Critical Passivation Potential Research Articles

Determining of the Critical Passivation Potential by a Step Potentiostatic Method

Determining of the Critical Passivation Potential by a Step Potentiostatic Method

The passivation mechanism of Fe in the acidic AlCl3-EMIC ionic liquid

The passivation behavior of Fe in the acidic AlCl3-1-ethyl-3-methyl-imidazolium chloride (AlCl3-EMIC) ionic liquid was studied by linear sweep voltammetry and chonopotentiometry. Various approaches were used to characterize the composition and morphology of passive film formed on the Fe electrode, such as scanning electron microscopy (SEM), Raman spectra and X-ray Photoelectron Spectroscopy (XPS). The results showed that the critical passivation potential of Fe shifted to more negative when the molar ratio of AlCl3:EMIC changing from 2 to 1.3. A film with a light yellow color was observed on the surface of the Fe electrode after passivation. The composition of the passive film was demonstrated to be FeCl2. The passive film was composed of particulate FeCl2 with average diameter of about 500 nm. The formation of FeCl2 film was attributed to the variation of the electrolyte Lewis acidity from acidic to neutral at the interface during the dissolution process of Fe. The reason caused the variation of the electrolyte acidity was discussed.

Initial Stages of Iron-Chromium Alloys Passivation in Acid Sulfate Solutions

Upon jumpwise applied “maximal critical passivation potential” (that is, the most positive potential at which Fe-Cr alloys still remain in the state of active-passive transition), chromium can continuously accumulate at the surface because of the selective dissolution of iron atoms. This probability can actualize at a chromium content in the alloy of up to 27.7% and pH < 1.2. Oxygen-containing compounds of bivalent chromium are the passivating agents at pH < 1.2 those of trivlent chromium, at pH ≥ 1.2.

Characterisation of anodic oxide films formed on titanium and two ternary titanium alloys in hydrochloric acid solutions

The purpose of this paper is to characterise the anodic oxide films formed on titanium and two ternary titanium alloys, Ti-15Mo-5Al and Ti-10Mo-10Al in 1%, 10% and 20% hydrochloric acid solutions at 25, 50, and 75°C. The anodic film on titanium in hydrochloric acid is stable between + 0.6 V (SCE) and + 2.0 V (SCE). For our new ternary titanium alloys, the passive film is formed at about + 0.6 V (SCE) and is stable to + 2.0 V (SCE). The anodic polarization curves for alloys differ from the base metal curve, presenting two peaks for the critical passivation current density in the active-passive potential range. At the first current peak (the first critical passivation potential E cr1 ) a porous titanium pentaoxide (Ti 3 O 5 ) is formed. When the potential reaches the second current peak (the second critical passivation potential E cr2 ) the compact and protective titanium dioxide (TiO 2 ) is formed. The impedance spectra exhibit the typical behaviour for a passive film i.e. a near capacitive response illustrated by a phase angle close to - 90 over a wide frequency range. The oxide film on titanium and its alloys in hydrochloric acid solutions exhibits a high resistance and a low capacitance (with the increase of the potential) attributed to the surface roughness decrease as the oxide layer thickens.

Accelerative Effect of Wear on Corrosion of High-Alloy Stainless Steel

The corrosive wear behavior of high-alloy stainless steels (SS) under sliding conditions was investigated in 20 wt% sulfuric acid (H2SO4) solution. Experiments showed a negative shift of the corrosion potential, positive shift of critical passivation potential, and steady passivation potential could be regarded as accelerative effects of wear on corrosion. For 18-8 austenitic SS (UNS S30302), it was concluded that the accelerative effect of wear on corrosion increased with increases of the corrosion rate.

