Rate determining processes anodic dissolution

It follows from equation 1.45 that the corrosion rate of a metal can be evaluated from the rate of the cathodic process, since the two are faradai-cally equivalent thus either the rate of hydrogen evolution or of oxygen reduction may be used to determine the corrosion rate, providing no other cathodic process occurs. If the anodic and cathodic sites are physically separable the rate of transfer of charge (the current) from one to the other can also be used, as, for example, in evaluating the effects produced by coupling two dissimilar metals. There are a number of examples quoted in the literature where this has been achieved, and reference should be made to the early work of Evans who determined the current and the rate of anodic dissolution in a number of systems in which the anodes and cathodes were physically separable. 

Cathodic corrosion inhibitors reduce the corrosion rate indirectly by retarding the cathodic process which is related to anodic dissolution. In this process, access to the reducible species such as protons, to electroactive site on the steel, is restricted. Reaction products of cathodic inhibitors may not be bonded to the metal surface as strongly as those used as anodic inhibitors. The effectiveness of the cathodic inhibitor is related to its molecular structure. Increased overall electron density and spatial distribution of the branch groups determine the extent of chemisorption on the metal and hence its effectiveness. Commonly used cathodic inhibitor materials are bases, such as NaOH, Na2C03, or NH4OH, which increase the pH of the medium and thereby also decrease the... 

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-... 

The etch rate of anodic oxide can be determined by methods similar to those for thermal oxide or deposited oxides. It may also be estimated from the anodic current of the oxidized electrode. The anodic i-V curve of a silicon electrode typically shows a passivation-like peak above which the dissolution occurs through a two-step process the formation of oxide film followed by the chemical dissolution of the oxide. The steady-state anodic current measured at an anodic potential above the peak potential indicates the dissolution rate of the anodic oxide. Thus, the passivation current, /p, listed in Table 5.5 can be used for estimation of the etch rate of the oxide film formed at the anodic potentials. (A current density of 1 mA/cm corresponds to a silicon etch rate of 3.1A/S or to a silicon oxide etch rate of about 7 A/s.) For example, in 1% HF solution, ip is 5mA/cm, and thus the etch rale of the oxide film formed at a potential anodic of the first current peak is about 35 A/s. hi 2M KOH solution at room temperature, ip == 0.002mA/cm equivalent to an etch rate of about 0.014 A/s. These numbers appear to be in general agreement with the data in Table 4.1. 

The electroless deposition of metals on a silicon surface in solutions is a corrosion process with a simultaneous metal deposition and oxidation/dissolution of silicon. The rate of deposition is determined by the reduction kinetics of the metals and by the anodic dissolution kinetics of silicon. The deposition process is complicated not only by the coupled anodic and cathodic reactions but also by the fact that as deposition proceeds, the effective surface areas for the anodic and cathodic reactions change. This is due to the gradual coverage of the metal deposits on the surface and may also be due to the formation of a silicon oxide film which passivates the surface. In addition, the metal deposits can act as either a catalyst or an inhibitor for hydrogen evolution. Furthermore, the dissolution of silicon may significantly change the surface morphology. 

Electroless deposition of Au in KAu(CN)2 -I- HF can be controlled by both the kinetic process and the diffusion process. The deposition is a two-step process, with initial diffusion-limited deposition of the intermediate species, followed by surface-limited reduction of this species. For electroless deposition of Pt, it has been reported that the rate-determining step is the deposition on n-Si, whereas it is the dissolution of silicon on p-Si. Electroless copper deposition does not occur on Si02-covered silicon surface due to the lack of anodic dissolution of silicon In a non-HF solution, the deposition of copper on a bare silicon surface results in the formation of oxide aroimd the metal particles. In HF solutions, the deposition of copper proceeds very slowly in the dark on both p-Si and n-Si samples due to the lack of carriers. The... 

Fundamentals. The composition of liquids with respect to both identity and concentration of dissolved species can be determined with inductively coupled plasma atomic emission spectrometry (ICP-AES) [972]. The employed spectrometer can be coupled directly with an electrochemical cell wherein processes like corrosion or anodic dissolution occur. Continuous aspiration of very small liquid volumes transferred into the spectrometer allows determination of rates of dissolution as a function of various experimental parameters like electrode potential [973]. 

