Initial stages of nickel passivation and dissolution in acidic sulfate solutions

Regularities of the electrochemical dissolution of freshly formed surface (FFS) of iron and nickel are similar as a whole, but exhibit a number of distinctions. At an oxidation stage Me⁰-Me⁺, a water molecule chemisorbs to form a surface complex with charge transfer; however, a mean fraction of transferred charge in the complex of nickel (0.8) is significantly higher than that of iron (0.5). In weakly acid sulfate solutions (pH 1.7–3.2), the iron FFS dissolution is predominantly inhibited by hydrogen atoms formed by the discharge of hydroxonium ions and adsorbed at the dissolution centers. On nickel at pH > 2.7, the inhibition is caused by the formation of adsorbed oxygen corresponding to more positive potentials in a range of active nickel dissolution. Original Russian Text ? A.N. Podobaev, I.I. Reformatskaya, 2006, published in Zashchita Metallov, 2006, Vol. 42, No. 1, pp. 73–75.     

A study of the initial stages of iron passivation in neutral solutions using the quartz crystal resonator technique

The initial period of growth of a passive film of iron in borate solutions (pH 7.4 and 6.7) is studied using the quartz crystal resonator technique (EQSN) and pulsed chronoamperometry. Dependences of the surface layer thickness on time are obtained at the metal passivation and prepassivation potentials. Regions corresponding to different stages of passive layer formation are found in anodic current transients, which allowed the ambiguous effect of atomic hydrogen on kinetics of hydrogenated iron dissolution to be explained. It is shown that the iron hydrogenation promoter prevents formation of a primary passive film by accelerating iron dissolution at prepassivation potentials.     

The effect of sorbed hydrogen on the corrosion potential of iron in acidic sulfate solutions

By using a bipolar electrode, i.e., a membrane, it is shown that hydrogen absorbed by a metal does not virtually affect the cathodic evolution of hydrogen. The change in the iron corrosion potential is determined by the effect of sorbed hydrogen on the anodic process.Translated from Zashchita Metallov, Vol. 41, No. 1, 2005, pp. 52–55.Original Russian Text Copyright © 2005 by Nenasheva, Marshakov.     

Anodic dissolution and passivation of lead in sodium sulfate solutions

The anodic behavior of lead depending on the sulfate solution concentration is studied. The relation between oxide-hydroxide passivation and salt repassivation is found. The main characteristics of the forward and reverse branches of the polarization curves and cyclic voltammograms, as well as anodic chronoammograms and cathodic chronopotentiograms, are determined. The time dependence of the phase-formation current is constructed and analyzed.     

On intensification of iron passivation by benzotriazole in aqueous solutions

The passivation of iron and mild steel by benzotriazole (BTA) and its equimolar composition with sodium phenyl undecanoate (SFU) in a borate buffer solution with pH 7.36 is studied. BTA is shown to be an inefficient passivator compared to SFU. However, the passivating effect of BTA can be increased by means of the preliminary formation of its adsorption layer at a cathodic potential E = −0.65 V and its combined use with SFU. The adsorption of the inhibitors studied can be described formally by the Frumkin isotherm. An analysis of the attraction interactions between the adsorbing particles of the inhibitors enabled us to suppose that, at the adsorption of the mixture on the iron free of oxides (E = −0.65 V), a layer adjacent to the electrode surface consists mainly of SFU anions, while the outer layer is formed by BTA. The adsorption of BTA on the iron electrode surface, where the SFU monolayer was preliminarily adsorbed, is much stronger compared to that on the reduced surface of the same electrode. Based on this observation, a two-stage treatment of the surface of a mild steel with SFU followed by BTA passivating solutions is proposed. Corrosion tests under severe conditions of everyday abundant moisture condensation on the steel surface confirmed the high efficiency of the method.     

Corrosion of iron in highly acidic hydro-organic solutions

In acidic water-organic solvents of ethylene glycol (EGOH), propylene glycol (PGOH), methanol (MeOH) and ethanol (EtOH), iron corrosion was studied by monitoring the corrosion potential, the potentiodynamic polarization curves and electrochemical impedance diagrams. In these aqueous glycols and alcoholic solutions containing HCl having concentrations of 0.5 up to 9 M, it has been shown by electrochemical analysis (I-E curves) that dissolution mechanism of iron is similar to that one in pure acidic aqueous solutions if only we take into account the relative amount of water in the medium. Based in our experimental data, water has an important role in the transfer kinetics of protons to the metallic electrode and limits electro dissolution rate of iron. When water quantity is sufficient at the metal surface, the acidity is a governing factor in the evolution of corrosion process.     

Anodic dissolution of iron in acidic chloride solutions

The dissolution of iron in acidic chloride solutions was studied by both rotating disc and impedance techniques. Two regions of different slope were observed in the Tafel graphs after the effects of diffusion were eliminated. The Tafel slope was seen to change from ca. 44 to ca. 85 mV/decade at more positive potentials. The lower slope was confirmed by impedance measurements. An electrode reaction sequence is proposed for the dissolution reaction in the region where the Tafel slope is ca. 44 mV/decade.The addition of decylamine reduced the rate of the dissolution reaction, but the form of the polarization curves and the mechanism of dissolution remained unchanged. It was also established that decylamine is not involved in the electrode reaction of the dissolving polycrystalline iron.     