NaCl-HCl系溶液中におけるTi-0.14 mass%Pd合金の腐食特性

Corrosion characteristics of the open surface of a Ti-Pd(0.14) alloy have been studied in NaCl-HCl solutions of various concentrations. It has been confirmed that the Ti-Pd alloy shows markedly lower corrosion rates and improved corrosion resistance in comparison with pure titanium in these environments. The electrochemical behavior of the Ti-Pd alloy in these solutions suggests that palladium accumulates on the surface of the Ti-Pd alloy during the initial period of corrosion, the accumulation of palladium on the alloy surface being confirmed by GDS analysis. The corrosion protection is explained by the fact that a palladium enrichment on the surface accelerates the cathodic reaction due to lowering the hydrogen overvoltage, and consequently, shifts the corrosion potential of the alloy into the passive region of titanium. Therefore, the corrosion potential of the alloy is more noble than the critical potential for passivity of titanium.

Radiotracer and electrochemical studies of the mechanisms of the passivating action of inhibitive anions

Following a brief description of types of chemicals used to prevent corrosion in neutral aqueous solutions, radiochemical and electrochemical studies that have been conducted at N.P.L. on the mechanisms of inhibitor action are reviewed. The non-oxidising type of inhibitor, e.g. phosphates or carboxylates, requires the presence of dissolved oxygen to maintain steel in the passive state. Anion adsorption on the active iron surface can retard metal dissolution and the coverage can reach an appreciable fraction of a monolayer at the critical passivation potential where a 3-dimensional passivating film begins to grow. Phosphate ions can become incorporated in the growing passivating film by ionexchange processes or by localised formation of iron salts at points of film breakdown; the latter effect may also be obtained with carboxylate ions. Chromate ions, representing the class of inhibitors that can lead to passivation in the absence of oxygen, participate — following an initial adsorption — in the growth of the iron oxide (passive) film by producing Cr 2O 3, the proportion of which in the film is dependent on time, pH, dissolved oxygen concentration and thickness of the initial air-formed oxide film. Surface spectroscopic techniques that will be used in future studies are briefly reviewed.

The corrosion behaviour and rutherford backscattering analysis of palladium-implanted titanium

The corrosion behaviour of Pd-implanted Ti in boiling 1M H 2SO 4 has been investigated by means of high resolution Rutherford backscattering (RBS) profiles of the Pd concentration as a function of corrosion time, and by electrochemical methods consisting of corrosion potential vs time curves and potentiostatic polarization data. Employing ion-implanted Xe as a surface ‘marker’, absolute corrosion rates were determined by RBS analysis and compared to corrosion rates determined from the polarization curves. The rate of Ti dissolution is found to be reduced by about three orders of magnitude for Pd-implanted samples. The corrosion potential-time curves and high resolution RBS data both show that soon after immersion, the Pd surface concentration rises to 20 at.% because of preferential dissolution of Ti and that the Pd is retained in the surface for corrosion times of up to 9 h. The steady state corrosion potential of implanted samples is close to that of pure Pd and 800 mV anodic with respect to the corrosion potential of pure Ti. The protection is explained by the fact that the mixed electrode potential is more noble than the critical potential for passivity of Ti, resulting in a markedly reduced dissolution rate.

Ellipsometric study of the formation of films on iron in sodium arsenate solution

Using the ellipsometric method, the formation of films on iron in sodium arsenate solution of pH 8·5 was studied. Depending upon the experimental conditions, different films, mixtures of iron oxides, elementary arsenic and, probably, iron hydroxide, manifesting various indices of refraction, are formed. The kinetics of film growth at open circuit potential in aerated solutions are logarithmic and the films are protective. Below the critical passivation potential, film growth follows a parabolic or cubic growth rate.

Einfluss Der Verformung Bei 77°K Auf Die Elektrochemischen Eigenschaften Der Oberfläche Von Nickeleinkristallen

Monocrystalline nickel with a (110 surface orientation was deformed during rolling in liquid nitrogen in a low-temperature rolling installation. The degree of deformation constituted 9, 23, 40 and 83 per cent. Plarization curves of deformed single crystal nickel showed that anodic oxidation processes depend on the degree of metal deformation. With the increase in degree of deformation the transition of monocrystalline nickel to the passive state is facilitated. It is evident from the deceleration of the dissolution rate in the region of the active state of the surface, from the decrease of critical passivation current as well as from the shifting of critical passivation potential towards the less noble values. Electron microscopic investigation has shown that the growth of the deformation degree results in an accumulation of faults in the crystal lattice. The growth of single crystal nickel susceptibility to passivation with the increase of deformation degree can be explained by the fact that growth of dislocation density leads to the increase of the number of nickel-oxygen compounds formation centres.

