Anodic process dissolution rates

This is essentially a corrosion reaction involving anodic metal dissolution where the conjugate reaction is the hydrogen (qv) evolution process. Hence, the rate depends on temperature, concentration of acid, inhibiting agents, nature of the surface oxide film, etc. Unless the metal chloride is insoluble in aqueous solution eg, Ag or Hg ", the reaction products are removed from the metal or alloy surface by dissolution. The extent of removal is controUed by the local hydrodynamic conditions. 

Two observations relevant to ECM can be made. (/) Because the anode metal dissolves electrochemicaHy, the rate of dissolution (or machining) depends, by Faraday s laws of electrolysis, only on the atomic weight M and valency of the anode material, the current I which is passed, and the time t for which the current passes. The dissolution rate is not infiuenced by hardness (qv) or any other characteristics of the metal. (2) Because only hydrogen gas is evolved at the cathode, the shape of that electrode remains unaltered during the electrolysis. This feature is perhaps the most relevant in the use of ECM as a metal-shaping process (4). 

Film-free conditions It has been observed for many metals that the magnitude of / i, (see Section 1.4) increases with temperature and that the activation energy for dissolution is low, suggestive of a diffusion-limited anode process when the migration of corrosion products away from the surface is rate controlling. Some examples of the value of the activation energy for this process are given in Table 2.4. 

The surface of the base metal is anodically polarized under the effect of local cells. For a graphical analysis of the phenomena, one must construct the polarization curves for the partial currents at the base metal as well as the overall anodic 4 vs. E curve reflecting the effective rate of dissolution of this metal under anodic polarization. The rate of the cathodic process, 4, at the inclusions is described by the corresponding cathodic polarization curve (since the surface areas of anodic and cathodic segments differ substantially, currents rather than current densities must be employed here). At open circuit the two rates are identical. 

The potential-decay method can be included in this group. Either a current is passed through the electrode for a certain period of time or the electrode is simply immersed in the solution and the dependence of the electrode potential on time is recorded in the currentless state. At a given electrolyte composition, various cathodic and anodic processes (e.g. anodic dissolution of the electrode) can proceed at the electrode simultaneously. The sum of their partial currents plus the charging current is equal to zero. As concentration changes thus occur in the electrolyte, the rates of the partial electrode reactions change along with the value of the electrode potential. The electrode potential has the character of a mixed potential (see Section 5.8.4). 

In contrast to the equilibrium electrode potential, the mixed potential is given by a non-equilibrium state of two different electrode processes and is accompanied by a spontaneous change in the system. Besides an electrode reaction, the rate-controlling step of one of these processes can be a transport process. For example, in the dissolution of mercury in nitric acid, the cathodic process is the reduction of nitric acid to nitrous acid and the anodic process is the ionization of mercury. The anodic process is controlled by the transport of mercuric ions from the electrode this process is accelerated, for example, by stirring (see Fig. 5.54B), resulting in a shift of the mixed potential to a more negative value, E mix. 

Anode material In aqueous solutions the anodic processes are either breakdown of the electrolyte solution (with oxygen evolution at an inert anode being favored) or the use of soluble anodes. The use of soluble anodes is limited by the passivation of many metals in aqueous solutions. In ionic liquids, however, the first option is not viable due to the cost and the nature of the anodic breakdown products. New strategies will therefore have to be developed to use soluble anodes where possible or add a sacrificial species that is oxidized to give a benign gaseous product. Preliminary data have shown that for some metals the anodic dissolution process is rate limiting and this affects the current distribution around the cathode and the current density that can be applied. 

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-... 

Experimental studies usually yield good agreement between the rates of corrosion obtained from polarization resistance measurements and those derived from weight-loss data, particularly if we recall that the Tafel slopes for the anodic and the cathodic processes may not be known very accurately. It cannot be overemphasized, however, that both methods yield the average rate of corrosion of the sample, which may not be the most critical aspect when localized corrosion occurs. In particular it should be noted that at the open-circuit corrosion potential, the total anodic and cathodic currents must be equal, while the local current densities on the surface can be quite different. This could be a serious problem when most of the surface acts as the cathode and small spots (e.g., pits or crevices) act as the anodic regions. The rate of anodic dissolution inside a pit can, under these circumstances, be hundreds or even thousands of times faster than the average corrosion rate obtained from micro polarization or weight-loss measurements. 

Corrosion current is a measure of the rate of the anodic reactions at (XP. The corrosion current may or may not be equivalent to the dissolution rate of sihcon in a solution since silicon may dissolve chemically and/or electrochemically, and the corrosion current is a measure of only the electrochemical part of the dissolution. It is a useful parameter as a measure of the electrochemical reactions in processes such as... 

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. 

In KOH solutions, the rate-limiting process at OCP is of chemical nature, that is, only r(, is involved. The electrochemical processes r4 and, -5 are increasingly involved as potential is increased from OCP to Vp. At potential larger than Vp the anodic reaction is limited by the dissolution rate of the oxide rio,... 

During ECM, electrochemical dissolution of anode and cathodic evolution of hydrogen proceeds on the electrodes (the WP and TE, respectively). Along with these basic reactions, parallel reactions proceed concurrently, for example, oxygen anodic evolution, cathodic reduction of nitrate ions, if NaNC>3 electrolyte is used. It is important to note that electrochemical reactions in a narrow IEG result in gas evolution. The temperature of the electrolyte in the IEG and the void fraction increase as the electrolyte flows along the gap. This leads to a variation in the electrolyte conductivity that has an effect on the distributions of current and metal dissolution rate over the WP surface. The electrode processes and the processes in... 

In the quasi-steady-state approximation, which is also known as the step method [9], it is assumed that the rate of variation in the WP shape, that is, the anodic dissolution rate, is small compared with the rates of transfer processes in the gap therefore, for calculating the distribution of the current density, the WP surface can be considered as being immobile. This approximation can be used at not very high current densities. At very high current densities, ignoring the WP surface motion during anodic dissolution and the hydrodynamic flow induced by this motion causes a considerable error in the calculated distribution of current density [33]. 

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