The effects of added impurities upon the electrochemical behaviour of iron in liquid ammonia solutions

Polarization data are presented for an iron electrode in liquid ammonia solutions containing ammonium salts and small amounts of the impurities: water, oxygen, nitrogen, ammonium carbamate, urea and boron trifluoride at — 40°C. Additions of water up to a limiting concentration increase the iron dissolution currents in the active region. Additions of oxygen have a similar effect. The presence of either oxygen or water assists the onset of passivation and gives the phenomenon a more irreversible character.In ammonia, one mole of oxygen with two moles of Fe(II) forms a complex which can be cathodically reduced on vitreous carbon (1.0F per mol of Fe(II)) or on iron (2.0F per mol of Fe(II)). On an iron cathode molecular oxygen is reduced at a rate determined by the mass transfer of oxygen, except at values of electrode potential more positive than the iron passivation potential.Nitrogen and urea do not affect the polarization behaviour of iron. Ammonium carbamate increases the iron dissolution currents obtained with nitrate or fluoroborate electrolytes but considerably inhibits the process in chloride electrolytes. Boron trifluoride profoundly increases the activity of iron towards dissolution.     

Kinetics of Pit Initiation at the Alloy Fe5Cr

Pit initiation at the passive alloy Fe5 Cr was studied and compared to the results obtained with pure iron. At constant electrode potential the addition of chloride to the solution increases the dissolution rate of Fe(III). Above the critical pitting potential the first pits develop during a time of incubation after addition of chloride. The total current density and the dissolution rate of Fe(II) rise simultaneously and suddenly. The logarithm of the times of incubation increase linearly with the reciprocal of the difference between the actual electrode potential and the pitting potential. Galvanostatic experiments show that, with Fe 5 Cr as with iron, chloride catalyzes the transfer reaction of oxygen ions at the interface between oxide and electrolyte more strongly than the dissolution of Fe(III). Addition of chloride. The total current density and the dissolution rate of Fe(II) rise simultaneously and suddenly. The logarithm of the times of incubation increase linearly with the actual electrode potential and the pitting potential. Galvanostatic experiments show that, with Fe5Cr as with iron, chloride catalyzers the transfer reaction of oxygen ions at the interface between oxide and electrolyte more strongly than the dissolution of Fe(III). Addition of Chromium lowers the current efficiency of Fe(III) dissolution and favors oxide growth. The ionic conductivity of the passivating oxide is not significantly changed by chloride but is lowered by chromium. Fluctuations connected to the onset of pitting are slower with Fe 5 Cr than with iron.     

Das Korrosionsverhalten von Eisen and Stahl in flüssigem Ammoniak

Corrosion behaviour of iron and steel in liquid ammonia Polarization curves were measured with iron in liquid ammonia containing various electrolytes of different acidities at - 40°C and + 20°C. The dissolution rate of iron by formation of iron(II) is independent of the electrolyte composition and follows a Tafel-line of the slope RT/2.0.4 F. At positive potentials iron becomes passive. The steady state current densities are nearly independent of the electrode potential and grow with acid concentration and temperature. At very positive electrode potentials the current densities again increase exponentially with the potential. The current efficiencies for nitrogen evolution and iron(III) dissolution are of equal magnitude. Iron oxidized in air exhibits an electrochemical behavior distinctly different from the one of iron passivated in liquid ammonia. Very probably the passivating layer is composed of iron nitride with a thickness of the order of nm. Passive steel FG 32 in the soft state after heat treatment corrodes at about the same rate as pure iron, but approximately 8 times faster in the cold worked state.     

