Changes on iron electrode surface during hydrogen permeation in borate buffer solution

Hydrogen interaction with oxide films grown on iron electrodes at open circuit potential (E_oc) and in the passive region (+0.30 V_ECS) was studied by chronopotentiometry, chronoamperometry and electrochemical impedance spectroscopy techniques. The results were obtained in deaerated 0.3 mol L⁻¹ H₃BO₃ + 0.075 mol L⁻¹ Na₂B₄O₇ (BB, pH 8.4) solution before, during and after hydrogen permeation. The iron oxide film modification was also investigated by means of in situ X-ray absorption near-edge spectroscopy (XANES) and scanning electrochemical microscopy (SECM) before and during hydrogen permeation. The main conclusion was that the passive film is reduced during the hydrogen diffusion. The hydrogen permeation stabilizes the iron surface at a potential close to the thermodynamic water stability line where hydrogen evolution can occur. The stationary condition required for the determination of the permeation parameters cannot be easily attained on iron surface during hydrogen permeation. Moreover, additional attention must be paid when obtaining the transport parameters using the classical permeation cell.     

Influence of the dissolution of MnS inclusions under free corrosion and potentiostatic conditions on the composition of passive films and the electrochemical behaviour of stainless steels

The influence of the dissolution of MnS inclusions at the OCP value and 400 mV versus SCE on the chemical composition of passive films and the electrochemical behaviour of resulfurized austenitic stainless steel was studied in 1 M NaClO₄, pH 3, solution using SIMS, XPS, electrochemical impedance spectroscopy and the electrochemical microcell technique. The electrochemical dissolution (at 400 mV versus SCE) of inclusions occurred uniformly along the interface and this process was almost complete after 25 min immersion. A small amount of sulfur was detected on the matrix that exhibited a wide passive range. By contrast, the dissolution process under free corrosion was not uniform along the interface and numerous inclusions were partly dissolved. Grains close to the completely dissolved inclusions showed an active behaviour. This was connected with the adsorption of a large number of sulfur on the surface. In the two cases, the presence of sulfur species on the surface was also found to enhance the kinetics of cathodic reactions.     

Micropatterning of active enzyme with a high-resolution by scanning electrochemical microscopy coupled with a nanometer-sized carbon fiber disk tip

We developed a new method for fabrication of nanometer-sized carbon fiber disk electrodes and applied them to micropattern active horseradish peroxidase (HRP) with a high-resolution by scanning electrochemical microscopy (SECM). In order to pattern active HRP, except for active HRP micropatterns predesigned other regions on a HRP-immobilized substrate was deactivated by a reactive species generated at the electrode as the tip of SECM held at 1.7 V through oxidation of Br⁻ in 0.20 mol/L phosphate buffer (PB) containing 2.5 × 10⁻² mol/L KBr and 2.0 × 10⁻³ mol/L BQ (pH 7.0). The micropatterns of active HRP were characterized using the feedback mode of SECM in PB containing 2.0 × 10⁻³ mol/L BQ and 2.0 × 10⁻³ mol/L H₂O₂, when the tip potential was held at −0.2 V.     

Reactivity at the film/solution interface of ex situ prepared bismuth film electrodes: A scanning electrochemical microscopy (SECM) and atomic force microscopy (AFM) investigation

