Polarization of the H2, CO, CO2|Ni and H2S, H2, CO, CO2|Ni electrodes in molten sodium carbonate

Reactions at the H₂,CO,CO₂,Ni and H₂S,H₂,CO,CO₂|Ni electrodes in molten sodium carbonate at 1000°C have been studied in detail by means of computer-assisted analysis of potentiostatic polarization curves. Calculated curves accounting for charge transfer polarization, concentration polarization, ohmic loss and passivation arc matched to experimental curves by a series of successive approximations. Kinetic parameters thus determined are interpreted via rate theory and hypotheses concerning the identity of individual electrode reactions. Previous work with the CO,CO₂|Ni electrode was satisfactorily modelled by five anodic reactions: oxidation of physically dissolved CO, oxidation of chemically dissolved CO, oxidation of Ni to Ni²⁺, oxidation of NiO to Ni₂O₃ and oxidation of carbonate, occurring at progressively increasing overpotentials. At the H₂,CO,CO₂|Ni electrode, in addition to these reactions (oxidation of physically dissolved CO was not seen at the P_co values employed) oxidation of H₂ was observed at low anodic overpotentials. These experiments also clearly delineated an additional reaction postulated to be oxidation of Ni₂O₃ to NiO₂. Reactions at the H₂S,H₂,CO,CO₂|Ni electrode were identical to those at the H₂,CO,CO₂|Ni electrode save that the diffusion-limited current for oxidation of chemically dissolved CO increased linearly with . This is attributed to the occurrence of chemically dissolved CO as a sulfide species, e.g. COHS⁻, in addition to the CO₂^2- found under CO, CO₂ atmospheres. In support of this mechanism, the reaction did not display diffusion control when 0.5% Na₂S was added to the electrolyte, suggesting a high level of, e.g., COS^2-. Total cell pressure was 1 atm; in the sulfide experiments P_H2S varied from 0.000027 to 0.027 atm, with P_{H2 = 0.5 atm} and P_co = P_co2 = 0.25 atm.     

Reactions at the CO,CO2/Ni electrode in molten sodium carbonate

Reactions at the nickel electrode in molten sodium carbonate at 1000°C under CO/CO₂ atmospheres were studied by electrode polarization. The polarization curves were analysed by means of a previously described computer model, which in turn was interpreted in terms of the absolute rate expressions for postulated reactions. In agreement with previous workers' observations for inert electrodes, the dominant anodic reactions were found to be oxidation of CO at low and intermediate anodic overpotentials, and oxidation of CO₃^2? at high anodic overpotentials. The cathodic reaction was found to be reduction of CO₃^2?. The form of the polarization curves was described by activation, concentration, and resistance polarization of these reactions; however, anomalous anodic behaviour was observed which was attributed to corrosion reactions at the electrode and accurately described through the use of additional anodic reactions displaying passivity.     

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.     

Passive film formed on nickel in a neutral solution

A bright nickel was passivated in the pH 8·39 boric acid-borate buffer solution. The oxide formation reactions were mainly investigated by means of polarization experiments, alternating anodic and cathodic polarization, potential decay experiments and colorimetric analysis. From the results, the chemical composition and structure of passive films formed on nickel were discussed. The experimental results are summarized as follows.1. A considerable amount of nickel ions dissolved during the anodic formation of passive films. The dissolution ceased because of the formation of NiO₂. 2. The passive films on bright nickel has a duplex structure consisting of NiO and Ni₃O₄. At higher potentials, NiO₂ was produced on them. NiO and Ni₃O₄ were formed directly from Ni, and NiO₂ was transformed from Ni₃O₄. 3. NiO and NiO₂ were reversibly produced and reduced, but Ni₃O₄ was very difficult to be reduced. 4. It was observed that there was a reversible charging and discharging layer at the potential where Ni₃O₄ formed. The electric capacity was calculated to be about 100 μF/cm², assuming that roughness factor was 2. It would be reasonable to think that a space charge was established on nickel surface.     

