Electrochemical impedance spectroscopy of anodic processes on cobalt disilicide in sulfuric acid solutions

The frequency dependences of the impedance of a CoSi₂ electrode in solutions of 0.5 M H₂SO₄ and 0.05 M H₂SO₄ + 0.45 M Na₂SO₄ are measured in the range of potentials of the active-passive transition, passive state, and transpassive state. Impedance spectra are described using an equivalent circuit with regard to the presence of an oxide layer on the electrode surface. The dependence of equivalent circuit parameters and time constants on the electrode potential is analyzed. It is found that the high anodic stability of cobalt disilicide in sulfuric acid solutions is due to the barrier properties of the oxide film. The variations in the thickness of the oxide layer with increasing electrode potential in the range of passive dissolution are estimated. It is revealed that the time constant related to the oxide layer increases before the transpassive transition.     

Impedance of NiSi electrode in sulfuric acid in the active anodic dissolution range

Anodic behavior of NiSi electrode in 0.5 M H₂SO₄ in a potential range of active dissolution is studied. A conclusion about the selective dissolution of nickel from nickel silicide at a small anodic polarization (up to 0.15 V) is drawn based on the impedance data. The solid-phase diffusivity of nickel and the thickness of the diffusion zone are estimated.     

Untersuchungen über den Einfluß von Schwefelwasserstoff und Schwefeldioxyd auf die Korrosion chemisch beständiger Chrom-Nickel-Stähle in schwefelsaurer Natriumsulfatlösung

Investigations into the influence of hydrogen sulphur dioxide on the corrosion of chemically resistant chrome nickel steels in a sulphuric acid chrome nickel steels in a sulphuric acid solution of sodium sulphate By measuring the current/potential curves and determining the weight losses, the influence of H₂S and SO₂ on the corrosion of an 18/8 Cr? NI steel and of an 18/8 Cr? Ni stell with 2pCMo and 2.8pC Cu in sodium sulphate solution has been investigated. H₂S and SO₂ have the effect of shifting the rest potential towards the electronegative side, compared with a solution flushed with nitrogen, enlarging the potential range of active dissolution, and greatly increasing the dissolution, and greatly increasing the corrosion rate in the active zone. From the results of the measurements, it may be concluded that the metal dissolution is catalyzed by hydrogen sulphide ions and probably by reduction products of the sulphur dioxide. This catalyzing effect is not confined to the zone of activation overpotential but also occurs in the zone of the active plateau, the shape of which is largely determined by the migration phenomena. SO₂also has the effect of increasing the dissolution rate in the passive condition.     

Effects of solution pH and Cl on electrochemical behaviour of an Aermet100 ultra-high strength steel in acidic environments

In this work, the effects of solution pH and Cl⁻ on the electrochemical behaviour of an Aermet100 ultra-high strength steel in 0.5 M (Na₂SO₄ + H₂SO₄) solution were studied by polarization curve and electrochemical impedance spectroscopy (EIS) measurements, combined with scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) characterization. The results show that, when solution pH is below 4, the steel is in the active dissolution state, and corrosion current decreases with the increase of pH. There exists a critical pH value, above which the steel is passivated. Moreover, the oxides and hydroxides of Fe, Co, Ni and Cr are the primary components of the passive film. With addition of Cl⁻, pits are initiated on the steel electrode.     

Technical note: Transpassive anodic dissolution of nickel in sulphuric acid

Abstract

Anodic dissolution behaviour of nickel in 3 and 10M H₂SO₄ solutions in the transpassive region has been investigated. Nickel dissolution current efficiency has been determined by splitting potentiostatic polarisation curves into oxygen evolution and nickel dissolution curves by carrying out solution analysis. Nickel dissolution current efficiency is minimum at 1800 mV(SCE) in 3M H₂SO₄ and primary passivation is not observed in 10M H₂SO₄, To study the stability fo the passive film formed on the surface of nickel in 3 and 10M H₂SO₄ solutions, potential decay curves have been recorded from various anodic potentials.     