Corrosion-electrochemical behavior of Ni-P coatings in deaerated acidic sulfate solutions

The corrosion-electrochemical behavior of Ni-P coatings in acidic sulfate solutions (a pH of 0.5–2.0) has been studied. It has been found that the hydrogen evolution reaction occurs through the discharge-electrochemical desorption route. Ni-P coatings are more efficient cathode materials in acidic sulfate media than nickel is. Preliminary cathodic polarization and an inert atmosphere have little effect on the behavior of the polarization curves of Ni-P coatings containing 13.4% phosphorus and significantly accelerate the anodic dissolution of coatings containing 8.0 and 11.1% phosphorus.     

The effect of atomic hydrogen on the kinetics of iron passivation in neutral solutions

Current-time dependences are recorded on iron in borate buffer (pH 7.4 and 6.7) and its mixture with NS4 solution (pH 6.7) at the potentials of passivity, active-passive transition, and prepassivation of iron. Hydrogenation of the metal is found to accelerate the dissolution of iron in steady passive state, produce no effect on the growth of a barrier layer, and prevent the formation of a primary passivating film. Atomic hydrogen decelerates the active dissolution of iron, which determines the anodic current at the initial stage of the metal passivation.     

On the passivation of iron in aqueous solutions of zinc 1-hydroxyethane-1,1-diphosphonate

This work is devoted to studying the passivating ability of the zinc complex of the 1-hydroxyethane-1,1-diphosphonic acid (HEDP) in a borate buffer solution. For the first time, we used the in situ ellipsometric method to study the mechanism of formation of a protective film on iron in the presence of HEDP, HEDPZn, and ZnSO₄ in the course of the cathodic polarization of the electrode. The investigations of adsorption of HEDPZn on iron (at E = −0.65 V) in combination with X-ray photoelectron spectroscopy (XPS) have shown that on the metal surface there is formed a multilayer protective film consisting of an internal layer of Zn(OH)₂ and an outer layer consisting of HEDP complexes with Fe²⁺ and/or Zn²⁺. It has been found that the thickness of the passivating film does not exceed 60 ?, of which 7–10 ? correspond to the low-soluble zinc hydroxide. Original Russian Text ? Yu.I. Kuznetsov, G.V. Zinchenko, L.P. Kazanskii, N.P. Andreeva, Yu.B. Makarychev, 2007, published in Korroziya: Materialy, Zashchita, 2006, No. 9, pp. 19–26.     

The effect of sulfate and nitrate ions on the passivation and activation of silver in alkaline solutions

Based on the concept of a competitive adsorption of hydroxide ions with SO₄²⁻ and NO₃⁻ activating anions, some peculiarities of the anodic behavior of silver in alkaline solutions containing sulfates or nitrates are explained. At the ascending branches of the anodic peaks, a co-adsorption of OH⁻ and SO₄²⁻ or NO₃⁻ results in the formation of mixed adsorption complexes, which are soluble better than hydroxide ones. An increase in the part of the soluble oxidation products of silver is observed if a rotating disc electrode with a ring is used. Passivation of silver is explained by a change in the electronic structure of the adsorption complexes when certain potential values are reached, while a local activation, by the destruction of these complexes at the passive state potentials between the anodic peaks. At the depassivation, pH value of the solution in pits decreases, which results in the formation of Ag₂SO₄ or AgNO₃ salts. The presence of the salts in a deposit on the electrode is confirmed by the appearance of a C ₃ cathodic peak. Original Russian Text ? N.N. Lesnykh, N.M. Tutukina, I.K. Marshakov, 2008, published in Fizikokhimiya Poverkhnosti i Zashchita Materialov, 2008, Vol. 44, No. 5, pp. 472–477.     

Electrochemical characteristics of iron in acidic solutions containing some organic sulphur compounds

Corrosion inhibition of bis-n-butyl-sulphinylmethane, bis-n-butyl-sulphonylmethane and ethylene oxide condensates of 2,2′-dihydroxy-n-hexylsulphide on iron in sulphuric and hydrochloric acids was measured by using electrochemical methods. These compounds were excellent inhibitors for acidic corrosion of iron and inhibited the cathodic and the anodic corrosion reactions.     

Effect of atomic hydrogen on the anodic dissolution of iron in a weakly acidic sulfate electrolyte

Using a membrane electrode, atomic hydrogen is shown to decelerate the dissolution of iron in sulfate and sulfate-citrate electrolytes (pH 5.5) in a potential range of the active metal dissolution and accelerates the process at the prepassivation potentials. Impedance spectra of iron at a controlled degree of surface coverage with hydrogen atoms are recorded. Rate constants of elementary stages of the anodic process are calculated and the reaction scheme of the iron dissolution in sulfate environments is made more accurate.     

