The dissolution of iron under cathodic polarization

It has been shown that the dissolution rate for iron at cathodic potential is higher than expected on the grounds of the Wagner-Traud mixed potential theory. This observation cannot be explained by a parallel chemical process as suggested by Kolotyrkin et al. The dissolution appeared to be unaffected by potential, pH, cations (Li⁺, Na⁺, K⁺), anions (SO^2?₄, Cl^?, ClO^?₄) and solvent (water-dioxan mixtures). However, the dissolution rate was decreased by an order of magnitude when the electrode was connected to a strong magnet, suggesting that the mechanical degradation of the surface under the influence of the cathodically evolved hydrogen is the main cause of the measured effects.     

Investigations of the hydrogen evolution kinetics and hydrogen absorption by iron electrodes during cathodic polarization

The kinetics of the electrochemical reduction of hydrogen ions or water molecules on iron electrodes have been investigated using the electropermeation technique. The variation of permeation current density and cathodic potential with constant cathodic current density has been measured in various solutions, without or with promoters. The hydrogen activity on the cathodic side of the iron membranes was calculated from the H-permeation current. The exchange current densities of the three partial reactions of the cathodic hydrogen evolution were calculated from the obtained experimental data. In 0.1N H₂SO₄ without added promoters the chemical Tafel recombination predominates up to high cathodic current densities whereas in 0.1N NaOH or nearly neutral solutions without added promoters the electrochemical Heyrovsky recombination predominates already at comparatively low cathodic current densities. This is mainly due to the different exchange current densities of the electrochemical recombination reaction with hydrogen ions or with water molecules. The promoter used enhances the hydrogenation of iron by inhibiting the recombination reactions stronger than the Volmer reaction.     

Investigations of the hydrogen evolution kinetics and hydrogen absorption by iron electrodes during cathodic polarization

The kinetics of the electrochemical reduction of hydrogen ions or water molecules on iron electrodes have been investigated using the electropermeation technique. The variation of permeation current density and cathodic potential with constant cathodic current density has been measured in various solutions, without or with promoters. The hydrogen activity on the cathodic side of the iron membranes was calculated from the H-permeation current. The exchange current densities of the three partial reactions of the cathodic hydrogen evolution were calculated from the obtained experimental data. In 0.1N H₂SO₄ without added promoters the chemical Tafel recombination predominates up to high cathodic current densities whereas in 0.1N NaOH or nearly neutral solutions without added promoters the electrochemical Heyrovsky recombination predominates already at comparatively low cathodic current densities. This is mainly due to the different exchange, current densities of the electrochemical recombination reaction with hydrogen ions or with water molecules. The promoter used enhances the hydrogenation of iron by inhibiting the recombination reactions stronger than the Volmer reaction.     

Mechanistic aspect of near-neutral pH stress corrosion cracking of pipelines under cathodic polarization

This paper investigates mechanistically stress corrosion cracking (SCC) of an X70 pipeline steel that is under cathodic protection (CP) in a near-neutral pH solution. It was found that there is a critical potential range, i.e., ?730 and ?920 mV_SCE, where the steel is in a non-equilibrium electrochemical state, and anodic dissolution (AD) reaction may occur when the steel is polarized cathodically. When the applied potential is more positive than this range, SCC is AD-based; while the applied potential is more negative, SCC of pipelines is under hydrogen embrittlement (HE) mechanism. When the polarization potential is within the range, SCC of the steel is under the combined effect of AD and HE. Therefore, AD may still occur on pipeline steel that is under CP with the potential within this critical range, contributing to the cracking process.     

Deposits formed on iron during cathodic polarization in the presence of arsenic and their effect on hydrogen permeation

Deposits produced on iron and steel during cathodic polarization in the presence and absence of arsenic in acid and alkaline solutions have been studied microscopically, by electron microprobe analysis, and by double layer capacity measurements. The results of these studies are compared with permeation rates of hydrogen through rotating membranes. It is shown that the compactness of the deposits and their effect on the permeation depend on following major factors: surface condition and pre-treatment of the cathode, pH of the electrolyte, As concentration, electrode potential. Elemental arsenic deposited on the cathode does not promote the permeation of hydrogen. On the contrary, adherent and compact films impede the entry of hydrogen into the metal phase. Arsine adsorbed on the metal surface is the main promoting species.     

