Anodic polarization of nickel

Anodic polarization of nickel-chromium binary alloys in 1 N H2S04 + 1 N NaCl. Redrawn from Ref 1 3... 

Kazantsev and co-workers studied the anodic polarization of nickel and manganese electrodes in the melt at different temperatures. The ionic species in the melt were suggested to be Ni +, Ni+, and Mn +. The fraction of Ni + ions increased with increasing current density and with decreasing temperature. The claim for the presence of Ni+ ions is not in agreement with the emf measurements on the NF+/Ni couple and the voltammetric results. ... 

Such an activation of the electrode surface can, on the one hand, take place in situ by the continuous formation of the active redox agent on the electrode surface during the electrolysis. This is valid, for example, for the nickel(III)oxide hydroxide electrode which is spontaneously formed during anodic polarization of a... 

The anodic polarization of a given alloy base metal such as iron or nickel is sensitive to alloying element additions and to heat treatments if the latter influences the homogeneity of solid solutions or the kinds and distribution of phases in the alloy. The effect of chromium in iron or nickel is to decrease both EpP and icrit and hence to enhance the ease of placing the alloy in the passive state. The addition of chromium to iron is the basis for a large number of alloys broadly called stainless steels, and chromium additions to nickel lead to a series of alloys with important corrosion-resistant properties. 

Anodic Polarization of Iron-Chromium-Nickel Alloys... 

The effects of acid concentration and temperature on the anodic polarization of a commercial nickel-base alloy (Hastelloy C, nominal composition 54 wt%Ni, 2.5 wt% Co, 15.5 wt% Cr, 16 wt% Mo, 4 wt% W, 5.5 wt% Fe, 0.06 wt% C maximum) are shown in Fig. 5.32 (Ref 29). Qualitative conclusions from these curves indicate that the changes in corrosion rate on increasing the acid concentration from 1 to 10 N should be relatively small but that the effect of increasing the tempera-... 

Several investigators have used combined approaches, particularly in the in situ precipitation of active material in the pores of sintered substrates, using cathodic polarization and caustic precipitation in simultaneous or nearly simultaneous steps. A considerable amount of the reported information on the chemistry, electrochemistry, and crystal structure of the nickel electrode has been obtained on thin films (qv) made by the anodic corrosion of nickel surfaces. However, such films do not necessarily duplicate the chemical and/or crystallographic condition of active material in practical electrodes. In particular, the high surface area, space charge region, and lattice defect structure are different. Some of the higher (3.5+) valence state electrochemical behavior seen in thin films has rarely been reproduced in practical electrodes. 

Just as in the limiting case of ohmic control, the depth of the corrosion pit increases with the square root of the time. However, the proportionality constant here differs, it includes the saturation concentration rather than the potential and the diffusion coefficient rather than the electrolyte conductivity. Figure 7.64 shows experimental results for potentiostatic anodic dissolution of nickel in chloride solution. The geometry of the electrochemical cell corresponds to the one-dimensional pit model represented in Figure 7.62. The results show that, after a certain time, the current density decreases according to the square root of the polarization time, independent of potential. Mass-transport controlled growth of corrosion pits is favored by a highly electrolyte conductivity and weakly soluble dissolution products. 

Kuzin BL, Neuymin AD, Pal guyev SF (1973) Anodic polarization of thin nickel electrodes in contact with solid electrolyte in H2-1-H2O gas variable compositions. Elektrokhimiya (Rus)... 

Anodic polarization of active/passive metals - alloys of nickel, iron, chromium, titanium, and stainless steel in weak-to-extiemely corrosive environments, where economy in consumption of protective currents is required. 

Unlike the cathodic reaction, anodic oxidation (ionization) of molecular hydrogen can be studied for only a few electrode materials, which include the platinum group metals, tungsten carbide, and in alkaline solutions nickel. Other metals either are not sufficiently stable in the appropriate range of potentials or prove to be inactive toward this reaction. For the materials mentioned, it can be realized only over a relatively narrow range of potentials. Adsorbed or phase oxide layers interfering with the reaction form on the surface at positive potentials. Hence, as the polarization is raised, the anodic current will first increase, then decrease (i.e., the electrode becomes passive see Fig. 16.3 in Chapter 16). In the case of nickel and tungsten... 

De Souza et al. (1997) used spectroscopic ellipsometry to study the oxidation of nickel in 1 M NaOH. Bare nickel electrodes were prepared by a series of mechanical polishing followed by etching in dilute HCl. The electrode was then transferred to the spectroelectrochemical cell and was cathodicaUy polarized at 1.0 V vs. Hg/HgO for 5 minutes. The electrode potential was then swept to 0.9 V. Ellipsometry data were recorded at several potentials during the first anodic and cathodic sweep. Figure 27.30 shows a voltammogram for Ni in l.OM NaOH. The potentials at which data were recorded are shown. Optical data were obtained for various standard materials, such as NiO, a -Ni(OH)2, p-Ni(OH)2, p-NiOOH, and y-NiOOH. 

Fig. 11-13. Anodic polarization curve of a metallic nickel electrode in a sulfuric add solution transpassivation arises at a potential relatively dose to the flat band potential because of p-type nature of the passive oxide film. [From Sato, 1982.]...

