Active Dissolution Region Research Articles

Effect of pH on the Barrier Layer of TiO2 Nanoporous Films Potentiostatically Grown in Aqueous Media Containing Fluoride Ions

The effect of pH on the potentiostatic anodization growth of TiO2 film in 0.1 M HClO4/0.05 M NH4F solution was electrochemically characterized by EIS measurements. The increase in electrolyte pH modified considerably the voltammetric behavior of Ti electrode by diminishing the active dissolution region of the material and giving rise to two current plateaus that indicate modification of properties of the formed oxide. Five different formation potentials (EF) were selected from this study to potentiostatically grow oxide films, and to evaluate their morphology, the modification of their properties, and their interaction with the electrolyte. SEM images of the formed films showed that the EF increase resulted in a larger diameter of the pores formed, but an increase in pH led to a decrease in this parameter. In-situ characterization by EIS indicated that pH increase in the electrolyte used in anodization causes lower interaction between the formed oxide and F− ions in the solution due to the change in the surface charge excess of Ti oxide, resulting in an increase in the compact layer resistance and a decrease in both the time constant associated with F− ion adsorption and F− ion diffusion coefficient within the compact oxide layer.

On the Development of Thermo-Kinetic Eh-pH Diagrams

It is shown that thermo-kinetic Eh-pH diagrams can be generated through electrochemical measurements. These diagrams offer an accurate method of determining stability regions for leaching without relying on thermodynamic calculations, which may be inaccurate or for which data may be difficult to obtain. The Fe-NH3-CO3-H2O system is studied here. Potentiodynamic polarization experiments were performed on an iron sample in ammoniacal solution in de-aerated condition at different temperature and pH. Polarization plots show that both active anodic dissolution and passive regions are present for pure iron in ammoniacal solution depending on the potential. The electrochemically obtained potential-pH data were used to generate the thermo-kinetic Eh-pH diagrams for the Fe-NH3-CO3-H2O system.

The Effect of Trace Boron on Corrosion Resistance of the Casted 0.04C-16Cr Ferritic Stainless Steels in H<sub>2</sub>SO<sub>4</sub> Medium

Three groups of 0.04C-16Cr ferritic stainless steels, with boron quality score of 0ppm, 16ppm and 26ppm, were smelted by a vacuum induction furnace. The chemical soak method, the tafel polarization curve method and the anode circular polarization curve method were used to research their general corrosion and intergranular corrosion resistance. The results showed that both general corrosion and intergranular corrosion of the casted 0.04C-16Cr ferritic stainless steels happened in dilute H2SO4 medium. The resuluts of electrochemical and chemical soak indicate that the addition of B improves intergranular corrosion resistance of 0.04C-16Cr stainless steel, moreover, the intergranular corrosion resistance increases with an increasing content of B. The addtion of B makes the general corrosion rate and self-corrosion current density of the 0.04 C-16Cr stainless steel reduced at active dissolution region in dilute sulphuric acid medium, so the general corrosion resistance of the 0.04 C-16Cr stainless steel is improved. The blunt-dimensional current density of the steel with 26ppm B is the lowest and the passivation zone of it is wider, so the stability of passive film and protection capacity of the steel with 26ppm B are the best. The electrochemical characteristics of general corrosion coincided with the results of soak corrosion.

Corrosion behavior of shape memory stainless steel in acid media

The corrosion behavior of three Fe–Mn–Si–Cr–Ni–(Co) shape memory stainless steels (SMSS) in 0.5 M H 2SO 4 solution was studied through electrochemical and immersion tests. The test results were compared with that of a type 304 (SS 304) austenitic stainless steel. The three SMSSs exhibited a passive behavior in 0.5 M H 2SO 4 solution; however, their anodic behavior in the active dissolution region was markedly different. The passive current densities of the SMSSs were similar to that of SS 304, although the critical anodic current required for passivation was higher. The corrosion rate of the SMSSs was much higher than that of SS 304. It was observed that the amount of Cr and Mn plays an important role in the corrosion behavior of SMSSs. The best corrosion behavior in acid media was shown by the SMSS that contained the highest amount of Cr and the lowest amount of Mn.

