Anode portion

In most aqueous systems, the corrosion reaction is divided into an anodic portion and a cathodic portion, occurring simultaneously at discrete points on metallic surfaces. Flow of electricity from the anodic to the cathodic areas may be generated by local cells set up either on a single metallic surface (because of local point-to-point differences on the surface) or between dissimilar met s. 

During the anodic portion of the scan, B is reoxidized to A at the electrode surface, triggering a progressive reestablishment of the initial concentration profiles. However, these are not completely restored when the potential goes back to its initial value. The charge used up to convert A into B during the first part of the scan is not recovered completely during the... 

Voltammograms of Ptdll) with and without COad adsorption in 0.5 M perchloric add are shown in Fig. 2-25. The voltammogram without CO was considerably different from those in sulfuric acid. The symmetric features in the range from 600 to 800 mV correspond to the anodic portion of the two split area of hydrogen adsorption-desorption in sulfuric add. Hydrogen adsorption-desorption features did not change after the oxidation peak at 1050 mV and its reduction while the further oxidation removes the feature irreversibly. Therefore the peak at 1050 mV is considered as a formation of a weak interaction with water. 

Estimate the corrosion rate of the copper under freely corroding conditions and when it is coupled to the steel by extrapolating the anodic portion of the copper polarization curve to ZTcoupie- Compare these oxidation rates. Why can t either rate be measured directly ... 

In analyzing the polarization data, it can be seen that the cathodic reaction on the copper (oxygen reduction) quickly becomes diffusion controlled. However, at potentials below -0.4 V, hydrogen evolution begins to become the dominant reaction, as seen by the Tafel behavior at those potentials. At the higher anodic potentials applied to the steel specimen, the effect of uncompensated ohmic resistance (IRohmk) can be seen as a curving up of the anodic portion of the curve. 

Jones and Bradshaw [J. Am, Chem, Soc. 54, 138 (1932)] passed a current of approximately 0.025 amp. for 8 hours through a solution of lithium chloride, using a silver anode and a silver chloride cathode 0.73936 g. of silver was deposited in a coulometer. The original electrolyte contained 0.43124 g. of lithium chloride per 100 g. of water, and after electrolysis the anode portion, weighing 128.615 g., contained 0.35941 g. of salt per 100 g. water, while the cathode portion, weighing 123.074 g., contained 0.50797 g. of salt per 100 g. of water. Calculate the transference number of the chloride ion from the separate data for anode and cathode solutions. 

The anodic portion of the potentiodynamic electrode curve shown in Fig. 6 likewise exhibits a special effect, i.e., the development of a current density maximum. This type of curve resembles those of metals that can be passivated. The cause of the current density maximum is examined in greater detail in Sec. 4.5.6. 

Fig. 6. The anodic portion of the potentiodynamic current-potential curve for El 11/A (400 rpm) [186, 187], exhibiting a characteristic current density maximum.

The procedure to be described is a determination of a transference number by the Hittorf1 method. The solution can be arbitrarily divided into three portions, as shown in the diagram, called respectively the anode, middle, and cathode portions. On passing a current the anode portion will become more concentrated, and the cathode portion more dilute. The middle portion will retain its original concentration. After the electrolysis the separate portions may be drawn off one after... 

It is evident that the changes at the cathode are, in this case, the precise reverse of the changes at the anode The net effect of the changes at both electrodes is the transfer of f oa equivalent, per faraday, of silver nitrate from the cathode portion to the anode portion, since the anode portion gains and the cathode portion loses that amount of substance. 

Another way of arriving at the result is as follows. The passage of 0.002 faraday will result in the formation of this fraction of an equivalent of silver ion in the anode portion. However, the observed increase is only 0.001064 therefore, 0.000936 equivalent must have migrated out of the portion carrying a proportion of the current equal to 0.000936/0.00200 = 0.468, which is the transference number of the silver ion at this concentration. 

One example will illustrate the method of computation. Take, for instance, the figures for the anode portion at 1.0 normal. Here 121.41 grams of anode portion was found to be 6.5100 per cent potassium chloride, that is to say, 7.9039 grams KC1 and 113.51 grams water. This water originally contained the same proportion of potassium chloride as the middle portions, or 7.1479 per cent. The amount of salt, xf originally associated with the water may therefore be obtained from the proportion... 