Electrolysis prior to substrate addition with 1 mA/cra by changing the polarity of the nickel net from anode to cathode (period 5-10 sec) in a 0.1 N nickel sulfate, 0.1 N sodium acetate, 0.005 N sodium hydroxide solution until 0.5 Cb/cm are consumed. 

The other important factor to affect the operational conditions of the cell is the voltage increase between the carbon and copper lead. This problem has been solved individually in industry. For example, a 250 pm thick layer of nickel can be coated onto the upper part of the carbon anode using the atmospheric plasma spraying method.7 This electrode has been operated at 15 to 17 A dm-2 in a 1000 A scale industrial cell for 19 months. The cell voltage was 9.5 V and polarization did not occur with this electrode. Characteristic points of this new carbon electrode are low polarizability and no anode effect, and the concentration of carbon tetrafluoride contaminating the fluorine is below 2 ppm. 

The behavior of nickel on anodic polarization is matched by the behavior of iron and cobalt on the surface of which oxygen is also liberated at higher current densities. Chromium anode dissolves at low current densities to form bivalent cations. When it becomes passive its potential increases by about 1 Volt. With further inorease of potential chromium enters a state called transpassive state in which instead of bivalent ions hexavalent ions are formed which reaot with the hydroxyl ions present in the electrolyte to form chromate ions according to equation ... 

The properties of the interface metal/solution. Cast iron corrodes because of exposure of its graphite to the surface (graphitic corrosion), which is cathodic to both low-alloy and mild steels. The trim of a valve must always maintain dimensional accuracy and be free of pitting and hence it should stay cathodic to the valve body. Hence, in aggressive media, valve bodies are frequently chosen of steel rather than cast iron. Because of increased anodic polarization, low-alloy steel (Cr and Ni as noble components) is cathodic to normal steel in most natural media. Accordingly, steel bolts and nuts coupled to underground mild steel pipes, or a weld rod used for steel plates on the hull of a ship, should always be of a low-nickel, low chromium steel or from a similar composition to that of the steel pipe.7... 

Comparative anodic polarization data (Fig. 27) obtained by Conway and Liu (285-287) for chemically and anodically formed nickel oxide show a Tafel slope of 33 mV on anodically formed nickel oxide, lower than that for the chemically formed film (60 mV), with better activity than the former. Pseudocapacitance profiles obtained from an analysis of the potential relaxation data are shown in Fig. 28. The initial descending part of the C o versus rj profiles of Fig. 28a appears to be connected with the positive end of the well-known cyclic-voltammetry peak for Ni(II) - Ni(III) oxidation in nickel oxide. This peak goes into an ascending current versus potential line for O2... 

Most often, it is the anodic polarization behavior that is useful in understanding alloy systems in various environments. Anodic polarization tests can be conducted with relatively simple equipment and the scans themselves can be done in a short period of time. They are extremely useful in studying the active-passive behavior that many materials exhibit. As the name suggests, these materials can exhibit both a highly corrosion-resistant behavior or that of a material that corrodes actively, while in the same corrodent. Metals that commonly exhibit this type of behavior include iron, titanium, aluminum, chromium, and nickel. Alloys of these materials are also subject to this type of behavior. 

Some papers have appeared that deal with the use of electrodes whose surfaces are modified with materials suitable for the catalytic reduction of halogenated organic compounds. Kerr and coworkers [408] employed a platinum electrode coated with poly-/7-nitrostyrene for the catalytic reduction of l,2-dibromo-l,2-diphenylethane. Catalytic reduction of 1,2-dibromo-l,2-diphenylethane, 1,2-dibromophenylethane, and 1,2-dibromopropane has been achieved with an electrode coated with covalently immobilized cobalt(II) or copper(II) tetraphenylporphyrin [409]. Carbon electrodes modified with /nc50-tetra(/7-aminophenyl)porphyrinatoiron(III) can be used for the catalytic reduction of benzyl bromide, triphenylmethyl bromide, and hexachloroethane when the surface-bound porphyrin is in the Fe(T) state [410]. Metal phthalocyanine-containing films on pyrolytic graphite have been utilized for the catalytic reduction of P anj -1,2-dibromocyclohexane and trichloroacetic acid [411], and copper and nickel phthalocyanines adsorbed onto carbon promote the catalytic reduction of 1,2-dibromobutane, n-<7/ 5-l,2-dibromocyclohexane, and trichloroacetic acid in bicontinuous microemulsions [412]. When carbon electrodes coated with anodically polymerized films of nickel(Il) salen are cathodically polarized to generate nickel(I) sites, it is possible to carry out the catalytic reduction of iodoethane and 2-iodopropane [29] and the reductive intramolecular cyclizations of 1,3-dibromopropane and of 1,4-dibromo- and 1,4-diiodobutane [413]. A volume edited by Murray [414] contains a valuable set of review chapters by experts in the field of chemically modified electrodes. 

FIGURE 22.24 Anodic polarization curves for passivation and transpassivation of metallic iron and nickel in 0.5 kmol m-3 sulfuric acid solution with inserted sketches for electronic energy diagrams of passive films [32] /ip = passivation potential, TP = transpassivation potential, fb = flat band potential, /Fe = anodic dissolution current of metallic iron, Nl = anodic dissolution current of metallic nickel, and io2 — anodic oxygen evolution current. 

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