Passivation Of Iron Research Articles

Corrosion and passivation of iron and its nitrided layer in borate buffer

The aim of this work was to gain better understanding of the reasons of enhanced resistance to pitting corrosion of nitrogen-containing iron. Gas nitrided (570 °C, 4 h) and untreated Armco iron were examined in a borate solution of pH 8.4 without and with chlorides or ammonia. Enhanced pitting resistance and enhanced anodic currents were exhibited by nitrided Fe and also by untreated Fe in the solution with added ammonia. XPS showed that anodic films on nitrided Fe contained much larger amounts of iron oxides, in particular of magnetite, than those on untreated Fe. It is suggested that the anodic behaviour of nitrided Fe is determined mainly by the effect of evolving ammonia on corrosion products. Increased anodic dissolution can be explained by the formation of soluble complexes with ammonia, whereas increased amounts of magnetite can be due to the ammonia-promoted conversion of FeOOH + Fe(II) to Fe 3O 4. It is proposed that the enhanced pitting resistance of nitrided Fe results mainly from the formation of large amounts of iron oxides and from binding of chloride anions into a Fe–NH 3–Cl complex.

Effects of initial iron corrosion rate on long-term performance of iron permeable reactive barriers: Column experiments and numerical simulation

Column experiments and numerical simulation were conducted to test the hypothesis that iron material having a high corrosion rate is not beneficial for the long-term performance of iron permeable reactive barriers (PRBs) because of faster passivation of iron and greater porosity loss close to the influent face of the PRBs. Four iron materials (Connelly, Gotthart-Maier, Peerless, and ISPAT) were used for the column experiments, and the changes in reactivity toward cis-dichloroethene (cis-DCE) degradation in the presence of dissolved CaCO 3 were evaluated. The experimental results showed that the difference in distribution of the accumulated precipitates, resulting from differences in iron corrosion rate, caused a difference in the migration rate of the cis-DCE profiles and a significant difference in the pattern of passivation, indicating a faster passivation in the region close to the influent end for the material having a higher corrosion rate. For the numerical simulation, the accumulation of secondary minerals and reactivity loss of iron were coupled using an empirically-derived relationship that was incorporated into a multi-component reactive transport model. The simulation results provided a reasonable representation of the evolution of iron reactivity toward cis-DCE treatment and the changes in geochemical conditions for each material, consistent with the observed data. The simulations for long-term performance were also conducted to further test the hypothesis and predict the differences in performance over a period of 40 years under typical groundwater conditions. The predictions showed that the cases of higher iron corrosion rates had earlier cis-DCE breakthrough and more reduction in porosity starting from near the influent face, due to more accumulation of carbonate minerals in that region. Therefore, both the experimental and simulation results appear to support the hypothesis and suggest that reactivity changes of iron materials resulting from evolution of geochemical conditions should be considered in the design of iron PRBs.

The pretreatment by the Fe–Cu process for enhancing biological degradability of the mixed wastewater

The Fe–Cu process in combination with cyclic activated sludge system (CASS) was used to treat the mixed wastewater composed of industry wastewater and urban sewage in this work. The results showed that the pretreatment by the Fe–Cu process removed 20% of COD cr and 32% of total phosphorus (TP), which reduced the loading rate of the subsequent biological treatment. Mean while, biodegradability of the wastewater was enhanced, which created favorable condition for the subsequent biological treatment. The formation of heavy, lumpy or granular, absorbent, enriched with microorganisms bio-ferric activated sludge with good setting performance promoted degradation of various refractory organic contaminants. The increase by 10 times of nitrifying and denitrifying bacteria counts (total biofilm biomass increased by 59%) and 0.4 of pH value enhanced the biological nitrification and denitrification to ensure the final effluent NH 3–N and TN to be 8 and 20 mg/L, respectively. Agglomeration, passivation and clogging of iron were not observed in three months of continuous operation. Furthermore, the consumption of iron was low. All these led to an easy maintenance and low operating cost.