Bismuth film electrodes (BiFEs) prepared ex situ with and without complexing bromide ions in the modification solution were investigated using scanning electrochemical microscopy (SECM) and atomic force microscopy (AFM). A feedback mode of the SECM was employed to examine the conductivity and reactivity of a series of thin bismuth films deposited onto disk glassy carbon substrate electrodes (GCEs) of 3 mm in diameter. A platinum micro-electrode (? = 25 μm) was used as the SECM tip, and current against tip/substrate distance was recorded in solutions containing either Ru(NH₃)₆³⁺ or Fe(CN)₆⁴⁻ species as redox mediators. With both redox mediators positive feedback approach curves were recorded, which indicated that the bismuth film deposition protocol associated with the addition of bromide ions in the modification solution did not compromise the conductivity of the bismuth film in comparison with that prepared without bromide. However, at the former Bi film a slight kinetic hindering was observed in recycling Ru(NH₃)₆³⁺, suggesting a different surface potential. On the other hand, the approach curves recorded by using Fe(CN)₆⁴⁻ showed that both types of the aforementioned bismuth films exhibited local reactivity with the oxidised form of the redox mediator, and that bismuth film obtained with bromide ions exhibited slightly lower reactivity. The use of SECM in the scanning operation mode allowed us to ascertain that the bismuth deposits were uniformly distributed across the whole surface of the glassy carbon substrate electrode. Comparative AFM measurements corroborated the above findings and additionally revealed a denser growth of smaller bismuth crystals over the surface of the substrate electrode in the presence of bromide ions, while the crystals were bigger but sparser in the absence of bromide ions in the modification solution.     

Local electrochemical impedance measurements on inclusion-containing stainless steels using microcapillary-based techniques

MnS inclusions are good precursor sites for pitting corrosion of stainless steel. The objective of this paper was to quantify the passive properties of resulfurized stainless steel after immersion in chloride media. This was done by combining microcapillary techniques with electrochemical impedance spectroscopy and numerical analysis (specific equivalent circuit). It was shown that sulfur species produced in the electrolyte during the dissolution of inclusions react with the native passive film to CrS and FeSO₄. Local electrochemical impedance spectroscopy measurements provided data describing the behaviour of the affected matrix at the microscale. For example, the value of the charge transfer and migration of point defects resistance decreases from 51,700 Ω cm², in sites free of any metallurgical heterogeneity down to 12,200 Ω cm², in sites containing a high density of inclusions. It was also shown that the integrity of the microcapillary can be altered by the presence of high quantity of sulfur in the electrolyte. Local impedance data allowed the detection of such problems.     

Quantitative study of pH change during LaNi5−xAlx (x = 0, 0.3) discharge process by SECM

Oxygen reduction reaction (ORR) on Pt microelectrode was used for developing a micro pH sensor for scanning electrochemical microscopy (SECM) study in this work. When the potential of Pt microelectrode was held constant in ORR region, the ORR current (cathodic current) increased with decreasing solution pH and vice versa. The response time of the ORR current to pH changes was measured to be ca. 30 ms which implies that the pH response is fast enough for monitoring the temporal pH changes. Furthermore, a fine linear relationship was found to exist between the half wave potential of ORR (E_1/2) and the solution pH value, and the slope is −46 mV/pH. The Pt micro pH sensor was located 1 μm above the LaNi_5−xAl_x (x = 0, 0.3) substrate electrode surface in pH = 9 KOH solution to perform the tip-substrate voltammetry of SECM. In tip voltammogram, the ORR tip current qualitatively reflects the transit solution pH changes during LaNi_5−xAl_x discharge reaction. Also, the minimum values of the solution pH near LaNi₅ and LaNi_4.7Al_0.3 surface during the discharge reaction were quantitatively detected; they were 7.17 and 7.57, respectively. The result indicates that Al partial substitution for Ni degrades the maximum discharge ability of the alloy and decreases the hydrogen diffusion coefficient in alloy bulk.     

Effects of nanocrystallization on the corrosion behavior of 309 stainless steel

Nanocrystallized (NC) 309 stainless steel (309SS) coating has been fabricated on glass substrate by DC magnetron sputtering. The coating, with an average grain size less than 50 nm, had ferritic (bcc) structure rather than the austenitic (fcc) structure of the bulk steel. The electrochemical corrosion behavior of the NC coating and the bulk steel in solutions of 0.25 M Na₂SO₄ + 0.05 M H₂SO₄ and 0.5 M NaCl + 0.05 M H₂SO₄ was investigated by using potentiodynamic polarization, potentiostatic polarization and AC impedance techniques. The results showed that the corrosion behavior of the NC 309SS coating and 309SS bulk steel depended on the composition of the solutions. In the Na₂SO₄ solution there was only a little difference between the corrosion resistance of the passive films on the NC coating and the bulk steel. However, in the solution with chloride ions, the localized corrosion resistance of 309SS was greatly enhanced by nanocrystallization due to the formation of a compact and stable passive film on the NC coating. The electronic structure of the passive film formed on the NC coating and on the bulk steel was analyzed by means of capacitance measurements, and a corrosion mechanism is proposed.     

