Metal dissolution corrosion current density

For the corresponding equations in alkaline solutions, see Chapter 9. The metal surface attains a mixed potential corrosion potential, such that the anodic current of the metal dissolution is exactly balanced by the cathodic current of one or more reduction reactions. The corrosion potential is given by Eq. (11.41), and the corrosion current density by Eq. (11.42). 

If one of the partial electrode reactions is the dissolution of the electrode (i.e. metal, semiconductor, etc.), the open circuit potential is a corrosion potential and the system undergoes corrosion at a rate given by the corrosion current density (/corr), which is a measure of the corrosion rate of the system. The magnitude of corr of corroding systems... 

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-... 

Corrosion current density — Anodic metal dissolution is compensated electronically by a cathodic process, like cathodic hydrogen evolution or oxygen reduction. These processes follow the exponential current density-potential relationship of the - Butler-Volmer equation in case of their charge transfer control or they may be transport controlled (- diffusion or - migration). At the -> rest potential Er both - current densities have the same value with opposite sign and compensate each other with a zero current density in the outer electronic circuit. In this case the rest potential is a -> mixed potential. This metal dissolution is related to the corro-... 

Electroless metal deposition at trace levels in the solution is an important factor affecting silicon wafer cleaning. The deposition rate of most metals at trace levels depends mainly on the metal concentration and some may also depend on the interaction with other species as well. For copper the deposition rate at trace levels in HF solutions is different for n and p types. It depends on illumination for p-Si but not for n-Si. It is also different in HF and BHF solutions. In a HF solution the deposition process is controlled by both the supply of minority carriers and the kinetics of cathodic reactions. Thus, a high deposition rate occurs on p-Si only when both and illumination are present. In the BHF solution, the corrosion process is limited by the supply of electrons for p-Si whereas for n-Si it is limited by the dissolution of silicon because the reaction rate is indepaidmt of concentration and illumination. The amount of copper deposition does not correlate with the corrosion current density, which may be attributed to the chemical reactions associated with hydrogen reduction. More information on trace metal deposition can be found in Chapters 2 and 7. 

A somewhat alternative analysis of pitting attributes pit initiation to the activation of defects in the passive film, defects such as those induced during film growth or those induced mechanically due to scratching or stress. The pit behavior is analyzed in terms of the product, xi, a parameter in which x is the pit or crevice depth (cm), and i is the corrosion current density (A/cm2) at the bottom of the pit (Ref 21). Experimental measurements confirm that, for many metal/environment systems, the active corrosion current density in a pit is of the order of 1 A/cm2. Therefore, numerical values for xi may be visualized as a pit depth in centimeters. A defect becomes a pit if the pH in the pit becomes sufficiently low to prevent maintaining the protective oxide film. Establishing the critical pH, for a specific oxide, will depend on the depth (metal ions trapped by diffiisional constraints), the current density (rate of generation of metal ions) and the external pH. In turn, the current density will be determined by the local electrochemical potential established by corrosion currents to the passive external cathodic surface or by a potentiostat. Once the critical condition for dissolution of the oxide has been reached, the pit becomes deeper and develops a still lower pH by further hydrolysis. 

The first term in Equation 52 is proportional to the rate of metal dissolution at the corrosion potential, i.e., the corrosion rate. Therefore, a measure of the corrosion rate is the corrosion current density. 

The total oxidation and reduction current densities will be equal at the point at which the anodic line for the metal dissolution reaction intersects the cathodic line for hydrogen evolution. The potential at which these lines intersect is the corrosion potential. The rate of the anodic reaction at the corrosion potential is the corrosion rate (corrosion current density). The corrosion potential always takes a value between the reversible potentials for the two partial reactions. 

Fig. 3 Schematic diagram of a passivated metal showing the corrosion current densities of metal dissolution and redox reactions at the surface of the passive layer, a pit, and an electron conducting inclusion.

From Figure 2.1 it can be understood that corrosion rate also can be expressed by corrosion current density. The dissolution rate (the corrosion rate) is the amount of metal ions removed from the metal per unit area and unit time. This transport of ions can be expressed as the electric current la per area unit, i.e. anodic current density ia = corrosion current density icon -... 

