The electrodissolution and passivation of mild steel in alkaline sulphide solutions

The electrodissolution and passivation of mild steel in alkaline sulphide solutions at 25°C is investigated by the use of steady state and transient electrochemical techniques and by scanning electron microscopy and energy dispersive X-ray analysis. The electrodissolution and passivation processes are explained by the initial competitive adsorption of OH^?, HS^? and H₂O and the subsequent charge transfer and proton transfer processes involving the OH and HS adsorbed species. The former is responsible for the passivation of the metal by an oxide metal layer while the second species undergoes two different reactions yielding in one case a precipitated mackinawite film on the metal, which is a poorly protective layer, and in the other a soluble Fe(II) species which can be detected by chemical analysis. The contribution of the reactions starting from each adsorbed species depends on the HS^?/OH^? concentration ratio in the solution. The initiation of pitting on iron is also explained through the postulated reaction pattern.     

Chelant enhanced passivation of carbon steel in deoxygenated alkaline aqueous solutions

Abstract

The passivation characteristics of carbon steel surfaces at 473 K in the presence of certain chelating agents such as NTA DTP A, and HEDTA are described. The relative impacts of these chelants on the passivation process are evaluated by using base metal loss, soluble and insoluble iron concentrations in the mediums SEM studies of the topography of the surface coatings, and electrochemical investigations of the protection afforded by the oxide coating. The results have been compared with the passivation behaviour obtained under simple alkaline pH (LiOH) treatment (without chelating agents) and that previously obtained in the presence of EDTA. It is found that at 473 K, the presence of these complexing agents greatly increases the base metal loss, although a lower base metal loss is observed with LiOH. but the oxide film formed is more protective than the one formed under LiOR or EDTA treatments. The morphology of the coatings formed under complexing conditions has revealed highly developed crystallite faces on the outer surface layer. It is concluded that at 473 K passivation by LiOR is preferred to that given by the chelants.     

Electrochemical adsorption and passivation on gold surface in alkaline thiourea solutions

An electrochemical method was used to examine Au surface absorption and passivation under the most commonly employed pH conditions in an alkaline Au leaching system.The polarization at different pH values was obtained through the current step curve.At pH 10,no absorption layer was formed.When the pH was increased to 11,an absorption layer was formed through the Au electrode reaction.At pH 12,the entire system could not be stabilized,even after long durations because of the thiourea oxidation and...     

Effect of electrolyte concentration on the passivation of Zn in alkaline solutions

In 0,01–0.3 N NaOH solutions and under galvanostatic and potentiostatic conditions Zn(OH)₂ is formed and is later transformed in oxide of higher valence state. In solutions above 0.3 N ZnO is precipitated after hydrolysis of Zn(OH)₂ an this ZnO forms a passivating surface layer. This layer does not undergo any further thickening nor transformation into ZnO₂.     

Effect of halide ions on passivation and pitting Corrosion of copper in alkaline solutions

The passivity of copper in NaOH and borate buffer solutions containing chloride, bromide and fluoride ions was studied by using cyclic voltammetry and potentiostatic current transient measurements. At scan rates ≥ 20 mVs^?1, the addition of halide ions does not nearly affect the cyclic voltammograms in the absence of pitting. But they differ considerably in presence of pitting. The pitting potential was found to depend on the solution pH and decreased linearly with increase of logarithm of halide ion concentration. The current transients in the passivity as well as in the pitting potential regions were analyzed. Before the pitting, i-time curves were rather similar to those obtained in the plain solutions. At times > 20 seconds, the current varies linearly with the reciprocal of the square root of time indicating diffusional characteristics of the metal corrosion through the passive layer. In presence of pitting, the pitting current versus time relations fit the Engell-Stolica equation.     

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.     

The Hg (Mercury) system

An erratum to this article is available at .     

