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.     

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.     

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.     

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.     

Environmental and inherent factors that affect hydrogen cathodic evolution on silicides of the iron family metals

Scientifically proven search for new electrode materials with different functions is an actual problem of applied electrochemistry. Promising are intermetallic and metal-like compounds of various composition, a lot of which exhibits extremely high corrosion resistance and low hydrogen overpotential. Original Russian Text ? V.S. Povroznik, A.B. Shein, 2007, published in Zashchita Metallov, 2007, Vol. 43, No. 2, pp. 216–221.     

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.     

Measurements of hydrogen activity in iron during cathodic protection using a potentiometric concentration cell

A solid - state potentiometric sensor, based on the well - tried principle of the thermodynamic concentration cell, has been constructed. The sensor uses a conventional bi-electrode design with an entry side and an exit side at which the hydrogen is detected. The sensor has been used for the quantitative determination of the hydrogen activity (equivalent H₂ pressure) generated during cathodic protection. Thus, the background hydrogen pressure in uncharged steel is 10^?17.7 (2 × 10^?18 atm.) with a variation of about 20 times. During cathodic protection this value rises by large amounts. Thus, using a zinc anode, the equivalent hydrogen pressure rose by 10^10.3 (to 4.4 × 10^?8 atm.) in 3.5% NaCl and by 10^16.9 (to 0.16 atm.) in artificial sea water, indicating the much larger amounts of hydrogen present in the latter case. This compares, for example, with literature data of 0.11 μA cm^?2 and 0.60 μA cm^?2 for hydrogen permeation at ?1000 mV (SCE) in 3.5% NaCl and artificial sea water respectively. Hydrogen entry and exit was also considerably slowed in sea water. These differences are caused by local surface pH buffering and deposit formation.     

On the participation of hydrogen sulfide in the cathodic reaction on iron

The immediate cathodic reduction of H₂S molecules on iron is substantiated by the use of voltammetric curves. Phase films of iron sulfide and hydroxide are formed on iron upon reaching the limiting current of hydrogen ions.     

Study on hydrogen absorption of pipeline steel under cathodic charging

The hydrogen electrochemical oxidation method was used to determine the total amount of hydrogen absorbed by pipeline steels under cathodic charging. The oxidation currents were obtained for the hydrogen pre-charged specimen maintained at a certain positive potential in dilute NaOH electrolyte. The area under the current curve demonstrated a relationship with the hydrogen concentration. Hydrogen concentrations in pre-charged X70 and 16Mn steels were estimated using this method. It is shown clearly that hydrogen concentration in specimens increase as the applied current density increased, but the sensitivities of different materials to hydrogen are different. The effect of solution pH and pre-charging time on absorbed hydrogen is also studied.     

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 uptake of cathodic hydrogen by shim steel

The electropermeation of hydrogen through mild shim steel has been studied under galvanostatic mode of charging in 0.1 M Na₂SO₄ solutions of different pH values. The dependence of permeation current and hydrogen concentration in steel on pH, charging current density and hydrogen overpotential has been examined. The results are discussed on the basis of electrodic factors governing hydrogen evolution reaction on steel.     

Effect of the interface properties on the kinetics of the hydrogen electrode reaction I: The cathodic region

The effect of thiourea adsorption on the kinetics of the hydrogen electrode reaction was studied for Pd-Ag membranes polarized cathodically in aqueous sulphuric acid.

The equilibrium relationship between the amount of hydrogen in the electrode and its open-circuit potential was established in order to interpret the kinetic data.

The influence of thiourea adsorption on the rates of the elementary steps involved in the overall process was examined and the results were interpreted in terms of interactions between metal and sulphur atoms.     

The kinetics of hydrogen evolution and oxygen reduction on Alloy 22

The kinetics of hydrogen evolution and oxygen reduction on Alloy 22 in 5 M NaCl + xHCl + yNaOH solutions as a function of pH at temperatures ranging from 20 to 95 °C have been investigated. The hydrogen evolution reaction proceeds via two basic mechanisms; the reduction of H⁺ at pH < 4 and the reduction of H₂O at higher pH values. Analytical expressions for the exchange current density for the hydrogen evolution reaction on Alloy 22 are also developed for the two mechanisms, in a form that they can be used in corrosion models for assessing the performance of this alloy in high level nuclear waste (HLNW) repositories. The kinetics of oxygen reduction have also been explored over wide ranges of pH and temperature. However, this reaction is complicated by the formation of H₂O₂ as an intermediate in a two-electron transfer reaction. Nevertheless, kinetic parameters have been obtained for this reaction and an expression has been developed that allows calculation of the exchange current density over a wide range of conditions. Finally, the kinetic data are used to identify probable mechanisms for hydrogen evolution and the reduction of oxygen on Alloy 22.     

Formation of hydrogen blister on AZ91 magnesium alloy during cathodic charging

The process of hydrogen blister formation on as-cast AZ91 magnesium alloy was investigated in 0.1 M NaOH solution using galvanostatic test. The results showed that cathodic charging of AZ91 alloy resulted in hydrogen blister formation, and blister rupture caused the nucleation and propagation of transgranular micro-cracks around the ruptured blister.     

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.     

An ambiguity of the cathodic polarization effect on crack growth during stress corrosion cracking of aluminum alloys

The kinetics of crack growth under the conditions of stress corrosion cracking of aluminum alloys in electrolytes with different anion composition and pH values from 0 to 13 has been studied. It is shown that for the majority of alloy-electrolyte systems anodic polarization accelerates whereas cathodic polarization decelerates crack growth. For “chromic acid – sodium chloride” electrolyte it is shown that cathodic polarization under the conditions of stress corrosion cracking (SCC) has an ambiguous effect on crack growth. The behaviour of A127-;1 alloy is taken as an illustration of such SCC cracking growth in these electrolytes.     

ScMn1.6V0.4合金吸氢热力学和动力学研究

利用XRD研究了ScMn1.6V0.4合金及其氢化物的晶体结构,利用氢气反应装置测量了合金的吸放氢性能.结果表明:ScMn1.6V0.4合金为理想的密排C14型Laves相结构.合金在实验条件下生成的氢化物晶体结构与原始合金相同,体积膨胀28％.合金在115 kPa和298 K条件下可快速吸氢,孕育期非常短,表现出优异...     

Boundary conditions during the electropermeation of hydrogen through pure iron

The boundary conditions prevailing at the entry side of pure iron during galvanostatic (G) or potentiostatic (P) mode of charging in 0.1 N H₂SO₄ have been studied by comparing the normalised permeation transients with the dimensionless theoretical curves constructed for different cases. In the case of vacuum-annealed pure iron, irrespective of the mode of charging, the condition for constant hydrogen concentration at the entry side is satisfied, whereas in the case of hydrogen-annealed specimens, the constant hydrogen flux condition is valid. The diffusivity values for hydrogen in pure iron, pre-treated by different methods, have been computed and compared.