Electrooxidation of stainless steel AISI 304 in carbonate aqueous solution at pH 8

The electrochemical behaviour of stainless steel AISI 304 (SS304) has been investigated in deaerated 0.1–1 m NaHCO₃ solutions at pH 8 using a rotating disc electrode. The polarization curves are characterized by a broad range of passivity at low potentials (–0.8 to 0.3 V), a depassivation region at 0.4 V vs SCE and, at high potentials (0.5 to 0.85 V), a passive region before oxygen evolution. In the low potential range, the SS304 electrode behaves like a Cr-rich metallic phase, and the dissolution of Fe²⁺ ions into the solution is hindered by the formation of a Cr₂O₃ layer. As the potential reaches 0.4V, the oxidation-dissolution of Cr(iii) oxide/hydroxide to CrO₄ ² ions occurs, with the participation of bicarbonate/carbonate as a catalyst in the dissolution reaction. Since the chromium oxide/hydroxide dissolution and subsequent surface enrichment of iron oxides occur, the applied potential, exposure time and oxidation charge have a considerable effect on the passive film properties. At high potentials, the presence of a passive film of iron oxides/hydroxides or oxyhydroxides plays a key role in the SS304 passivity with the presence of Fe(vi) species incorporated or adsorbed into the passive films. Colouration of the SS304 surface is observed in the second passive region. A film of a uniform gold colour formed on SS304, mild steel 1024 and iron in carbonate and borate solutions at pH 8. The colour of the electrode surfaces remain unchanged in air and in solutions at positive potential but it disappears at open-circuit potential or is easily reduced in the first negative-going potential scan.     

Nature and stability of anodic oxide films formed on molybdenum in chloride solutions

The formation and stability of anodic oxide films on molybdenum in chloride solutions was tested using impedance and polarization measurements. The efficiency of oxide formation increases as the acidity of the formation medium increases. The film is highly stable and resistive to attack and dissolution regardless of the chloride concentration, pH or film thickness. The corrosion potential did not vary with variation of immersion time, solution composition or film thickness and recorded 0.000V vs SCE indicating the high insulating properties of the film. Polarization measurements on previously anodized molybdenum electrode showed that the electrode is ideally polarized over a potential region not less than 2V. The magnitude of that potential region increases as the film thickness increases. The anodic film cannot be reduced or removed by galvanostatic cathodic polarization. The impedance behaviour of the anodized molybdenum electrode was found to be purely capacitive and the oxide film may be treated approximately as a perfect dielectric material.     

EIS study of potentiostatically formed passive film on 304 stainless steel

Passive film was potentiostatically grown on Type 304 stainless steel at potentials between −0.4 and 0.3 V vs. Hg/HgSO₄ in 50 mV intervals. Electrochemical impedance spectroscopy was used to study properties of the grown passive films. Relevant circuit analogs were selected to fit the electrochemical impedance data obtained at each potential. The resultant parameters were used to calculate film thickness through both film capacitance and resistance. The calculated thicknesses were compared with ellipsometry thickness measurement results. An overestimation occurred when the film resistance was used to calculate the film thickness. On the other hand, when the CPE parameter Q was used, the film thickness was underestimated. Available approaches were employed to calculate the effective capacitance of the film. A comparison between the film thickness calculated from the effective capacitance and ellipsometry measurements suggested a surface distribution of time constants on the surface.     

The application of polycarbazole,polycarbazole/nanoclay and polycarbazole/Zn-nanoparticles as a corrosion inhibition for SS304 in saltwater

Polycarbazole (PCz), polycarbazole/nanoclay and polycarbazole/Zn-nanocomposites were chemically and electrochemically synthesized on a stainless steel (SS304) electrode. The modified electrodes were characterized by electrochemical methods (CV and chronoamperometry), Fourier transform infrared spectroscopy (FTIR)-attenuated transmission reflectance (ATR), scanning electron microscopy (SEM)-energy dispersive X-ray analysis (EDX), four point probe, electrochemical impedance spectroscopy (EIS), and equivalent circuit model of R_s(Q_c(R_c(Q_pR_ct))). The electrochemical behavior of the modified films on SS304 was assessed by open circuit potential monitoring, potentiodynamic polarization and EIS measurements to test the corrosion protection efficiency against 3.5% NaCl solution. PCz, PCz/nanoclay and PCz/nanoZn films obtained by chemical method coated on SS304 electrode exhibited better corrosion protection performance compared to the films obtained by the electrochemical method. This result may be attributed to the effective formation of a thin and protective layer. The highest protection efficiency (PE = 99.81%) was obtained for chemically synthesized PCz films.     

