Transformation of green rust (Cl−) into different oxyhydroxides in aqueous solution containing molybdenum ions

To investigate the influence of the addition of molybdenum ions on the transformation of green rust GR(Cl^?) comprising Fe(II) and Fe(III) into different oxyhydroxides, suspensions of GR(Cl^?) containing molybdenum ions were oxidized by passing nitrogen gas containing oxygen through the suspensions. X‐ray diffraction (XRD) was used for identifying the solid particles formed by oxidation. The results showed that in the presence of molybdenum ions, the formation of β‐FeOOH and α‐FeOOH was enhanced as compared to that of γ‐FeOOH in the GR(Cl^?) suspensions. This implied that the molybdenum ions interacted with the chloride ions obtained from GR(Cl^?) and induced the formation of β‐FeOOH that contained chloride ions. Transmission electron micrographs of the solid particles showed that the particle size of the resulting oxyhydroxides was considerably reduced because of the addition of the molybdenum ions. The pH and oxidation–reduction potential (ORP) of the aqueous solution were also measured during the oxidation. The addition of molybdenum ions was found to cause characteristic changes in the pH and ORP in the initial stage of the oxidation.     

Influence of tungstate ions on transformation of green rust to ferric oxyhydroxide via aqueous solution investigated by in situ X-ray absorption spectroscopy

Influence of tungstate ions on the transformation of chloride-containing green rust (GR(Cl⁻)) to fine goethite (α-FeOOH) particles due to the oxidation reaction via aqueous solution at about 300 K was investigated by in situ measurements of X-ray absorption spectra. Results showed that the transformation rate of GR(Cl⁻) in the suspension containing 5 mol% W was lower than that in the suspension without tungstate ions. Almost all tungstate ions in the suspension were adsorbed on GR(Cl⁻) and α-FeOOH. It is probable that the adsorption of tungstate ions reduces the transformation rate of GR(Cl⁻) and also leads to the precipitation of fine particles.     

SCC of X‐52 and X‐60 weldements in diluted NaHCO3 solutions with chloride and sulfate ions

Stress corrosion cracking tests were performed in both X‐52 and X‐60 weldments in sodium bicarbonate (NaHCO₃) solutions at 50°C using the Slow Strain Rate Testing (SSRT) technique. Solution concentrations varied between 0.1 to 0.0001 M, and to simulate the NS‐4 solution, chloride (Cl^?) and/or sulfate ( ) ions were added to the 0.01 M solution. Tests were complemented with hydrogen permeation measurements and polarization curves. It was found that the corrosion rate, taken as the corrosion current, I_corr, was maximum in 0.01 M NaHCO₃ and with additions of ions. Higher or lower solution concentrations or additions of Cl^? alone decreased the corrosion rate of the weldment. The SSC susceptibility, measured as the percentage reduction in area, was maximum in 0.01M NaHCO₃. Higher or lower solution concentrations of additions of Cl^? or decreased the SCC susceptibility of the weldment. The amount of hydrogen uptake for the weldment was also highest in 0.01 M NaHCO₃ solution, but it was minimum with the addition of Cl^? or ions. Thus, the most likely mechanism for the cracking susceptibility of X‐52 and X‐60 weldments in diluted NaHCO₃ solutions seems to be hydrogen‐assisted anodic dissolution.     

Die Bedeutung der pH-abhängigen Durchbruchsreaktionen bei NiCrMo-Werkstoffen in Medien der Chemie- und Umwelttechnik mit hohem Redoxpotential

The importance of pH-dependent break-through reactions of NiCrMo-materials in media of chemical and environmental engineering with a high redox potential Compared to CrNiMo-steels NiCrMo-alloys show a specific property at relatively positive potentials. As a result of breakthrough reaction which is not connected to the presence of Cl^?-ions Ni-alloys show a stable and uniform dissolution in media with pH-values between 4 and 9, whereas steels were not attacked. This phenomenon was studied for steels 1.4563 (Nicrofer 3127 LC), 1.4562 (Nicrofer 3127 hMo) and Ni-alloys 2.4856 (Nicrofer 6020 hMo) and 2.4605 (Nicrofer 5923 hMo) in Cl^? -and SO-solutions in the pH-range of pH = 3–10. In the break-through reaction Cr and Mo dissolve 6-valently and Ni 2-valently. The break-through potential is the same in Cl^?- and SO-solutions and is hardly influenced by the Mo-content. The reaction rate increases with increasing Mo-content of the alloy. This material property of Ni-alloys has to be considered if they should be used in slightly acid to slightly alkaline media having a high redox potential.     

