Anodic behaviour of tin in maleic acid solution and the effect of some inorganic inhibitors

The anodic behaviour of a tin electrode in maleic acid solutions was investigated by potentiodynamic and chronopotentiometric methods. Measurements were conducted under different experimental conditions. The results demonstrated that the polarization curves exhibit active/passive transition. In active regions, tin dissolves as Sn²⁺ which is subsequently oxidized to Sn⁴⁺ and the dissolution process is controlled partly by diffusion of the solution species. The passivity is due to the presence of thin film of SnO₂ on the anode surface formed by dehydration of precipitated Sn(OH)₄. The active dissolution of tin increases with increasing acid concentration, temperature and scan rate. The potential transients showed that the passivation time decreases with increasing applied current density. The effect of adding increasing concentrations of CrO₄²⁻, MoO₄²⁻ and NO₂⁻ ions on the anodic behaviour of tin in maleic acid was studied. These ions inhibit the active dissolution of tin and promote the attainment of passivity. The extent of these changes depends upon the type and concentration of the inhibitor.     

Effect of underpotential deposition of lead on polarization behavior of nickel in acidic perchlorate solutions at room temperature

The effect of Pb²⁺ on polarization behavior of nickel has been investigated in 0.1 M NaClO₄ + 10⁻² M HClO₄ + x M PbO solutions (x = 0, 10⁻⁵, 10⁻⁴, 10⁻³) at room temperature. The cyclic voltammogram has suggested that Pb²⁺ degrades the stability of the passive film on Ni. The corrosion potential of Ni shifted to the more noble direction and the anodic current peak of Ni dissolution decreased with increasing Pb²⁺ concentration in solution, indicating that Pb²⁺ suppresses significantly the anodic dissolution. The underpotential deposition (UPD) of lead on Ni in the potential range more noble than −0.215 V (SHE) corresponding to the equilibrium potential of the Pb²⁺ (10⁻³ M)/Pb electrode was confirmed by XPS and GDOES analyses. The anodic Tafel slope, b⁺, of Ni dissolution changed from b⁺ = 40 mV decade⁻¹ in the absence of Pb²⁺ to b⁺ = 17 mV decade⁻¹ in the presence of 10⁻⁴ or 10⁻³ M Pb²⁺, which was ascribed to the increase in active sites of Ni surface emerged as a result of electrodesorption of Pb adatoms. The roles of Pb adatoms in active dissolution and active/passive transition of Ni were discussed from the above results.     

Galvanostatic study of the breakdown of Zn passivity by sulphate anions

The anodic behavior of Zn electrode in Na₂SO₄ solutions was studied by galvanostatic polarization technique. The polarization curves are characterized by a distinct arrest corresponding to ZnO formation, after which the potential rises linearly with time up to a well-defined value, the breakdown potential, at which the potential drops with time, down to more negative direction. This denotes the destruction of the passive film and initiation of pitting corrosion. It was found that, the breakdown potential, the time consumed till the breakdown potential and the rate of potential rise with time depend on the sulphate anions concentration, solution temperature and magnitude of the imposed current density. Addition of increasing concentration of phosphate, molybdate, tungstate or chromate anions causes a shift of the breakdown potential into the noble direction, indicating the inhibitive effect of these anions. The inhibitive effect of these inhibitors decreases in the order: CrO₄²⁻ > WO₄²⁻ > MoO₄²⁻ > HPO₄²⁻.     

Preparation of a one-dimensional polymer film on passivated iron by modification of a carboxylate ion self-assembled monolayer with octyltriethoxysilane for preventing passive film breakdown

A self-assembled monolayer (SAM) of 16-hydroxyhexadecanoate ion HO(CH₂)₁₅CO₂⁻(HOC₁₆A⁻) has been prepared on an iron electrode passivated in a borate buffer solution (pH 8.49) in the preceding paper. In this work, the HOC₁₆A⁻ SAM on the passivated electrode was modified with octyltriethoxysilane C₈H₁₇Si(OC₂H₅)₃ to form a film composed of one-dimensional polymer. Prevention of passive film breakdown was examined by anodic polarization measurements of the electrodes uncoated and coated with the modified SAM in the borate buffer containing 0.1 M of Cl⁻. The pitting potential, E_pit of the coated electrode shifted from that of the uncoated electrode in the positive direction, indicating prevention of passive film breakdown. The anodic current density was decreased in the passive and transpassive regions by coverage with the modified SAM. Neither current spikes nor E_pit was observed in the curve of the passive region in some cases, demonstrating complete protection of the passive film against breakdown in the Cl⁻ solution. The modified SAM on the electrode was characterized by X-ray photoelectron and FTIR reflection spectroscopies and contact angle measurement.     