Electrochemical Behavior Assessment of Micro- and Nano-Grained Commercial Pure Titanium in H<Subscript>2</Subscript>SO<Subscript>4</Subscript> Solutions

In this study, the electrochemical behavior of commercial pure titanium with both coarse-grained (annealed sample with the average grain size of about 45 µm) and nano-grained microstructure was compared by potentiodynamic polarization, electrochemical impedance spectroscopy (EIS), and Mott-Schottky analysis. Nano-grained Ti, which typically has a grain size of about 90 nm, is successfully made by six-cycle accumulative roll-bonding process at room temperature. Potentiodynamic polarization plots and impedance measurements revealed that as a result of grain refinement, the passive behavior of the nano-grained sample was improved compared to that of annealed pure Ti in H₂SO₄ solutions. Mott-Schottky analysis indicated that the passive films behaved as n-type semiconductors in H₂SO₄ solutions and grain refinement did not change the semiconductor type of passive films. Also, Mott-Schottky analysis showed that the donor densities decreased as the grain size of the samples reduced. Finally, all electrochemical tests showed that the electrochemical behavior of the nano-grained sample was improved compared to that of annealed pure Ti, mainly due to the formation of thicker and less defective oxide film.     

Corrosion and time dependent passivation of Al 5052 in the presence of H<Subscript>2</Subscript>O<Subscript>2</Subscript>

Corrosion and time–dependent oxide film growth on AA5052 Aluminum alloy in 0.25M Na₂SO₄ solution containing H₂O₂ was studied using electrochemical impedance spectroscopy, potentiodynamic polarization, chronoamperometric and open circuit potential monitoring. It was found that sequential addition of H₂O₂ provokes passivation of AA5052 which ultimately thickens the oxide film and brings slower corrosion rates for AA5052. H2O2 facilitates kinetics of oxide film growth on AA 5052 at 25° and 60 °C which is indicative of formation of a thick barrier film that leads to an increment in the charge transfer resistance. Pitting incubation time increases by introduction of H₂O₂ accompanied by lower pitting and smoother surface morphologies. At short exposure (up to 8 h) to H₂O₂–containing solution, the inductive response at low frequencies predominantly determined the corrosion mechanism of AA5052. On the other hand, at prolonged exposure times (more than 24 h) to 0.25M Na₂SO₄+1vol% H₂O₂ solution, thicker oxide layers resulted in the mixed inductive–Warburg elements in the spectra.     

Impedance of NiSi electrode in sulfuric-acid solution in the region of passive and transpassive states

The behavior of an NiSi electrode in 0.5 M H₂SO₄ in the region of passive and transpassive state (from 0.50 to 2.1 V (versus SHE)) is studied. The conclusion is made about formation of oxide film close to SiO₂ by composition on the electrode surface in the passivation process. Equivalent electric circuit modeling passive and transpassive state of nickel silicide is proposed. Thickness of oxide film and its specific resistance depending on electrode potential are calculated basing on the impedance data.     

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.     

Methanol oxidation on aluminium-copper-silicon alloys coated with polyprrole

Methanol oxidation on the aluminum-copper-silicon alloys which are coated with polyprrole were investigated in 1 N H₂SO₄ by using electrochemical method. For this purpose, first the current densitypotential curve of alloys were obtained in 1 N H₂SO₄ + x M pyrrole solutions, determined passive zones are coated with polyprrole alloys in H₂SO₄ solutions. Then the current-potential curves were obtained in 1 N H₂SO₄ + x M methanol with aluminum alloys and these are expected at +1.5 V against to the standard calomel electrode (SCE) at the different times in 1 N H₂SO₄ + 10⁻³ M pyrrole that were obtained with different scan rates. The extreme methanol oxidation was seen on the E110 and E140 alloys which are coated with polyprrole. Embedded pH electrode solutions which were coated with polypyrrole in 1 N H₂SO₄ and 1 N H₂SO₄ + 0.5 M methanol solutions were measured after 30 minutes at +2.1 V.     

An ultrathin polymer coating of carboxylate self-assembled monolayer adsorbed on passivated iron to prevent iron corrosion in 0.1 M Na2SO4

For preparing an ultrathin two-dimensional polymer coating adsorbed on passivated iron, a 16-hydroxyhexadecanoate ion HO(CH₂)₁₅CO₂⁻ self-assembled monolayer (SAM) was modified with 1,2-bis(triethoxysilyl)ethane (C₂H₅O)₃Si(CH₂)₂Si(OC₂H₅)₃ and octadecyltriethoxysilane C₁₈H₃₇Si(OC₂H₅)₃. Protection of passivated iron against passive film breakdown and corrosion of iron was investigated by monitoring of the open-circuit potential and repeated polarization measurements in an aerated 0.1 M Na₂SO₄ solution during immersion for many hours. The time required for passive film breakdown of the polymer-coated electrode was markedly higher in this solution than that of the passivated one, indicating protection of the passive film from breakdown by coverage with the polymer coating. The protective efficiencies of the passive film covered with the coating were extremely high, more than 99.9% in 0.1 M Na₂SO₄ before the passive film was broken down, showing prominent cooperative suppression of iron corrosion in the solution by coverage with the passive film and polymer coating. The polymer-coated surface was characterized by contact angle measurement and electron-probe microanalysis (EPMA). Prevention of passive film breakdown and iron corrosion for the polymer-coated electrode healed in 0.1 M NaNO₃ was also examined in 0.1 M Na₂SO₄.     