Effects of an applied magnetic field on the dissolution and passivation of iron in sulphuric acid

The effects of an applied magnetic field (MF) on the electrochemical state, anodic dissolution and passivation of iron in sulphuric acid solution were studied by potentiodynamic scanning polarisation measurements, potentiostatic polarisation measurements and scanning electron microscopy observation. The magnetic field reduced the fractional surface film coverage on the electrode by enhancing the film dissolution process. This made the electrode prone to active dissolution. With increasing applied potentials the magnetic field accelerated the anodic dissolution at relatively low potentials, changed the oscillation or passivation to permanent active dissolution at intermediate potentials, and maintained the passive state at high potentials. Potentials for the onset of passivation moved in the noble direction when the magnetic field was imposed. An electrode kinetics formulation for the effects of the magnetic field on the dissolution and passivation is proposed. In the presence of a magnetic field and at specific anodic potentials, scalloping occurred due to accelerated localized dissolution. The scalloping areas were on both sides of the electrode and oriented parallel to the direction of the earth’s gravitation field. The ratios of the scalloping area caused by a 0.4 T magnetic field on the whole electrode surface were 0.69 (at 200 mV), 0.66 (at 350 mV) and 0.75 (at 400 mV), respectively. In contrast, uniform electrode surfaces were observed at these anodic potentials in the absence of the magnetic field. Uneven dissolution of iron in the presence of a magnetic field was related to the relative configuration between the magnetic field direction and the electrode surface and also to the special concentration gradient of reactive species at the electrode circumferential area.     

The dissolution and passivation of Fe and Fe-Cr alloys in acidified sulphate medium: Influences of pH and Cr content

The steady state polarization curves of ferritic FeCr alloys, containing 7–12% Cr and immersed in 1M H₂SO₄, exhibit two current maxima, i.e. two passivating processes take place at the electrode interface within two different voltage ranges. The origin of these two passivation phenomena was investigated by plotting the steady state polarization curves and also by measuring the electrode impedance along with different polarization points. These results are then compared to those obtained with Fe electrode immersed in sulphate solutions of various pH values (0–5) since the steady state polarization curves of this electrode exhibit also two current maxima in a weakly acid medium, i.e. at solution pH greater than 4. However, even in a strongly acid solution in which only one current maximum is observed for the Fe electrode, the electrode impedance showed two passivation processes are occurring at the electrode interface. Therefore, the Fe dissolution involves at least two dissolution paths and the current at which one dissolution path overtakes the other is dependent upon the solution pH. The impedance diagrams of FeCr alloy electrode are similar to those of Fe electrode especially when the comparison is made with the Fe electrode immersed in a less acid solution, i.e. the addition of Cr in Fe enhances the appearance of two passivation phenomena with increase of solution pH.     

Film Growth and Dissolution of Iron Ions at passive iron in neutral solutions containing chloride

The passivating oxide layer on iron grows by transfer of oxygen ions from the solution into the oxide. As expected theoretically, the dissolution rate of iron ions increases with the growth rate of the layer. In neutral solution the current efficiency for oxide growth is larger than in acid solutions. By addition of chloride to the solution the current efficiency is further enhanced, because chloride catalyzes the transfer of oxygen ions more strongly than the transfer of iron ions. Chloride in the solution does not change the ionic conductivity of the oxide to a measurable extent. During galvanostatic polarization in solutions containing chloride, fluctuations of the potential and of the dissolution rate of Fe(II) are observed. The frequency of the fluctuations increases with current density and chloride concentration.     

Untersuchung des Passivierungsmechanismus von Eisen in Phosphatelektrolyten an erosiv beanspruchten Oberflächen

Investigation into the passivation mechanism of iron in phosphate electrolytes on surfaces exposed to erosive attack On iron electrodes in neutral phosphate electrolytes by continuous solid particle impingement a reaction layer is formed within the transition potential region under anodic polarization. XPS and AES investigations show that the reaction layer formed under impingement will be replaced by an oxide layer during the transition into the passive state under simultaneous decrease of the layer thickness. The active/passive transition in phosphate electrolytes may be attached to the equilibrium potential of the reaction . According to this thermodynamic interpretation of both the active/passive transition and the passive/active transition, respectively on iron in presence of phosphate ions may be described as the Fe(II)/Fe(III)-redox reaction with two solid phases, the iron(II)-phosphate phase and the iron(III)-oxide phase. The increase of the interfacial dynamic processes at the solid/liquid interface causes in consequence of the solid particle impingement that thermodynamic laws govern the course of reactions.     