Self-assembled monolayers of carboxylate ions on passivated iron for preventing passive film breakdown

Self-assembled monolayers (SAMs) of carboxylate ions C_n−1H_2n−1CO₂⁻ (C_nA⁻) with the carbon number, n=12-18 and 16-hydroxyhexadecanoate ion HO(CH₂)₁₅CO₂⁻ (HOC₁₆A⁻) were prepared on an iron electrode previously passivated in a borate buffer at pH 8.49 by treatment in aqueous solutions of their sodium salts for many hours. Breakdown of the passive film on the electrode coated with the SAM was examined by anodic polarization measurement in the borate buffer containing 0.1 M of Cl⁻. The pitting potentials of the passivated electrodes coated with the SAMs of C_nA⁻ and HOC₁₆A⁻ shifted toward a more positive potential than that of the uncoated electrode, indicating prevention of passive film breakdown by blocking diffusion of Cl⁻ through the SAM to defects of the passive film. No breakdown was observed over the potential range of the passive region by coverage of the passive film with the SAM in some cases. The SAMs on the passive film were characterized by contact angle measurements and X-ray photoelectron and Fourier transform infrared reflection spectroscopies.     

Breakdown of passive film on nickel in borate solutions containing halide anions

The effect of Cl⁻, Br⁻ and I⁻ anions as aggressive agents on the anodic behaviour of nickel electrode in deaerated Na₂B₄O₇ solutions have been investigated by galvanostatic polarization technique. Lower concentrations of the halide anions have no effect on the mechanism of nickel passivation. An increase in the halide anions concentration causes oscillation of the potential in the oxygen evolution region. This could be attributed to the destruction of the passivity by halide anions and repassivation of the film by anodic current and/or OH⁻ anions. Higher aggressive anion concentrations cause breakdown of the passive film and initiated pitting corrosion. As the temperature increases, the breakdown potential is shifted towards the more negative direction. On the other hand, as the pH of the solution increases, the breakdown potential is shifted toward more positive direction, indicating increased protection of the passive film. The activation energy, , of the oxide film formation in the presence of Cl⁻ anions was calculated and was found to be 21 kJ/mol.     

The effect of cysteine on the corrosion of 304L stainless steel in sulphuric acid

The effect of cysteine on the corrosion of 304L stainless steel in 1 mol l⁻¹ H₂SO₄ was studied using open-circuit potential measurements, anodic polarization curves, electrochemical impedance spectroscopy (EIS) and scanning electron microscopy (SEM). All the electrochemical measurements obtained in the presence of low cysteine concentration (10⁻⁶-10⁻⁵ mol l⁻¹) presented the same behaviour as those obtained in the absence of cysteine, a passivated steel surface. However, for higher cysteine concentrations (10⁻⁴-10⁻² mol l⁻¹), a different behaviour was observed: the corrosion potential stabilized at a more negative value; an active region was observed in the anodic polarization curves and the electrochemical impedance diagrams showed an inductive loop at lower frequencies and a much lower polarization resistance. These results show that the presence of cysteine at high concentration turns the surface of 304L stainless steel electrochemically active, probably dissolving the passivation layer and promoting the stainless steel anodic dissolution. SEM experiments performed after immersion experiments at corrosion potential were in good agreement with the electrochemical results.     

