Nitrate ion effect on the passive film breakdown and current oscillations at iron surfaces polarized in chloride-containing sulfuric acid solutions

Current oscillatory phenomena have been used to study the effect of nitrates on pitting corrosion of passive iron surfaces in chloride-containing sulfuric acid solutions. From the quasi steady-state current-potential and potentiostatic current-time curves of the Fe | 0.75 M H₂SO₄+10 mM Cl⁻ system it is deduced that at lower potentials nitrates stimulate pitting acting as activators of the oxide dissolution. At higher potentials nitrates act as passivators causing a sudden passivation of Fe during a mass-transport controlled process across a salt film. Current oscillations appear over a wide potential region. The oscillation period as well as the induction period of time occurring before the onset of oscillation, both decrease by increasing the nitrate concentration. The effect of nitrates at lower and higher potentials is discussed in terms of the electrochemical and redox reactions of nitrate ions.     

Ozone-oxygen gas enhanced passivation of pure iron

Pure iron samples were passivated in borate buffer solution and were exposed to ozone-oxygen gas to study the possibility of improving the surface nanostructure such as atomically flat terrace structure. The treated samples were analyzed with spectroscopic ellipsometry and scanning tunneling microscopy, and the test of corrosion resistance was also carried out. The thickness of the oxide film increased by passivation at 800 mV (Ag/AgCl) and increased by ozone-oxygen gas exposure, but the oxidation film thickness decreased in air due to the reconstruction after rapid growth of oxides by passivation. The reconstruction of the oxide film was estimated by the change of the film thickness and compositions which were analyzed by spectroscopic ellipsometry using several layer models of Fe(OH)₃, γ-Fe₂O₃ and Fe₃O₄ and so on. The widest terrace width of the oxidation film was obtained on large particles based on the reconstruction after the combination treatment of passivation at 800 mV as well as subsequent ozone-oxygen gas treatment. The terrace width of the oxide film after passivation and ozone-oxygen gas treatment was three times larger than that of air-formed oxide film. The terrace width with atomic scale flatness was correlated with the corrosion resistance except for the increase in oxide film thickness.     

The corrosion behavior of Fe-10Cr nanocrystalline coating

Fe-10Cr nanocrystalline (nc) coatings with a grain size of 20-30 nm were synthesized on glass substrates by magnetron sputtering. The corrosion behavior was investigated in 0.05 mol/L H₂SO₄ + 0.25 mol/L Na₂SO₄ and 0.05 mol/L H₂SO₄ + 0.5 mol/L NaCl solution by polarization curves, EIS and Mott-Schottky analysis. The results showed that compared to Fe-10Cr cast alloy, the active dissolution of the coating was accelerated; the passive film contained more Cr and therefore the coating was easier to passivate. The passive films formed on Fe-10Cr nc and cast alloy exhibited n-type semiconducting behavior in acidic solutions without Cl⁻ and p-type semiconducting behavior in acidic solutions with Cl⁻. The lower breakdown potential for both materials in the solution with Cl⁻ is related to the p-type passive film formed on them. For Fe-10Cr nc, lower donor density and increased Cr content were responsible for the chemical stability of the passive film.     

Nucleation and growth of anodic films on stainless steel alloys I. Influence of minor alloying elements and applied potential on passive film growth

The influence of minor alloying elements (Mo, V, W) when added to a Fe18Cr alloy on the ability of a passive film to nucleate and grow on a freshly generated metal surface, and on the subsequent stability of the film was investigated as a function of electrolyte composition and applied potential using a scratch chronoamperometric technique. Mo and V decreased the rate of active dissolution prior to passivation, allowing the onset of passivation to occur more rapidly, and also improved the stability of the passive film, especially to attack by Cl^– in acidic (H₂SO₄, HClO₄) solutions. W additions had a detrimental effect on the repassivation behaviour of Fe18Cr. Repassivation of the scratch scars was evaluated, from the current transients, in terms of the number of layers of surface film formed.     