The dissolution and passivation of Fe and FeCr alloys in acidified sulphate medium: Influences of pH and Cr content

The steady state polarization curves of ferritic FeCr alloys, containing 7–12% Cr and immersed in 1M H₂SO₄, exhibit two current maxima, i.e. two passivating processes take place at the electrode interface within two different voltage ranges. The origin of these two passivation phenomena was investigated by plotting the steady state polarization curves and also by measuring the electrode impedance along with different polarization points. These results are then compared to those obtained with Fe electrode immersed in sulphate solutions of various pH value (0–5) since the steady state polarization curves of this electrode exhibit also two current maxima in a weakly acid medium, i.e. at solution pH greater than 4. However, even in a strongly acid solution in which only one current maximum is observed for the Fe electrode, the electrode impedance showed two passivation processes are occurring at the electrode interface. Therefore, the Fe dissolution involves at least two dissolution paths and the current at which one dissolution path overtakes the other is dependent upon the solution pH. The impedance diagrams of FeCr alloy electrode are similar to those of Fe electrode especially when the comparison is made with the Fe electrode immersed in a less acid solution, i.e. the addition of Cr in Fe enhances the appearance of two passivation phenomena with increase of solution pH.     

Interfacial reactions of chalcopyrite in ammonia–ammonium chloride solution

The interfacial reactions of chalcopyrite in ammonia–ammonium chloride solution were investigated. The chalcopyrite surface was examined by scanning electron microscopy and X-ray photoelectron spectroscopy (XPS) techniques. It was found that interfacial passivation layers of chalcopyrite were formed from an iron oxide layer on top of a copper sulfide layer overlaying the bulk chalcopyrite, whereas CuFe_1–xS₂ or copper sulfides were formed via the preferential dissolution of Fe. The copper sulfide layer formed a new passivation layer, whereas the iron oxide layer peeled off spontaneously and partially from the chalcopyrite surface. The state of the copper sulfide layer was discussed after being deduced from the appearance of S^2–, S^2?₂, S^2?_n, S⁰ and SO^2?₄. A mechanism for the oxidation and passivation of chalcopyrite under different pH values and redox potentials was proposed. Accordingly, a model of the interfacial reaction on the chalcopyrite surface was constructed using a three-step reaction pathway, which demonstrated the formation and transformation of passivation layers under the present experimental conditions.     

铜-氨-硫代硫酸盐体系金溶出影响因素的电化学研究

铜-氨-硫代硫酸盐浸金体系中，金的溶出本质上是电化学过程。采用交流阻抗和Tafel曲线分析硫代硫酸盐、铜离子、氨浓度对金的溶出影响。结果表明:在金的溶出过程中，硫代硫酸盐和铜离子可能会导致钝化，尤其使铜离子浓度增大会导致钝化加剧，这种钝化会交替出现；氨水不会导致钝化；对金溶出速率的影响，硫代硫酸盐最大，铜离子最为复杂，氨影响相对最小。     

Untersuchungen zum Aktiv/Passiv-Übergang von nichtrostenden Chromnickelstählen in organisch wäßrigen Medien. Teil 3. Resultate der Impedanzmessungen und Passivierungsmodell einer Legierung