Der Einfluß des Molybdängehaltes auf die Korrosion austenitischer Cr-Ni-Stähle im Aktivzustand

The influence of the molybdenum content on the corrosion rate of austenitic Cr-Ni steels in the active condition The corrosion rate of steels X 5 CrNi 189, X 5 CrNoMo 18 10, X 5 CrNiMo 18 12 and X 5 CrNiMo 17 13 has been investigated under potentiostatic conditions in the active zone in 2 n H₂SO₄ flushed with nitrogen and sulphur dioxide. The active rest potential of the steels is in the vicinity of the active-passive transition, and the corrosion rate increase at cathodic polarisation. With increasing Mo content, the corrosion rate is reduced in the active condition, but the passivation potential and the corrosion rate in the passive condition are not influenced. In the acid flushed with SO₂, the corrosion rate is increased in the active range, and the latter is extended in the direction of the electronegative potentials. With these steels, even a pre-activation of the specimens has an influence on the test results. In the active-passive transition zone, the steels in the test solution containing SO₂ are partially passive and subject to local corrosion attacks.     

Der Einfluß von Kupfer auf das Korrosions und Passivierungsverhalten von Eisen-Chrom-Legierungen Teil I: Elektrochemisches Verhalten in schwefelsauren Elektrolyten

The influence of copper on corrosion and passiviation of iron-chromium-alloys Part I: Electrochemical behavior in sulfuric acid solutions The kinetic of dissolution and passivation of alloys type FeCrXCuY (X = 5; 7; 9; 11%; Y = 0; 0.5; 1%) was investigated in 1N H₂SO₄ (pH 0.3) and 0.5 M Na₂SO₄ (pH 3), using rotating disc electrodes (RDE) and hydrodynamically modulated rotating ring disc electrodes (HMRRDE). Instationary current density-potential curves (10 mV/s) were measured. For both types of electrodes the transients of current density after pulse polarisation into the passive range were investigated. The selfactivation of one hour prepassivated electrodes (open circuit breakdown) was investigated by potential scans. The dissolution of Cu bearing samples led to the formation of a visible film of copper lowering the current densities in the active and prepassive range as well as the critical current and charge densities of passiviation. Copper accelerates similar to chromium the decline of current transients after pulse passivation, whereas the partial current for film formation was increased as deduced from studies with the HMRRDE, leading to the conclusion of a larger film thickness. However, according to the accelerated open circuit breakdown of copper bearing samples, films on these alloys are less stable than expected, which is related to a faster removal of the oxide film. The results demonstrate an ambivalent effect of copper in the passive state of these alloys.     

Impedance of NiSi electrode in sulfuric-acid solution in the active-passive transition range

The behavior of NiSi electrode in 0.5 M H₂SO₄ in a potential range of active-passive transition (from ?0.02 to 0.40 V (N.H.E.)) is studied with the use of impedance spectroscopy. Nyquist plots with an arc in the second quadrant (Z′ < 0 and Z″ > 0) in the lowest frequency range were constructed for the beginning of the active-passive transition. The shape of impedance plots is explained by the effect of the solid-phase diffusion of selectively dissolved nickel on the character of passivation of nickel silicide.     

Die Inhibition der Aktivkorrosion nichtrostender Stähle in Säuren durch Kohlenmonoxid

The inhibition of the active corrosion of stainless steels in acids due to carbon monoxide The active corrosion of stainless austenitic chrome-nickel steels in H₂SO₄ is effectively inhibited by CO below the boiling temperature of the acid. The potential range of active corrosion is narrowed down, and the passivation current density as well as the maximum dissolution rate are reduced. In the boiling solution, pitting corrosion occurs in certain potential ranges, which can be reduced or completely prevented by increasing the Mo-content of the steels.     

