Inhibition of the anodic dissolution of iron in alkaline solution by metal complex ions

The possible inhibiting effects of GaO₃^3?, GeO₃^2?, CrO₃^2?, and MoO₄^2? ions on anodic and cathodic reactions on iron in 5 M KOH were studied. It was shown that a concentration of these ions of 10^?3 M inhibits the anodic iron dissolution reaction 2–3 times, while no effect on the hydrogen evolution reaction was observed. The effect was explained by the underpotential deposition of adatoms of the metal complexing ions on the iron electrode.     

The influence of ion implantation on the corrosion behaviour of iron in acid solution

The influence of ion implantation on the aqueous corrosion of pure iron in IN H₂SO₄ was studied. The iron was bombarded with 5 × 10¹⁵–10¹⁷ ions.cm^?2 of Ne, Ar, Cu, Pb and Au. The current density-potential curves of the implanted samples were measured and compared with that of untreated pure iron. Ne⁺ and Cu⁺ bombardments lead to a slightly higher corrosion rate in comparison with untreated iron. Pb⁺ depressed the corrosion rate by orders of magnitude, Au⁺ enhanced it by a factor of more than ten. The effect is attributed to a reduction or an increase of the activity of the electrode surface with respect to the cathodic hydrogen evolution reaction, i.e. the ion implantation influences strongly the exchange current density of the hydrogen evolution reaction. A marked influence of the implantation on the anodic behaviour of the corroding metal could also be observed.     

The inhibiting effect of underpotential metal deposits on iron corrosion

The polarization behaviour of iron in perchlorate, chloride and sulphate base electrolytes at pH values ranging from 0.3 ≤ pH ≤ 3.5 was found to be changed drastically by the addition of small amounts of the heavy metal ions Me^z+ = Pb²⁺, Tl⁺ to the electrolyte solutions. At potentials positive to the corresponding Me/Me^z+-electrode a strong inhibition of the active iron dissolution process as well as the charge transfer controlled hydrogen evolution was observed. This effect can be explained by an underpotential deposition of Me^z+ leading to a complete coverage of Me-adsorbate on the iron surface. In deaerated solution an inhibition factor of 98% can be obtained, whereas in aerated solutions especially under rotating conditions the corrosion rate was not decreased. In the latter case the corrosion process is dominantly determined by the limiting diffusion controlled oxygen reduction reaction which was found to be not markedly inhibited by the metal adsorbates. In acid chloride solutions a competetive adsorption of Cl^? must be taken into account.     

Hydrogen penetration rates of iron. Effect of halide ions,hydrogen sulfide and hexynol

The effect of the halides (Cl^?, Br^? and I^?) on the rates of the hydrogen evolution reaction (h. E. R.) and hydrogen penetration (h. P.) of iron in H₂SO₄ is determined. The H. E. R. and H. P. of Fe in these same electrolytes with the addition of H₂S are also studied. The inhibition of corrosion and h. p. in the H₂S environment by hexynol are compared. With halide addition her., i_corr and h. p. decreased in the following order, Cl^?> Br^?> I^?. H₂S catalyzed both the h. e. r. and h. p. The catalytic activity does not appear dependent on the H₂S concentration for the H. E. R. but does for H. P. The fraction of hydrogen generated which is absorbed by Fe is greater during corrosion when a new surface is formed continuously than during cathodic polarization. The addition of the acetylenic alcohol, hexynol, in H₂SO₄ containing H₂S and halides inhibited the h. e. r. corrosion and h. p.     

Kinetics of hydrogen evolution reaction on iron and its diffusion through membrane from ethylene glycol solutions

The article discusses the effect of guanidine concentration on the rate of hydrogen evolution reaction (HER) on iron in ethylene glycol-water (0.1 and 50 wt. % H₂O, respectively) solutions of HCl with constant ionic strength. In an ethylene glycol environment, guanidine does not vary the rate of HER and the essence of the delayed stage, but rather increases the hydrogen diffusion rate through steel membrane. In the environment with binary solvent, containing 50 wt. % C₂H₄(OH)₂ and electrolyte of the same composition, the discharge rate of (NH₂)₂CNH₂⁺ ions determines the first stage of HER, the second stage occurs by the Tafel reaction. An increase in the initial concentration of guanidine stimulates HER and slightly varies the hydrogen diffusion flow into metal.     