Formation of ‘ferric green rust’ and/or ferrihydrite by fast oxidation of iron(II-III) hydroxychloride green rust

Fast oxidation processes of iron(II-III) hydroxychloride green rust GR(Cl⁻), Fe^II₃Fe^III(OH)₈Cl · 2H₂O, were simulated by two different methods. The first one consisted in using a strong oxidiser, namely H₂O₂. The main end product, analysed by X-ray diffraction and Mössbauer spectroscopy, is an iron(III) compound, designated as “ferric GR(Cl⁻)”, characterised by a layered structure identical to that of normal GR(Cl⁻). The second method consisted in decreasing the initial concentrations of reactants, thus increasing the proportion of dissolved O₂. Suspensions of GR(Cl⁻), obtained by mixing 4 × 10⁻³ M NaOH with FeCl₂ solutions for various R^′=[Cl⁻]/[OH⁻] values, oxidised rapidly into ferrihydrite-like compounds.     

Evaluation of corrosion inhibitors for cooling water systems operating at high concentration cycles

The present work aimed at evaluating AISI 1020 carbon steel corrosion resistance of a 6:4:1:1 (MoO/HEDP/PO/Zn²⁺) inhibitor mixture present in a solution which simulates an industrial cooling water system operating at high concentration cycles (1050 ppm Cl⁻ and 450 ppm Ca²⁺). High concentration cycles are desirable, because system purge and treated water consumption are decreased. On the other hand, a high number of concentration cycles can increase the concentration of salts and dissolved impurities, causing corrosion, incrustations, and deposits inside the pipes, heat exchangers, and cooling towers. Thus, the chloride (Cl⁻) and calcium (Ca²⁺) ions aggressiveness was studied on the proposed inhibiting mixture, at the temperatures of 40 and 60 °C, through electrochemical techniques like open circuit potential measurements, anodic and cathodic polarization, and weight loss. The results showed that the inhibitor mixture conferred adequate protection to carbon steel in low concentrations, even in high aggressive media.     

Eigenschaften von Elektrolyt-Filmen bei der atmosphärischen Korrosion

Properties of electrolyte films formed through atmospheric corrosion An investigation has been carried out into the composition of the electrolyte films which are formed on non-metallic materials (glass) as well as on metals (Cu, Zn, Fe) in an atmosphere containing SO₂. Fresh as well as pre-corroded specimen were used. It was found that the SO₂ absorbed in the solution is very rapidly oxidized into SO if the electrolyte film contains dissolved particles of the corrosion products. With 1 to 55 ppm SO₂ in the atmosphere, the change in the pH value of the electrolyte is but small and does not vary with the SO₂ partial pressure.     

Influence of the sulphate reducing bacteria on API‐X70 steel corrosion

The corrosion behaviour of API X70 immersed in a specific medium with a strain of thermophilic sulphate reducing bacteria (SRB) was analysed. Anaerobic corrosion test was carry out for 32 days at 50 °C. During the exposure time, pH, sulphate (SO) and hydrogen sulphide (H₂S) concentration were measured. Corrosion potential, linear polarization resistance and potentiodynamic polarization curve were used in order to get the influence of the SRB in the corrosion phenomenon. Scanning electron microscopy was used to determine corrosion morphology. Results show that the SRB activity influenced the overall corrosion process. The anodic branches in the polarization curves show a passivity feature, whereas, the cathodic branches were not affected. A localized corrosion attack was found.     

Oxidation of green rust suspensions containing different chromium ion species

The oxidation of hydrosulphate green rust (GR2(SO₄²⁻)) suspension containing different chromium ion species was investigated by X-ray diffraction, X-ray absorption spectroscopy and transmission electron microscopy. The pH, oxidation-reduction potential and amount of dissolved oxygen in aqueous solutions were measured during the reactions. The results show that the addition of Cr(III)₂(SO₄)₃ solution suppresses the transformation of GR2(SO₄²⁻) into iron oxyhydroxides and oxides in aqueous solution, while the addition of Na₂Cr(VI)O₄ solution promotes the transformation of GR2(SO₄²⁻) in which Cr(VI) is reduced to Cr(III); α-FeOOH particles were refined by the addition of the chromium ions.     