Localized corrosion of 1024 mild steel in slightly alkaline bicarbonate solution with Cl− ions

The passive film breakdown of 1024 mild steel induced by the presence of 0.05m chloride ions had been investigated in 0.075–0.75m bicarbonate solutions at pH8.9–9.7. A rotating disc electrode with a Kel-F holder was used in conjunction with a rotating ring-disc electrode. The resistance to localized attack is closely linked to the preanodization potential (Eox) applied in the absence of Cl- ions. For Eox below about 0.2–0.3V vs SCE, the resistance to localized attack provided by the passive film is independent of Eox; above the breakdown potential, the localized attack is manifested by the formation of pits at the mild steel surface. The breakdown potential increases linearly with NaHCO3 concentration and pH. Passive film breakdown for Eox below about 0.2–0.3V vs SCE most likely begins with a surface film dissolution prior to the penetration of the aggressive anions through the film. For Eox above about 0.3V vs SCE under the same conditions, no pitting is noticed and the potential associated with localized attack shifts considerably in the anodic direction due to interstitial (formation of crevices) corrosion at the mild steel/Kel-F interface.     

Localized corrosion of 1024 mild steel in slightly alkaline bicarbonate solution with Cl? ions

The passive film breakdown of 1024 mild steel induced by the presence of 0.05m chloride ions had been investigated in 0.075–0.75m bicarbonate solutions at pH8.9–9.7. A rotating disc electrode with a Kel-F holder was used in conjunction with a rotating ring-disc electrode. The resistance to localized attack is closely linked to the preanodization potential (Eox) applied in the absence of Cl- ions. For Eox below about 0.2–0.3V vs SCE, the resistance to localized attack provided by the passive film is independent of Eox; above the breakdown potential, the localized attack is manifested by the formation of pits at the mild steel surface. The breakdown potential increases linearly with NaHCO3 concentration and pH. Passive film breakdown for Eox below about 0.2–0.3V vs SCE most likely begins with a surface film dissolution prior to the penetration of the aggressive anions through the film. For Eox above about 0.3V vs SCE under the same conditions, no pitting is noticed and the potential associated with localized attack shifts considerably in the anodic direction due to interstitial (formation of crevices) corrosion at the mild steel/Kel-F interface.     

Electrochemical behaviour of cadmium in NaOH solution

The electrochemical behaviour of cadmium in 1 M NaOH solution was studied. Different electrochemical methods such as potentiodynamic measurements, polarization techniques and electrochemical impedance spectroscopy (EIS) were used. During the course of polarization, several characteristic features were observed, including prepassivation and a relatively wide passive range extending over 1.4 V. Equivalent circuit models for the electrode–electrolyte interface under different applied potentials were proposed. The impedance data measured at steady state were fitted to calculated data according to proposed equivalent circuits. The relevance of the proposed equivalent circuits to the different phenomena occurring at the electrode–solution interface was discussed. The passive film formed on Cd is composed of two layers: a barrier layer in contact with metal and a deposit on the barrier layer. The barrier layer appears to form directly from the metal through a solid state reaction, rather than by a dissolution–precipitation mechanism. For an applied potential >+0.7 V the relative passive film thickness and resistance decrease to lower values indicating transpassive dissolution at this potential.     

Passivation behavior of Type 304 stainless steel in a non-aqueous alkyl carbonate solution containing LiPF6 salt

Passivation behavior of type 304 stainless steel in a non-aqueous alkyl carbonate solution containing LiPF₆ salt was studied using electrochemical polarization, X-ray photoelectron spectroscopy (XPS) and time of flight-secondary ion mass spectroscopy (ToF-SIMS). Cathodic polarization to 0 V vs. Li/Li⁺ resulted in most but not complete reduction of the air-formed film from oxides to metal on the stainless steel, as confirmed by XPS. For complete elimination of the air-formed film, the surface of the stainless steel was scratched under anodic polarization conditions. At 3 V vs. Li/Li⁺ where an anodic current peak appeared, only an indistinct layer was recognized on the newly scratched surface, according to ToF-SIMS analysis. Above 4 V vs. Li/Li⁺, substantial passive films were formed, which were composed of oxides and fluorides of iron and chromium. The origin of oxide was due to water contained in the non-aqueous alkyl carbonate solution, and that of fluorides were the result of the decomposition of electrolytic salt, LiPF₆, especially at higher potential. The resultant passive films were stable in the non-aqueous alkyl carbonate solution containing LiPF₆ salt.     