阴极极化对不同时效状态7003铝合金应力腐蚀行为的影响

采用电化学腐蚀试验与不同阴极极化电位下的慢应变速率拉伸试验,并结合XRD衍射图谱分析及SEM断口形貌观察,研究了阴极极化对峰时效(PA),双峰时效(DPA)以及回归再时效(RRA)7003铝合金在3.5%Na Cl溶液中应力腐蚀类型的影响。结果表明,在开路电位下7003铝合金的应力腐蚀主要由阳极溶解控制,应力腐蚀敏感性ISCC(PA)ISCC(DPA)ISCC(RRA)。而外加阴极电位的情况下XRD衍射谱中出现Al H3衍射峰,断口形貌则表现出沿晶及解理倾向,说明应力腐蚀转变为由氢脆控制,且氢脆敏感性(IHE)随着极化电位的负移而不断增加,其中3种时效状态的敏感性顺序为:IHE(PA)IHE(DPA)IHE(RRA)。     

Anomalous surface morphology of iron generated after anodic dissolution under magnetic fields

The effects of applied magnetic fields on the anodic dissolution of iron in a sulfuric acid solution were studied by potentiodynamic polarization measurements and electrode morphology observations. Uneven anodic dissolution occurs in the presence of magnetic field and the extent of the electrode surface inhomogeneity increases with magnetic flux density. Severely local dissolution at two edge areas of the iron electrode in the presence of magnetic fields is caused by the inhomogeneous distribution of the magnetic flux density at the ferromagnetic iron electrode and the resultant enhancement of the mass transport rate of interfacial film at local areas.     

The behaviour of copper-zinc alloys in alkaline solutions upon alternate anodic and cathodic polarization

Galvanostatic cyclic anodic and cathodic polarization curves for four CuZn alloys are traced in alkaline solutions of different concentrations. The anodic behaviour of the first three alloys, with 15.9, 46.8 and 50.5 wt% Zn, resembles that of pure Cu. Oxidation arrests, corresponding to the formation of Cu₂O, Cu(OH)₂, HCuO₂? and Cu₂O₃, are recorded before the evolution of O₂ on the passive electrode. The Zn of the electrodes does not develop its oxidation arrest. It affects, however, the behaviour of the alloys in a number of ways. The results are explained on the basis of kinetic interactions, and in relation to the phase diagram of the Cu-Zn system.The fourth alloy, with 85.9 wt% Zn (ε + η phases), yields upon oxidation in concentrated alkali solutions a series of five or six arrests. The first two of these represent the oxidation of the Zn of the η- and ε-phases, respectively. Calculations have shown that the activity of the Zn in the ε-phase is ～ 4.6 × 10^?10 times that of the free η-phase. The other oxidation steps correspond to the oxidation of the Cu of the alloy. In 0.1N NaOH the same alloy behaves as if it was pure Zn.Critical current densities for the passivation of Cu, Zn and the four alloys are determined in 0.1M Na₂SO₄. The ability of the tested materials to withstand electrochemical dissolution decreases in the succession: Alloy I > Alloy III > Cu > Alloy II > Alloy IV > Zn.     