Note, however, that there are conditions under which inhibitors can give rise to detrimental local corrosion, that is, pitting corrosion. This is the case when the amount of inhibitor is insufficient. Under these conditions, only part of the surface can be covered, thus giving rise to a local element. Corrosive attack is particularly extensive at the uncovered anode areas because of increased corrosion current density and deep cavities penetrating into the material. Similarly, if the inhibitor is too readily reduced at the cathodic areas of the metal surface, increased corrosion can result because compact protective films are not formed. Since there are no universally applicable inhibitors, they must be carefully selected and examined for each specific case. In doing so, inhibition of metal dissolution is not the only point to be considered—there is also hydrogen absorption. 

The detection of ions released from metallic implants is dependent on the technique used. Very minute amounts of ions can be detected by electrothermal atomic absorption spectroscopy (ET-AAS), which goes down to concentrations of the order of ng/g. Electrochemical methods enable the detection of extremely low corrosion current densities, below 1 xA/cm, corresponding to dissolution in the passive state. These rates of corrosion do not modify the aspect of the surface and are not normally considered as surface attack. 

T] = E-Eq. a semi-logarithmic Tafel plot yields the lines of the current densities of anodic metal dissolution and cathodic reduction of the redox system, as presented for iron dissolution in 0.5 M H2SO4 in Fig. 1-30 (Kaesche, 1979). The intersection of both lines yields Er and the related corrosion current density 4 within the electrolyte. In the case of iron corrosion in sulfuric acid, the corrosion rates determined by the electrochemical evaluation of the Tafel plot and the chemical analysis of the dissolved species or the weight loss of the specimen for simple immersion tests agree sufficiently well (Kaesche, 1979). 

Figure 1-29. Superposition of the current density potential curves of an Me/Me " and a redox electrode, which yields the polarization curve of anodic metal dissolution and cathodic reduction of the redox system Eq.m nd Fq, redox t Nernst potentials, r is the rest potential, i o,m

The complex impedance of the anodic metal dissolution Z e in parallel to the overall transport impedance and the electrode capacitance (Fig. 7-17) is approximated by a charge transfer resistance / Me related to the corrosion current density. 

In this equation n is the charge number (dimensionless), which indicates the number of electrons exchanged in the dissolution reaction, and F is the Faraday constant, F = 96,485 C/mol. In the absence of an external polarization a metal in contact with an oxidizing electrolytic environment acquires spontaneously a certain potential, called the corrosion potential, The partial anodic current density at the corrosion potential is equal to the corrosion current density / Equation (4) thus becomes ... 

Superimposed metal dissolution and hydrogen evolution with equilibrium potentials Ej (Me/Me +) and Ejq(H2/H+), respectively, leading to the rest or corrosion potential Eg for disappearing current in the external circuit and the corrosion current density i(. Solid curves correspond to the partial current densities of anodic metal dissolution and cathodic hydrogen evolution and to the sum curve of both reactions. 

If the oxygen reduction is the counter reaction of metal dissolution, its kinetics is often determined by diffusion due to the small solubility of O2 gas and long diffusion distances especially in unstirred solution. In this case, the cathodic reaction gets potential independent at negative potentials with a diffusion-limited cathodic current density Ido2 the vicinity of the rest potential (Figure 1.42). In this situation, the corrosion current density ic is equal to the limiting cathodic current density ip,02/ and Equation 1.159 simplifies to Equation 1.164, which becomes Equation 1.165 for n -> 0 (E —> Er). Hence ic can be calculated from Rp and the anodic Tafel slope (Equation 1.166). 

As mentioned above, inhibitors may block anodic metal dissolution or the cathodic reduction reaction or both processes simultaneously. If the cathodic reaction is inhibited, the related Tafel line of Figure 1.45a is shifted to negative potentials. As a consequence, the rest potential Er shifts to a more negative value Er, and the related corrosion current density from z c to the smaller value i i- If the anodic metal dissolution is inhibited, the related... 

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