Pitting corrosion of AZ91D and AJ62x magnesium alloys in alkaline chloride medium using electrochemical techniques

Pitting corrosion of AZ91D-DC (die cast), AZ91D-ESTC (electromagnetically-stirred billets; thixocast), AZ91D-SFTC (billets solidified freely; thixocast) and AJ62x-DC (die cast) specimens was studied in alkaline chloride medium (0.1 M NaOH + 0.05 M NaCl + 2 ml H₂O₂) at 25 °C and pH 12.3. Electrochemical noise (EN) measurements have confirmed to some extent the polarization results (passive zone, pitting current and average corrosion rate). AZ91D-ESTC specimens have shown the best corrosion resistance followed by AZ91D-SFTC and AZ91D-DC. Intense corrosion rate was observed at the beginning of experiment and it decreased with immersion period. Localized corrosion with dense pitted areas was observed during a 16 h immersion period for AZ91D-SFTC and AZ91D-ESTC specimens. The best passive zone was observed for AJ62x-DC because of the corrosion products formed at the surface. After a 6 h of immersion, EN analyses in the frequency domain indicated a change in the sub-mode of pitting, becoming a classical pitting type, for AJ62x and AZ91D die cast specimens. Analysis with the scanning reference electrode technique (SRET) has showed that AJ62x specimen presented the biggest potential difference between the most active anode and the most active cathode and more numerous zones of intense localized corrosion. It was also found that the lifetime of the pit appeared after 8:20 h of immersion was longer for AJ62x and AZ91D die cast specimens being associated to a classical pitting.     

Electrochemical impedance spectroscopy study of the passive films of alloy 22 in low pH nitrate and chloride environments

Utilizing electrochemical impedance spectroscopy (EIS), we characterize the passive film properties of alloy 22 during immersion in low pH nitrate and chloride solutions. In pure HCl, the passive film grows thinner with increasing acid concentration. In contrast, in HNO₃, the passive film corrosion protection properties are enhanced, which leads to low corrosion rates, even at pH < −0.5. The combined influence of both HCl and HNO₃ in contact simultaneously with the alloy 22 surface shows multiple phases in the passive film properties depending on the pH. EIS results show that the passive film changes either thickness and/or composition as the system is driven chemically through different corrosion states, including: active, passive, active/passive and transpassive.     

Corrosion effect of allylthiourea on bulk nanocrystalline ingot iron in diluted acidic sulphate solution

The effect of allylthiourea (ATU) on the corrosion behaviors of bulk nanocrystalline ingot iron (BNII) fabricated from conventional polycrystalline ingot iron (CPII) by severe rolling technique was studied. In the corrosive solution-0.1 mol L⁻¹ H₂SO₄ + 0.25 mol L⁻¹ Na₂SO₄ inhibited by ATU at ambient temperature, for CPII, when immersed for as short as 5 min, an inductive loop appears at different concentrations at the Nyquist complex planes; for BNII, only a simple capacitance loop appears in the Nyqusit plot at the same period. This difference on the time effect is attributed to the higher volume fractions of defects, which are caused by severe rolling technique compared with its coarse counterpart-CPII. With the elongation of the immersion time, for BNII, two time constants appear in the Nyquist plot; for CPII, the impedance spectrum shows a slightly depressed semicircular shape, and a critical concentration is observed. The corrosion effect of ATU for BNII is really negative in comparison with CPII.     