Effect of carbon on stress corrosion cracking and anodic oxidation of iron in NaOH solutions

Anodic behaviour of decarburised iron and of quenched Fe–C materials with up to 0.875 wt% C was examined in 8.5 M NaOH at 100 °C to explain the role of carbon in caustic stress corrosion cracking (SCC) of plain steels. Removal of carbon from Armco iron strongly reduced its intergranular SCC. Slip steps on grains did not initiate cracks. It has been shown that carbon at low contents deteriorates the passivation of iron, whereas at high contents it promotes the formation of magnetite. High resistance to SCC of high carbon steels can be explained by an intense formation of magnetite on these steels.

On the passivation of iron in aqueous solutions of zinc 1-hydroxyethane-1,1-diphosphonate

This work is devoted to studying the passivating ability of the zinc complex of the 1-hydroxyethane-1,1-diphosphonic acid (HEDP) in a borate buffer solution. For the first time, we used the in situ ellipsometric method to study the mechanism of formation of a protective film on iron in the presence of HEDP, HEDPZn, and ZnSO4 in the course of the cathodic polarization of the electrode. The investigations of adsorption of HEDPZn on iron (at E = −0.65 V) in combination with X-ray photoelectron spectroscopy (XPS) have shown that on the metal surface there is formed a multilayer protective film consisting of an internal layer of Zn(OH)2 and an outer layer consisting of HEDP complexes with Fe2+ and/or Zn2+. It has been found that the thickness of the passivating film does not exceed 60 Å, of which 7–10 Å correspond to the low-soluble zinc hydroxide.

Cobalt and Molybdate Ions Effects on the Passivation of Iron Based FeCoC Ternary Alloys, in non Deaerated Solution of 10-3 M NaHCO3 + 10-3 M Na2SO4, at 25 °C

In the present paper, the electrochemical behaviour of four based iron FeCoC ternary alloys was studied in non deaerated solution of 10 -3 M NaHCO 3 + 10 -3 M Na 2SO 4, at 25 °C, and compared to that of pure iron and cobalt. The inhibition effect of sodium molybdate has also been considered for different concentrations. It was noted that the oxygen oxidizes the FeCoC ternary alloys except that with the most important cobalt content. For this reason, a potentiodynamic polarisation was carried out at different temperatures (25, 35, 45 and 55°C) in which the current growth versus temperature indicates really that molybdate oxidizes this alloy.

Electronic barriers in the iron oxide film govern its passivity and redox behavior: Effect of electrode potential and solution pH

We have measured in situ the electronic conductance spectra of the passive film formed on an Fe electrode immersed in a borate buffer solution using electrochemical tunneling spectroscopy (ECTS) and electrochemical impedance spectroscopy (EIS) techniques, and we have followed their changes as the electrode is electrochemically oxidized and reduced. We demonstrate that pre-passive Fe(II) oxide and the passive Fe(II)/Fe(III) film, behave as p- and n-type semiconductors, respectively and that their reversible inter-conversion is mediated by the availability of free charge carriers on the electrode surface. ECTS spectra have been also modeled to obtain the main electrochemical kinetic parameters of the electron transfer through both p-Fe(II) and n-Fe(III) oxides at different sample potentials and pHs values. We find that the electronic energy barrier in the oxide and its dependence with electrode potential and solution pH, determine the reactivity and passivity of iron.

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.

