Corrosion current reduction reactions

Anode The region of the electrical cell where positive current flows into the electrolyte. Anode is the site where oxidation occurs. In a corrosion cell anode is the region which is dissolving. Cathode is the region of an electrical cell where positive electric current enters from the electrolyte. In a corrosion cell reduction reaction takes place at the cathode. 

Corrosion occurs at the anode, where metal dissolves. Often, this is separated by a physical distance from the cathode, where a reduction reaction takes place. An electrical potential difference exists between these sites, and current flows through the solution from the anode to the cathode. This is accompanied by the flow of electrons from the anode to the cathode through the metal (Fig. 8). 

A sheet of steel of thickness 0.50 mm is tinplated on both sides and subjected to a corrosive environment. During service, the tinplate becomes scratched, so that steel is exposed over 0.5% of the area of the sheet. Under these conditions it is estimated that the current consumed at the tinned surface by the oxygen-reduction reaction is 2 X 10 A m -. Will the sheet rust through within 5 years in the scratched condition The density of steel is 7.87Mg m . Assume that the steel corrodes to give Fe " ions. The atomic weight of iron is 55.9. 

Although Table 2.16 shows which metal of a couple will be the anode and will thus corrode more rapidly, little information regarding the corrosion current, and hence the corrosion rate, can be obtained from the e.m.f. of the cell. The kinetics of the corrosion reaction will be determined by the rates of the electrode processes and the corrosion rates of the anode of the couple will depend on the rate of reduction of hydrogen ions or dissolved oxygen at the cathode metal (Section 1.4). 

Fig. 12.4 Corrosion diagram for a zinc diecasting in a nickel plating bath, pH 2-2. There are two possible cathodic reactions, hydrogen evolution (H) and nickel ion reduction (AO. The corrosion current is the sum of the partial cathode currents. Even with live entry the potential is still too high to suppress corrosion, though the rate is reduced to...

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). 

The potential of a mixed electrode at which a coupled reaction of charge transfer proceeds is called the mixed electrode potential , this mixed electrode potential is obviously different from the single electrode potential at which a single reaction of charge transfer is at equilibrium. For corroding metal electrodes, as shown in Fig. 11—2, the mixed potential is often called the corrosion potential, E . At this corrosion potential Eemt the anodic transfer current of metallic ions i, which corresponds to the corrosion rate (the corrosion current ), is exactly balanced with the cathodic transfer current of electrons for reduction of oxidants (e.g. hydrogen ions) i as shown in Eqn. 11-4 ... 

When the cathodic reaction is the reduction of oi n molecules for which the equilibrium potential is relatively high (much more anodic than the corrosion potential), the corrosion current is frequently controlled by the diffusion of hydrated o Q en molecules towards the corroding metal electrode thus, the corrosion ciurent equals the diffusion current of o en molecules as shown in Fig. 11-8. For this mode of diffusion-controlled corrosion of metals the cathodic Tafel constant is... 

They found that a Cu electrode, pretreated by immersing it in a 0.1M BTA solution for 15 seconds, inhibited the 0 reduction reaction initially and that on subsequent cycles the currents Increased to that of bare Cu in a short time. A similar effect was observed when a Cu electrode was cycled in a ImM solution of BTA. They discovered that a solution of 0.1M BTA produced a lasting effect, indicating that a reservoir of BTA is necessary for continuous protection of the copper against corrosion. We found that bare Cu gives the same voltammogram in the 0 reduction region in both acetate buffer and phosphate buffer therefore, McCrory-Joy et. al. s results can be directly compared to the results reported here. 

The cathodic reaction during corrosion of iron in sea water is oxygen reduction. Solubility of 02 from the air in sea water is 0.189 mol m 3 and the diffusion coefficient of 02 is 2.75 x 10 9 m2 s 1. The diffusion layer thickness in an unstirred solution is about 0.5 mm. (a) Estimate the corrosion current density of iron in sea water, (b) If iron is connected to the negative pole of an external... 

The Tafel expressions for both the anodic and the cathodic reaction can be directly incorporated into a mixed potential model. In modeling terms, a Tafel relationship can be defined in terms of the Tafel slope (b), the equilibrium potential for the specific half-reaction ( e), and the exchange current density (70), where the latter can be easily expressed as a rate constant, k. An attempt to illustrate this is shown in Fig. 10 using the corrosion of Cu in neutral aerated chloride solutions as an example. The equilibrium potential is calculated from the Nernst equation e.g., for the 02 reduction reaction,... 

While an ovapotential may be applied electrically, we are interested in the overpotential that is reached via chemical equilibrium with a second reaction. As mentioned previously, the oxidation of a metal requires a corresponding reduction reaction. As shown in Figure 4.34, both copper oxidation, and the corresponding reduction reaction may be plotted on the same scale to determine the chemical equilibrium between the two reactions. The intersection of the two curves in Figure 4.34 gives the mixed potential and the corrosion current. The intersection point depends upon several factors including (the reversible potential of the cathodic reaction), cu2+/cu> Tafel slopes and of each reaction, and whether the reactions are controlled by Tafel kinetics or concentration polarization. In addition, other reduction and oxidation reactions may occur simultaneously which will influence the mixed potential. 

In the absence of copper ions in the slurry or copper metal on the wafer, / , is the only means by which titanium is removed. As copper ions are added to the slurry, either as a by-product of copper polishing or by the addition of a copper salt such as Cu(N03)2, titanium removal also occurs via the galvanic couple with Cu ions, / 2- Increasing the concentration of Cu, increases Ecu2f/cu and moves the copper reduction curve in Figure 4.44b in the noble (positive) direction. Consequently, the corrosion current density for the Cu -Ti exchange reaction increases, increasing R2... 

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. 

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