Pitting of Al and Al–Si alloys in KSCN solutions and the effect of light

The pitting corrosion susceptibility of pure Al and three Al-Si alloys, namely (Al-6%Si), (Al-12%Si) and (Al-18%Si) has been studied in 0.04M KSCN solution. Measurements were carried out under the effect of various experimental conditions using cyclic polarization, potentiostatic and galvanostatic techniques. In all cases, the potentiodynamic anodic polarization curves do not exhibit active dissolution region due to spontaneous passivation. The passivity is due to the presence of a thin film of Al2O3 on the anode surface. The passive region is followed by pitting corrosion, at a certain critical potential, pitting potential (Epit), as a result of breakdown of the passive film by SCN− anions. Cyclic polarization measurements allowed the determination of the pitting corrosion parameters, namely the pitting potential and the repassivation potential (Erp). Alloyed Si decreased the passive current (jpass) and shifted both Epit and Erp towards more positive values. Thus alloyed Si suppressed pitting attack. The effect of illumination on passivity and the initiation of pitting corrosion on Al in KSCN solutions was also studied. It is observed that illumination of Al leads to an increase in its pitting corrosion resistance-apparent from jpass, Epit, and Erp measurements in aggressive KSCN solutions.

Electrochemical impedance spectroscopic studies of copper dissolution in glycine–hydrogen peroxide solutions

Anodic dissolution of copper in glycine solution at various hydrogen peroxide concentrations was investigated. The dissolution rate increases, reaches a maximum, and then decreases with hydrogen peroxide concentration. Anodic polarization studies and electrochemical impedance spectroscopy (EIS) studies were carried out to determine the mechanistic pathway of anodic dissolution of copper in glycine system at three different hydrogen peroxide concentrations: one at low hydrogen peroxide concentration in the active dissolution region, another in the maximum dissolution region, and the third at the high hydrogen peroxide concentration in the post-peak-dissolution region. The EIS data in complex plane plots show presence of two capacitance loops and one negative capacitance loop. The impedance plot patterns strongly depend on the hydrogen peroxide concentration in solution. Reaction mechanism analysis technique was employed to model the EIS data. A three-step mechanism with two intermediate adsorbates and a parallel dissolution by catalytic mechanism simulates EIS patterns which match the experimental trends. The intermediates are likely to be cupric and cuprous oxides. The essential features of impedance spectra at various overpotentials at three different hydrogen peroxide concentrations are captured by the proposed mechanism. The results also show that the film present on the copper surface in glycine and hydrogen peroxide solutions does not passivate the surface.

Electrochemical Behavior of Tantalum in Anhydrous Ethanol

The electrochemical behaviors of Ta in ethanol solutions were investigated using potentiodynamic polarization, cyclic voltammetry, and potentiostatic current–time transient techniques, complemented with a scanning electron microscope (SEM). The potentiodynamic anodic polarization curves did not exhibit an active dissolution region due to the presence of a thin oxide film on the electrode surface, which was followed by pitting corrosion as a result of passivity breakdown by the aggressive attack of anions. SEM images confirmed the existence of pits on the electrode surface. The pitting potential decreased with the increase in solution temperature, but increased with increasing potential scan rate. The potentiostatic current–time transients showed that the incubation time slightly decreased with increasing concentrations. The pitting growth current density agreed well with Hills’s model.

Effect of bromide ions on the corrosion behavior of tantalum in anhydrous ethanol

The electrochemical behaviors of Ta in Et 4NBr ethanol solutions were investigated using potentiodynamic polarization, cyclic voltammetry, potentiostatic current–time transient and impedance techniques. The potentiodynamic anodic polarization curves did not exhibit active dissolution region due to the presence of thin oxide film on the electrode surface, which was followed by pitting corrosion as a result of passivity breakdown by the aggressive attack of Br − anions. The pitting potential ( E b) decreased with the increase of solution temperature and Br − concentration, but increased with increasing potential scan rate and water concentration. The incubation time derived from potentiostatic current–time transients decreased with increasing potentials. The impedance spectra exhibited two time constants for all the potentials and the resistance of passive layer decreased with increasing potential.