Effects of a magnetic field on the anodic dissolution, passivation and transpassivation behaviour of iron in weakly alkaline solutions with or without halides

The effects of a 0.4 T horizontal magnetic field on the anodic dissolution, passivation and transpassivation behaviour of iron in bicarbonate solutions of various concentrations and in dilute bicarbonate solutions with or without halides are investigated by electrochemical polarisation measurements. The applied magnetic field does not affect the activation-controlled anodic current, the steady passive current and the transpassive current, but significantly affects the activation–passivation transition processes for iron in bicarbonate solutions without halides. The effects of magnetic field are strongly dependent on passivation mechanisms that result in different types of surface films and corresponding rate determining steps of film dissolution. There is a synergistic effect between the applied magnetic field and halides, chlorides or bromides, on attacking the passivation of iron in dilute bicarbonate solutions. The effects of the magnetic field are analysed based on the previously proposed electrochemical kinetics equations. The magnetic field affects the anodic polarisation behaviour through its enhancing effects on mass transport processes at the precipitation–dissolution type surface film/solution interfaces. The magnetic field shows little or no effects on continuous and steady passivation films where the oxidation rate is controlled by mass transport processes within surface films. Magnetoelectrochemistry measurements are suggested as a prospective method for researches on corrosion or passivation mechanisms.

SYNTHESIS OF NANO-SIZED IRON FOR REDUCTIVE DECHLORINATION. 2. Effects of Synthesis Conditions on Iron Reactivities

Nano-sized iron was synthesized using borohydride reduction of in aqueous solution. A wide range of synthesis conditions including varying concentrations of reagents, reagent feeding rate, and solution pH was applied in an aqueous system under anaerobic condition. The reactivity of nano-sized iron from each synthesis was evaluated by reacting the iron with TCE in batch systems. Evidence obtained from this study suggest the reactivity of iron is strongly dependent on the synthesis solution pH. The iron reactivity increased as solution pH decreased. More rapid TCE reduction was observed for iron samples synthesized from higher initial concentration, which resulted in lower solution pH during the synthesis reaction. Faster feeding of solution to the solution resulted in lower synthesis solution pH and the resultant iron samples gave higher TCE reduction rate. Lowering the pH of the solution after completion of the synthesis reaction significantly increased reactivity of iron. It is presumed that the increase in the reactivity of iron synthesized at lower pH is due to less precipitation of iron (hydr)oxides or less surface passivation of iron.

Adsorptive passivation of iron by organic acid anions

Studies on the iron passivation by organic acid anions in aqueous solutions are briefly reviewed. It is shown that the passivation can be caused only by their adsorption, retarding both the iron dissolution and the formation of oxide films. Earlier, it had been believed that oxide films play a dominant role in the iron passivation in neutral solutions. The recent viewpoint is that such nonoxide iron passivation can occur in solutions of salts of not only aromatic amino acids (sodium phenylantranilate and its substitutes), but other carboxylic acids as well. An important role of chemisorption and hydrophobic properties of anions for the formation of adsorption passive films is emphasized. New possibilities for inhibitor protection of iron against corrosion, which is based on adsorptive passivation, are pointed out.

Passivity of iron in red mud?s water solutions

Red mud suspensions are studied as passivating agents for iron in alkaline chloride media. Red mud particles in alkaline media are negatively charged, and can absorb considerable amounts of protons without significant changes in pH. These particles adhere easily to an iron surface forming aggregates of heterogeneous shape, smaller than 10μm×10 μm. These adhered particles recrystallise on potential cycling, and hinder metal dissolution and magnetite formation. Carbon steel samples passivated in red mud suspensions become resistant to corrosion in alkaline chloride media. Corrosion potential versus time records and impedance spectroscopy measurements allow formulating some hypotheses about the inhibiting mechanism.

Passivity of iron in red mud’s water solutions

Red mud suspensions are studied as passivating agents for iron in alkaline chloride media. Red mud particles in alkaline media are negatively charged, and can absorb considerable amounts of protons without significant changes in pH. These particles adhere easily to an iron surface forming aggregates of heterogeneous shape, smaller than 10 μm×10 μm. These adhered particles recrystallise on potential cycling, and hinder metal dissolution and magnetite formation. Carbon steel samples passivated in red mud suspensions become resistant to corrosion in alkaline chloride media. Corrosion potential versus time records and impedance spectroscopy measurements allow formulating some hypotheses about the inhibiting mechanism.

