Nickel dissolution

Datha, M. and Landolt, D., Stoichiometry of Anodic Nickel Dissolution in NaCl and NaC103 Under Active and Transpassive Conditions , Corros. Sci., 13, 187 (1973)... 

Figure 45. Semilog plot of the pit-growth current, J vs. V = 50 mV, [NiCU = 5 mol in 3, (NaCIJ = 1000 mol m 3. T=300 K, J0 is the current component shown in Eq. (Ill), which becomes unstable at the minimum state and Iq is the growth factor of the pits expressed by Eq. (112). (Reprinted from M. Asanuma and R. Aogaki, Nonequilibrium fluctuation theory on pitting dissolution. III. Experimental examinations on critical fluctuation and its growth process in nickel dissolution, J. Chem. Phys. 106,9944, 1997, Fig. 14. Copyright 1997, American Institute of Physics.)...

Rubisov, D. H. Krowinkel, J. M. Papangelakis, V. G. Sulphuric acid pressure leaching of laterites—universal kinetics of nickel dissolution for limonites and limonitic/saprolitic blends. Hydrometallurgy 2000, 58, 1-11. 

Figure 11-15 shows the corrosion rate observed for a metallic nickel electrode in aerated aqueous sulfate solutions as a function of pH. In addic solutions, nickel corrodes in the active state at a rate which is controlled by the diffusion of hydrated oi en molecules (oxidants). In solutions more basic than pH 6, however, nickel spontaneously passivates by hydrated oiQ n molecules and corrosion is negligible. As shown in the inserted sub-figures in Fig. 11-15, the maximum current of anodic nickel dissolution in the active state is greater in the range of addic pH however, the Tnaximnm current of anodic nickel dissolution is smaller in the range of basic pH than the current of cathodic reduction of os en molecules (dashed curve) which is controlled by the diffusion of hydrated oiQ gen molecules. Consequently, metallic nickel remains in the active state in addic solutions but is spontaneously passivated by hydrated ojQ n molecules in basic solutions. It... 

The structure of complex behavior in anodic nickel dissolution was analyzed and... 

In [57], investigations of nickel dissolution in sulfuric acid solution have been performed by using an electrochemical microgravimetry (EQCM). On the basis of... 

EDS study at location B, at the bottom of a pit showed that location was mainly composed of Cu and Ni with a small amount of Fe, which could be attributed to contamination since Fe was not detected in some other pits. The EDS result indicates no denickelification inside the pit since both Ni and Cu were found, and the crystals appear to be compact with no evidence of any copper crystal deposit or selective nickel dissolution leaving a porous structure. It should also be noted that there was no corrosion product at the bottom of the pit, and there was clear evidence of copper redeposit at the edge of the pit, as indicated in EDS of the copper and oxygen peaks. 

For example, it is possible to find with the help of the STM images that the highly crystalline nature of the oxide film on the nickel surfaces is Ni(100) in the alkaline solutions [46]. At low potentials, a well-ordered rhombic structure is formed, which is resistant to reduction and is assigned to the irreversible Ni(OH)2 formation. At higher potentials, it is possible to see a quasi-hexagonal structure consistent with NiO(lll). In other words, a crystalline oxide is formed of the NiO(lll) order independently of the crystal orientation of nickel. Moreover, it is very useful and practical to obtain a reduced surface by the cathodic treatments, where the hydrogen evolution produces the chemical and electrochemical reductions of nickel. However, this has to be done with careful attention and in light alkaline solutions to avoid nickel dissolution. When this is performed, monoatomic steps mostly oriented in the (100) and (111) directions are observed [47]. 

In comparison with the ionic solid corrosion discussed earlier, it is also worth noting that nickel ion transfer from the film into the solution is assumed to control the transpassive nickel dissolution whose rate increases with increasing interfacial potential. We may also see that the rate-determining process changes from the metal ion transfer to the oxide ion transfer near the oxygen evolution potential, beyond which the dissolution rate of the transpassive oxide film decreases with increasing interfacial potential, AH. 

The slow pH increase has been attributed to nickel dissolution due to the acidification of the medium following ... 

Area Ratio Cr/Ni (Cr area = 6.3 cm ) Anodic Current Density for Nickel Dissolution (mA/cm )... 

One of the major problems with the MCFC is that the nickel oxide state-of-the-art cathode material has a small, but significant, solubility in molten carbonates. Through dissolution, some nickel ions are formed in the electrolyte. These then tend to diffuse into the electrolyte towards the anode. As the nickel ions move towards the chemically reducing conditions at the anode (hydrogen is present from the fuel gas), metallic nickel can precipitate out in the electrolyte. This precipitation of nickel can cause internal short-circuits of the fuel cell with subsequent loss of power. Furthermore, the precipitated nickel can act as a sink for nickel ions, which promotes the further dissolution of nickel from the cathode. The phenomenon of nickel dissolution becomes worse at high CO2 partial pressures because of the reaction... 

With state-of-the-art nickel oxide cathodes, nickel dissolution can be minimised by (1) using a basic carbonate (2) operating at atmospheric pressure and keeping the CO2... 

Passive and Transpassive Dissolution of Nickel in Acidic Solutions The kinetics of nickel dissolution in the passive and transpassive ranges M remained totally unclear until the application of a very low frequency impedance technique. A general model was proposed on the basis of an extensive study of anion effects [143]. In the passive state, the fiequency domain had to be extended far below ImHz and long-term stability was obtained only by using single-crystal electrodes [144]. 

However, closer examination showed that the Cu-Ni system, immediately after the interruption of current, acquires an open circuit potential (or a mixed potential) which hes above the nickel reversible potential and below the copper reversible potential. In addition, the pH of these solutions, i.e. pH = 4, does not allow the formation of nickel oxides on the surface of the nickel. Therefore, in accordance with the mixed potential theory, copper continues to deposit at the expense of nickel dissolution, forming a classic galvanic corrosion cell. This process does not... 

Further research was carried out to determine whether Ni passivated quickly when a passive potential was imposed on it. This was performed by driving the nickel potential to the regime where nickel oxide is ejqtected to form (Meuleman et al, 2002). It was found that indeed nickel dissolution was blocked after one or two monolayers of nickel were lost, due to the faster passivation kinetics (Meuleman et al, 2002). It was found that the interfaces between the copper and nickel layers were sharper, which also corroborates that passivation was rapid (Meuleman et al, 2004), and it has been estimated that almost 50% less nickel was lost by oxidation (Meuleman et al, 2002). 

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