Breakdown of cadmium passivity in alkaline solution

The electrochemical behaviour of a Cd anode was investigated in 0.05–2 M NaOH solutions by the potentiodynamic technique. The polarization curves exhibit active to passive transition prior to oxygen evolution. The threshold potential of the active dissolution is very close to the reversible potential of the system Cd/Cd(OH)₂. X-Ray diffraction and XPS measurements reveal that the passive layer is composed of both Cd(OH)₂ and CdO. The influence of increasing amounts of Cl^?, Br^? or I^? ions on the anodic behaviour of Cd in NaOH solution has been investigated. The halides stimulate the anodic dissolution in the active region and tend to break-down the passive layer in the passive region, leading to pitting attack. The pitting potential shifts to more negative values with increasing halide ion concentration but to the reverse direction with increasing alkali concentration.     

Direct modification of a passivated iron electrode with alkyltriethoxysilanes for preventing passive film breakdown in a borate buffer containing 0.1 M of chloride ion

A passive film on an iron electrode was modified with alkyltriethoxysilanes directly. In order to examine the protective ability of the modified passive film against breakdown, the pitting potential, E_pit was measured by anodic polarization of the modified electrode in a borate buffer solution (pH 8.49) containing 0.1 M of Cl⁻. The value of E_pit for the modified electrode shifted in the positive direction from that of the unmodified electrode, indicating prevention of passive film breakdown. The modified passive film was not broken down in the passive and transpassive regions of polarization curve in some cases. However, many current spikes appeared in the all curves of the modified electrodes. The modified surface of passivated electrode was characterized by X-ray photoelectron and FTIR reflection spectroscopies and contact angle measurement. There were defects and clusters of associated water within the modified film and hence, Cl⁻ could permeate through the defects, leading to appearance of current spikes and occurrence of breakdown.     

Prevention of passive film breakdown on iron by coverage with one-dimensional polymer films of a carboxylate ion self-assembled monolayer modified with alkyltriethoxysilanes

A film composed of a one-dimensional polymer was fabricated by modification of a 16-hydroxyhexadecanoate ion HO(CH₂)₁₅CO₂⁻ self-assembled monolayer (SAM) adsorbed on a passivated iron electrode with octadecyltriethoxysilane C₁₈H₃₇Si(OC₂H₅)₃. The pitting potential, E_pit of the passivated electrode coated with this film was measured by anodic polarization in a borate buffer solution containing 0.1 M of Cl⁻. The E_pit value of the electrode coated with the film was markedly shifted from the value of the bare electrode in the positive direction, indicating prevention of passive film breakdown. No breakdown occurred over the potential range of passive region in some cases. Structure of the modified SAM was discussed by X-ray photoelectron and FTIR reflection spectroscopies and contact angle measurement of the electrode surface covered with the film. Suppression of Cl⁻ accumulation at a defect of the passive film was revealed by electron-probe microanalyses of the surfaces uncoated and coated with the SAM modified with octyltriethoxysilane C₈H₁₇Si(OC₂H₅)₃ after anodic polarization in the borate buffer containing Cl⁻.     

Passivity and passivity breakdown of zinc anode in alkaline medium

The electrochemical behaviour of zinc in NaOH solutions has been investigated by using potentiodynamic technique and complemented by X-ray analysis. The E/i curves exhibit active, passive and transpassive regions prior to oxygen evolution. The active region displays two anodic peaks. The passivity is due to the Formation of a compact Zn(OH)₂ film on the anode surface. The transpassive region is assigned to the electroformation of ZnO₂. The reverse sweep shows an activation anodic peak and one catholic peak prior to hydrogen evolution. The influence of increasing additives of NaCl, NaBr and Nal on the anodic behaviour of zinc in NaOH solutions has been studied. The halides stimulate the active dissolution of zinc and tend to break down the passive film, leading to pitting corrosion. The aggressiveness of the halide anions towards the stability of the passive film decreases in the order: I^? > Br^? > Cl^?. The susceptibility of zinc anode to pitting corrosion enhances with increasing the halide ion concentration but decreases with increasing both the alkali concentration and the sweep rate.     