Effect of the Scan Rate on the Kinetic Parameters of Active Dissolution and Passivation of Iron in a Neutral Solution

The effect of the potential scan rate (V = 0.2–100 mV/s) of a rotating disk electrode (rotational speed ν = 6000 rpm) on the kinetics of active anodic dissolution and active-passive transition of Armco iron in a deaerated borate buffer (pH 7.40) was studied by cyclic voltammetry. The rates of active dissolution and the formation of a prepassive film were found to be determined, under free-diffusion conditions, by slow electrochemical steps of electron transfer throughout the V range studied; the cyclic voltammogram is a transient, thermodynamically nonequilibrium curve. The anodic current peak linearly grows and its potential is shifted in the positive direction with an increase in logV. The apparent coefficients of electron transfer for active anodic dissolution depend on V in the whole range studied. This can provide explanation to a large scatter of literature data on the active anodic dissolution and active-passive transition of iron in neutral media.

Effects of an applied magnetic field on the dissolution and passivation of iron in sulphuric acid

The effects of an applied magnetic field (MF) on the electrochemical state, anodic dissolution and passivation of iron in sulphuric acid solution were studied by potentiodynamic scanning polarisation measurements, potentiostatic polarisation measurements and scanning electron microscopy observation. The magnetic field reduced the fractional surface film coverage on the electrode by enhancing the film dissolution process. This made the electrode prone to active dissolution. With increasing applied potentials the magnetic field accelerated the anodic dissolution at relatively low potentials, changed the oscillation or passivation to permanent active dissolution at intermediate potentials, and maintained the passive state at high potentials. Potentials for the onset of passivation moved in the noble direction when the magnetic field was imposed. An electrode kinetics formulation for the effects of the magnetic field on the dissolution and passivation is proposed. In the presence of a magnetic field and at specific anodic potentials, scalloping occurred due to accelerated localized dissolution. The scalloping areas were on both sides of the electrode and oriented parallel to the direction of the earth’s gravitation field. The ratios of the scalloping area caused by a 0.4 T magnetic field on the whole electrode surface were 0.69 (at 200 mV), 0.66 (at 350 mV) and 0.75 (at 400 mV), respectively. In contrast, uniform electrode surfaces were observed at these anodic potentials in the absence of the magnetic field. Uneven dissolution of iron in the presence of a magnetic field was related to the relative configuration between the magnetic field direction and the electrode surface and also to the special concentration gradient of reactive species at the electrode circumferential area.

Electrochemical behavior of surface films formed on Fe in chromate solutions

The stability of passive films formed on Fe in K 2Cr 2O 7 solutions during exposure at open-circuit potential or by potential cycling is studied in a chromate-free solution. The electrochemical behavior of chromate-passivated Fe is investigated with cyclic voltammetry combined with LASER light reflectance measurements which allow an in situ determination of the thickness of the iron oxide film. The electrochemical behavior of chromate-passivated Fe in chromate-free solutions strongly depends on passivation treatment. Passivation of iron by immersion at open-circuit in chromate solution leads to a passive film, in which both Fe and Cr species dissolve almost independently of the presence of the other one: Fe oxide by reductive dissolution and Cr oxide by oxidative dissolution in the corresponding potential regions. Passivation of iron by potential cycling in chromate solutions leads to much less loss of the otherwise soluble oxidized chromate and reduced ferrous species in subsequent electrochemical experiments (trapping in a protective film). Concerning the dissolution behavior, the film formed on iron by cycling in chromate solution behaves similarly as the passive film on Fe–17Cr alloy. However, the remnant passive film after reductive or oxidative dissolution on the Fe–Cr alloy is of truely protective nature as compared to the films formed on iron in chromate solutions, which show only a small contribution to the potential drop.