Anodic Polarization of Titanium and Titanium Alloys in Hydrochloric Acid

Commercially pure titanium and the alloys studied exhibited active to passive transitions in . Increasing the acid concentration increased the critical current density for passivity and the dissolution current density in the passive range and shifted the critical potential for passivity in the noble direction. Increasing the temperature served only to increase the critical current density for passivity and the dissolution current density in the passive range. Activation energies for the anodic polarization process were the right order of magnitude for a reaction controlled by reactivity at the metal surface. For the α‐β alloy 6Al‐6V‐2Sn the critical current density for passivity increased as the ratio of amounts of beta to alpha phase decreased with increasing strength level. The dissolution is accelerated probably, by the galvanic effects and the unfavorable area ratio. Ti 75A, a commercially pure metal, had a smaller critical current density for passivity value than did the alloys, which was expected. Secondary current density increases were observed for all the alloys at potentials above + 1.0v. For the 13V‐11Cr‐3Al alloy such behavior was attributed to transpassivity. For the other alloys the phenomena was attributed to pitting or localized corrosion. The commercially pure metal remained passive in this potential range. The addition of Fe3+ and Cu2 + (0.03M) to the facilitated passivation of the metal and its alloys. Pitting of several of the alloys occurred in but not in .

Anodic Behavior of Titanium and Commercial Alloys in Sulfuric Acid

Abstract The anodic polarization behavior of commercially pure alpha titanium 75A (annealed), beta alloy 13V-llCr-3Al (annealed) and alpha-beta alloy 6Al-6V-2Sn (heat treated to strength levels of 140, 160 and 180 ksi) in hydrogen saturated 5, 12 and 20 percent sulfuric acid solutions was investigated at 20, 35, 50 and 65 ± 1 C, using a potentiostatic technique. All specimens exhibited active to passive transition at temperatures above 20 C and acid concentrations greater than 5 percent Titanium alloy 13V-11Cr-3Al showed transpassive behavior which was attributed to the 11 percent chromium content Increasing acid concentration increased me critical current for passivity and shifted the critical potential for passivity in the noble direction. Increasing the temperature served only to increase the critical current for passivity. Temperature coefficients and activation energies were the right order of magnitude for a reaction controlled by reactivity at the metal surface. For the alpha beta alloy 6Al–6V–2Sn the critical current for passivity increased as the ratio of amount of alpha to beta phase increased with increasing strength level. The dissolution is accelerated, probably, by the galvanic effects and the unfavorable area ratio. Ti 75A, a commercially pure metal and Ti 13V-11Cr-3Al, a homogeneous single phase alloy, had a smaller critical current for passivity values man did Ti 6Al-6V-2Sn, a multiphase alloy, which was expected. The effect of Fe3+, Cu2+ and Cl− on polarization of Ti 6Al-6V-2Sn (180 ksi) was investigated also. Fe3+ and Cu3+ inhibited the anodic dissolution reaction and Cl− facilitated passivation.

The Anodic Passivation of Iron in Solutions Of Inhibitive Anions*

The anodic passivation of iron in air-saturated and de-aerated solutions of a number of inhibitive anions has been studied by the determination of potentiostatic polarisation curves. The critical passivation potential, critical passivation current density and leakage currents through the passive films have been found to depend on pH and on the nature of the anion present in the solution. The critical passivation potential (N.H.E. scale) in solutions of anions, where passivation occurs relatively easily, varies with pH in the range 6–12 according to Ep = + 0.09 – 0.06 pH volt For the anions studied, the critical passivation current densities in de-aerated 0·1 M-solution increase (and the difficulty of passivation increases) in the order: nitrite < hydroxide < chromate < borate < phosphate < carbonate < benzoate < bicarbonate < nitrate.