Chemische und elektrochemische Reaktionen von Eisensulfid und Mangansulfid in sauren und neutralen Lösungen

Chemical and electrochemical reactions of iron sulfide and manganese sulfide in acid and neutral solutions The reactions which occur upon corrosion of massive iron sulfide and manganese specimens in perchloric acid and in neutral sodium chloride solution were elucidated by measurements of current-potential curves and by coulometric and analytical investigations on the processes. In acids the sulfides are dissolved by prevailing chemical reaction under evolution of H₂S. Upon applying anodic overpotentials electrochemical reactions occur simultaneously, however, with such low velocity that the contribution to corrosion of the sulfides is insignificant. Upon applying cathodic overpotentials some hydrogen discharge is observed on iron sulfide but not on manganese sulfide. In 3% sodium chloride solution both sulfides corrode very slowly upon anodic polarization, forming elementary sulfur according to MeS = Me²⁺ + S + 2e^? (Me = Fe or Mn). At high anodic potentials additional oxidation reactions occur in which three-valent iron and tetravalent manganese ions as well as sulfite and sulfate ions are formed. Iron sulfide and manganese sulfide inclusions can he isolated from steels only by electrochemical dissolution in neutral or weakly basic electrolytes, the potential during electrolysis must not be more positive than the corrosion potential of the sulfides.     

The polarization behavior of the iron, 304 and 316 stainless steel electrodes in molten sodium carbonate under CO/CO2/H2/H2S atmospheres

The electrode polarization behavior of iron and austenitic stainless steels 304 and 316 was studied in molten sodium carbonate at 1000°C under an atmosphere of 24% CO, 24% CO₂, 49.3% H₂ and 2.7% H₂S at 1 atm. The empirical polarization data were fitted to a previously described analytical model incorporating the effects of electron transfer polarization, concentration polarization and ohmic loss. Three distinct anodic reactions were seen. Comparison of curve parameters with results from a previous study of the nickel electrode together with analyses of anodic scales and expected equilibria in the system indicate that these reactions, in order of increasing anodic overpotential, are the oxidation of iron to the Fe²⁺ state with the formation of FeO, the oxidation of iron and chromium to the 3+ state with the formation of the spinel oxide FeFe_2−xCr_xO₄, and the oxidation of CO₃²⁻ with formation of O₂. The only distinction in the behavior of the iron and stainless steel electrodes is in the composition of the spinel oxide and a Nernstian displacement of the reaction Fe = Fe²⁺ + 2e. In addition to these significant electrode reactions, carburization of iron was seen at cathodic potentials, with oxidation of the carbide at anodic potentials.     

Microelectrochemical surface and product investigations during electrochemical machining (ECM) in NaNO3

The structure of the sample surface during electrochemical machining (ECM) of iron in neutral NaNO₃ solutions was investigated. From former experiments, we expected a duplex structure: a solid oxide film of some nm and, above, a meta-stable, highly soluble supersaturated iron nitrate film for ECM in NaNO₃ (current densities up to 100 A/cm², electrolyte flow). The total current (and charge) and the dissolution products during anodic pulses was measured. The formation of Fe³⁺ and Fe²⁺ was monitored by UV-VIS spectroscopy and enabled (for the first time) a quantitative product determination.A transition from predominant oxygen evolution to predominant iron dissolution was found in the range from 5 to 30 A/cm². At current densities >35 A/cm² the oxygen evolution is reduced to some percent of the total current. The Fe³⁺/Fe²⁺ formation ratio increases to 2, corresponding to a dissolution of Fe₃O₄.     

Das anodische und kathodische Verhalten des Eisens in sulfathaltigen Elektrolyten. Chaire d'Electrochimie,Faculté des Sciences,Strasbourg, France

The anodic and cathodic behaviour of iron in sulphate containing electrolytes The formation of Fe₂(SO₄)₃ on passive iron at p_H = 1 appears probable from a thermodynamical point of view. At high SO₄^2? concentrations the equilibrium system contains but low concentrations of Fe³⁺, and no Fe²⁺ ions, a fact showing the relatively elevated stability of the Fe₂(SO₄)₃ layer on passive iron. In slightly acid solution (p_H = 4) the passivity of the iron is determined by iron oxide layers. The formation of FeSO₄ from metallic iron and sulphate ions is restricted to the transpassive zone (p_H 4 to 7), in alkaline solutions even to the active zone. In the p_H region 2 to 14 the passive layer on iron has about the same composition in the systems Fe|H₂O + SO₄^2? and Fe|H₂O.     