The region of stability of green rust II in the electrochemical potential-pH equilibrium diagram of iron in sulphate medium

The oxidation of Fe(OH)₂, by slow air bubbling, can lead to the formation of oxyhydroxides. In sulphate medium the following succession was observed: Fe(OH)₂ → green rust II → FeOOH. Green rust II is unstable in sulphate-free solutions; its composition is 2Fe(OH)₃.4Fe(OH)₂.FeSO₄. × H₂O.From the values of electrode potential and pH corresponding to the equilibrium between the solution and the solids, GR II and α-FeOOH, the value of free enthalpy of GR II has been evaluated to — 1,020,400 ± 4,600 calories/mole.The probable area of stability of the GR II in the tension—pH equilibria diagram for iron has been established.     

Effect of Si on the Corrosion behaviour of Fe-B and Fe-Ni-B glassy alloys

The effect of substituting Si for B on the electrochemical behaviour of the glassy alloy families Fe-B and Fe-Ni-B has been investigated in solutions with pH ranging from 4 up to 8.4. The influence of the aggressive ions (Cl^? and SOu) has also been studied. AES and XPS have been used to further characterize the alloys in order to interpret their behaviour in the various solutions.     

Calciumnitrit als Korrosionsinhibitor – Auswirkung einer teilweisen Umwandlung zu Nitrat

Calcium nitrite as corrosion inhibitor – effect of a partial conversion into nitrate The effectiveness and harmlessness of a calcium nitrite based corrosion inhibitor, i.e. passivator had to be proven within the scope of a German approval. Hence, an extensive investigation programme was agreed upon. Two of these investigations will be presented here in detail: Nitrites (NO₂) may be converted by oxidation into nitrates (NO₃). In the case of a risk due to chloride induced corrosion the inhibiting effect may be reduced. This aspect was investigated in corrosion tests performing current‐potential‐measurements in synthetic pore solutions containing different NO₃/NO₂‐ratios with pH‐values of 12.5, 13.0 and 13.5 and corresponding chloride concentrations. A sufficient effectiveness was assessed for the following degrees of oxidation: – 70% for pH = 12.5 and 0.126 g Cl⁻/l – 50% for pH = 13.0 and 0.710 g Cl⁻/l – 20% for pH = 13.5 and 3.995 g Cl⁻/l A conversion of nitrite does not only reduce the passivating effect, but the nitrate may furthermore cause stress corrosion cracking (SCC) of low carbon steels. In the consequence the maximum conversion rate of nitrite into nitrate in pore water solutions of mortars of different constitution and treatment was searched for. The maximum nitrate concentration was found in mortars produced with limestone powder as aggregate being subjected to carbonation. The tests revealed that approximately 26% of the totally added nitrite content were converted into nitrates. For the upper limit of the passivator dosage according to the approval demands, it is therefore assumed that a critical nitrate concentration which may cause SCC of low carbon steels has not to be expected.     

Identification and formation of green rust 2 as an atmospheric corrosion product of carbon steel in marine atmospheres

Green rust 2 (GR2(SO^2?₄ )) was detected amongst the products formed on a carbon steel rod exposed to atmospheric corrosion using X‐ray diffraction (XRD). The presence of green rust 2 has been related to sulphate reducing bacteria (SRB) in the sea spray. These were detected using test catch rods made out of inert material and posterior lab identification using Starkey culture. Likewise, after the exposure of said rods to sea environmental conditions, SRBs have been isolated from among the carbon steel corrosion products. The evolution of GR2(SO^2?₄ ) from GR1(SO^2?₄ ) was ruled out due to the tendency of this compund to produce GR2(SO^2?₄ ) in the presence of sulphate ions, as is the case here. Likewise, the evolution from GR1(Cl^?) has also been ruled out since in such case as this compound should be formed (it has not been detected in any of the 39 stations studied), the enormous affinity of the GRs with divalent anions (such as is the case with SO^2?₄ ) as opposed to the monovalent ions (such as is the case of the Cl^?) makes GR1(Cl^?) transform into GR2 (SO^2?₄ ).     