Pitting corrosion studies on Al and Al–Zn alloys in SCN − solutions

Pitting of Al and Al–6%Zn and Al–12%Zn alloys in KSCN solutions was studied by means of potentiodynamic anodic polarization, cyclic voltammetry, potentiostatic and impedance techniques. Measurements were conducted under different experimental conditions, complemented by ex situ scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDXA). The potentiodynamic anodic polarization curves do not exhibit active dissolution region due to spontaneous passivation. The passivity is due to the presence of thin film of Al 2O 3 on the anode surface (in case of Al) and the formation of ZnO on the Al 2O 3 matrix, in case of the two Al–Zn alloys (as evidenced from EDXA). The passive region is followed by pitting corrosion as a result of passivity breakdown by the aggressive attack of SCN − anions. SEM images confirmed the existence of pits on the electrode surface. Alloyed Zn was found to enhance pitting attack. The pitting potential ( E pit) decreases with an increase in SCN − concentration and temperature, but increases with increasing potential scan rate. The current/time transients show that the incubation time for passivity breakdown decreases with increasing applied positive potential, SCN − concentration, and temperature. Impedance measurements showed that Nyquist plots are characterized by a depressed charge-transfer semicircle, the diameter of which is a function of SCN − concentration, applied potential, solution temperature and sample composition.

Anodic behavior of tin, indium, and tin–indium alloys in oxalic acid solution

The anodic behavior of tin, indium, and tin–indium alloys was studied in oxalic acid solution using potentiodynamic technique and characterized by X-ray diffraction and scanning electron microscopy. The E/I curves showed that the anodic behavior of all investigated electrodes exhibits active/passive transition. In the case of tin, the active dissolution region involves two anodic peaks (I and II) prior to permanent passive region. On the other hand, the active dissolution of indium involves four peaks (I–IV) prior to permanent passive region. The first (I) can be associated with the active dissolution of indium to InOOH, the second peak (II) to the formation of In(OH)3, the third peak (III) to partially dehydration of In(OH)3, and the peak (IV) to complete dehydration of In(OH)3 to In2O3. When the surface is entirely covered with In2O3 film, the anodic current falls to a small value (I pass) indicating the onset of passivation. The active dissolution potential region of the first three tin–indium alloys involves a net anodic contribution peak, and this is followed by a passive region. It is expected that the investigated peak is related to the formation of In2O3 and SnO (mixed oxides). When the formation of oxides (the oxides of In and Sn) exceeds its dissolution rate, the current drops, indicating the onset of passivation precipitation of In2O3/SnO and SnO2 on the surface which blocks the dissolution of active sites. The alloys IV and V showed small second peak at about −620 mV which may be related to oxidation of In to In2O3 due to high In content in the two examined alloys. The active dissolution and passive current are increase with increasing temperature for all investigated metals and their alloys.

OPEN-CIRCUIT AND POTENTIODYNAMIC STUDY ON THE ELECTROCHEMICAL BEHAVIOR OF TIN ELECTRODE IN SULPHURIC ACID AND SODIUM HYDROXIDE SOLUTIONS

