Nickel potential diagram

Similar current-potential diagrams were obtained for nickel. The current-potential curve for Ni is shown in Figure 10.16. The passive layer of Ni behaves like a p-semiconductor. At 0.85 V NiO is oxidized to NijOj and is thus connected with a steady increase in corrosion rate (Figure 10.16). At higher potentials a further oxidation to Ni02 with a new decrease in current is observed. Additionally oxygen evolution starts. 

Fig. 1.18 Modified potential-pH diagram for the Ni-H20 system the curves showing the stability of the nickel oxides have been extrapolated into the acid region to indicate the formation of metastable oxides (after De Gromoboy and Shreir")...

Nickel occupies an intermediate position in the electrochemical series Ni2 /Ni = -0-227 V, so that it is more noble than Zn and Fe but less noble than Sn, Pb and Cu. Figure 4.21 shows a revised potential-pH equilibrium (Pourbaix) diagram for the Ni-H O system at 25°C. The existence of the higher anhydrous oxides Nij04, NijO, and NiOj shown in an earlier diagram appears doubtful in aqueous systems in the absence of positive identification of such species. It is seen that ... 

Fig. 12.4 Corrosion diagram for a zinc diecasting in a nickel plating bath, pH 2-2. There are two possible cathodic reactions, hydrogen evolution (H) and nickel ion reduction (AO. The corrosion current is the sum of the partial cathode currents. Even with live entry the potential is still too high to suppress corrosion, though the rate is reduced to...

Figure 3.2. Potential energy diagram of chemisorption for the adsorption of hydrogen on nickel (after Le Page, 1987).

It is important to recognize some of the limitations of the Pourbaix diagrams. One factor which has an important bearing on the thermodynamics of metal ions in aqueous solutions is the presence of complex ions. For example, in ammoniacal solutions, nickel, cobalt, and copper are present as complex ions which are characterized by their different stabilities from hydrated ions. Thus, the potential-pH diagrams for simple metal-water systems are not directly applicable in these cases. The Pourbaix diagrams relate to 25 °C but, as is known, it is often necessary to implement operation at elevated temperatures to improve reaction rates, and at elevated temperatures used in practice the thermodynamic equilibria calculated at 25 °C are no longer valid. 

In other papers by the same group, the effects of sulfur adsorbed or segregated on the Ni surface on corrosion or passivation were described, including the sulfur-induced enhancement of dissolution and the blocking of passivation. It was shown how the conditions of stability of adsorbed sulfur monolayers could be predicted on thermodynamical grounds and this was illustrated by a potential-pH diagram for adsorbed sulfur on nickel in water at 25 °C. (See Refs. 22, 25-29 and papers cited therein.)... 

Of the Group 10 elements, nickel, palladium and platinum, only the +2 states of Ni and Pd are well characterized in aqueous acid solutions. Their + 2/0 standard reduction potentials in acid solution are given in the Latimer diagram ... 

Figure 32 Schematic electron density-of-states diagrams for electrochromic, EC, multilayer design. The materials include ln203 Sn (ITO), nickel oxide (presiunably hydrous), tungsten oxide (also presumably hydrous) prepared so that the EC and chemically protective (PR) properties are emphasized, and an electrol)de. The Fermi energy is denoted Ep, with Epi and Ep2 pertaining to the case of an applied potential, Ucoi fiUed states are denoted by shadings. (Ref 235. Reproduced by permission of Springer Verlag)...

FIGURE 21.18 Potential-pH equilibrium diagram for the nickel-water system at 25 °C (from Ref 30). 

In terms of the Pourbaix potential/pH diagrams, the theoretical scale compares the potentials of immunity of the different metals, while the practical scale compares the potentials of passivation. But this is not enough either. The real scale depends on the environment with which the structure will be in contact during service. Passivity, as we have seen, depends on pH. It also depends on the ionic composition of the electrolyte, particularly the concentration of chloride ions or other species that are detrimental to passivity. Finally, one must remember that construction materials are always alloys, never the pure metals. The tendency of a metal to be passivated spontaneously can depend dramatically on alloying elements. For example, an alloy of iron with 8% nickel and 18% chromium (known as 304 stainless steel) is commonly used for kitchen utensils. This alloy passivates spontaneously and should be ranked, on the practical scale of potentials, near copper. If... 

Potential energy diagram for the chemisorption of hydrogen on nickel. 

They measured the zero potential point of the system, which depends on the halide concentration and type as described by the equations above. They were able to measure the effect of diffusion of species to the electrode and were able to determine a mixed potential of the electrode where the forward transfer of electrons from the silver was equal to the rate of reduction of the oxidant. This is shown diagram-matically in Figure 31. Nickel et al. found that the adsorption of halide to the silver surface did not influence the potential if the halide layer was less than 5 pm thick, but they did note that at a clean silver surface the silver halide formation began at a higher potential than expected, which they attributed to the high solubility of microscopic silver on the surface. 

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. 

Figure 1 Energy diagrams showing the relative positions of the silicon bandgap and the chemical potential of platinum ions (left) and nickel ions (right) in fluoride solutions.

Based on thermodynamic quantities, derived from the experimental investigation of the solubility of nickel sulphide performed by Thiel and Gessner [14TH1/GES], solubility constants as well as Pourbaix diagrams (redox potential plotted V5. pH) have been calculated. 

The potential-pH diagram of nickel is shown in Fig. 20. In acidic solutions, dissolution to Ni " " can appear at potentials above the equilibrium potential. In alkaline solutions, HNi02 ions can be formed. Various oxides and hydroxides can be formed on nickel, for example, Ni(OH)2, Ni304, Ni203, and Ni02. In contradiction to the thermodynamic expectations, nickel... 

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