To the Low-Temperature Passivity of Iron at the Gaseous Oxidation

The low-temperature passivation of the oxide growth on iron is studied with the use of digital ellipsometry at a temperature of 300°C and an oxygen pressure from 10–3 to 1 mmHg. The maximum increment in the oxide thickness as a function of the oxygen pressure P a–p is observed in an hour of exposure, which indicates the active–passive transition. This passivity of iron and other metals can be caused by the multilayer and multiphase structure of the oxide film formed. As the oxygen pressure is increased, an external protective hematite layer appears on iron, and the inner quickly growing magnetite layer has no time to reach its limiting thickness. Shortening of the period of a rapid growth of magnetite, which is observed upon the increase in the oxygen pressure at the same exposure, results in the maximum of the summary thickness of the layer at a certain pressure P a–p. Hematite over the magnetite layer is usually formed as laterally spreading islets, the coalescence of which sharply decelerates the oxidation. In the time–pressure–oxide thickness plot, the areas of the low-temperature passivation of iron can be distinguished in wide ranges of temperature and pressure. The electrophysical treatment in the range of the active–passive transition sharply intensifies the oxidation of iron-based alloys and leads to the formation of layers with a substantial thickness and protective ability.

Passivation of Chrome-Plated Sheet Iron in Electrolytes Based on Chromium(III) Salts

Passivation of chrome-plated sheet iron in electrolytes based on Cr(III) salts and capable of completely replacing highly toxic and aggressive electrolyte based on Cr(VI), is studied.

Three-dimensional modelling of selective dissolution and passivation of iron–chromium alloys

A three-dimensional model has been developed for modelling the selective dissolution and passivation of alloys. The model has been used to simulate the passivation of iron–chromium alloys. The real structure of the alloy is taken into account (bcc in the present case), as well as the structure of the initial surface. The passivation is modelled in considering the formation of “oxide” nuclei, resulting from the presence of local chromium-rich clusters. During the dynamic evolution of the model, based on the Monte Carlo method, surface diffusion and dissolution of atoms occur according to probabilities dependent on the nature of the atom (Cr or Fe) and on its chemical environment. The conditions of simulation can be changed through a set of parameters defining the rules for surface diffusion, selective dissolution and number of Cr atoms in the Cr clusters required to initiate locally the passivation. The effects of these parameters on the simulation have been tested for an alloy containing 22 at.% Cr and compared with experimental data. The results show that the diffusion of Fe has little influence on the course of passivation while the diffusion of Cr has a marked effect. When the number of surface chromium atoms required to form a nucleus of passive film increases, the passivation becomes less rapid, with a marked effect on the composition of the passivated layer. The extent of the chromium enrichment in the passivated surface obtained in the model for the initial stages of passivation is not as high as the one measured experimentally in the stationary state of passivity. Other simulations have then been performed with various chromium contents in the alloy. The results show the existence of a transition, which is not sharp but progressive, between alloys that cannot be passivated to alloys that are passivated.

Prediction of the extraction energy of an atom from the surface of Fe–Cr alloys using topological descriptors

A 3D model for simulating the passivation of iron–chromium alloys has been developed previously. This paper describes the attempts to estimate the probabilities of dissolution used in the model from the energy of extraction of Fe or Cr atoms according to their chemical environment. Quantum chemistry calculations have been performed to estimate the energy of extraction of Fe or Cr from the cluster for various topological environments of the atom to be extracted. Using the obtained results, a simple multilinear correlation between the energy of extraction of an atom and its environment has been developed. Such a relation will allow us to do more rapid calculations of the energy of extraction for any topology, which can be included in the simulation. The correlation used topological descriptors describing, in a relevant way, the environment of an atom to be extracted. At first, we tested the descriptors connected to the location of the extracted atom in relation with its neighbors. Then, we took into account the parameters describing only the distribution of the neighbors among them, which leads to the autocorrelation function of the cluster. Finally, we combined these two types of descriptors.