Chemical and phase composition of nanosized oxide and passive films on Ni-Cr alloys. II. XPS analysis of films produced by anodic passivation of alloys in 1 N H<Subscript>2</Subscript>SO<Subscript>4</Subscript>

XPS data of thin (1 to 2 nm) oxide films formed by the anodic passivation of Ni-2 at % Cr and Ni-6 at % Cr alloys in 1 N H₂SO₄ are discussed. Thermodynamic calculations of the solid-phase chemical reaction 3NiO + 2Cr = Cr₂O₃ + 3Ni are carried out taking into account the changes in the surface energy at the alloy-oxide film interface along with the Gibbs energy change in the alloy oxidation reaction.     

Passivity of polycrystalline NiMnGa alloys for magnetic shape memory applications

The corrosion and passivation behaviour of bulk polycrystalline martensite Ni₅₀Mn₃₀Ga₂₀ and austenite Ni₄₈Mn₃₀Ga₂₂ alloys was compared in electrolytes with different pH values. Linear anodic and cyclic potentiodynamic polarisation methods and anodic current transient measurements have been conducted for the alloys and their constituents to analyze free corrosion, anodic dissolution and passive layer formation processes. Electrochemically treated alloy surfaces were characterized with scanning electron microscopy (SEM) and angle-resolved x-ray photoelectron spectroscopy (XPS). The electrochemical response of both alloys is in principal similar and is dominated by the Ni oxidation. In acidic solutions (pH 0.5 and 5) a slightly higher reactivity is detectable for the martensitic alloy which is mainly attributed to enhanced dissolution processes at the multiple twin boundaries. In weakly acidic to strongly alkaline solutions (pH 5-11) both alloys exhibit a low corrosion rate and a stable anodic passivity. While air-formed films comprise NiOOH, Ga₂O₃ and MnO₂, passive films formed in near neutral media (pH 5-8.4) are composed of Ni(OH)₂, NiOOH and Ga₂O₃ in the outer region and of NiO, MnO₂ and MnO in the metal-near region.     

Studies on the anodic polarization of amalgams: The behaviour of nickel amalgams in alkaline solutions

Nickel amalgams of varying composition were oxidized in 0.1, 2, 4 and 6N NaOH solutions. Two different oxidation patterns were distinguished. The first describes the behaviour of concentrated nickel amalgams, and the second the behaviour of dilute nickel amalgams in NaOH solutions. The anodic curve for the concentrated amalgams showed regions for the charging of the anodic double layer, then oxidation of nickel and mercury, then oxygen evolution. NiO is first formed which is oxidized to Ni₃O₄, then to Ni₂O₃. For dilute amalgams the oxidation curves showed only one arrest corresponding to the system NiO/Ni₃O₄ before oxygen evolution. The relation between the polarizing current and the time of passivation was found to fit the equation

$\text{log}\text{τ = A ? n}\text{log}\text{i}$

, where A and n are constants. With cathodically pretreated electrodes the oxidation curves showed regions for the dissolution of the sodium amalgam formed during precathodization, the charging of the anode double layer and a prolonged indefinite arrest at the potential corresponding to the reaction

$\text{NiO + H}{}_2\text{O = Ni}{}_3\text{O}{}_4\text{+ 2H}{}^{\mathrm{+}}\text{+ 2e?}$

.     

The mechanism of high-temperature oxidation of directionally solidified Ni3AlNi3Nb eutectic

The isothermal oxidation behaviour of directionally solidified Ni₃AlNi₃Nb eutectic in 1 atm flowing air at high temperature has been examined. The alloy oxidizes relatively rapidly and develops an NiO external scale which thickens with time. At the same time, internal oxidation takes place, penetrating deeper into the Ni₃Nb lamellae than the Ni₃Al lamellae. Eventually, the alloy at the internal oxide/alloy interface becomes sufficiently depleted in aluminium and niobium that a single phase, nickel-rich alloy zone develops. Further internal oxide no longer has a direct relationship with the alloy lamellar structure. After long periods, sufficient aluminium and niobium diffuse to the internal oxide/alloy interface to enable complete, healing Al₂O₃ layers to develop. These layers retard further penetration of internal oxide into the alloy although they do eventually break down and allow this to occur. This oxidation behaviour is found to be reasonably independent of lamellar orientation.     