Whether the total corrosion process is determined by the kinetics of anodic metal dissolution or the cathodic process depends on the size of the cathode and the kinetics of the partial electrode processes. The slowest reaction is the rate-determining step, as is usual in kinetics. In the case of a well-passivated valve metal, this is most probably the cathodic reaction, whereas for metals with semiconducting oxides, the rate-determining step win he anodic metal dissolution. In order to study the partial reactions of pitting corrosion separately, potentiostatic experiments are preferred. The cathodic process is replaced hy the electronic circuit of the potentiostat to investigate the anodic metal... 

For applications in adhesive bonding research or technology, ellipsometry is useful for the quantitative determination of film thicknesses. Especially aluminum is a metal that has been studied extensively. It lends itself well to oxide thickness measurements because AI2O3 is transparent, whieh is a requirement. The thickness of the oxide formed in certain media can be determined [92]. Other studies reported on the use of ellipsometry to investigate the corrosion or rate of oxide film dissolution in certain environments in situ. As the film dissolves, the formation of pores and differences between the densities of different layers in the oxide film ean be distinguished and related to the conditions of the anodizing process [93]. 

Corrosion occurs at a rate determined by equilibrium between opposing electrochemical reactions. The rate of any given electrochemical process depends on the rates of two conjugate reactions proceeding at the surface of the metal. Transfer of metal atoms from the lattice to the solution (anodic reaction) with the liberation of electrons and consumption of these electrons by some depolarisers (cathodic reaction). When these two reactions are in equilibrium, the flow of electrons from each reaction of balanced and no net electron flow (current) occurs. Various methods are available for the determination of dissolution rate of metals in corrosive environments but electrochemical methods employing polarisation techniques are by far most widely used. The corrosion rate (CR) is evaluated by mass loss method considering uniform corrosion. The Corrosion rate is determined by the following formula as per standard [102]. 

It has been assumed that the anodic curve for M - M (aq.) conforms with the Tafel relationship, and that -log / is linear throughout the range of potentials under consideration. It follows, therefore, that charge transfer rather than mass transfer is rate determining, and that the linearity of the i -log i curve will be maintained until transport of metal ions away from the surface becomes significant. It is not proposed to consider metal dissolution in detail here, but it is appropriate to illustrate the complexity of the process by considering the anodic dissolution of iron ... 

Concerning the current-potential curves given in Figure 8.3, it is interesting to see that the current rises exponentially with increasing anodic potentials for a p-type Ge electrode. A quantitative evaluation of this current-potential curve yielded a slope of around 80 mV per one decade of current increase (not shown here). Since holes are required for the dissolution process one would expect theoretically that the rate-determining step is a one-hole process, that is, the current should be proportional to the hole density at the surface Ps)- Since Ps Po exp EA(p / kT) one would expect that the vs curve would rise with 60 mV/decade. Obviously, some film formation at the Ge surface influences the potential distribution. 

The same discussion with the same equations holds for anodic reactions with diffusion of the species Red as the rate-determining step. In the case of intense metal corrosion, the diffusion of cations from the electrode surface to the bulk may become the rate-determining step, with their aecumula-tion at the electrode surface and the final precipitation of a salt film. These processes are important for intense active metal dissolution and localized corrosion, as will be discussed in Sec. 1.5.4. At small current densities, a superposition of charge transfer and diffusion control is obtained. Therefore the current density increases exponentially in the vicinity of the Nernst potential ac-... 

Identification of the surface species taking part in anodic dissolution can be tentatively dealt with in the framework of the absolute reaction rate and activated complex theory [18]. A description of the activated state in metal dissolution is central to the imderstanding of corrosion and passivation. However, the identification of this activated state is difficult. For active metal dissolution the ionization is a very fast process (characteristic time estimated to be less than 10 ps). Following the chemical relaxation technique introduced by Eigen [19,20] for investigating fast homogeneous reactions, so-called scr e potential measiu ements were applied to the determination of the initial potential and of its relaxation time on fresh surfaces exposed to aqueous solution [21]. 

---