Investigations into the active/passive transition of 304 stainless steel in organic media containing water and hydrogen chloride Part 3. Results of the impedance measurements and passivation model of an alloy Steady state polarization curves and electrode impedances were measured during the active/passive transition of type 304 stainless steel in dearated ethanolic solution containing hydrogen chloride and different amounts of water. The passivation potential and the critical current density for passivation strongly depend on the water content of the solution. The impedance measurements in the active/passive transition show the same sequence of diagrams independent of the water content of the solution. They indicate the onset of passivation before the maximum current density and show two time constants related to two different passivating species on the alloy surface. The experimental results were interpreted on the basis of a reaction model with parallel dissolution and passivation mechanism of the iron and the chromium compound of the alloy. The resulting total surface composition (related to the steady-state polarization curves) can be described with a reaction model of iron–the alloy behaviour is that of pure metal. The fundamental passivation reaction is described as a potential dependent equilibrium between adsorbed Me(II)- and passivating Me(III)-hydroxide, water molecules being directly involved in the formation of this primary passivating film. In the case of stainless steel this primary passivating film mainly consists of chromium (III) adsorbates. Finally, a general model for the passivation is proposed: The passivation of a pure metal or of an alloy can be understood as the coupling of the stepwise deprotonation of the water molecules at the interface metal/solution and the formation of a high cation charge density in this adsorbed hydroxide/oxide film to build up the passive layer. The effect of water content, pH, adding of passivating species to the solution or the alloying with chromium on the passivation potential and the critical current density thus can be explained.     

Kinetics of Pit Initiation at Passive Iron

Pit initiation at passive iron in borate and phthalate buffer solutions was investigated by separately measuring dissolution rates of Fe(III) and Fe(II) at rotating ring-dis electrodes. The rate of Fe(III) dissolution grow linearly with the chloride concentration. Fe(II) appears simultaneously with the first pits. At a constant electrode potential positive to the critical pitting potential a stage of pit initiation is observed ending at the time t_i of incubation after addition of chloride to the solution. The time t_i is independent of the time of prior passivation. The potential dependence of t_i described by a relation known from the theory of two-dimensional nucleation. The physical meaning of this formal analogy is discussed. According to the dependence of t_i on pH and chloride concentration the observed mechanism of pit initiation is possible only above a critical chloride concentration c=0.3 _mM and below a critical _pH 10.4. The absolute values of critical pitting potentials and the dependence of the critical pitting potentials on chloride concentration were found to be a function of the nature and the concentration of the supporting electrolyte. The influence of the supporting electrolyte on the time of incubation was also investigated.     

Effect of products of nitric acid reduction on active dissolution and passivation of nickel

For nickel in HNO₃ solutions, the passivation process is impeded and the range of its active dissolution is broadened, as opposed to in solutions of nonoxidative acids, which is stipulated by the interaction processes between metal surface and the products of cathode reduction of the solution. It has been shown that the products of cathode reaction promote the occurrence of a new electronegative reaction of nickel dissolution in the form of a complex with ammonia, the ligand formed on the cathode. Moreover, the intermediate products of NO reduction to NO₂ by interacting with nickel surface displace passivating oxygen and, therefore, retard the process of electrode passivation. The kinetic control of the cathode reaction of HNO₃ reduction within the potential range of active dissolution of nickel is connected with the complicated NO desorption from nickel surface, which virtually eliminates the possibility of the development of homogeneous autocatalysis within a certain potential range.     

Der Einfluß von Wasser and gelösten Gasen auf das Korrosionsverhalten des Eisens in flüssigem Ammoniak

The influence of water and dissolved gases on the corrosion behaviour of iron in liquid ammonia Oxygen increases the corrosion rate of active iron in liquid ammonia at + 20°C and passivates iron at concentrations exceeding a critical concentration which increases with the acidity. Water acts as an acid and increases the corrosion rate of passive iron up to 0.1 wt. %. At higher concentrations of water, corrosion is inhibited probably because of the precipitation of hydrated iron (III) oxide. Carbon dioxide always enhances the corrosion rate of passive iron acting as a strong acid and a strong complexing agent for iron (III). At high concentrations of carbon dioxide or ammonium carbaminate iron cannot be passivated.     

全息显微摄影术在金属腐蚀研究中的一些应用

利用全息显微摄影术配合电化学现场测试,研究了酸性介质中铁的阳极溶解、振荡、钝化、去钝化过程以及在氯离子存在下形成孔蚀时电极/溶液界面发生的变化.结果表明:全息显微摄影术可以现场观察电极/溶液界面发生的动态过程,所获得的有关信息可以从固/液界面状态变化的角度深化对电极过程本质的认识.     