The corrosion of two NiNb alloys under 1 atm O2 at 600–800 °C

The oxidation of two NiNb alloys containing 15 and 30 wt% Nb has been studied at 600–800 °C in pure oxygen under 1 atm O₂ at 600–800 °C. The scales formed on both alloys under all conditions show an external scale, generally duplex, containing an outermost layer of nearly pure NiO and an innermost region of NiO mixed with the double NiNb oxide NiNb₂O₆. Moreover, the samples corroded at all temperatures also show a region of internal oxidation composed of a mixture of alpha nickel and niobium oxides (Nb₂O₅ or/and NbO₂), which formed from both alloy phases Ni₈Nb and Ni₃Nb. No important depletion of niobium was observed in the alloy close to the interface with the zone of internal oxidation, while the depth of this region is generally much higher than measured for the corrosion of the same alloys under low oxygen pressures at the same temperatures. The corrosion mechanism of these alloys is examined with special reference to the effects of the low solubility of niobium in nickel.     

In-Situ Electrochemical Corrosion Behavior of Nickel-Base 718 Alloy Under Various CO<Subscript>2</Subscript> Partial Pressures at 150 and 205 °C in NaCl Solution

The electrochemical corrosion behavior of nickel-base alloy 718 was investigated using electrochemical impedance spectroscopy and potentiodynamic polarization techniques at various partial pressures of CO₂ (\(P_{{{\text{CO}}_{2} }}\)s) in a 25 wt% NaCl solution at 150 and 205 °C. The passive films composed of FeCO₃ exhibit good corrosion resistance with a feature of Warburg impedance, Tafel plots show a complete passivation and the anodic reactions was dominated by a diffusion process at low \(P_{{{\text{CO}}_{2} }}\)s (1.8–9.8 MPa) at 150 °C. While numerous dented corrosion areas appeared on the sample surface for the \(P_{{{\text{CO}}_{2} }}\) of 11.6 MPa at 205 °C, the Tafel plot with three anodic peaks and the Nyquist diagram with an atrophied impedance arc were present. This dented corrosion attribute to the synergistic effects of stress, temperature, \(P_{{{\text{CO}}_{2} }}\) and Cl^?, the temperature and stress could play crucial roles on the corrosion of the alloy 718.     

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.     

Hot Corrosion Behavior of Inconel 625 Superalloy in Eutectic Molten Nitrate Salts

Corrosion resistance of Inconel 625 Ni-based superalloy was studied in a molten nitrate salt consisting of 40 KNO₃–60 NaNO₃ (wt%) at 500 and 600 °C. Open-circuit potential, potentiodynamic polarization, electrochemical impedance spectroscopy and gravimetric tests were used to evaluate the degradation mechanism and corrosion behavior of the alloy. Surface morphology and chemical analysis of corrosion products were characterized by means of scanning electron microscopy and energy-dispersive X-ray spectrometry. The weight-loss curves showed that with the increase in temperature, the oxidation rate and mass gain increased; the relationship between the mass gain and time was close to the parabolic oxidation law. The electrochemical corrosion results confirmed that during the exposure of Inconel 625 alloy to the molten salts, nickel dissolves as a result of non-protective NiO layer formed. The formation of a non-protective oxide layer with low barrier property was responsible for observing the weak corrosion resistance of the alloy at high temperatures (500 and 600 °C). Cyclic polarization tests showed a positive hysteresis confirming the nucleation and growth of stable pits on the surface of Inconel 625 at high anodic overpotentials. Sodium nitrite acts as an efficient pitting inhibitor for this case. In this way, the sodium nitrite with the concentration of 0.1 molal was found to have an optimum inhibition effect on pit nucleation at 600 °C.     