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.     

Über das Verhalten einiger CrNiMo-Stähle beim Einsatz im chemischen Betrieb

The behaviour of some CrNiMo steels in use at chemical plants A report is given about the behaviour of some highly alloyed CrNiMo steels in use in inorganic-chemical plants. The observations are supported and supplemented by results of potentiostatic tests. In the presence of mixed acids, the corrosion resistance of the steels greatly depends, e.g., on the SO: Cl^? ion ratio. In the presence of Cl^? ions and at higher temperatures, exceeding about 70°C, the resistance is largely influenced by the specifically orientated analytical and structural pattern of the steels. Attention is drawn to the detrimental influence, especially in cast metals and welds, of the concomitant element, silicon, which — if encountered in increasing quantitities — favours the segregation of several corrosion-promoting phases. Examinations of case of damage in practical operation, supported by potentiostatic tests with CrNiMo steels, with and without copper contents, have shown that the presence of copper is apt to reduce the corrosion resistance in media containing hydrochloric acid or chlorine ions. On the other hand, a copper content may be beneficial in sulphate solutions free from chlorine ions.     

The effect of the precipitation conditions on the morphology and the sorption properties of CuS particles

The effect of the nature of the copper salt precursor anion and the pH of a solution on the morphology and the phase composition of CuS particles has been studied upon precipitation from thiourea solutions. It has been established that, irrespective of the precipitation conditions, generation of the CuS phase and the chalcanthite impurity phase CuSO₄ ? 5H₂O is recorded according to the X-ray diffraction analysis data. At pH 8 and a c((NH₂)₂CS): c(Cu²⁺) ratio of 1: 1, from nitrate solutions a fine powder is formed that consists of spherical particles smaller than 100 nm in size. An increase in the c((NH₂)₂CS): c(Cu²⁺) ratio and in the precipitation pH results in coarser particles of up to 1.5 μm in size. The replacement of the salt precursor anion by SO₄^2- facilitates the decrease in the spherical particle size to 1 μm and that by Cl^– to 0.7 μm. The synthesis conditions of the CuS sorbent that control the latter’s morphological properties have an effect on the sorption of cadmium from aqueous solutions.     

The Effects of KCl,NaCl and K<Subscript>2</Subscript>CO<Subscript>3</Subscript> on the High-Temperature Oxidation Onset of Sanicro 28 Steel

The present study investigates the early stages in the oxidation process of Sanicro 28 (Fe31Cr27Ni) stainless steel when exposed to an alkali salt (KCl, NaCl or K₂CO₃) for 2 h at 450 and 535 °C. After the exposure, the oxidized samples were analyzed with a combinatory method (CA, XPS and SEM–EDX). It was found that all three salts were corrosive, and the overall oxidation reaction rate was much higher at 535 °C than at 450 °C. There were clear differences in terms of the impact of cations (Na⁺, K⁺) and anions (Cl^?, CO₃ ^2?) on the initial corrosion process at both temperatures. When focusing on the cations, the presence of potassium ions resulted in a higher rate of chromate formation than in the presence of sodium ions. When studying the effect of anions, the oxidation of iron and chromium occurred at higher rates in the presence of both chloride salts than in the presence of the carbonate salt, and chloride salts seemed to possess higher diffusion rate in the gas phase and along the surface than carbonate salts. Moreover, at the higher temperature of 535 °C, the formed chromate reacted further to chromium oxide, and an ongoing oxidation process of iron and chromium was identified with a significantly higher reaction rate than at 450 °C.     

The preparation and thermodynamic properties of Fe(II)---Fe(III) hydroxide-carbonate (green rust 1); Pourbaix diagram of iron in carbonate-containing aqueous media