Anomalous diffusion in dilute solid solutions

Diffusion in metals and alloys encapsulates many different physical phenomena and a range of time and length scales, and consequently, a hierarchical combination of simulation methods is required to study diffusion. We develop such methods to study the role interaction among defects and diffusants, and local association effects, play in diffusion in metals. We use Fe impurity diffusion in Al as an example. Using recently developed, accurate, interatomic potentials for the Fe–Al system, we calculate migration energies for atom–vacancy exchange in a variety of local atomic configurations, using lattice statics methods. These are used in a kinetic Monte-Carlo framework to calculate diffusivities. Two different activation regimes are observed at temperatures above and below 550 K. We explain this anomalous, non-Arrhenius behavior of the diffusion activation energy in terms of the interaction among vacancies and Fe atoms, and local association/ordering effects.     

The hydrogen evolution reaction on iron corroded in dilute and very dilute solutions of sulfuric acid

Permeation transients for hydrogen, generated by corrosion of iron in dilute solutions of sulfuric acid, were recorded with an improved electrochemical permeation apparatus.The mechanism of the hydrogen evolution reactions is coupled discharge hydrogen recombination for all methods of hydrogen generation, low current anodic polarization, cathodic polarization and corrosion.The experimental hydrogen transient was compared with that predicted theoretically.It is concluded that electrochemical hydrogen permeation is a sensitive tool for investigating processes on metal surfaces which are of engineering importance.     

The Al−Hg (Aluminum-Mercury) system

Journal of Phase Equilibria -     

Excess thermodynamic properties of dilute solutions

Appropriate equations for representation of excess thermodynamic properties of moderately dilute solutions are presented with discussions. Illustrative examples are given, based largely on the experimental results of Morita et al. It is shown that the statistical thermodynamic treatments with enumeration of configurations, and with the free volume concept yield the best existing equations. Future developments will depend on realistic enumeration of configurations with models having attractive and repulsive forces. The lack of adequate data on the physical properties, such as the excess volume, electronic configuration, etc., have discouraged further realistic developments.     

Effect of adsorption of some azoles on copper passivation in alkaline medium

We have studied the adsorption properties and the effect of some azoles such as benzotriazole (BTA), mercaptobenzothiazole (MBT), benzimidazole (BIMD), mercaptobenzimidazole (MBIMD), imidazole (IMD) and tetrazole on the growth of oxide film on copper in 0.1 M NaOH. Cyclic voltammetry and electrochemical impedance spectroscopy are used to characterise the passivation process. The relative inhibition efficiencies of the azoles in controlling the oxide film formation is found to be in the order MBT > BIMD ≈ MBIMD > BTA ≈ IMD. We find that the azoles follow the Langmuir adsorption isotherm. Study of coverage by the measurement of capacitance obtained from the imaginary component of impedance (Z^′′) as a function of time indicates that in about 800 s more than 95% of the coverage is complete in all the cases of inhibitor adsorption.     

Structure and magnetic properties of MnSb(Zn) and MnSb(Cu) solid solutions

The MnSb(Zn) and MnSb(Cu) solid solutions with NiAs-type of crystal structure have been obtained. The existence regions of mono-phase solutions have been determined. Zn and Cu can substitute 10% atomic for Mn. On the basis of magnetic measurements it was supposed that the Cu or Zn atoms preferable occupy the MeII sites of NiAs-structure if the amount of nonmagnetic substitution not exceed 10% atomic. The preferable occupation of MeII sites by nonmagnetic atom manifests in rising of the Curie temperature of solid solutions relatively unsubstituted alloy.     

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.     

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.     

The inhibition of the corrosion of iron in alkaline solutions

Abstract

The inhibition of the corrosion of iron in alkaline solutions is due to the anodic formation of a film of a ferrous compound, which is usually ferrous hydroxide, but in the presence of anions yielding less soluble ferrous salts, e.g. carbonate, the composition may be mixed. The ferrous compounds are subsequently oxidised to anhydrous cubic oxide; consequentty the film thickens until it prevents the passage of ferrous ions into solution.     

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.