Passivation of Nickel in NaOH Solutions

The passivation behavior of Ni electrode in NaOH solutions is studied by cyclic voltammetry technique. Different experimental factors, such as, electrolyte concentration, voltage scanning rate and the sweeping potential range are examined. The cyclic voltammgrams data, Cvs, indicated a correlation between two well–defined anodic oxidation peaks and only cathodic peak. The first anodic peak was attributed to the oxidation of Ni to Ni(OH)₂, followed by a passive region corresponding to the transformation of Ni(OH)₂ to β-NiOOH. The second anodic peak was suggested to correspond to the oxidation of Ni(OH)₂ or NiO to some higher oxides of nickel. The cathodic branch of the cyclic voltammogram was characterized by only one cathodic peak, which was splitted into two sub peaks on increasing the concentration of NaOH. These peaks are thought to correspond to the reduction of (or part of) the products formed during the second anodic peak.     

The early stages of oxidation of ion-implanted nickel at high temperature

The early stages of oxidation of nickel implanted with nickel, chromium, or lithium ions in oxygen at 1100°C have been studied using various electron-optical techniques. The unimplanted metal develops initially a fine-grained, convoluted scale having a ridged, cellular structure. Subsequently, the oxide grains increase in size significantly and oxidation becomes predominantly controlled by diffusion of Ni ²⁺ ions across a compact, columnar scale. Implantation of the surface with nickel ions has no significant effect on the initial oxidation behavior. However, after implantation with chromium or lithium ions, the development of the NiO scale is, in the early stages of oxidation, suppressed by formation of NiCr ₂O₄ or LiO₂ nodules, respectively. Subsequently, the implanted species are incorporated into the steady-state NiO scale where they dope the oxide and thus influence the diffusion rate of Ni ²⁺ ions through it. As would be predicted, the steady-state oxidation rate of chromium-implanted nickel is increased while that of lithium-implanted nickel is decreased compared with that of the unimplanted metal.     

Influence of Cr on the Oxidation of Fe3Al and Ni3Al at 500°C

The influence of chromium on the mechanical properties of the aluminides Fe₃Al and Ni₃Al has been studied extensively. In order to evaluate the role of Cr during the early stages of oxidation, Fe₃Al and Ni₃Al containing 2 and 4 at.% Cr were oxidized in dry air at 500°C for 6, 50, and 100 hr. The oxide scale on Fe₃Al consists of a layer of Fe₂O₃ mixed with FeAl₂O₄ on top of a continuous layer of (Al, Cr)₂O₃. Ni₃Al is covered with a mixed layer of (Al, Cr)₂O₃ and NiO/NiAl₂O₄ underneath a layer of NiO/NiAl₂O₄. Moreover, Cr induces the nucleation and growth of Fe₂O₃ and NiO particles at the oxide surface of Fe₃Al and Ni₃Al, respectively. This is due to enhanced cationic diffusion through the Cr-modified oxides. As a conclusion, additions of Cr up to 4 at.% are detrimental to the oxidation behavior of both aluminides at 500°C.     

Investigating the Significance of Bicarbonate with the Corrosion of High-Strength Steel in CO2-Saturated Solutions

This paper presents an electrochemical study on the corrosion behavior of API-X100 steel, heat-treated to have microstructures similar to those of the heat-affected zones (HAZs) of pipeline welding, in bicarbonate-CO₂ saturated solutions. The corrosion reactions, onto the surface and through the passive films, are simulated by cyclic voltammetry. The interrelation between bicarbonate concentration and CO₂ hydration is analyzed during the filming process at the open-circuit potentials. In dilute bicarbonate solutions, H₂CO₃ drives more dominantly the cathodic reduction and the passive films form slowly. In the concentrated solutions, bicarbonate catalyzes both the anodic and cathodic reactions, only initially, after which it drives a fast-forming thick passivation that inhibits the underlying dissolution and impedes the cathodic reduction. The significance of the substrate is as critical as that of passivation in controlling the course of the corrosion reactions in the dilute solutions. For fast-cooled (heat treatment) HAZs, its metallurgical significance becomes more comparable to that of slower-cooled HAZs as the bicarbonate concentration is higher.     