The structure and electronic properties of passive and prepassive films of iron in borate buffer

The films that form on pure iron during potentiodynamic anodic polarization in aqueous borate buffer were investigated by surface enhanced Raman spectroscopy (SERS), and by electrochemical impedance spectroscopy and Mott-Schottky analysis at selected potentials. According to SERS, the passive film is a bilayer film with an outer layer of an as yet undetermined Fe(III)oxide/hydroxide, identified by a strong Raman peak at 560 cm⁻¹. The inner layer was a spinel compound. The capacitances of passive iron were frequency dependent and a constant phase element (CPE) best described the frequency dispersion. Current increases in cathodic polarization scans confirmed the accuracy of flatband potentials calculated from Mott-Schottky tests at two different film formation potentials. Both films were found to be n-type and flatband potentials of −846 and −95 mV vs. SHE and carrier densities of 1.6 × 10²² and 8.3 × 10²⁰/cm³ were found for films grown at −500 and +1000 mV, respectively. The cathodic polarization curve of passivated iron exhibited a complex shape that was explained by the electronic properties of iron's passive and prepassive films. The reductive dissolution of the films abruptly began when the potential was lowered below their flatband potentials. It is suggested that the cathodic polarization behavior contributes to iron's susceptibility to localized corrosion.     

Synergic effect of benzotriazole and chloride ion on Cu passivation in a phosphate electrochemical mechanical planarization electrolyte

In this study, the effect of chloride ion (Cl⁻) in phosphate electrolytes of pH 2 containing benzotriazole (BTAH) developed for use in electrochemical mechanical planarization (ECMP) was investigated at various anodic potentials. According to D.C. and A.C. electrochemical analyses, the inhibition effect of the BTAH passive film formed in phosphate electrolyte containing both BTAH and Cl⁻ was superior to that formed in phosphate electrolytes containing BTAH alone, even at high anodic potential. The effective window for BTAH passivation reached ∼1.3 V vs. Ag/AgCl nearly three times that of the ∼0.5 V vs. Ag/AgCl recorded for electrolyte containing BTAH alone. According to analyses conducted by atomic force microscopy (AFM) and secondary ion mass spectrometer (SIMS), the thickness of the passive film grown from the BTAH-only electrolyte at 0.3 V vs. Ag/AgCl was ∼52 ± 7 nm and ∼55 nm, respectively. As for the passive film grown from the BTAH and Cl⁻ electrolyte, the thickness increased to ∼104 ± 18 nm and ∼106 nm, respectively. The mechanism for the enhanced inhibition capability was that the passive film grown from the BTAH and Cl⁻ electrolyte was thicker compared to that formed from the BTAH-only electrolyte due to the incorporation of Cl⁻ into the BTAH passive film. The ECMP polishing results also demonstrated an obvious step height reduction of ∼1000 nm in a patterned structure for only 60 s polishing at a high potential of 1.0 V vs. Ag/AgCl under a low downward pressure (∼0.5 psi). Subsequently, this study proposes that the control of Cl⁻ in a phosphate ECMP electrolyte of pH 2 may be useful in enhancing the passivation capability of BTAH passive film, thus expanding the operating potential window.     

Supramolecular assembly of porphyrin bound DNA and its catalytic behavior for nitric oxide reduction

A stable Fe(4-TMPyP)-DNA-PADDA (FePyDP) film was prepared on pyrolytic graphite electrode (PGE) through the supramolecular interaction between water-soluble iron(III) meso-tetrakis(N-methylpyridinium-4-yl)porphyrin (Fe(4-TMPyP)) and DNA template, where PADDA (poly(acrylamide-co-diallyldimethylammonium chloride)) is employed as a co-immobilizing polymer. Electronic absorption spectral and quartz crystal microbalance measurements revealed that Fe(4-TMPyP) interacted with DNA to generate a species with the molar ratio of 1:5 for Fe(4-TMPyP):DNA phosphate. Cyclic voltammetry of FePyDP film showed a pair of stable and reversible peaks corresponding to Fe^III/Fe^II redox potential of −0.13 V versus Ag|AgCl in pH 7.4 PBS. The electron transfer was expected across the double-strand of DNA by an “electron tunneling” mechanism. The modified electrode displayed an excellent catalytic activity for NO reduction at −0.61 V versus Ag|AgCl. The catalytic current was enhanced at lower pH. Chronoamperometric experiments demonstrated a rapid response to the reduction of NO with a linear range from 0.1 to 90 μM. The detection limit was 30 nM at a signal-to-noise ratio of 3.     

