Nickel anodic polarization

Fig. 11-10. Anodic polarization curves observed for metallic iron, nickel, and chromium electrodes in a sulfuric acid solution (0.5 M H 2SO 4) at 25°C solid curve = anodic metal dissolution current dot-dash curve s anodic oxygen evolution current [Sato-Okamoto, 1981.]...

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.]...

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... 

If, for instance, nickel is polarized in a solution of sulphuric acid with gradually increased anodic potential, the metal first dissolves in the form of bivalent ions in exact agreement with Faraday s law (Fig. 31, curve AB). At a definite moment... 

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. 

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. 

Anodic polarization curves determined potentiostatically for three low index faces cut from a nickel monocrystal grown paral-N H2S04 at 22-23 °C. Redrawn from Ref 15... 

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... 

Anodic polarization curves for nickel-chromium alloys in 1 N H2S04. Redrawn from Ref 1 3... 

The effect of pH on the polarization of iron is shown in Fig. 5.6. The effect ofpH on the polarization of type 304 stainless steel (nominally 18 to 20 wt% Cr, 8 to 10.5 wt%Ni, 0.08 wt% C maximum) in environments based on 1 M Na2SC>4 with additions of H2SO4 and NaOH to control the pH is shown in Fig. 5.31 (Ref 28). The influence of chromium and nickel in moving the anodic polarization curve of iron to lower current densities persists over the indicated pH range with the corrosion rates being very low for pH >4.0. 

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-... 

Fis. 5.38 Anodic polarization of chromium-nickel binary alloys in 1 N H2S04 + 1 N NaCl. Redrawn from Ref 13... 

The approximate anodic polarization curves for iron, nickel, chromium, and titanium in 1 N H2SO4 are shown in Fig. 5.42. The cathodic reactions are for the environments shown and are representative of curves obtained on platinum. Since they may be displaced significantly when the reactions occur on the other metal surfaces, particularly the shift of the oxygen curves to lower potentials and current densities, the following discussion is qualitative. The conclusions drawn, however, are consistent with observations on the actual metal/environment systems. 

Nickel has a very small effect on the anodic polarization behavior of iron, and hence, iron-nickel alloys are of minor significance as corrosion-resistant alloys. However, the addition of nickel to iron-chromium alloys (AISI 200 series) permits conversion of the latter as ferritic alloys to austenitic iron-chromium-nickel alloys (AISI 300 series). In... 

A schematic summary of the alloying metals that affect the anodic polarization curve of stainless steel is shown in Fig. 4.16. The addition of 8% nickel to an alloy containing 18% chromium forms austenitic structure SS Type 304. The addition of Mn and N increases the stability of austenitic steel. The chromium content of stainless steel affects the anodic polarization curves as shown in Fig. 4.16. Nickel promotes repassivation in a corrosive environment, but concentrations higher than 30% reduces the passivation current, the critical current density, and increases the critical pitting potential. Nitrogen... 

K. Osozawa, H.. Engel, The anodic polarization curves of iron-nickel-chromium alloys, Corros. Sci. 6 (1966) 389-393. 

One can consider that the equilibrium potential for nickel Cgq, N, = —0.35 V vs. SHE represents a point on the Fe-Ni couple anodic polarization hne. To estimate the total anodic current of the couple at this potential, it is necessary to calculate the iron anodic current contribution at this point. Thus, by applying an overvoltage, i] = — 0.35 — (—0.44) to Eq. (E6.9), one can calculate the iron contribution to the overall current density of the couple ... 

A passive metal is one that is active in the Emf Series, but that corrodes nevertheless at a very low rate. Passivity is the property underlying the useful natural corrosion resistance of many structural metals, including aluminum, nickel, and the stainless steels. Some metals and alloys can be made passive by exposure to passivating environments (e.g., iron in chromate or nitrite solutions) or by anodic polarization at sufficiently high current densities (e.g., iron in H2SO4). 

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