Anode reactions with film formation

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. 

Passivity the state of a metal in which a low corrosion rate is brought about by reaction with its environment under a high anodic driving force through formation of a surface barrier film, usually an oxide. 

The formation of colloidal sulfur occurring in the aqueous, either alkaline or acidic, solutions comprises a serious drawback for the deposits quality. Saloniemi et al. [206] attempted to circumvent this problem and to avoid also the use of a lead substrate needed in the case of anodic formation, by devising a cyclic electrochemical technique including alternate cathodic and anodic reactions. Their method was based on fast cycling of the substrate (TO/glass) potential in an alkaline (pH 8.5) solution of sodium sulfide, Pb(II), and EDTA, between two values with a symmetric triangle wave shape. At cathodic potentials, Pb(EDTA)2 reduced to Pb, and at anodic potentials Pb reoxidized and reacted with sulfide instead of EDTA or hydroxide ions. Films electrodeposited in the optimized potential region were stoichiometric and with a random polycrystalline RS structure. The authors noticed that cyclic deposition also occurs from an acidic solution, but the problem of colloidal sulfur formation remains. 

The formation condition for PS can be best characterized by i-V curves. Figure 2 shows a typical i-V curve of silicon in a HF solution.56 At small anodic overpotentials the current increases exponentially with electrode potential. As the potential is increased, the current exhibits a peak and then remains at a relatively constant value. At potentials more positive than the current peak the surface is completely covered with an oxide film and the anodic reaction proceeds through the formation and dissolution of oxide, the rate of which depends strongly on HF concentration. Hydrogen evolution simultaneously occurs in the exponential region and its rate decreases with potential and almost ceases above the peak value. 

Studies by Heinze etal. on donor-substituted thiophenes or pyrroles [33] such as methylthio (= methylsulfonyl) or methoxy-substituted derivatives provide further clear evidence for this reaction pathway. They found, for instance, that 3-methylthiothiophene or 3-methoxythio-phene (2) undergo a fast coupling reaction. However, deposition processes or insoluble film formation could not be detected in usual experiments with these compounds, even at high concentrations. Similarly, the corresponding 3,3 -disubstituted bithiophenes (2a) do not polymerize, but the anodic oxidation of 4,4 -disubstituted bithiophenes (2c) produces excellent yields of conducting polymers. 

The swelling of the cathode (CFx) influences the discharge through the formation of a LiF precipitate (23). A film of LiCl is formed on the Li anode by its reaction with the depolarizer SOCI2 this causes the voltage delay during discharge (24). 

The formation of an anodic film on alkali metal anodes is mentioned in (129b). If it can be confirmed that Li is not unique in its reaction with water, as it is claimed in (123-124), then Na may also develop such a film in contact with H2O or non-aqueous liquids and so be protected. Design variations in Na-H20 primary batteries are described (130-132). 

A number of workers have studied this phenomenon [124,168 -170] and have concluded that the proper formation of this film determines the efficacy and reproducibility of the system, and that experimental data are consistent with a reaction controlled by the amount of surface area available on the anode film [171]. Also, the corrosion rate of nickel anodes is much lower in the presence of the organic reactant than the rate of corrosion in pure hydrogen fluoride, indicating that the anode reaction is modified by the presence of the organic. 

Fig. 4 shows a simple phase diagram for a metal (1) covered with a passivating oxide layer (2) contacting the electrolyte (3) with the reactions at the interfaces and the transfer processes across the film. This model is oversimplified. Most passive layers have a multilayer structure, but usually at least one of these partial layers has barrier character for the transfer of cations and anions. Three main reactions have to be distinguished. The corrosion in the passive state involves the transfer of cations from the metal to the oxide, across the oxide and to the electrolyte (reaction 1). It is a matter of a detailed kinetic investigation as to which part of this sequence of reactions is the rate-determining step. The transfer of O2 or OH- from the electrolyte to the film corresponds to film growth or film dissolution if it occurs in the opposite direction (reaction 2). These anions will combine with cations to new oxide at the metal/oxide and the oxide/electrolyte interface. Finally, one has to discuss electron transfer across the layer which is involved especially when cathodic redox processes have to occur to compensate the anodic metal dissolution and film formation (reaction 3). In addition, one has to discuss the formation of complexes of cations at the surface of the passive layer, which may increase their transfer into the electrolyte and thus the corrosion current density (reaction 4). The scheme of Fig. 4 explains the interaction of the partial electrode processes that are linked to each other by the elec-... 

Nonaqueous electrolyte solutions can be reduced at negative electrodes, because of an extremely low electrode potential of lithium intercalated carbon material. The reduction products have been identified with various kinds of analytical methods. Table 3 shows several products that detected by in situ or ex situ spectroscopic analyses [16-29]. Most of products are organic compounds derived from solvents used for nonaqueous electrolytes. In some cases, LiF is observed as a reduction product. It is produced from a direct reduction of anions or chemical reactions of HF on anode materials. Here, HF is sometimes present as a contaminant in nonaqueous solutions containing nonmetal fluorides. Such HF would be produced due to instability of anions. A direct reduction of anions with anode materials is a possible scheme for formation of LiF, but anode materials are usually covered with a surface film that prevents a direct contact of anode materials with nonaqueous electrolytes. Therefore, LiF formation is due to chemical reactions with HF [19]. Where does HF come from Originally, there is no HF in nonaqueous electrolyte solutions. HF can be produced by decomposition of fluorides. For example, HF can be formed in nonaqueous electrolyte solutions by decomposition of PF6 ions through the reactions with H20 [19,30]. 

Since the reaction rate at potentials above Vp is limited by the dissolution of oxide and the dissolution rate depends on the nature of the oxides (see Chapter 4), the change of current with potential indicates that oxide composition/structure varies with the formation. The nonstoichiometric composition of an anodically formed oxide film can be expressed as SiO with a higher value of n close to the oxide/electrolyte interface and a lower value of n close to the Si/oxide interface.A thicker oxide film has a bulk composition closer to the stoichiometric SiOa. The dissolution rate of anodic oxide depends on the composition of the electrolyte as shown in Fig. 5.46. " (also Fig. 5.7 ). It depends little on the type of material and doping levels. ... 

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