Visible light photoelectrochemical responsiveness of self-organized nanoporous WO3 films

Visible light-responsive WO₃ nanoporous films with preferential orientation of the (0 0 2) planes were prepared by anodization in neutral F⁻-containing strong electrolytes. The pore diameter of the self-organized structure was estimated to be in the region of 70-90 nm. Voltages were applied by stepping, which positively influenced passivity breakdown and played a significant role in the formation of self-organized nanoporous films. Under visible light irradiation, the photocurrent density (at 1.6 V vs. Ag/AgCl) and maximum photoconversion efficiency generated by the annealed nanoporous film were 3.45 mA/cm² and 0.91%, respectively. The annealed nanoporous WO₃ films show maximum incident photon-to-current conversion efficiency of 92% at 340 nm at 1.2 V vs. Ag/AgCl. These values are higher than that of annealed compact WO₃ film due to the large interfacial heterojunction area. The photoelectrochemical activities and electronic conductivities were also enhanced by annealing crystallization, which removed the recombination centers.     

Evolution of oxygen reduction current and biofilm on stainless steels cathodically polarised in natural aerated seawater

The aim of a series of works recently performed at ISMAR was to provide new useful information for a better understanding of the mechanisms by which bacteria settlement causes corrosion on Stainless Steels (SS) and similar active-passive alloys exposed to seawater. In this work, the evolutions of cathodic current, bacteria population, and electronic structure of the passive layer were investigated on SS samples polarised at fixed potentials during their exposure to natural seawater. It was found that, during the first phase of biofilm growth, cathodic current increase is proportional to the number of settled bacteria at each fixed potential. However, the proportionality factor between settled bacteria and cathodic current depends on imposed potential. In particular, the proportionality factor strongly decreases when the potential is increased above a critical value close to −150 mV Ag/AgCl. This effect seems to be correlated with the electronic structure of the passive layer. Indeed, the outer part of the passive layer on tested SS was found to behave like a conductor at potentials more active than −150 mV Ag/AgCl, and like an n-type semiconductor at more noble potentials.     

Localized passivity breakdown of iron in chlorate- and perchlorate-containing sulphuric acid solutions: A study based on current oscillations and a point defect model

Current oscillations are used to study the effect of chlorate and perchlorate ions on the iron passivity in sulphuric acid solutions. Quasi steady-state current-potential and potentiostatic current-time curves show the emergence of complex current oscillations, besides the simple periodic ones attributed to general corrosion occurring across the passive-active transition of the Fe|0.75 M H₂SO₄ electrochemical system. The complex current oscillations arising at the iron passive state are indicative of pitting corrosion. Experimental results support that pitting is due to chlorides produced via the reduction of chlorates and perchlorates by ferrous ions either during the active phase of current oscillations or in the passive phase during the H⁺-catalyzed dissolution of the oxide. Thus, chloride is the aggressive ion that causes pitting corrosion and not chlorates and perchlorates themselves. Chloride production induced via the reduction of perchlorates is much slower than that induced by chlorates. A point defect model (PDM) is employed to explain the oxide growth and its breakdown induced by general and pitting corrosion.     

Development and application of a poly(2,2′-dithiodianiline) (PDTDA)-coated screen-printed carbon electrode in inorganic mercury determination

In this work, monomer solutions of aniline (ANI) and 2,2′-dithiodianiline (DTDA), an aniline derivative containing -S-S- links, were prepared and used in the electrochemical copolymerisation of ANI and DTDA by cyclic voltammetry on a screen-printed electrode (SPE) in 1 M HCl. Electropolymerisation of aniline on the surface of the screen-printed working electrode was performed by sweeping the potential between −500 and + 1100 mV (vs. Ag/AgCl) at a sweep rate of 100 mV/s. Electrocopolymerisation was performed with a mixture of ANI and DTDA by sweeping the potential between −200 and + 1100 mV (vs. Ag/AgCl) at a sweep rate of 100 mV/s [J.L. Hobman, J.R. Wilson, N.L. Brown, in: D.R. Lovley (Ed.), Environmental Microbe Metal Interactions, ASM Press, Herndon, Va, 2000, p. 177]. The cyclic voltammogram (CV) for each of the electrochemically deposited polyaniline (PANI) and the mixture of ANI and DTDA for the co-polymer polymerisation on SPCE were recorded for electrochemical analysis of the peak potential data for the mono and copolymer. Anodic stripping voltammetry (ASV) was used to evaluate a solution composed of (1 × 10⁻⁶ M HgCl₂, 0.1 M H₂SO₄, 0.5 M HCl), in the presence of the co-polymer sensor electrode. The Hg²⁺ ions were determined as follows: (i) pre-concentration and reduction on the modified electrode surface and (ii) subsequent stripping from the electrode surface during the positive potential sweep. The experimental conditions optimised for Hg²⁺ determination included the supporting electrolyte concentration and the accumulation time. The results of the study have shown the use of a conducting polymer modified SPCE as an alternative transducer for the voltammetric stripping and analysis of inorganic Hg²⁺ ions.     