Passivity and passivity breakdown of 304 stainless steel in alkaline sodium sulphate solutions

The passivity and passivity breakdown of 304 stainless steel were investigated in 0.25 M Na₂SO₄solutions of pH 10. The effect of applied potential and the presence of Cl^– ions in the electrolyte were also studied. Different electrochemical methods such as open circuit potential measurements, polarization techniques and electrochemical impedance spectroscopy (EIS) were used. The results showed that the steel electrode passivates under open circuit conditions and also under potentiostatic control. The rate of passive film thickening under open circuit conditions follows a simple logarithmic law. Addition of Cl^– ion shifts the polarization curves in the active direction and above a critical chloride concentration, [Cl^– ] 0.15 M, pitting corrosion occurs and the pitting potential, E _pit, decreases linearly with the logarithm of [Cl^–]. The addition of sulphate ions to the chloride-containing solutions was found to inhibit the pitting process, and at [SO^2- ₄] 0.25 M, a complete immunity to pitting corrosion was recorded. The impedance measurements provided support for film thickening and film breakdown reactions. An equivalent circuit model which consists of a pure resistor, R , in series with a parallel combination of a pure resistor, R _p, and a constant phase element, Q, was proposed to describe the electrode/electrolyte interface. The passive film thickness was found to increase with applied potential up to a critical value of 0.3 V. At higher voltages, breakdown of the passive film occured.     

Corrosion inhibition of stainless steel by polyaniline,poly(2-chloroaniline), and poly(aniline-co-2-chloroaniline) in HCl

Polyaniline (PANi), poly(2-chloroaniline) (PClANi), and poly(aniline-co-2-chloroaniline) (co-PClANi) films were synthesized by electrochemical deposition on 304-stainless steel (SS) from an acetonitrile solution. The structural properties of these polymer films were characterized by spectroscopic (FTIR and UV–vis) and electrochemical (cyclic voltammetry) methods. Open circuit potential–time (E_ocp–time) curves, potentiodynamic polarization, and electrochemical impedance (EIS) measurements showed that these films have significant protective performance against corrosion of SS in 0.5 M HCl solution. It was found that co-PClANi film has acted as a passivator as well as barrier for cathodic reduction reaction in a similar manner as PANi film. However, PClANi film has behaved only as barrier for corrosion protection of SS in 0.5 M HCl.     

The Influence of Ni Content on the Stability of Copper—Nickel Alloys in Alkaline Sulphate Solutions

The electrochemical behaviour of copper–nickel alloys with different Ni content (5–65%) in sulphate solutions of pH 12 was investigated. The effects of temperature, immersion time, and concentration of sulphate ions were also studied. Different electrochemical methods such as open-circuit potential measurements, polarization techniques and electrochemical impedance spectroscopy (EIS) were used. Potentiodynamic measurements reveal that the increase in nickel content increases the corrosion rate of the alloy in sulphate solution linearly. Nevertheless, an increase in the nickel content along with increase in immersion time improves the stability of the Cu–Ni alloys due to the formation of a stable passive film. An equivalent circuit model for the electrode/electrolyte interface under different conditions was proposed. The experimental impedance data were fitted to theoretical data according to the proposed model. The relevance of the model to the corrosion/passivation phenomena occurring at the electrode/solution interface was discussed.     

Corrosion protection of mild steel by polypyrrole phosphate composite coating

Electrodeposition of polypyrrole phosphate (PPy–P) on mild steel (ST₁₂) was achieved in oxalic acid medium using cyclic voltammetry (CV) technique. Adherent and homogeneous PPy–P films were obtained. The corrosion behavior of mild steel with phosphate (PPy–P) coatings in 3.5% NaCl solutions was investigated through potentiodynamic polarization technique, open circuit potential–time curves, and electrochemical impedance spectroscopy (EIS). Based on a physical model for the corrosion of mild steel composite, the Zview (II) software was applied to the EIS to estimate the parameters of the proposed equivalent circuit. It was found that the PPy–P coatings could provide much better protection than PPy. The effect of phosphate on the morphology and structure of the passive film was investigated by scanning electron microscopy and electron dispersion X-ray analysis (EDX). The results reveal that the PPy–P coated electrode provided a noticeable enhancement of protection against corrosion process.     