Tiの不動態について

Passivation and activation of titanium in deaerated non-oxidizing acid solutions were studied in order to investigate on the passivity of titanium. Critical potential for passivity and Flade potential shift towards noble direction with increasing chloride and hydrogen ion concentration, thus passivation becoming difficult. The critical potentials are expressed as E=E0+mlog[Cl-]-n(RT/0.4343F)pH, where R, the gas constant, T, absolute temperature, F, Faraday constant, E0 and m are constants. The constant n varies from 0.9 to 3.0 with environmental conditions such as temperature and kind of anion. Such different pH dependence of the critical potentials is attributed to the presence of different equilibrium surface reactions corresponding to passivation or activation for the respective corrosion conditions. The equilibrium reactions are considered to beTia++2H2O=TiO2+4H++(4-a)e, 2Tib++3H2O=Ti2O3+6H++(6-2b)e, Ti+2H2O=TiO2+4H++4e, 2Ti+3H2O=Ti2O3+6H++6e.When the critical potentials expressed as equilibrium potentials of two former reactions, passivation or stability of passivity is affected directly by titanium ion. In such cases, as demonstrated for titanium-palladium alloy, corrosion resistance of titanium depends on the concentration of titanium ion in corrosive media.

Sodium Nitrite as a Corrosion Inhibitor for Mild Steel

Abstract The effect of sodium nitrite on the corrosion rate of mild steel in chloride and sulfate media has been investigated. The results obtained support the view that passivity in these systems results from an oxide film maintained in a state of dynamic equilibrium, with protection dependent upon the rate of protective oxide film formation exceeding the rate of film deterioration. The data also indicate that the basic function of sodium nitrite is to assist the cathodic process and thereby provide that current necessary to form a protective oxide film, or expressed differently, to provide that current necessary to polarize above the critical potential for passivity.

The Electrochemical Behavior of Uranium

The potential behavior of uranium was studied in both oxidizing and nonoxidizing media. A low critical anodic current and a very high critical potential for passivity were observed. In the presence of oxygen especially interesting behavior was noted. Oxygen, while passivating the corroding electrode, was reduced to form hydrogen peroxide. At steady state the potential of the passive electrode was found to be slightly more active than that for the reversible oxygen‐peroxide couple. Under these conditions, changing the partial pressure of oxygen caused potential shifts in accordance with the Nernst equation.

Anodic Polarization and Passivity of Ni and Ni‐Cu Alloys in Sulfuric Acid

Potentiostatic data are reported for anodic polarization of Ni at 25° and 40°C and for Ni‐Cu alloys at 25°C in deaerated . The critical potential for passivity of Ni follows the relation: pH and the critical current density (ma/cm2) follows: . At 40° the critical potential in is the same as at 25° within experimental error; the critical current density for passivity or for maintaining the passive state is higher. A Tafel relation holds in the transpassive region with slope of 0.140 v (25°) independent of pH. The critical current density for passivity and the minimum current density to maintain passivity move to higher values as Cu is alloyed with Ni. Above about 70% Cu, vestiges of passivity are lost and the alloy behaves as does Cu. This critical alloy composition is related to a filled d band electronic structure corresponding to relatively short life of the adsorbed passive film above 70% Cu. Thermodynamic properties of the passive film are discussed.

The Influence of Noble Metal Alloy Additions on the Electrochemical and Corrosion Behavior of Titanium

Alloying of titanium with small amounts of noble metals markedly improves its corrosion resistance in reducing‐type acids without any impairment of its high resistance to oxidizing media. The mixed potentials of titanium noble metal alloys are more noble than the critical potential for passivity of titanium in the same environment. Passivity is observed in hydrogen‐saturated systems without the presence of a redox system more oxidizing than the hydrogen electrode. The mixed potential of titanium noble metal alloys is much more sensitive to traces of oxidizing agents than unalloyed metal.