Galvanostatic response of the passive film on iron in acid media

The galvanostatic technique applied to the passive system as it is described here enable access to the region between zero current and the steady state passivation current in the potentiostatic polarization diagram. It allows us to visualize the electric field applied to the passive film out of its steady state value. A good correlation with the impedance behaviour of the passive iron is observed in the entire frequency range. The long time domain of the potential/time curves reveals an exceptional degree of linearity in a wide range of current applied. In this region the passive film may be represented by a capacitor whose charge is defined not only by the applied potential but also by the actual film thickness. The hydrodynamic conditions affect the galvanostatic transient in the case of the Fe/H₃PO₄ passive system.     

Potential dependence of normalized friction coefficient of passive iron surface evaluated by nano-scratching in solution

Nano-scratching in solution was performed to the single-crystal iron (1 0 0) surface passivated at 0.0-1.0 V (SHE) in pH 8.4 borate buffer solution to evaluate the friction coefficient of the iron (1 0 0) surface kept in the passive state and its potential dependence. The friction coefficient obtained with nano-scratching for the passive iron surface depended on normal force, i.e., normal displacement, which resulted mainly from the geometry of the diamond tip. In order to avoid the effect of the tip geometry on friction coefficient, the normalized friction coefficient was newly defined with dividing friction coefficient by geometrical factor. The normalized friction coefficient obtained with nano-scratching in solution for the iron (1 0 0) surface kept in the passive sate was significantly larger than those obtained with nano-scratching in air after passivation. The normalized friction coefficient obtained with nano-scratching in air after passivation was almost independent of potential in the passive region. On the other hand, the normalized friction coefficient obtained with nano-scratching in solution increased with increasing potential in the passive region.The difference between normalized friction coefficients obtained with nano-scratching in solution and in air was discussed by taking into account a series of mechano-electrochemical reaction (film rupture, active dissolution and repassivation) which would take place at the moving front of the diamond tip during nano-scratching in solution. The large potential dependence of the normalized friction coefficient obtained with nano-scratching in solution was explained in terms of the increase in repassivation rate at the film rupture sites with increasing potential in the passive region.     

A microelectrochemical scanning flow cell with downstream analytics

The combination of a capillary based microelectrochemical flow cell system and downstream UV–vis analytics allows obtaining synchronized electrochemical and spectroscopic data in a fully automated mode. This method combination can be generally applied to microelectrochemical studies in which an electrochemical species is released or consumed during the electrochemical reaction. For the example of pure zinc surfaces, the characterization of the integrated spectroscopic system is presented with a Zn²⁺ detection limit below 0.1 μmol l⁻¹ using Zincon as complexing agent. A parameter screening of the effect of pH in the range of 6.6–9.0 in borate buffer reveals a linear increase in zinc dissolution with proton concentration but a distinct step in the open circuit potential from the active state (around −700 mV SHE, pH 6.6–7.1) to the passive state (around −300 mV SHE, pH 7.4–9.0) indicating the formation of a closed passive layer. This mechanism is strongly influenced by sulfate anions which increase the dissolution rate of the passive film and promote the active state as monitored by the dissolution profile and OCP (open circuit potential) values. Within the scope of this parameter variation, the congruency between OCP transients, potentiodynamic sweeps and time resolved dissolution profiles is discussed.     