Synthesis and corrosion property of pure Ni with a high density of nanoscale twins

Pure Ni with a high density of nanoscale twins (NT Ni) had been synthesized by electrodeposition technique. Its corrosion property was investigated in 0.1 mol/L H₃BO₃ + 0.025 mol/L Na₂B₄O₇ buffer solution with pH 8.4 by electrochemical techniques and compared with its cast counterpart. The results demonstrated that NT structure has little effect on the point defect concentration in the passive film. However, the NT structure can greatly influence the film thickness and the diffusion coefficient (D_d) of point defect, which are responsible for the formation of a thinner, perfect passive film and finally gives rise to a higher corrosion resistance for NT Ni. This indicates that the material may have great potential as a substitute of the nuclear steam generator tube material.     

Electrochemical corrosion behavior of nanocrystallized bulk 304 stainless steel

By employing Mott-Schottky analysis in conjunction with the point defect model (PDM), we compared donor density and donor diffusion coefficients in the passive films formed on the surface of nanocrystallized bulk 304 stainless steel (NB304ss) and cast 304 stainless steel (304ss) in 0.05 mol/L H₂SO₄ + 0.25 mol/L Na₂SO₄ solution. The donor density at the metal/film interface of the NB304ss was lower than that at the metal film interface of the cast 304ss. Based on the Mott-Schottky analysis, an exponential relationship between donor density and formation potentials of the passive films on the NB304ss and the cast 304ss was built up. The results showed that the donor diffusion coefficients in the passive film formed on the surface of NB304ss was lower than that in the cast 304ss. The lower donor density and the lower diffusion coefficient restrained the electrochemical reaction in the passive film and improved the stability of the passive film. That is the reason why the passive film formed on the NB304ss was more protective.     

In situ synchrotron radiation grazing incidence X-ray diffraction—A powerful technique for the characterization of solid-state ion-selective electrode surfaces

An in situ surface study of the iron chalcogenide glass membrane ion-selective electrode (ISE) in aqueous media has been undertaken using a tandem technique of mixed potential/synchrotron radiation grazing incidence X-ray diffraction (SR-GIXRD) and atomic force microscopy (AFM). This work has simultaneously monitored the mixed potential and in situ surface diffraction patterns of this crystalline glassy material, showing that the observed gradual shift of the electrode potential in the anodic direction is linked to the preferential dissolution of the GeSe (1 1 1), GeSe (1 0 1) and GeSe (1 4 1) and/or Sb₂Se₃ (0 1 3), Sb₂Se₃ (2 2 1) and Sb₂Se₃ (0 2 0) surfaces. Expectedly, these observations are internally consistent with preferential oxidative attack of the crystalline regions of the membrane comprising GeSe and/or Sb₂Se₃, as evidenced by AFM imaging of the electrode surface. Clearly, this work corroborates the results of previous ex situ surface studies on the iron chalcogenide glass ISE, whereby it was shown that alkaline saline solutions have a tendency to alter the surface chemistry and concomitant response characteristics of the ISE.     

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.     

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.     

Heterogeneous hydrogen evolution on corroding Fe-3 at.% Si surface observed by scanning electrochemical microscopy

Corrosion behavior of Fe-3 at.% Si alloy in 0.01 mol dm⁻³ HCl solution was investigated by using scanning electrochemical microscopy (SECM) as well as general electrochemistry. The rate of corrosion coupled with hydrogen evolution was initially 0.44 A m⁻² but decreased significantly with time. Localized hydrogen evolution on the specimen surface was probed by an SECM system in which a force sensor was mounted to determine the probe height from the specimen surface. SECM images revealed that hydrogen evolution took place heterogeneously on the specimen surface depending on crystallographic orientation of substrate single grains in the initial stage and then became relatively homogeneous. Finally, a heterogeneous hydrogen distribution corresponding to the appearance of localized corrosion sites was observed.     