Corrosion kinetics in low conductivity media—I. Iron in natural waters

Potentiodynamic polarization curves with automatic compensation of the ohmic drop and a.c. impedance spectra have been obtained for rotating cylinder iron electrodes in tapwater and ASTM corrosive water with and without addition of 1 M NaClO₄ and 0.1 w/o NaNO₂, and in 1 M NaClO₄ with and without 1.0 w/o NaNO₂. In tapwater and corrosive water free of additives, passivation is not observed up to 10 mA cm^?2. NaClO₄ leads to passivation, but in corrosive water containing NaClO₄ reactivation occurs in the passive region followed by repassivation. Addition of NaNO₂ leads to passivation at low current densities, but pitting occurs at about + 0.22 V vs SCE. In the presence of NaClO₄ and NaNO₂, the peculiar effect of ClO₄^? occurs again in the passive region. From the impedance data recorded at E_corr the polarization resistance R_p could be determined for solutions free of NaNO₂. Good agreement was found with the R_p values determined from linear sweeps at E_corr. In the presence of NO₂^?, only a lower estimate of R_p could be given since the impedance was capacitive at the lowest frequencies studied. The capacitance decreased in the presence of NO₂^? due to adsorption of the inhibitor. Similar values of R_p were obtained in tapwater and ASTM corrosive water on the one hand and in the same solutions with the various additions on the other hand despite the different anodic polarization behavior.     

Effect of tin on high-temperature sulphidation of Fe-Cr alloys in H2/H2S atmospheres

The high temperature sulphidation behaviour of Fe-46Cr-xSn (x = 0; 0.2; 0.5; 1; 2) alloys has been studied at temperatures of 1073, 1173 and 1273 K in H₂/H₂S mixtures with different sulphur vapour partial pressures of 10⁻¹, 10⁻³ and 10⁻⁵ Pa. Thermogravimetric studies in combination with scanning electron microscope (SEM), with energy dispersive spectrometer (EDS), and X-ray diffraction (XRD) techniques, have displayed a significant influence of the sulphur partial pressure on the composition and growth rate of the sulphide scale. The results have shown that addition of tin increases the sulphidation rate of Fe-46Cr alloys but not considerably (except at temperatures of 1073 and 1173 K combined with sulphur partial pressure of 10⁻⁵ Pa). The metallic core of the studied samples was enriched in tin and iron, moreover tin was found in the internal layer close to the metallic core as metallic Fe_xSn_y inclusions with tin concentrations of up to 12 at.%.     

The inhibition effect of 2,4,6-tris (2-pyridyl)-1,3,5-triazine on corrosion of tin, indium and tin-indium alloys in hydrochloric acid solution

The influence of 2,4,6-tris (2-pyridyl)-1,3,5-triazine (TPTZ) on the corrosion of tin, indium and tin-indium alloys in 0.5 M HCl solution at different temperatures was studied. Potentiodynamic cathodic polarization and extrapolation of cathodic and anodic Tafel lines techniques were used to obtained experimental data. In the case of tin, the percent inhibition efficiency (IE%) increases as both concentration of TPTZ and temperature are increased. The value of activation energy (E_a) is smaller in the presence of TPTZ than that in uninhibited solution, and decreases with increasing the concentration. However, the effect of TPTZ on indium and the investigated alloys exhibited similar behavior; so, the maximum inhibition efficiency is observed at lowest concentration (10⁻⁶ M) of TPTZ. Then, the value of inhibition efficiency starts to decrease gradually with increasing TPTZ concentration than that of 10⁻⁶ M. But at higher concentration (10⁻³ M) the corrosion current density (I_corr) is still lower than that in uninhibited solution. SEM photographs support that the higher inhibition efficiency is observed at 10⁻⁶ M of TPTZ.The plots of ln K versus 1/T in the presence of the TPTZ in the case of tin, the inhibitor showed linear behavior. The standard enthalpy, ΔH°_ads., entropy, ΔS°_ads. and free energy changes of adsorption ΔG°_ads. were evaluated using Frumkin adsorption isotherm.     