The electrochemical behavior of tin electrode in H2SO4 and NaOH solutions is studied in details using open- circuit potential measurements and potentiodynamic polarization techniques. The identification of the elements present in the surface of specimen after immersion in the two test solutions is performed using an energy dispersive X-ray analysis(EDX). Open- circuit potential measurements show that for tin electrode in low concentrations of H2SO4 and NaOH solutions the passivity was increased with dilution. At higher concentrations, dissolution of the pre-immersion oxide film occurs in which the corrosivety of NaOH is higher than that of H2SO4 solutions. Passivation of tin in NaOH solutions appears to take place in two steps. The first involves formation of SnO or Sn(OH)2, in the second step oxidation to Sn(OH)4 takes place. Potentiodynamic polarization techniques illustrate that the dissolution of tin in sulphuric acid occurs through the participation of both OH¯ and SO42- ions through an intermediate of (SnHSO4OH) species. The potentiodynamic curves for tin in sodium hydroxide solutions exhibits an active/passive transition. The active dissolution region involves two anodic peaks prior to permanent passivity. The first anodic peak is due to the formation of Sn(OH)2 whilethe second peak corresponded to the formation of Sn(OH)4 in addition to the formation of the soluble stannate ions. Dehydration of unstable Sn(OH)4 to the stable SnO2 can occur on the tin electrode during the potential sweep to positive direction. Increasing concentrations of H2SO4 and NaOH solutions led to enhance the corrosion current (icorr.), and corrosion potentials (Ecorr)shift towards more negative values. EDX analysis showed that Sn % present on the surface of tin electrode in 1M of each of NaOH and H2SO4 solutionswas 91.82 and 83.52 respectively.

Pitting corrosion of Al and Al–Cu alloys by ClO 4− ions in neutral sulphate solutions

The influence of various concentrations of NaClO 4, as a pitting corrosion agent, on the corrosion behaviour of pure Al, and two Al–Cu alloys, namely (Al + 2.5 wt% Cu) and (Al + 7 wt% Cu) alloys in 1.0 M Na 2SO 4 solution was investigated by potentiodynamic polarization and potentiostatic techniques at 25 °C. Measurements were conducted under the influence of various experimental conditions, complemented by ex situ energy dispersive X-ray (EDX) and scanning electron microscopy (SEM) examinations of the electrode surface. In free perchlorate sulphate solutions, for the three Al samples, the anodic polarization exhibits an active/passive transition. The active dissolution region involves an anodic peak (peak A) which is assigned to the formation of Al 2O 3 passive film on the electrode surface. The passive region extends up to 1500 mV with almost constant current density ( j pass) without exhibiting a critical breakdown potential or showing any evidence of pitting attack. For the three Al samples, addition of ClO 4 − ions to the sulphate solution stimulates their active anodic dissolution and tends to induce pitting corrosion within the oxide passive region. Pitting corrosion was confirmed by SEM examination of the electrode surface. The pitting potential decreases with increasing ClO 4 − ion concentration indicating a decrease in pitting corrosion resistance. The susceptibility of the three Al samples towards pitting corrosion decreases in the order: Al > (Al + 2.5 wt% Cu) alloy > (Al + 7 wt% Cu) alloy. Potentiostatic measurements showed that the rate of pitting initiation increases with increasing ClO 4 − ion concentration and applied step anodic potential, while it decreases with increasing %Cu in the Al samples. The inhibitive effect of SO 4 2− ions was also discussed.

Modeling dissolution in a filtration flow

The article considers mathematical models that describe in various approximations dissolution in a moving filtration flow. A more complete new model is developed and its results are compared with experimental data. The new model reflects the main features of mineral leaching by sulfuric-acid solutions. Self-similar solutions are obtained describing the motion of the solution leading front that displaces the natural water and the trailing front that tracks the progress of the active dissolution region. The effective dissolution zone width and the dissolution zone transport velocity are determined as functions of the filtration velocity, porosity, saturated solution concentration, and other parameters of the medium. The model is applied to construct the longitudinal concentration distribution of the leached substance for a one-dimensional constant-velocity flow.

