during the high-sulfur bauxite treatment by the Bayer process.     

Zinc treatment effects on corrosion behavior of 304 stainless steel in high temperature, hydrogenated water

Trace levels of soluble zinc(II) ions (30 ppb) maintained in mildly alkaline, hydrogenated water at 260 °C were found to lower the corrosion rate of austenitic stainless steel (UNS S30400) by about a factor of five, relative to a non-zinc baseline test [S.E. Ziemniak, M. Hanson, Corros. Sci. 44 (2002) 2209] after 10,000 h. Characterizations of the corrosion oxide layer via grazing incidence X-ray diffraction and X-ray photoelectron spectroscopy in combination with argon ion milling and target factor analysis, revealed that miscibility gaps in two spinel binaries—Fe(Fe_1−mCr_m)₂O₄ and (Fe_1−nZn_n)Fe₂O₄—play a significant role in determining the composition and structure of the corrosion layer(s). Although compositions of the inner and outer corrosion oxide layers represent solvus phases in the Fe₃O₄-FeCr₂O₄ binary, zinc(II) ion incorporation into both phases leads to further phase separation in the outer (ferrite) layer. Recrystallization of the low zinc content ferrite solvus phase is seen to produce an extremely fine grain size (∼20 nm), which is comparable in size to grains in the inner layer and which is known to impart resistance to corrosion. Zinc(II) ion incorporation into the inner layer creates additional corrosion oxide film stabilization by further reducing the unit cell dimension via the substitution reaction

0.2Zn²⁺(aq)+Fe(Fe_0.35Cr_0.65)₂O₄(s)?0.2Fe²⁺(aq)+(Zn_0.2Fe_0.8)(Fe_0.35Cr_0.65)₂O₄(s)