Effect of ball milling on the corrosion resistance of magnesium in aqueous media

The influence of the high-energy ball milling on the corrosion behavior of magnesium in aqueous media has been investigated through electrochemical experiments complemented by morphological, structural, chemical and surface analyses. The milling duration was varied from 0 to 40 h. Polarization curves show that the milling procedure improves the magnesium corrosion resistance in passive conditions (KOH solution) and in more active corrosion conditions (borate solution). This is illustrated by the corrosion potential which becomes nobler with milling. The variation of the polarization resistance and related corrosion current with milling time is also an indication of the improvement of the Mg corrosion resistance due to the milling. Moreover, the passive current is significantly lower for milled Mg. The Mg crystallite size is reduced from >100 to 34 nm after 10 h of milling and does not decrease significantly with further milling. The iron contamination of the Mg powder due to the erosion of the milling tools is very low (0.09 wt.% after 40 h of milling). In contrast, a significant oxygen contamination occurs during milling (2.6 wt.% after 40 h of milling). XPS and AES data indicate MgO enrichment in the bulk of the milled Mg without significant MgO increase at the powder surface. The corrosion improvement was attributed to the increase through the milling process of the density of surface defects and grain boundaries susceptible to increase the number of nucleation sites for Mg hydroxylation in aqueous media, leading to the rapid formation of a dense and protective Mg(OH)₂ layer.     

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.     

Effect of copolymer ratio on hydrolytic degradation of poly(lactide-co-glycolide) from drug eluting coronary stents

The in vitro hydrolytic degradation behavior of poly(d,l-lactide-co-glycolide) (PLGA) has been systematically investigated from the drug eluting coronary stents with respect to different copolymer compositions. The drug-polymer coated stents were incubated in phosphate buffer saline (pH 7.4) at 37 °C and 120 rpm up to 12 months to facilitate hydrolytic degradation. Gel permeable chromatography, differential scanning calorimetry and scanning electron microscopy were employed to characterize their degradation profiles. The study supports the bulk degradation behavior for PLGA from coated stents. Molecular weight of polymer decreased immediately after immersion in PBS but mass loss was not observed during first few days. The rate of hydrolytic degradation was influenced by copolymer ratio, i.e., degradation of 50:50 PLGA was fastest followed by 65:35 PLGA and 75:25 PLGA. The drug release from PLGA coated stent followed biphasic pattern which was governed by surface dissolution and diffusion of drug rather than polymer degradation.     

Co (II) porphyrin adsorbed on SiO2/SnO2/phosphate prepared by the sol-gel method: Application in electroreduction of dissolved dioxygen

This paper describes the immobilization procedure of 5,10,15,20-tetrakis(1-methyl-4-pyridyl)-21-H,23-H-porphyrin ion on SiO₂/SnO₂/phosphate, obtained by the sol-gel processing method. P 2p X-ray photoelectron and the ³¹P MAS NMR spectra revealed that dihydrogen phosphate is the species present on the surface. The porphyrin was adsorbed on the surface of the modified material and furthermore metallized in situ with Co (II) ion. The porphyrin metallation process was followed with UV-vis spectroscopy by inspecting the Q bands of the free and metallated porphyrin. The free porphyrin presented four Q bands associated to a D_2h local symmetry and the metallated one, two bands related to a D_4h local symmetry. The amount of electroactive species adsorbed on the material was estimated by integrating the area under the peak of Co (II) → Co (I) reduction by using the pulse differential voltammetric technique. The amount of the metallated porphyrin was 2.3 × 10⁻¹⁰ mol cm⁻². A carbon paste electrode of the modified material containing metallated porphyrin was used to study the electrocatalytic reduction of dissolved dioxygen by means of cyclic voltammetry, chronoamperometry and linear sweeping voltammetry. The modified electrode was very stable and exhibited the electrocatalytic reduction of dissolved dioxygen at −180 mV versus SCE by a two-electron mechanism, producing hydrogen peroxide at pH 5.4. The electroactive species was strongly retained on the material surface, presumably inside the pores of the material, since in a test of various oxidation-reduction cycles no significant decrease of the current densities was detected, indicating that it was not leached off during the experiment.