Über den Einfluß von Schwefelwasserstoff auf die Korrosion des Eisens in sauren Lösungen

The influence of hydrogen sulphide on the corrosion of iron in acid solutions A considerable change of the polarisation behaviour of iron electrodes in acidic perchlorate and sulphate solutions was observed if they where saturated with hydrogen sulfide gas. H₂S markedly increased the anodic current densities and increases the corrosion rates by a factor of ten. The anodic Tafel slopes d log i_A/dE are only about half as steep as those in the absence of hydrogen sulphide. It is probable that the SH^?-ions behave as catalysts of the iron dissolution reaction in a similar manner as the OH^?-ions, but are adsorbed almost to saturation.     

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.     

Corrosion of Mg alloys EV31A,WE43B,and ZE41A in chloride- and sulfate-containing solutions saturated with magnesium hydroxide

This study studied corrosion in 0.1 M Na₂SO₄, 0.1 M NaCl, and 0.6 M NaCl, all saturated with Mg(OH)₂, using weight loss, hydrogen evolution, and electrochemical measurements. Corrosion was similar in all cases. Nevertheless, the corrosion rates were alloy-dependent, were somewhat lower in 0.1 M Na₂SO₄ than in 0.1 M NaCl, and increased with NaCl concentration. The corrosion damage morphology was similar for all solutions; the extent correlated with the corrosion rate. The corrosion rates evaluated by the electrochemical methods were lower than those evaluated from hydrogen evolution, consistent with the Mg corrosion mechanism involving the unipositive Mg⁺ ion.     

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.     

Effect of the cathode material on the accumulation of hydrogen peroxide in a plasma-solution system

The accumulation of hydrogen peroxide in a plasma-solution system with an electrolyte cathode under glow discharge action in relation to the cathode material has been studied, as well as the kinetics of the accumulation of Cu²⁺, Ni²⁺, and Ag⁺ ions passing from the cathode into the solution at the discharge combustion. It is shown that the Cu²⁺ concentration is comparable with that of the H₂O₂, and the use of a silver cathode results in a considerable decrease of the initial formation rate and the highest concentration of hydrogen peroxide in the plasma-solution system under study.     

Moisture-induced delayed spallation and interfacial hydrogen embrittlement of alumina scales

While interfacial sulfuris the primary chemical factor affecting Al₂O₃ scale adhesion, moisture-induced delayed spallation appears as a secondary, but impressive, mechanistic detail. Similarities with bulk metallic phenomena suggest that hydrogen embrittlement from ambient humidity, resulting from the reaction Al_alloy+3(H₂O)_air=Al(OH)⁻ ₃+3H⁺ may be the operative mechanism. This proposal was tested on pre-oxidized René N5 by standard cathodic hydrogen charging in 1N H₂SO₄, as monitored by weight change, induced current, and microstructure. Cathodic polarization at −2.0 V abruptly stripped mature Al₂O₃ scales at the oxide-metal interface. Anodic polarization at +2.0V, however, produced alloy dissolution. Finally, with no applied voltage, the acid electrolyte produced neither scale spallation nor alloy dissolution. Thus, hydrogen charging was detrimental to alumina scale adhesion. Moisture-induced interfacial hydrogen embrittlement is concluded to be the cause of delayed scale spallation and desktop thermal barrier coating failures.     

