Anodic film formation

Duncan and Frankenthal report on the effect of pH on the corrosion rate of gold in sulphate solutions in terms of the polarization curves. It was found that the rate of anodic dissolution is independent of pH in such solutions and that the rate controlling mechanism for anodic film formation and oxygen evolution are the same. For the open circuit behaviour of ferric oxide films on a gold substrate in sodium chloride solutions containing low iron concentration it is found that the film oxide is readily transformed to a lower oxidation state with a Fe /Fe ratio corresponding to that of magnetite . 

Addition of anions to NaOH solution affects the anodic film formation kinetics and morphology. These films are mainly a-PbO, with the exception of chloride ions solution, where 8-PbO is formed [157]. Tknodic oxidation of Pb electrode in hot alkaKne solution (containing NaOH) facilitates selective growth of a-PbO, /3-PbO, and Pb02- (x = 0-1) phases, depending... 

A special kind of anodic film formation is called passivation and it is itself the best example of anodic protection. So important has passivation been over the years, and so puzzling its mechanism, that the subject (which implicitly presents anodic inhibition) will be treated in a separate section (12.4). 

A field-ion microscope study of anodic film formation on platinum in 0-05 M sulphuric acid between +0-5 and +2-2 V gave similar results (Schubert et al., 1973). 

Nucleation and growth kinetics — Nucleation-and-growth is the principal mechanism of phase transformation in electrochemical systems, widely seen in gas evolution, metal deposition, anodic film formation reactions, and polymer film deposition, etc. It is also seen in solid-state phase transformations (e.g., battery materials). It is characterized by the complex coupling of two processes (nucleation and phase growth of the new phase, typically a crystal), and may also involve a third process (diffusion) at high rates of reaction. In the absence of diffusion, the observed electric current due to the nucleation and growth of a large number of independent crystals is [i]... 

Figure 1.43 SEM cross-sections through Nb/Nb205/Mn02 capacitor structures proving the uniform, texture independent anodic film formation on Nb, dfCa. 90 nm [117].

Three main types of such systems are possible (a) anodic dissolution without anodic film formation, (b) unlimited accumulation of low-valence compounds at the anode up to complete breakdown of the whole technological process, and (c) the electrolytic process proceeding in the presence of an anodic film of a certain stationary thickness. 

Thompson G.E. and Wood G.C. (1981), Rorous anodic film formation on aluminium . Nature, 290,230-32. 

Factors influencing the morphology of anodic film. Formation of well-defined nanotubular struetures depends on a number of factors such as the applied potential, the temperature of the electrolyte, the concentration of fluorides, Regarding the fluoride concentration, if very little... 

J.K.G. Panitz, D.J. Sharp, The effect of different alloy surface compositions on barrier anodic film formation, J. Electrochem. Soc. 131 (10) (1984) 2227. 

The assessment for nonalloyed ferrous materials (e.g., mild steel, cast iron) can also be applied generally to hot-dipped galvanized steel. Surface films of corrosion products act favorably in limiting corrosion of the zinc. This strongly retards the development of anodic areas. Surface film formation can also be assessed from the sum of rating numbers [3, 14]. 

The anodes are generally not of pure metals but of alloys. Certain alloying elements serve to give a fine-grained structure, leading to a relatively uniform metal loss from the surface. Others serve to reduce the self-corrosion and raise the current yield. Finally, alloying elements can prevent or reduce the tendency to surface film formation or passivation. Such activating additions are necessary with aluminum. 

Measuring electrodes for impressed current protection are robust reference electrodes (see Section 3.2 and Table 3-1) which are permanently exposed to seawater and remain unpolarized when a small control current is taken. The otherwise usual silver-silver chloride and calomel reference electrodes are used only for checking (see Section 16.7). All reference electrodes with electrolytes and diaphragms are unsuitable as long-term electrodes for potential-controlled rectifiers. Only metal-medium electrodes which have a sufficiently constant potential can be considered as measuring electrodes. The silver-silver chloride electrode has a potential that depends on the chloride content of the water [see Eq. (2-29)]. This potential deviation can usually be tolerated [3]. The most reliable electrodes are those of pure zinc [3]. They have a constant rest potential, are slightly polarizable and in case of film formation can be regenerated by an anodic current pulse. They last at least 5 years. 