Carbonate-containing green rust 1, GR1(CO₃²⁻), is prepared by oxidation of Fe(OH)₂ in aqueous solution. Ferrous hydroxide is precipitated from NaOH and FeSO₄·7H₂O solutions and carbonate ions are added as a Na₂CO₃ solution. For sufficiently large concentrations of sodium carbonate, SO₄²⁻ ions do not play any role during the oxidation process and, at the end of the first stage of reaction, Fe(OH)₂ oxidizes into GR1(CO₃²⁻). In the second stage of reaction, GR1(CO₃²⁻) oxidizes into α-FeOOH goethite except when the transformation of ferrous hydroxide is partial, which leads to the formation of magnetite. From the X-ray diffraction analysis of GR1(CO₃²⁻), lattice parameters of its hexagonal cell are found to be a = 3.160 ± 0.005 Å and C = 22.45 ± 0.05 Å. From the Mössbauer analysis of the stoichiometric GR1(CO₃²⁻), which leads to a Fe²⁺:Fe³⁺ ratio of 2:1, the chemical formula is established to be: [Fe₄^(II)Fe₂^(III)(OH)₁₂][CO₃·2H₂O]. The 78 K Mössbauer spectrum of the compound can be fitted with three quadrupole doublets, two Fe²⁺ doublets d₁ and D₂ corresponding to isomer shifts (IS) of 1.27 and 1.28 mm s⁻¹ and quadrupole splittings (QS) of 2.93 and 2.67 mm s⁻¹, respectively, and one Fe³⁺ doublet D₃ with an IS of 0.47 mm s⁻¹ and QS of 0.43 mm s⁻¹. These three doublets were already used to fit the Mössbauer spectrum of chloride-containing GR1(Cl⁻) [see J.M.R. Génin et al., Mat. Sci. Forum8, 477 (1986) and J.M.R. Génin et al., Hyp. Int. 29, 1355 (1986)]and therefore are characteristic of GR1 compounds. From the recording of electrode potential E and the pH of the suspension versus time during the oxidation, the standard free enthalpy of formation of stoichiometric GR1(CO₃²⁻) is estimated to be ΔG °_f = − 966.250 cal mol⁻¹. Knowing the chemical formula and ΔG °_f of GR1(CO₃²⁻) the Pourbaix diagram of iron in carbonate-containing aqueous solutions is drawn.     

The preparation and thermodynamic properties of Fe(II)Fe(III) hydroxide-carbonate (green rust 1); Pourbaix diagram of iron in carbonate-containing aqueous media

Carbonate-containing green rust 1, GR1(CO₃²⁻), is prepared by oxidation of Fe(OH)₂ in aqueous solution. Ferrous hydroxide is precipitated from NaOH and FeSO₄·7H₂O solutions and carbonate ions are added as a Na₂CO₃ solution. For sufficiently large concentrations of sodium carbonate, SO₄²⁻ ions do not play any role during the oxidation process and, at the end of the first stage of reaction, Fe(OH)₂ oxidizes into GR1(CO₃²⁻). In the second stage of reaction, GR1(CO₃²⁻) oxidizes into α-FeOOH goethite except when the transformation of ferrous hydroxide is partial, which leads to the formation of magnetite. From the X-ray diffraction analysis of GR1(CO₃²⁻), lattice parameters of its hexagonal cell are found to be . From the Mössbauer analysis of the stoichiometric GR1(CO₃²⁻), which leads to a Fe²⁺:Fe³⁺ ratio of 2:1, the chemical formula is established to be: [Fe₄^(II)Fe₂^(III)(OH)₁₂][CO₃·2H₂O]. The 78 K Mössbauer spectrum of the compound can be fitted with three quadrupole doublets, two Fe²⁺ doublets d₁ and D₂ corresponding to isomer shifts (IS) of 1.27 and 1.28 mm s⁻¹ and quadrupole splittings (QS) of 2.93 and 2.67 mm s⁻¹, respectively, and one Fe³⁺ doublet D₃ with an IS of 0.47 mm s⁻¹ and QS of 0.43 mm s⁻¹. These three doublets were already used to fit the Mössbauer spectrum of chloride-containing GR1(Cl⁻) [see J.M.R. Génin et al., Mat. Sci. Forum8, 477 (1986) and J.M.R. Génin et al., Hyp. Int. 29, 1355 (1986)]and therefore are characteristic of GR1 compounds. From the recording of electrode potential E and the pH of the suspension versus time during the oxidation, the standard free enthalpy of formation of stoichiometric GR1(CO₃²⁻) is estimated to be ΔG °_f = − 966.250 calmol⁻¹. Knowing the chemical formula and ΔG °_f of GR1(CO₃²⁻) the Pourbaix diagram of iron in carbonate-containing aqueous solutions is drawn.