Influence of Pb ions on the dissolution and passivation of nickel and Ni–21Cr in acidic solutions

The deleterious effect of dissolved Pb⁺⁺ on passivity of Ni and Ni–21Cr has been explored at 90 °C using two solutions: 0.1 M perchloric acid, and a pH 3.5 acetic acid/sodium acetate buffer. Pb⁺⁺ strongly inhibits true (low-potential) active dissolution of pure Ni; this is consistent with underpotential deposition (UPD) of elemental Pb, apparently a new observation. The range of potentials where this occurs is greater for the more acidic solution. By inoculating Pb⁺⁺ at different potentials, the maximum underpotential can be shown to be at least 300 mV. In the less-aggressive acetic acid environment, a ‘pre-passive’ state of Ni is more noticeable than in the perchloric acid, and in contrast to the true active region, this region is subject to a strong activating effect of Pb. Dissolution of Ni proceeds rapidly on a surface that is trying to form its pre-passive film but is partly covered by Pb. The time-dependence of the passive current in the acetic solution is altered subtly by incorporation of Pb into the metal-film interface, such that the normal decay with time is truncated, leading to a higher long-term dissolution or film thickening rate. The presence of 21% Cr suppresses active dissolution, greatly ennobles the open-circuit potential, and eliminates (for these particular conditions) both the inhibiting and the activating effects of Pb. The relevance of these results to corrosion and SCC caused by lead in nuclear power systems is briefly discussed.     

Mechanism of the action of weak acids and their salts on the passivation of iron by oxygen

Oxygen containing solutions of salts of weak acids passivate iron only when the pH of the solution exceeds a critical value. The dependence of the critical pH on the nature (pK_a) and concentration of the salt (c₈) as well as dissolved oxygen (c₀₂) can often be represented by an equation of the following form: pH_c=log c_s-log c_o2+pK_a-const. A quantitative explanation for this relationship is derived in terms of the pH shift nearest the iron surface in course of the diffusion controlled corrosion process.     

Mechanism of the action of weak acids and their salts on the passivation of iron by oxygen

Oxygen containing solutions of salts of weak acids passivate iron only when the pH of the solution exceeds a critical value. The dependence of the critical pH on the nature (pK_a) and concentration of the salt (c_s) as well as dissolved oxygen (c₀₂) can often be represented by an equation of the following form: ${pH}_c=log{c}_B?log{c}_{0_2}+{pK}_a?const.$A quantitative explanation for this relationship is derived in terms of the pH shift nearest the iron surface in course of the diffusion controlled corrosion process.     

The passivation of zinc in slightly alkaline solutions

The inhibition of zinc corrosion by Na₂HPO₄ in a slightly alkaline electrolyte has been investigated by a.c. impedance measurements. A corrosion process controlled by the diffusion of ions through a passivating layer has been determined. The corrosion resistance has been observed to be increased by the healing of the passivating layer which always follows a short-time breakdown of the layer, due to an anodic potential jump.     

In situ gravimetry of nickel thin film during potentiodynamic polarization in acidic and alkaline sulfate solutions

The passivation process of nickel thin films during potentiodynamic polarization in acidic and alkaline sulfate solutions was analyzed by an electrochemical quartz crystal microbalance (EQCM) to separate the partial current density of nickel dissolution through the passive film, i_Ni²⁺, and the partial current density of film growth, i_O^2-. The values of i_Ni²⁺ and i_O^2- during potentiodynamic polarization (20 mV s⁻¹) in the passive potential region could be separated by comparing the mass change rate and net polarization current as a function of electrode potential. It is found that i_Ni²⁺ is larger than i_O^2- in pH 3.0, 0.5 M sulfate solution, while the situation reverses in pH 12, 0.5 M sulfate solution. The sulfate concentration dependences of i_Ni²⁺ and i_O^2- in pH 3.0, sulfate solution were significant as compared to those in pH 12, sulfate solution.     

Study of the chemical passivation of iron by phosphonates

The effect of some phosphonic acids (HMEP, NTMP, HEDP) on the chemical passivation of iron by dissolved oxygen in neutral aqueous solutions has been investigated. It has been shown that in the presence of phosphonic acids chemical passivation cannot be attained. This effect is due to the formation of soluble complexes as could be shown by the stability constants determined.     

Scanning electrochemical diagnostics of initial stages of anodic dissolution of carbon steel in chloride containing solutions

An optical PC scanner is used in combination with electrochemical methods of controlling the pitting destruction by an example of the initial states of the anodic dissolution of carbon steel St3 in 0.1 M sodium chloride solution. It is shown that under anodic polarization at the active dissolution conditions, the carbon steel surface exhibits noticeable formation of local defects looking as pits. Under free corrosion conditions in chloride-containing media, this pitting includes at least two stages, namely, the pits initiation and their development, where the first stage is completed in 60 min of tests. Original Russian Text ? M.A. Petrunin, L.B. Maksaeva, D.N. Tyurin, V.A. Kotenev, 2008, published in Fizikokhimiya Poverkhnosti i Zashchita Materialov, 2008, Vol. 44, No. 5, pp. 564–568.