Copper corrosion inhibition by Azadirachta indica leaves extract in 0.5 M sulphuric acid

Azadirachta indica leaves extract (AI) was investigated as a copper corrosion inhibitor in 0.5 M sulphuric acid. Inhibition efficiency of AI was compared to that of the already proven good inhibitors 2-acetamino-5-mercapto-1,3,4-thiadiazole (AAMTDA) and 1,2,3-benzotriazole (BTAH). The inhibition properties were studied using electrochemical polarization and weight loss techniques. In the region of active copper dissolution, the highest inhibition efficiency was exhibited by AAMTDA (92.7%). AI exhibited somewhat higher efficiency (86.4%) than the widely used BTAH (85.5%), showing that the extract could serve as a effective substitute for currently preferred copper corrosion inhibitors in sulphuric acid. The weight loss results were interpreted by means of the Frumkin isotherm of adsorption on the metal surface. The values of Δ G ads equal to − 41.96 kJ mol − 1 for AAMTDA and − 35.22 kJ mol − 1 for BTAH indicate strong spontaneous adsorption while the surface coverage dependence on the log c following the Frumkin isotherm is suggestive of chemisorption in case of all three tested inhibitors.

Corrosion behavior of Al/AlNp composite in alkaline solution

The corrosion behavior of Al/AlNp composite in alkaline solution is investigated by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). The potentiodynamic anodic polarization cyclic voltammetry curves exhibit an active/passive transition. An anodic current peak PI appears in the active dissolution region, which is correlated to the precipitation of Al2O3 and the attack of hydroxide ions. With the increase of immersion time, the peak PI increasingly vanishes, implying the slow down of the electrochemical dissolution of the Al/AlNp composite. Bolt plots for the composite reveals a shift of phase angle peaks towards low frequencies with the increase of immersion time. The Nyquist plot shows an ideal Warburg tail following a semi-circle in the high frequency range in the late time, suggesting that the corrosion of Al/AlNp composite is completely controlled by a diffusion process. In addition, in different corrosion stage, the hydrolysis of AlN particle is observed to play different roles. In the initial time, it makes the passive layer of the Al/AlNp composite poorly protective by causing crevices within the passive layer. While, in the late time, the hydrolysis of AlN particle increases corrosion resistance of the composite by limiting the formation and mass transfer of Al(OH)4− and preventing hydroxide ions from invasion.

AC and DC studies of the pitting corrosion of Al in perchlorate solutions

The pitting corrosion behaviour of Al in aerated neutral sodium perchlorate solutions was investigated by potentiodynamic, cyclic voltammetry, galvanostatic, potentiostatic and electrochemical impedance spectroscopy (EIS) techniques, complemented by ex situ scanning electron microscopy (SEM) examinations of the electrode surface. The potentiodynamic anodic polarization curves do not exhibit active dissolution region due to spontaneous passivation. The passivity is due to the presence of thin film of Al 2O 3 on the anode surface. The passive region is followed by pitting corrosion as a result of breakdown of the passive film by ClO 4 − ions. SEM images confirmed the existence of pits on the electrode surface. Cyclic voltammetry and galvanostatic measurements allow the pitting potential ( E pit) and the repassivation potential ( E rp) to be determined. E pit decreases with increase in ClO 4 − concentration, but increases with increase in potential scan rate. Potentiostatic measurements showed that the overall anodic processes can be described by three stages. The first stage corresponds to the nucleation and growth of a passive oxide layer. The second and the third stages involve pit nucleation and growth, respectively. Nucleation of pit takes place after an incubation time ( t i). The rate of pit nucleation ( t i −1) increases with increase in ClO 4 − concentration and applied step anodic potential ( E s,a). EIS measurements showed that at E s,a < E pit, a charge-transfer semicircle is obtained. This semicircle is followed by a Warburg diffusion tail at E s,a > E pit. An attempt is made to compare the values of E pit and E rp obtained through different methods and to determine the factors influencing these values in each particular method.