Synthesis and characterization of poly(aniline) and poly(o-anisidine) films in sulphamic acid solution and their anticorrosion properties

Poly(o-anisidine) (POA) and polyaniline (PANI) coatings were synthesized on platinum (Pt) surface and stainless steel (SS) in monomer containing 0.50 M sulphamic acid (SA) solution by means of cyclic voltammetry (CV) technique. Meanwhile, poly(o-anisidine) film was also deposited with a different scan rate on SS electrode. The behaviour of PANI and POA films obtained on stainless steel examined by CV was different from the one obtained for PANI and POA on Pt electrode. The corrosion performances of PANI and POA coatings in 3.5% NaCl solution were investigated with anodic polarization technique and electrochemical impedance spectroscopy (EIS). EIS measurements verified the effect of monomers and that of scan rate on corrosion inhibition of coatings on SS electrode. The results showed that POA film synthesized at low scan rate exhibited an effective anticorrosive property on SS electrode. POA synthesized at low scan rate and PANI coatings provided a remarkable anodic protection to SS substrate for longer exposure time than the one observed for POA coating produced at high scan rate as well as that of bare SS electrode.     

Electrochemical study of the chromium electrode behaviour in borate buffer solution

The electrochemical behaviour of the chromium electrode in borate buffer solution (pH 9.3) was studied by cyclic voltammetry and electrochemical impedance spectroscopy. Chromium passivity was observed over a broad potential region, from –1.0 to 0.5 V vs SCE. The passivation process took place in two steps: formation of a chromium oxide monolayer and transition of chromium to a higher valence state. The anodic film exhibited the properties of a p-type semiconductor. Transpassive dissolution of chromium occurred at 0.5 V vs SCE, with two reaction intermediates present, Cr_Cr ^III and Cr_ad ⁴⁺.     

Voltammetric and galvanostatic studies of hydrous and anhydrous iridium oxide films

Electrolytically grown hydrous oxide films on iridium wire electrodes have been thermally treated from 473 to 773 K. Anhydrous oxide films formed by this treatment have been subjected to cathodic polarization at the potential of the hydrogen evolution reaction, square-wave pulsing of potential from –0.25 to +1.25 V with respoect to SCE and to anodic galvanostatic polarization in 0.5 mol dm^–3 H₂SO₄. Cathodic pretreatment caused an increase of the voltammetric charge in the oxide formation region while the square-wave pulsing formed a hydrous oxide film whose voltammetric charge was superimposed on the charge of the anhydrous oxide film. Both procedures restored the hydrophilic nature of the electrode/solution interface. Potential-time curves during anodic galvanostatic polarization served as a diagnostic criterion for the stability and the state of the oxide film.     

Analysis of electrochemical parameters in time domain during the passive layer cracking occurring on the 304L stainless steel in chlorides solution under tensile stresses

Dynamic electrochemical impedance spectroscopy (DEIS) has been applied for detailed analysis of the passive–active transient region during the passive layer cracking process. The effect of applied potential and tensile stresses on the passive layer rupture of type 304L stainless steel (SS) immersed in 0.5 M NaCl solution at room temperature was examined. This paper presents instantaneous impedance spectra obtained for 304L stainless steel samples at the different potential values that refer to the transition from passive into active state while crack of the passive layer took place. Besides, differential dependencies of electrochemical parameters versus relative elongation have been presented to illustrate the system's dynamics changes.     

DEIS evaluation of the relative effective surface area of AISI 304 stainless steel dissolution process in conditions of intergranular corrosion

The results presented in this paper contribute further information concerning examinations of AISI 304 stainless steel dissolution process in conditions of proceeding intergranular corrosion (IG) which have been determined on the basis of dynamic electrochemical impedance spectroscopy (DEIS) measurements. For the first time changes of the relative effective surface area versus time of AISI 304 stainless steel dissolution process in conditions of proceeding intergranular corrosion (IG) in 0.5 M H₂SO₄ + 0.01 M KSCN solution within the range of reactivation polarization scan have been demonstrated. The assessment of the effective surface area of the investigated process was based on approximation to theory of iron dissolution in sulfuric acid medium according to the shape of instantaneous impedance spectra recorded by means of DEIS technique. As a result, it was possible to evaluate that initially changes of the employed equivalent circuit parameters are not only caused by the changes of the relative effective surface area but also by the changes of the AISI 304 SS dissolution process kinetics. Further progress of the examined process implied that changes of the equivalent circuit parameters are only affected by the changes of the relative effective surface area during proceeding IG. Moreover, it was found that the character of changes of the relative effective surface area in conditions of proceeding IG can be described by exponential function.     