The electrochemical behaviour of the bright chromium deposits plated with direct- and pulse-current in 1 M H2SO4

The electrochemical behaviour of bright Cr-deposits was studied in 1 M H₂SO₄ at 27 °C. The bright Cr-deposits were electroplated by using direct- or pulse-current (DC or PC) at 50 °C in a sulfate-catalyzed chromic acid bath with plating current densities of 30, 40, 50 and 60 A/dm² respectively. The results show that the surface-crack density of bright Cr-deposits plated with either DC or PC decreased with increasing the plating current density, whereas the passive current densities in their anodic polarization curves increased when plating current densities increased. From the results of electrochemical tests, it was found that passive and active dissolution rates of the Cr-deposits were not affected by the amount of the surface cracks developed on the Cr-deposit. Corrosion resistance of the Cr-deposit plated with PC was better than that with DC. The critical current density in the anodic polarization curve of the former is approximately one order less than that of the latter. The Cr-deposit plated with PC can be passivated more easily than that with DC. The rate of active dissolution of Cr deposits is much higher than that of passive dissolution. The aforementioned results can be recognized with AC impedance test polarized in passive and open-circuit potentials.     

The dissolution behaviour of iron,chromium, molybdenum and copper from pure metals and from ferritic stainless steels

Dissolution rates of iron, chromium, molybdenum and copper as pure metals and as components of three ferritic stainless steels in neutral solutions (distilled H₂O and 0.5 M NaCl) and acid solutions (1 M and 9.2 M of HNO₃ and H₂SO₄) are presented. The relative position of dissolution rates of the elements is in general the same in pure metal and alloy dissolution. Selective dissolution of iron and simultaneous surface enrichment of chromium is observed under all the investigated conditions. A slight surface enrichment of molybdenum is found upon exposure in the neutral and sulfuric acid solutions but not in nitric acid solution. Large amounts of copper are found in the surface of the copper-containing steel exposed to 1 MH₂SO₄. Evidence is presented which shows that an increase in electrolyte volume decreases the thickness and changes the composition of the passive film. It is believed that this hardly ever mentioned effect is due to an increased amount of dissolution of the film components upon increased volume of electrolyte.     

The corrosion behaviour of Fe-15Mn-7Si-9Cr-5Ni shape memory alloy

The corrosion behaviour of Fe-15Mn-7Si-9Cr-5Ni (mass%) shape memory alloy at 25 °C in 0.5 M H₂SO₄ and 3.5% NaCl solutions has been studied using potentiodynamic polarization and electrochemical impedance techniques. Three different microstructures viz., single-phase γ, γ-δ and γ-Fe₅Ni₃Si₂, were produced by heat-treating the alloy in different equilibrium phase fields. The corrosion behaviour in 0.5 M H₂SO₄ solution is almost same for all three microstructures, barring a slight difference in the passivation range. Although, the passivation current in 0.5 M H₂SO₄, is in the same range as that of SS 304, the critical current required for onset of passivation is almost three orders higher and the passivation range is much shorter. In 3.5% NaCl solution the corrosion behaviour of all three microstructures of the Fe-15Mn-7Si-9Cr-5Ni shape memory alloy was that of general dissolution without passivity or localized attack (pitting). The best corrosion resistance in both H₂SO₄ and NaCl solutions is shown by the single-phase γ microstructure.     

The role of Ce ions in electrochemical corrosion of 304 SS in Ce(NO<Subscript>3</Subscript>)<Subscript>3</Subscript>.6H<Subscript>2</Subscript>O

Effects of temperature and potential on the electrochemical corrosion behavior of alloy AISI 304 (UNS S30400) Stainless steel were investigated in 3 wt.% cerium nitrate (Ce[NO₃]₃.6H₂O) solution. With an increase in electrolyte temperature from ambient temperature to 90°C, the corrosion potential of the alloy shifted towards the noble direction, and the resistance to polarization increased due to the formation of Ce-oxide on the electrode surface. The oxide films formed at the open circuit potential (OCP) and a passive potential of 0.4 V_SCE were examined by x-ray photoelectron spectroscopy (XPS). The oxide film formed at 50°C and a passive potentialof 0.4 V_SCE consists of mixed oxides of Ce and Cr, whereas that at OCP consists of only Cr oxide. The formation of Cr oxides on the electrode surface was primarily due to the nitrate (NO₃ ⁻) ions in Ce(NO₃)₃.6H₂O electrolyte.     

Anodic behavior of cobalt silicides in potassium hydroxide solutions

Anodic behavior of a CoSi₂ electrode in 0.1–4 M KOH solutions and a Co₂Si electrode in 1 M KOH was studied in a range from the corrosion potential (E _cor) to the oxygen evolution potentials with the use of polarization and impedance measurements. A number of results indicate enrichment of electrode surface layers in cobalt. At potentials from E _cor to E _tp transpassivation potential, impedance spectra correspond to the passive state of the silicides. At potentials from E _tp to the onset potential of oxygen evolution, impedance spectra can be described taking into account the inclusion of hydroxide ions in the passivating film. Equivalent electric circuits are proposed for different electrode potential ranges.