Prevention of passive film breakdown and corrosion of iron in 0.1 M KClO4 with and without Cl by covering with an ultrathin two-dimensional polymer coating and healing treatment in 0.1 M NaNO3

A two-dimensional polymer coating, the self-assembled monolayer of 16-hydroxy hexadecanoate ion HO(CH₂)₁₅ modified with 1,2-bis(triethoxysilyl)ethane (C₂H₅O)₃Si(CH₂)₂Si(OC₂H₅)₃ and octadecyltriethoxysilane C₁₈H₃₇Si(OC₂H₅)₃ was prepared on the passivated iron electrode and further, the passive film was healed by additional treatment in 0.1 M NaNO₃. This electrode was immersed in oxygenated 0.1 M KClO₄ solutions with and without 1 × 10⁻⁴ to 1 × 10⁻² M of Cl⁻. Protection of passive film against breakdown by covering the electrode with the polymer coating was examined by monitoring the open-circuit potential during immersion in the solutions for many hours to determine the time for passive film breakdown, t_bd. Repeated polarization measurements were carried out during immersion in these solutions for obtaining the protective efficiency, P. The t_bd value of the passivated, polymer-coated and healed electrode in 0.1 M KClO₄ solutions with and without Cl⁻ increased with a decrease in the concentration of Cl⁻. No breakdown occurred on the electrode during immersion in 0.1 M KClO₄ solutions with and without 1 × 10⁻⁴ of Cl⁻ for 360 h. The P values were extremely high, more than 99.9% before t_bd, indicating complete protection of iron from corrosion. The effect of healing treatment in 0.1 M NaNO₃ on passive film breakdown was investigated by electron-probe microanalysis.     

Characterisation of anodic layers on Cu-10Sn bronze (RDE) in aerated NaCl solution

The anodic surfaces formed on Cu-10Sn (wt.%) alloy (α-bronze) are investigated in aerated 0.1 M aqueous chloride solution, using electrochemical reduction and characterisation methods such as scanning electron microscopy (SEM), energy dispersive spectrometry (EDS), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). On the whole anodic domain, investigations performed on a bronze rotating disk electrode (RDE) reveal the systematic formation of a uniform oxidation layer. It is evidenced that the chemical composition of the layer varies with the applied anodic potential, but also that the latter always exhibits a poorly crystallised (probably nanocrystalline) hydrated and hydroxylated nature. Close to E_oc, the compounds are mainly (hydroxide) oxides of tin and copper, incorporating very low amounts of chlorides. At intermediate oxidation potentials corresponding to the active-passive transition, the first oxidation peak corresponds to the formation of hydrated tin oxyhydroxide chloride species which transforms in a more stable one - probably related to the Sn(II) → Sn(IV) oxidation. At higher anodic potential, on the current plateau, the layer contains hydrated tin (IV) oxyhydroxide and copper chloride (mainly CuCl). However, XRD and XPS results reveal that the barrier layer has a complex nature, including unidentified products and different spatially distributed charged surface zones. The corrosion mechanism involves an internal oxidation of the alloy linked to a preferential dissolution of copper, namely a decuprification. A decuprification factor f_Cu is defined and calculated. Both f_Cu and the layer thickness increase with the applied potential. We show unambiguously that the tin compounds remain in the corrosion layer, acting as stabilizing species. It is suggested that the tin species promote the formation of a network as for tin oxide xerogel, through which copper ions and anions migrate. Both the layer microstructure and the decuprification factor (f_Cu) are in agreement with those found in Type I patina of ancient bronzes.     

Electrochemical behaviour of nickel anode in H2SO4 solutions and the effect of halide ions

The electrochemical behaviour of pure nickel in H₂SO₄ solutions has been potentiodynamically investigated. The effects of the following factors on the anodic dissolution and passivation of the metal are discussed: potential scan rate, successive cyclic voltammetry and progressive additions of Cl^?, Br^? and I^? ions. Increasing the potential scan rate increases the critical current density i_cc, denoting that the active dissolution of nickel in H₂SO₄ is a diffusion controlled process. Cyclic voltammetry shows that the reverse excursion does not restore the anode to its active state. On successive cycling, the height of i_cc decreases; this could be attributed to the decrease in the reduction efficiency of passivating oxide film during the cathodic half cycles. The presence of the halogen ions below a certain concentration specific to each anion inhibits the anodic dissolution both in the active and passive states. The inhibitive action of these additives decreases in the order I^?, Br^?, Cl^?. Beyond the specific concentrations, the halogen ions accelerate the anodic dissolution and shift the active passive transition to more positive values. The aggressiveness of these anions decreases in the sequence Cl^?, Br^?, I^?, Further increase in the halogen ion concentrations can lead to breakdown of the passive film and initiate pitting. The susceptibility of nickel to pitting attack enhances with increasing H₂SO₄ concentration.     