Investigation of calcareous deposits formation on copper and 316L stainless steel under cathodic polarization in artificial seawater

The formation and growth of calcareous deposits on 316L stainless steel and copper under cathodic polarization in artificial seawater were investigated by electrochemical tests and analytical techniques such as SEM, EDX and XRD. The deposits mineral compositions were related to the types of metallic materials and were different on each substrate. On 316L stainless steel at potentials less negative than the water reduction potential (−1100 mV/SCE), the deposits were composed of aragonite with low amounts of brucite; at the potentials more negative than the water reduction potentials, only of brucite. Around the water reduction potentials, the deposits were composed of both aragonite and brucite together. However the formation of brucite was noted before activating the water reduction processes. The results were moderately similar to that on mild steel and the type of deposited phases depended on potentials, at which the cathodic protection was carried out. The deposits formed on copper differed from those on 316L stainless steel and the types of deposited phases were independent from water reduction potential on copper (−1150 mV/SCE). Hence, the deposits were composed only of aragonite at all potential ranges. Due to the low current densities observed in chronoamperometric curves during cathodic polarization and in sufficient alkalinity, it seemed that the brucite could not deposit on this metallic substrate. The article is published in the original.     

Effect of cathodic polarization on the corrosion cracking rate in pipe steels

Cathodic polarization can both inhibit and promote the growth of corrosion cracks in pipe steels. The promoting effect indicates that corrosion cracking follows the mechanism of hydrogen embrittlement. The critical current of hydrogen penetration across a steel membrane can serve as a criterion for the risk of embrittlement. Critical currents of hydrogen penetration for 14Γ2CAΦ and X70 steels were determined.     

Hydride formation during the cathodic polarization of titanium in artificial sea water

Hydride formation was studied in pure titanium under cathodic polarization in artificial sea water. The existence of a hydride in a pure titanium matrix was identified by X-ray diffraction, optical microscopy and transmission electron microscopy (TEM). Experimental results indicated that cathodic polarization of pure titanium in artificial sea water at temperatures even below 75 °C would induce hydride precipitation. The phase formed was identified as the face-centred cubic γ hydride.     

The electrochemical dissolution of cobalt in carbonate-bicarbonate solutions from EIS and steady polarization data

The cobalt electrodissolution process in carbonate-bicarbonate containing solutions 8.9 ? pH ? 10.5 covering relatively wide ranges of ionic strength, pH, and hydrodynamic conditions, has been investigated using electrochemical impedance spectroscopy (EIS) and steady polarization data. To distinguish the competing stages of the metal electrodissolution mechanism a cobalt rotating disc electrode was utilized. The proposed mechanism model consists of the formation of adsorbed Co(I) intermediate species followed by mass-transport contributions of soluble Co(II) ions as final products. The coherence of the proposed mechanisms with EIS and steady polarization data is presented.     

Prediction of pathways for the dissolution of iron

Previous mechanisms for the anodic dissolution of iron have been reviewed, and experimental evidence to distinguish between them is given. Of the various values found in the literature for the anodic Tafel slope, the values of 30 and 40 mV at (30°C) seem to be the most accurate. These two values have been taken as criteria for the possible mechanisms. New reaction pathways have been suggested which also involve the formation of Fe(OH)₂, FeOH⁺, and Fe₂OH⁺ as intermediates, and both the Tafel slope and the reaction order with respect to OH^? for these pathways have been deduced from steady state kinetics. The data thus obtained show that many of the proposed rate-determining steps give rise to the same results derived for previous mechanisms, thus indicating that such previous mechanisms are not the only possible ones. Difficulties associated with the work on iron, and with the present state of steady state kinetics are also outlined.     

High potential cathodic polarization of oxidized aluminium electrodes in neutral and acid media

The cathodic polarization of an oxidized 99.9995% aluminium electrode at potentials from −2 to −2.5 V(SCE) in several electrolytes of neutral and acid pH values has been studied. Potentiodynamic oxidation of cathodically polarized aluminium electrodes following the technique of Rozenfel'd et al. have been performed and analyzed. In solutions of pH 1–2.1, the oxide film present on the aluminium surface is partially removed by means of a continuous cathodic polarization at those potentials; there is evidence of a small amount of oxide formation following oxide removal and metal etching. The cathodic behaviour in acid and neutral media is interpreted on the basis of a local alkalization resulting from the H₂ evolution. A possible mechanism for the oxide removal is discussed.     