Effects of nitrogen on the passivation of nickel-free high nitrogen and manganese stainless steels in acidic chloride solutions

The role of nitrogen on the passivation of nickel-free high nitrogen and manganese stainless steels was investigated in 0.5 M H₂SO₄, 3.5% NaCl and 0.5 M H₂SO₄ + 0.5 M NaCl solutions using potentiodynamic polarization, electrochemical impedance spectroscopy and X-ray photoelectron spectroscopy techniques. The passive film stability was enhanced in 0.5 M H₂SO₄ and the pitting resistance was improved in 3.5% NaCl solution by more nitrogen addition. The influence of nitrogen extended the whole anodic polarization region in 0.5 M H₂SO₄ + 0.5 M NaCl solution, as demonstrated by the enhanced dissolution resistance, promoted adsorption and passivation process, improved film protection and pitting resistance with increasing nitrogen content. Possible mechanisms relating to the role of nitrogen in different potential regions were discussed.     

Strategy for the syntheses of isolated fine silver nanoparticles and polypyrrole/silver nanocomposites on gold substrates

In this work, isolated fine silver nanoparticles and polypyrrole/silver nanocomposites with diameters of about 10 nm on gold substrates were first prepared by electrochemical methods. First, an Ag substrate was cycled in a deoxygenated aqueous solution containing 0.1 M HCl from −0.30 to +0.30 V versus Ag/AgCl at 5 mV/s with 30 scans. Subsequently the Ag working electrode was immediately replaced by an Au electrode and a cathodic overpotential of 0.2 V was applied under controlled sonication to synthesize Ag nanoparticles on the Au electrode. Then pyrrole monomers were encouragingly found to be polymerized on the deposited Ag nanoparticles. This polymerization is distinguishable from the known chemical or electrochemical one, due to the electrochemical activity of unreduced species of Ag_n⁺ clusters inside the nanoparticles. Also, this polymerization may be ascribed to the oxidizing agent of AuCl₄⁻, which is present on the Au electrode.     

A study of ion exchange at the poly(butyl viologen)-electrolyte interface by SECM

In this work, the ion exchange characteristics of poly(butyl viologen) (PBV) thin films on a platinum electrode has been investigated by cyclic voltammetric (CV) scans. Since ferrocyanide anions (Fe(CN)₆⁴⁻) were added during the polymerization of the PBV thin-film for its stability, Fe(CN)₆⁴⁻ could form charge transfer complex with monomer and co-deposited with polymer. Scanning electrochemical microscopy (SECM) was used to probe the released Fe(CN)₆⁴⁻ ions from PBV film with Os(bpy)₃Cl₂ as a mediator for the approaching process in 0.5 M KCl medium. Mass changes during the redox process of the film were also monitored in-situ by electrochemical quartz crystal microbalance (EQCM). The ion exchange and transport behavior was observed during CV cycling of the film of the SECM and EQCM. The insertion and extraction of anions were found to be potential-dependence. Moreover, the decrease in tip current of released Fe(CN)₆⁴⁻ with increasing cycle number accounted for the ion exchange between Fe(CN)₆⁴⁻ and Cl⁻ in the KCl electrolyte. However, the Fe(CN)₆⁴⁻/Fe(CN)₆³⁻ redox couple was found to be highly stable between 0.0 and 0.5 V (vs. Ag/AgCl/saturated KCl) in the phosphate buffer solution. Therefore, the electrochemical property of Fe(CN)₆⁴⁻/Fe(CN)₆³⁻ redox couple was studied at different scan rates using CV technique. The peak currents were directly proportional to the scan rate as predicted for a surface confined diffusionless system. The surface coverage (Γ) and the concentration of Fe(CN)₆⁴⁻ were determined to be 1.88 × 10⁻⁸ mol/cm² and 0.641 mol/dm³, respectively. By neglecting cations incorporation during redox reaction of the PBV film and also based on the results obtained from energy-dispersive X-ray spectroscopy for the films of as-deposited, reduced and oxidized states, an ion exchange mechanism was proposed.     