Passivity and passivity breakdown of a zinc electrode in aerated neutral sodium nitrate solutions

The passivation and pitting corrosion behaviour of a zinc electrode in aerated neutral sodium nitrate solutions was investigated by cyclic voltammetry and chronopotentiometry techniques, complemented by ex situ scanning electron microscopy (SEM), X-ray diffraction (XRD) and energy dispersive X-ray (EDX) examinations of the electrode surface. Measurements were conducted under different experimental conditions. The potentiodynamic anodic polarization curves do not exhibit active dissolution region due to spontaneous passivation. The passivity is due to the presence of thin film of ZnO on the anode surface. The passive region is followed by pitting corrosion as a result of breakdown of the passive film. SEM images confirmed the existence of pits on the electrode surface. The breakdown potential decreases with an increase in NO 3 − concentration and temperature, but increases with increasing potential scan rate. Addition of SO 4 2− ions to the nitrate solution accelerates pitting corrosion, while addition of WO 4 2− and MoO 4 2− ions inhibits pitting corrosion. The chronopotentiometry measurements show that the incubation time for pitting initiation decreases with increasing NO 3 − concentration, temperature and applied anodic current density. Addition of SO 4 2− ions decreases the rate of passive film growth and the incubation time, while the reverse changes produced by addition of either WO 4 2− or MoO 4 2− ions.

Properties and preparation of amorphous chromium carbide electroplates

The electrochemical and corrosion behavior of chromium electroplates formed in sulfuric acid solutions of Cr(III) in the presence of oxalates was studied by measuring steady-state polarization curves in 0.5 M H 2SO 4 solution. These electroplates demonstrate no active dissolution region and their open-circuit potentials are located in the passivity region, i.e. shifted substantially in the positive direction as compared with those of metallurgical chromium and electroplates from standard chrome-plating baths. The results of X-ray photoelectron spectroscopy (XPS) studies evidence that the passive metal surface layer several nanometers thick consists of chromium oxides with incorporated carbides formed during electroplating. It is supposed that the peculiarities of the corrosion and electrochemical behavior of the deposits under study can be attributed to the presence in them of chromium carbides, which operate as cathodic agents. At the same time, the formation of these carbide compounds during the cathodic deposition of chromium electroplates from sulfuric acid Cr(III) solutions in the presence of sodium oxalate is a result of electrocatalytic activity of metal chromium. The latter assumption is confirmed by XPS analysis of surface layers formed during the exposure of chromium to sulfuric acid solutions containing organic substances.

Impedance investigation of the anodic iron dissolution in perchloric acid solution

The anodic iron dissolution in 0.5 mol dm −3 perchloric acid (HClO 4) was investigated by the electrochemical impedance spectroscopy, the potentiodynamic sweep and the scanning electron microscopy measurements. The anodic polarization behavior of iron in HClO 4 solution showed that the strong current oscillations occurred in a narrow potential region, particularly the pitting corrosion was observed in the active dissolution region. These characteristics were quite different from those of iron in the sulfuric acid (H 2SO 4). At the potentials 82 and 132 mV more positive than the corrosion potential (−482 mV vs. SCE), the impedance spectra for the iron in HClO 4 displayed two inductive arcs; however, by gradually increasing potential the lower frequency inductive arc disappeared at −300 mV at first, and then the higher frequency inductive arc changed into a capacitive arc at −250 mV. Based on the impedance display of iron at various potentials, a reaction model involving two adsorbed intermediate species was proposed, in terms of which the impedance behavior at different potentials were described. Occurrence of the pitting corrosion in active dissolution region was explained.

Observation of Species of Intermediate Oxidation Degrees, Formed during Anodic Dissolution of Chromium, by the Rotating Ring–Disk Electrode Method

The possibilities and limitations of the RRDE method in recording and identifying intermediate and target products of chromium dissolution in sulfuric acid solutions of pH 0.3–2 are analyzed on the basis of own and literature data. From the experimental data, partial currents of chromium dissolution to Cr(II) ions in the region of active dissolution of chromium and its transition into a passive state are computed. The Cr(III) ions can be discovered through the reaction of their reduction on an Hg(Au) ring electrode only at pH ≥ 1.8. The current efficiency of reactions that lead to the formation of Cr(II) and Cr(III) ions near the anodic-dissolution peak is other than 100%. Assumptions as to the causes of this effect are made. In the solutions studied, Cr(VI) ions are recorded with a 100-% current efficiency on a gold ring electrode in a transpassive region of the chromium disk potentials.