Anodic dissolution of iron silicides in alkaline electrolyte

Anodic dissolution of iron and its silicides (FeSi, FeSi₂, as well as the eutectic alloy FeSi₂-Si) and pure Si, in 0.1 to 5.0 N NaOH solutions is studied by cyclic voltammetry and x-ray photoelectron spectroscopy. Principal characteristic features of the silicide anodic dissolution are revealed and the composition of surface films investigated. It is shown that, despite an increase in Si solubility at higher pHs, the iron silicides are highly resistant to anodic dissolution due to especial protective properties of the complex oxide surface film. Original Russian Text ? A.B. Shein, I.L. Rakityanskaya, S.F. Lomaeva, 2007, published in Zashchita Metallov, 2007, Vol. 43, No. 1, pp. 59–63.     

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.     

The change of pH under a paint film due to cathodic protection

The diffusion of OH⁻-ions and H₂ formed during cathodic protection through a paint film is studied. The diffusion equation is solved for non-stationary conditions and from this the steady state is also derived. It is shown that under usual operating conditions of cathodic protection of ships the stationary state is reached in about 15 days. The danger of paint deterioration through high alkalinity and blistering is found to increase with current density and paint film thickness and with decreasing diffusion coefficient in the paint. For practical calculations a better knowledge of diffusion coefficients in paints is needed.     

The effect of sorbed hydrogen on the kinetics of active dissolution of iron

The effect of hydrogen adsorbed or absorbed by iron (0.009% C) on the iron dissolution is studied on a bipolar electrode-membrane in 0.5 M SO ₄ ²⁻ solutions (pH 1.30) by cyclic potential pulses. Expressions that allow one to calculate the hydrogen coverage on the iron surface (θ) as a function of the potential variation in a cyclic stepwise manner and also the hydrogen concentration in the near-surface metal layer (C) as a function of variations in the intensity of the diffusion flow of hydrogen atoms in the membrane are given. The method of cyclic potential pulses together with the analysis of solutions for metal ions shows that the iron dissolution rate substantially decreases as θ increases. A bipolar electrode-membrane allowed the determination of the C intervals corresponding to the inhibition of iron dissolution (at C < C _c ≈ 3 × 10⁻⁸ g-at/cm³), the activating effect of hydrogen absorbed by the metal on the anodic process (for C > C _c), and the metal destruction (for C ≫ C _c). The absorbed hydrogen is assumed to accelerate the ionization of iron due to the formation of new dissolution sites as a result of plastic deformations of the metal. Thus, the effects of two forms of sorbed hydrogen on the iron dissolution are separated. Original Russian Text ? A.I. Marshakov, A.A. Rybkina, T.A. Nenasheva, 2007, published in Korroiya: Materialy, Zashchita, 2006, No. 5, pp. 2–14.     

The effect of cathodic polarization on the corrosion behavior of X65 steel in seawater containing sulfate-reducing bacteria

Sulfate-reducing bacteria (SRB) are one of the main reasons for the accelerated corrosion of steel. Cathodic polarization has been reported as an effective and economic method against marine corrosion, including microbiologically induced corrosion. However, the interaction between cathodic polarization and microbial activity has not been well defined. In this study, a fluorine-doped tin oxide electrode is used to study the effect of cathodic current on SRB cells. Fluorescence microscopy results clearly show that the attachment degree of SRB is dependent on the electric quantity and current intensity. The large electric quantity and high cathodic current (400 mA/m² × 30 h) can effectively inhibit bacterial attachment and subsequent biofilm formation. Furthermore, the effect of cathodic potential on the corrosion behavior of X65 steel in the presence of SRB is systematically investigated. Results show that the impressed charges, the increase of pH, and the formation of calcareous deposits on the electrode surface at the cathodic potential of −1,050 mV/SCE inhibit the attachment of SRB. In turn, the presence of SRB also interferes with the electrochemical reactions that occur during the polarization process, thus increasing the cathodic current. The interaction between SRB-induced corrosion and the process of preventing corrosion by various cathodic potentials is discussed.