Passivity and pitting of carbon steel in chromate solutions

The passivity and pitting behavior of A516-70 carbon steel in chromate solutions were studied using electrochemical measurements. The anodic Tafel slopes in the active region show that carbon steel dissolution involves two mechanisms in this range: formation and further oxidation of a pre-passive film of Fe(OH)₂. The first current peak at −0.228 V (Ag | AgCl) in cyclic voltammograms is caused by the oxidation of the pre-passive film and the formation of a stable passive film of Cr³⁺+Fe³⁺. The second peak at 0.612 V is ascribed to the oxidation of Cr³⁺ in passive film to Cr⁶⁺. The charge-transfer step at the electrode/solution interface controls the film formation and dissolution; the role of diffusion is negligible. Chromate ions play a prominent role in the formation of passive film, but hardly affect the stability of the passive state. More chromate ions in solution enhance the dissolution of Cr³⁺ at the second peak potential. Upon addition of chloride ions metastable pits are initiated, as indicated by a typical current transient: a quick current rise followed by a slow recovery. A maximum exists in the potential dependence of the pit initiation rate. Metastable pit growth is controlled by the ohmic potential drop mainly across the cover over the pits. Increasing potential is beneficial to the repassivation of metastable pits, as indicated by the decreasing average repassivation time. A pit stabilization criterion, the ratio of peak pit current to pit radius, must exceed 6×10⁻² A cm⁻¹ during pit growth to avoid repassivation in the present system.     

Electrochemical characterization of chemical species formed during the electrochemical treatment of chalcopyrite in sulfuric acid

The dissolution mechanism of chalcopyrite, and the potential range in which its passivation phenomenon takes place, were studied on carbon paste electrodes with chalcopyrite (99.46% purity, +300 mesh, 53 μm size) (CPE-CP) in 1.7 mol/dm³ H₂SO₄. A sequence of anodic potential pulses was applied to the CPE-CP to characterize its electrochemical behavior. Copper ions, dissolved by the potential pulses, were determined using a mercury film electrode (MFE) and the anodic stripping voltammetry (ASV) on a vitreous carbon disk. In addition, the modified surface of CPE-CP was characterized, before and after the potential pulses, by cyclic voltammetry (CV). The characterization of the final surface state of each electrochemically modified CPE-CP and the amount of dissolved copper showed five potential regions where the chalcopyrite dissolution mechanism changed. The initial dissolution occurs at 0.615 V ≤ E_anod < 1.015 V versus SHE forming a non-stoichiometric polysulfide (Cu_1−rFe_1−sS_2−t). The absence of copper ions in the solution indicates a passive sulfide. However, at 1.015 V ≤ E_anod < 1.085 V versus SHE, the passive product decomposes forming porous layers of non-stoichiometric polysulfide (Cu_1−xFe_1−yS_2−z) that allow the diffusional transport of charged species and the dissolution of the mineral. In the region of 1.085 V ≤ E_anod < 1.165 V versus SHE, formation covellite (CuS) was identified. At E > 1.165 V versus SHE, CuS is unstable and gives rise to complete dissolution of the chalcopyrite. Due to the experimental conditions, the mineral dissolution is inhibited by possible jarosite precipitation.     

Potential dependent adsorption behaviour of thiothymine derivatives on the Au(1 1 1) electrode

The adsorption behaviour of 2-thiothymine and 4-thiothymine on a Au(1 1 1) single crystal electrode has been studied using cyclic voltammetry and X-ray photo electron spectroscopy. For both thio derivatives the adsorption region is restricted due to the onset of reversible oxidization to 2,2′-bis(1H-5-methylpyrimidin-4-one-2-yl)-disulphide or 4,4′-bis(1H-5-methylpyrimidin-2-one-4-yl)-disulphide at anodic potentials. Two different orientations of adsorbed 2-thiothymine have been observed. Between −350 mV and −700 mV versus Ag/Ag⁺ the molecule is solely chemisorbed via its sulphur atom and adopts an upright orientation towards the surface. However at more negative potentials 2-thiothymine is reoriented into a slightly tilted position interacting via its S, N and O atoms with the surface. In contrast, 4-thiothymine exhibits only one adsorption geometry. Between −300 mV and −700 mV versus Ag/Ag⁺ it is chemisorbed via sulphur and nitrogen adopting a slightly tilted position. At −950 mV versus Ag/Ag⁺ 4-thiothymine is irreversibly reduced. The sulphur substituent is eliminated and covers the substrate.