Study of pyrite oxidation by cyclic voltammetric, impedance spectroscopic and potential step techniques

Pyrite oxidation in chloride solutions was investigated with cyclic voltammetry, a.c. impedance and potential step techniques. The oxidation reactions of pyrite were examined by cyclic voltammetric technique and a two-step reaction with a passivation film forming as a first-step product was proposed. An equivalent circuit was then postulated based on the oxidation reactions. Parameters indicated in the equivalent circuit such as reaction resistance and pseudo-capacitance caused by the passivation film, were determined by a.c. impedance measurements. A mathematical formula was derived from the concept of the equivalent circuit to explain the depression of the semicircle in the complex plane plot. When the semicircle is depressed, the mathematical formula indicates that the reaction resistance should be obtained from the intersection of the semi-circle with Z-axis instead of the semicircle diameter. Potential step chronoamperometric technique was then applied to measure the charging current, which is caused by the pseudo-capacitance of the passivation film, to examine the proposed equivalent circuit. The peak charging current densities at 1.10 and 0.90 V vs SHE obtained from the equivalent circuit and the a.c. impedance measurements are 110 and 75 mA cm^–2, respectively. They are consistent with the peak current densities of 105 and 69 mA cm^–2 at 1.10 and 0.90 V, respectively, determined by the potential step chronoamperometric measurements.     

碳化钨纳米晶薄膜电极在对硝基苯酚电化学还原中的催化性能

采用磁控溅射物理气相沉积和等离子体增强化学气相沉积技术,在金属镍基体上制备具有纳米晶结构的碳化钨薄膜;采用循环伏安、准稳态极化和恒电位阶跃等电化学方法研究了对硝基苯酚(PNP)在碳化钨纳米晶薄膜电极上电化学还原的特性和机理.研究表明,采用磁控溅射物理气相沉积技术制备得到的薄膜是由直径为20nm的WC1-X构成,在这种薄膜电极上,PNP电化学还原在电位为(0.95V(Vs. SCE)时,出现一个电流密度为6.0mA(cm(2还原峰,还原反应的表观活化能为12.0kJ(mol(1;而采用等离子体增强化学气相沉积技术制备得到的薄膜是由直径为35nm的纯相WC构成,PNP在该薄膜电极上电化学还原峰电位为(1.05 V(Vs. SCE),还原电流达10.0 mA(cm(2,表观活化能为10.9 kJ(mol(1. PNP在这两种碳化钨薄膜电极上都经过两步不可逆的电化学反应还原成对氨基苯酚,控制步骤为电极反应的电荷传递过程.     

Comparative EIS study of the adsorption and electro-oxidation of thiourea and tetramethylthiourea on gold electrodes

A comparative study of the electrochemical behaviour of thiourea (TU) and tetramethylthiourea (TMTU) on gold in sulphuric acid was performed using a gold rotating disc electrode. The electrochemical impedance spectra are interpreted through an equivalent circuit involving the electrolyte resistance, a constant phase element for the capacity double layer, a charge transfer resistance and, depending on the electrode potential, a second parallel impedance element whose interpretation depends on the potential region considered. Thus, for both thioureas, the Nyquist plots for E < ?0.6 V (vs. MSE) exhibit a single capacitive time constant related to the adsorption of the molecule on the electrode surface. As E increases, the Nyquist plots exhibit a new time constant assigned to the formation of a soluble complex species. This time constant appears in the potential region also related to the electro-oxidation of the thioureas to the corresponding formamidinium disulphide. This means that these processes are coupled and, therefore, only one time constant can be observed. An inductive loop at low frequencies is associated with the pitting of the gold electrode for E ≥ ?0.20 V, in agreement with SEM micrographs. The value of the corresponding charge transfer resistance decreases markedly with the electrode potential, indicating the increase in the rate of the electrochemical processes. Electrodissolution of gold results more importantly in the presence of TMTU. At potential values associated with the formation of the anodic oxide layer on gold, a negative resistance is recorded, indicating the passivation of the electrode surface. Eventually, at E > 0.6 V, the electrode surface passivation disappears and the Nyquist plots exhibit two strongly overlapped capacitive constants assigned to the oxide film growth at the monolayer level and the second electro-oxidation process of thioureas, the latter resulting in the formation of carbon dioxide and sulphate ions according to FTIRRAS data.