Electrochemical behavior of tin in sodium borate solutions and the effect of halide ions and some inorganic inhibitors

The corrosion of tin electrode in sodium borate (Na₂B₄O₇) solutions was investigated using cyclic voltammetry and potentiostatic current transient techniques. In absence of halide ions, the E/j response exhibits active/passive transition. The active region involves one anodic peak corresponding to the formation of Sn(OH)₂ and/or SnO. Addition of Cl⁻, Br⁻ or I⁻ (C ? 0.01 M) ions inhibits the active dissolution of tin, but higher concentrations enhance the active dissolution and tend to breakdown the passive film and induce pitting attack. The effect of , , and as inorganic inhibitors on the pitting corrosion of tin in (0.1 M Na₂B₄O₇ + 0.1 M NaCl) solution has also been studied. The presence of these anions (except ) inhibits pitting corrosion. Chronoamperometry measurements showed that nucleation of pit takes place after an incubation time (t_i). The rate of pit nucleation () increases with increasing halide ions concentration and applied potentials, but decreases with increasing the concentration of the inorganic inhibitors (except ). The inhibition efficiency of these inhibitors decreases in the order:

     

Electrochemical study on hot corrosion of Si-modified aluminide coated In-738LC in Na2SO4-20 wt.% NaCl melt at 750 °C

The corrosion performance of the slurry Si-modified aluminide coating on the nickel base superalloy In-738LC exposed to low temperature hot corrosion condition has been investigated in Na₂SO₄-20 wt.% NaCl melt at 750 °C by combined use of the anodic polarization and characterization techniques.The coated specimen showed a passive behavior up to −0.460 V vs. Ag/AgCl (0.1 mol fraction) reference electrode, followed by a rapid increase in anodic current due to localized attack in the higher potential region. In the passive region, the anodic dissolution of constituents of the coating occurred through the passive film, probably SiO₂, at slow rate of 20-30 μA/cm². The passive current for the Si-modified coating was two orders of magnitude smaller than that for bare In-738LC, which is known as Cr₂O₃ former in this melt. This indicates that the SiO₂ film is chemically more stable than Cr₂O₃ film under this condition. However, pitting-like corrosion commenced around −0.460 V and proceeded at the high rate of 100 mA/cm² in the higher potential region than +0.400 V. The corrosion products formed on the coating polarized in different anodic potentials were characterized by SEM, EDS and XRD. It was found from the characterization that oxidation was dominant attack mode and no considerable sulfidation occurred at 750 °C. The SiO₂ oxide was not characterized in the passive region because the thickness of the passive film was extremely thin, but was detected as the primary oxide in the localized corrosion region, where the selective oxidation of Al was observed by further progress of the corrosion attack front into the inner layer of coating.     

Prevention of passive film breakdown on iron in a borate buffer solution containing chloride ion by coverage with a self-assembled monolayer of hexadecanoate ion

Breakdown of a passive film on iron in a borate buffer solution (pH 8.49) containing 0.1 M of Cl⁻ was suppressed by coverage of the passive film surface with a self-assembled monolayer (SAM) of hexadecanoate ion C₁₅H₃₁CO₂⁻ (C₁₆A⁻). The pitting potential of an iron electrode previously passivated in the borate buffer at 0.50 V/SCE increased by treatment in an aqueous solution of sodium hexadecanoate for many hours, indicating protection of the passive film from breakdown caused by an attack on defects of the film with Cl⁻. No breakdown occurred over the potential range of the passive region by coverage with the SAM of C₁₆A⁻ in some cases. Structures of the passive film and the monolayer were characterized by X-ray photoelectron and Fourier transform infrared reflection spectroscopies and contact angle measurement with a drop of water.