Fig. 40a, b. Anodic portions of the cyclic voltammetric responses exhibited, in 1,2-Dichloroethane solution, by a) Pd2W2(CO)6 (C5H5)2(PPh3)2 b) Pt2W2(CO)6 (C5H5)2(PPh3)2. Platinum working electrode. Scan rate 0.2 Vs 1 [from Ref. 95]... 

In cyclic voltammetry, the anodic portion on the reverse scan is not affected as much as the forward response by the coupled reaction (Figure 12.3.2). The ratio of (with /pa measured from the extension of the cathodic curve as described in Section 6.5) increases with increasing scan rate as shown in the working curve in Figure 12.3.6 (25). The actual i-E curves can be drawn using series solutions or a table given by Nicholson and Shain (25) or by digital simulation. 

In aHittorf experiment to determine the transference numbers in KCl solution, the following data were obtained. (D. A. Macinnes and M. Dole. J.A.C.S. 53, 357 [1931].) Mass of the anode solution, 117.79 g mass of the cathode solution, 120.99 g. Percent KCl in anode portion, 0.10336% percent KCl in cathode portion, 0.19398 %. The percent KCl in the middle portion was 0.14948 %. Calculate from the amounts of KCl transferred from the anode compartment and to the cathode compartment and the average value of t+. (Note 0.16034 g of silver was deposited in a silver coulometer in series with the cell. The concentration of KCl was 0.2 mol/L.) Silver-silver chloride electrodes were used. 

Unlike the cathodic portion of the polarization curve, the anodic portion of the curve in Fig. 3(b) does not exhibit clear Tafel-type behavior. The mechanism for Fe dissolution in acids is quite complex. A line can be drawn in the region just above the corrosion potential, giving a Tafel slope of 34 mV decade k Extrapolation of this line intersects the zero-current potential at 7 X 10 A cm , a considerably different value than the extrapolation of the cathodic portion of the curve. This is not uncommon in practice. When this happens, it is usually considered that the anodic portion of the curve is affected by changes on the electrode surface, that is, surface roughening or film formation. The corrosion rate is typically determined from the extrapolated cathodic Tafel region. 

Table 4.3 Activity of extended surface intermetallics, from anodic portion of cyclic voltammogram in 0.125 M formic acid and 0.1 M HCIO4 (except PtsSn and PtSb in 0.25 M formic acid) at 10 mV s ...

Avoid contact with hot surfaces within the equipment. The anode portion of most power tubes are air-cooled. The external surface normally operates at a high temperature (up to 250°C). Other portions of the tube may also reach high temperatures, especially the cathode insulator and the cathode/heater surfaces. AH hot surfaces may remain hot for an extended time after the tube is shut off To prevent serious burns, avoid bodily contact with these surfaces both during and for a reasonable cool-down period after tube operation. 

ASTM G 3 (Practice for Conventions Applicable to Electrochemical Measurements in Corrosion Testing) [74] and Refs 49, 55, and 73 show the schematics for the apparatus for corrosion measurements and schematic drawings for cathodic and anodic polarization diagrams and polarization plots. ASTM G 5 (Standard Reference Test Method for Making Potentiostatic and Potentiodynamic Anodic Polarization Measurements) [74] and ASTM F 4 [55] test methods and practices describe the setup and procedures for making potentiostatic and potentiodynamic anodic polarization measurements. A cyclic polarization curve that contains both the cathodic and anodic portions provides data that can be used to describe corrosion behavior in terms of passivity, breakdown, corrosion rate, and susceptibility to pitting. 

With well dialyzed or concentrated solutions a precipitation of the gold on the anode in the form of a black powder may be obtained.t This black powder after it has been dried has the glance of gold. Here traces of impurities may act as protective colloids, and no precipitation occurs on the anode, but instead a concentration in the anode portion may take place. The author was enabled to observe such a condition of affairs during the electrolysis of gold solutions containing gelatin. 

As T] is increased, the rate of cathodic reduction (cathodic polarization) increases. The slope of the anodic portion of the curve is given by /la — —0.0591zf and that of the cathodic portion by pc = -0.059lz l-f). 

---