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.     

The corrosion and passivation of iron in the presence of halide ions in aqueous solution

The anodic dissolution behaviour of iron in halide solutions has been studied with both stationary and rotating electrodes. With stationary electrodes active dissolution kinetics are observed, whereas with rotation a pronounced active/passive transition occurs. A distinct pitting potential (E_c) was noted in each solution, the value of E_c increasing in the order I>Br>Cl>F. Halide ion concentration and electrode velocity did not have any effect on the value of E_c, indicating that the kinetics of pit initiation are independent of mass-transfer effects.During anodic dissolution at potentials more negative than E_c, an inhibiting effect was noted, the degree of which depended on the atomic radius of the anion. A model is suggested which involves three electrode reactions of iron with the electrolyte: (1) Active dissolution involving the well-known FeOH⁺ (ads) rate-determining step. (2) Above the passivation potential, increased reaction of the metal surface with hydroxyl ions causes passivation due to the enhanced access of OH^? to the surface and accelerated removal of solvated protons caused by rotation and a thinning of the diffusion layer. (3) At the pitting potential, direct reaction of the metal with electro-adsorbed halide ions produces pit initiation and growth by a complex ion formation reaction not possible at lower electrode potentials.     

The corrosion and passivation of iron in the presence of halide ions in aqueous solution

The anodic dissolution behaviour of iron in halide solutions has been studied with both stationary and rotating electrodes. With stationary electrodes active dissolution kinetics are observed, whereas with rotation a pronounced active/passive transition occurs. A distinct pitting potential (E_c) was noted in each solution, the value of E_c increasing in the order I>Br>Cl>F. Halide ion concentration and electrode velocity did not have any effect on the value of E_c, indicating that the kinetics of pit initiation are independent of mass-transfer effects.During anodic dissolution at potentials more regative than E_c, an inhibiting effect was noted, the degree of which depended on the atomic radius of the anion. A model is suggested which involves three electrode reactions of iron with the electrolyte: (1) Active dissolution involving the well-known FeOH⁺ (ads) rate-determining step. (2) Above the passivation potential, increased reaction of the metal surface with hydroxyl ions causes passivation due to the enhanced access of OH^? to the surface and accelerated removal of solvated protons caused by rotation and a thinning of the diffusion layer. (3) At the pitting potential, direct reaction of the metal with electro-adsorbed halide ions produces pit initiation and growth by a complex ion formation reaction not possible at lower electrode potentials.     

A Mechanism of the Anodic Dissolution of Armco Iron and High-Strength Ferritic Cast Iron in an Oxalate Medium

The kinetics of the anodic dissolution of Armco iron and high-strength ferritic cast iron is investigated in an oxalic-acid medium at different pH values. It is shown that passivation occurs with the participation of secondary and electrochemical mechanisms and the formation of ferrous oxalate film. It is ascertained that hydro-oxalate ions participate in the electrode process with the reaction order equal to 0.5. The proposed mechanism of anodic dissolution of Armco iron and high-strength ferritic cast iron includes the stages of the formation of passivating film and Fe²⁺ions. The latter, accumulating in a near-electrode layer, promote the passivation of metal by the mechanism of secondary crystallization.     

Zur Kinetik der Reaktionsschichtbildung an Reinsteisen bei mechanischer Bearbeitung (Simulation von Erosionskorrosion)

Contribution to the kinetics of the formation of reaction layers on high purity iron during mechanical machining (simulation of erosion-corrosion) Under the effect of working with impact bodies passivation kinetics of high purity iron in neutral phosphate electrolytes is considerably modified. Under potentio-dynamic conditions a reaction layer is formed by the mechanical treatment prior to the passivation layer. The kinetics of formation of this layer is studied in terms of current density vs. time at constant potential. In this connection the time dependence is derived for the different partial currents. The mechanical working initiates a two-dimensional proressive nucleation which can be interpreted in terms of reactions between freshly generated iron surfaces with the electrolyte.