The influence of different aggressive anions on the electrochemical behaviour of aluminium in sodium nitrate aqueous solutions

The effects of different experimental parameters influencing the determination of critical pitting and protection potentials of aluminium and its alloys have been studied by potentiostatic and potentiodynamic methods. The resistance of aluminium against corrosion in aqueous media can be attributed to a rapidly formed surface oxide film. The addition of the aggressive anions like: chloride, thiocyanide, hydroxyl, sulphide, formate, and acetate (Cl⁻, SCN⁻, OH⁻, S²⁻, HCOO⁻ and CH₃‐ COO⁻) lead to extensive localized attack in all of the cases. The breakdown of the passive film takes into account the migration of aggressive anions through the film. Breakdown occurs when aggressive anions reach the metal‐film interface. E_π is the critical pitting potential, E_p ist the protection potential and the pitting can be formed only in the E_π–E_p polarization range as it was proved in many experiments [1–3]. The most likely action mechanism of aggressive anions is not a complete dissolution of the film, nor penetration of aggressive anions through solid oxide as suggested for nickel and iron [4]. It is more likely to be somewhere in between the two i.e. action of aggressive anions is that of complexing aluminium ions and pulling in water to hydrate the layer in a way similar to that occurring at cathodic hydrogen evolution, where such dramatic increase of hydrogen evolution rate is observed after a certain cathodic potential is reached. Localized corrosion can be prevented by the action of adsorptive inhibitors which prevent the adsorption of the aggressive anions or by the formation of a more resistant oxide film on the metal surface. The corrosion mechanism is not modified by the addition of ammonium rhodanide but only slowed down.     

Transient state scale formation on Fe 32 Ni 20 Cr and Fe 20 Cr in H2-H2O-H2S atmospheres

The transient state of simultaneous oxidation and sulfidation of Fe-32 Ni-20 Cr and Fe-20 Cr was studied at 700°C for short time exposures in H₂-H₂O-H₂S. After heating the specimens in pure, dry hydrogen they were corroded by introduction of the oxidizing and sulfidizing atmosphere for 2, 4 or 15 min. After quenching the layer was investigated by SEM, AES, X-ray and electron diffraction. Four different gas compositions were applied: pS₂ = 10^?12 bar and pO₂ = 10^?25, 10^?26, 10^?27, 10^?28 bar, all within the thermodynamic stability range of Cr₂O₃. After the short time exposures oxides and sulfides were present on the surface, Cr₂O₃ and Cr₃S₄ had grown side by side and in case of the alloy Fe-32 Ni-20 Cr Fe- and Ni-containing sulfides formed patches on top of the scale. The amount of sulfides was higher for the lower oxygen pressures. After a longer time exposure, 120 min, all sulfides had vanished. Simultaneous formation of oxides and sulfides occurs in the transient state during phase boundary reaction or transport control. Upon transition to diffusion control the sulfides vanish by dissolution into the alloy and reaction with the gas atmosphere. This is valid for low pS₂ where no iron and nickel sulfides are stable.     

Uniform corrosion of titanium in alkaline hydrogen peroxide conditions: influence of transition metals and inhibitors calcium and silicate

Uniform corrosion of titanium was studied in alkaline hydrogen peroxide environments simulating pulp bleaching conditions. Corrosion rates of unalloyed Grade 2 and alloyed Grade 5 were determined as a function of hydrogen peroxide anion (HOO^?) concentration. Influences of calcium and silicate inhibitors and iron and manganese were investigated. Without inhibition titanium corroded at HOO^? content of 200 mg/l: Grade 2 0.4 mm/y and Grade 5 1.4 mm/y. Addition of calcium (Ca²⁺) and silicate (SiO₃^2?) diminished the corrosion of Grade 2 to critical anion level 400 mg/l, but could not protect Grade 5 even at the HOO^? concentration of 300 mg/l. Presence of iron and manganese raised the critical levels of the both grades. High HOO^? anion level was observed as a notable potential difference between titanium and platinum.     

The inhibiting action of Pb2+ on stress corrosion cracking of austenitic stainless steels in acidic chloride media

The influence of the presence of traces of Pb²⁺ in acidic chloride media promoting s.c.c. on Type 304 stainless steel has been evaluated. Pb²⁺ even in traces can inhibit the occurrence of s.c.c. and the inhibiting effect is mainly due to the inhibition of the cathodic process of hydrogen evolution. Underpotential deposition of Pb²⁺ is invoked as an explanation of the inhibition of the reaction step involving the adsorption of hydrogen at the alloy surface. Some influence of Pb²⁺ to decrease the role of sulphide inclusions in the initiation stage of localized corrosion is also postulated.