The influence of silicon on the high-temperature oxidation of nickel

An investigation of the oxidation of nickel-silicon alloys has been carried out in order to ascertain the mode of development of partially or fully protective SiO₂ layers. The addition of 1% Si has little effect on the oxidation rate of nickel at 1000°C but is sufficient for partial-healing layers of amorphous SiO₂ to be established. These layers are incorporated into the inner part of the duplex NiO scale but do not react with the oxide to form a double oxide. Increasing the silicon concentration to 4% or 7% facilitates the development of apparently continuous amorphous SiO₂ layers at the base of the NiO scale, resulting in reduced rates of oxidation. However, these layers develop imperfections, possibly microcracks resulting from oxide growth stresses, and are unable to prevent some continued transport of Ni²⁺ ions into the NiO scale and oxygen into the alloy, particularly for Ni-4% Si. Although the formation of SiO₂-healing layers can reduce the rate of oxidation of nickel, they provide planes of weakness that result in considerable damage under the differential thermal contraction stresses during cooling. In particular, severe scale spalling occurs for Ni-4% Si and Ni-7% Si as failure occurs coherently within the SiO₂ layer.     

Anodic passivity of glassy Ni83B17 alloy in sulphuric acid

The anodic passivation of Ni₈₃B₁₇ glassy was investigated in sulphuric acid solutions with respect to the degree of proton hydration and the structure of acid. The possibility of passivation in methanolic solutions of sulphuric acid was also examined. The investigations involved electrochemical measurements, XPS and sulphur dioxide analyses. When acid concentration is low, the passivity of Ni₈₃B₁₇ is a result of the formation of an oxide layer in the presence of “free” water molecules (i.e. not bound to protons). In concentrated sulphuric acid, the passivation may occur as a result of the oxidative ability of acid connected with S⁶⁺ ion reduction. At intermediate acid concentrations a mixed salt-oxide layer is formed. The passivity of Ni₈₃B₁₇ was also observed in methanolic solutions of sulphuric acid, as a result of the oxidising ability of the acid.     

Material characterization and corrosion performance of nickel electroplated in supercritical CO2 fluid

The surface roughness, microhardness, crystal structure and corrosion performance of nickels electroplated at atmospheric pressure and in supercritical CO₂ fluid, respectively, are compared. The experimental results showed that nickel electroplated in the latter case exhibited a lower surface roughness and a higher hardness. In HCl solution, the nickel deposited in supercritical CO₂ fluid had a lower anodic current density and a higher polarization resistance. The advantage of emulsified supercritical CO₂ fluid for electroplating was demonstrated.     

Das Oxydationsverhalten von Nickel-Vanadium-Legierungen in Luft

The oxidation behaviour of nickel-vanadium alloys in air With oxidation tests carried out on pure nickel in air at 1000°C, a simple oxide film of NiO is formed. With the oxidation of nickel-vanadium alloys, however, several layers of different composition are formed. the outermost layer contains mainly NiO and a small content of nickel vanadate, Ni(VO₃)₂. Below it is a second oxide layer which has, on the outside, a strong concentration of vanadium. On the side facing the metal, this second layer has a low content of either metal, and is porous. This is followed by an inner oxidation zone which projects into the matrix in the form of conic islands with concentrations of V₂O₃. In the temperature range from 800 to 1200 °C, the scale constants indicating the reactions of the nickel-vanadium alloys are of an order of magnitude above that of unalloyed nickel. The oxidation reactions obey parabolic laws for the formation of the outer NiO layers with nickel and of NiO and Ni(VO₃)₂ with the nickel-vanadium alloys. The growth of the inner oxidation zones is governed by a logarithmic law. The activation energy of the oxidation in air, for nickel and for the nickel-vanadium alloys investigated, is of the order of magnitude of 50kcal/Mol.