In addition, with high solid content of the cooling water and at high flow velocities, severe corrosive conditions exist which continuously destroy surface films. Cathodic protection alone is not sufficient. Additional measures must be undertaken to promote the formation of a surface film. This is possible with iron anodes because the anodically produced hydrated iron oxide promotes surface film formation on copper. 

Passivating inhibitors act in two ways. First they can reduce the passivating current density by encouraging passive film formation, and second they raise the cathodic partial current density by their reduction. Inhibitors can have either both or only one of these properties. Passivating inhibitors belong to the group of so-called dangerous inhibitors because with incomplete inhibition, severe local active corrosion occurs. In this case, passivated cathodic surfaces are close to noninhibited anodic surfaces. 

Anodizing—the formation of a hard, corrosion-resistant oxide film on metals via anodic oxidation of the metal in an electrolytic solution. 

It must be emphasised that although, the rate of anodic dissolution of iron increases with,increase in. pH this will not necessarily apply to the corrosion rate which will be dependent On a number of other. factors, e.g. the thermodynamics and kinetics of the cathodic reaction, film formation, etc. 

In practice the danger of aerated systems becomes apparent when the temperature is above. a certain minimum, for there is no passive film formation, and it is clear that anodic protection cannot be effective in these circumstances. 

As indicated when discussing anodic behaviour the mechanism of film formation is complex, involving adsorption of OH ions to form a prepassive layer followed by either dissolution or film formation as alternative processes. 

It is now well established that in lithium batteries (including lithium-ion batteries) containing either liquid or polymer electrolytes, the anode is always covered by a passivating layer called the SEI. However, the chemical and electrochemical formation reactions and properties of this layer are as yet not well understood. In this section we discuss the electrode surface and SEI characterizations, film formation reactions (chemical and electrochemical), and other phenomena taking place at the lithium or lithium-alloy anode, and at the Li. C6 anode/electrolyte interface in both liquid and polymer-electrolyte batteries. We focus on the lithium anode but the theoretical considerations are common to all alkali-metal anodes. We address also the initial electrochemical formation steps of the SEI, the role of the solvated-electron rate constant in the selection of SEI-building materials (precursors), and the correlation between SEI properties and battery quality and performance. 

CV of solutions of lithium bis[ salicy-lato(2-)]borate in PC shows mainly the same oxidation behavior as with lithium bis[2,2 biphenyldiolato(2-)-0,0 ] borate, i.e., electrode (stainless steel or Au) passivation. The anodic oxidation limit is the highest of all borates investigated by us so far, namely 4.5 V versus Li. However, in contrast to lithium bis[2,2 -biphenyl-diolato(2-)-0,0 Jborate based solutions, lithium deposition and dissolution without previous protective film formation by oxidation of the anion is not possible, as the anion itself is probably reduced at potentials of 620-670 mV versus Li, where a... 

According to Sato et al.,6,9 the barrier-layer thickness is about 1.5 to 1.8 nm V-1, and increases to 3 nm around the oxygen-evolution potential. In Fig. 5, the scale of the electrode potential, Vrhe, is that of the reversible hydrogen electrode (RHE) in the same solution. The electrode potentials extrapolated from the linear plots of the potentials against the film thickness suggested that the potential corresponding to the barrier thickness equal to zero is almost equal to 0.0 V on the RHE scale, independent of the pH of the solution, and approximately agrees with the equilibrium potential for the oxide film formation of Fe or Fe. Therefore it is concluded that the anodic overpotential AE applied from the equilibrium potential to form the oxide film is almost entirely loaded with the barrier portion. 

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