Cathode portion

The sohd line in Figure 3 represents the potential vs the measured (or the appHed) current density. Measured or appHed current is the current actually measured in an external circuit ie, the amount of external current that must be appHed to the electrode in order to move the potential to each desired point. The corrosion potential and corrosion current density can also be deterrnined from the potential vs measured current behavior, which is referred to as polarization curve rather than an Evans diagram, by extrapolation of either or both the anodic or cathodic portion of the curve. This latter procedure does not require specific knowledge of the equiHbrium potentials, exchange current densities, and Tafel slope values of the specific reactions involved. Thus Evans diagrams, constmcted from information contained in the Hterature, and polarization curves, generated by experimentation, can be used to predict and analyze uniform and other forms of corrosion. Further treatment of these subjects can be found elsewhere (1—3,6,18). 

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. 

There are several factors that can lead to non-Tafel behavior. Diffusion limitations on a reaction have already been introduced and can be seen in the cathodic portion of Fig. 27. Ohmic losses in solution can lead to a curvature of the Tafel region, leading to erroneously high estimations of corrosion rate if not compensated for properly. The effects of the presence of a buffer in solution can also lead to odd-looking polarization behavior that does not lend itself to direct Tafel extrapolation. 

From extrapolation of the cathodic portion of the copper to EOMlAi, ... 

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. 

Although corrosion is favored by a large difference of potential between the anodic and cathodic portions of a system, even the smallest of such differences is sufficient to stimulate corrosion in the presence of a depolarizer. In an apparently uniform piece of metal, any portion which has been subjected to strain is less noble than an unstrained portion and small crystals are less noble than large ones further, minute inclusions of noble material are often found in relatively pure metals. These differences permit local voltaic cells to be set up and, in the presence of a depolarizer, corrosion of the baser (anodic) regions will occur. 

In the foregoing discussion different types of corrosion have been considered separately in practice the situation is complicated by the simultaneous occurrence of two or more forms of corrosion, by the production of adherent films which result in passivity, and by loose deposits, such as rust, which arise from the interaction of the alkali produced at the cathodic portions of the metal with the cations formed at the anodic regions. In addition to these possibilities, due to chemical or electrochemical action, physical factors, such as surface forces, often play a part a film which would normally be protective may be drawn up into the solution-air interface and thus be prevented from covering the surface of the metal. It is because of these complicating factors that the phenomena of corrosion are sometimes difficult to explain, but it is believed that the principles enunciated in this and the preceding sections represent the fundamentals of electrochemical corrosion. 

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. 

The equilibrium, I2 + I = Ia, exists in these solutions, but. this does not affect the stoichiometrical relations. After the electrolysis is completed, a delivery tube is connected to D, and the anode and cathode portions of the electrolytes are drawn over into separate flasks. The two portions are then titrated for iodine with arsenious acid solution which lias been standardized against carefully purified iodine. By comparison with the silver coulometer, Bates and Vinal8 found the electrochemical equivalent of iodine to be 0.00131505 gram per coulomb, leading to a value of the faraday of 96,514. The accuracy of the experimental work may be judged from the results for the different experiments given in Table I. 

To demonstrate that the TCNQ-BLM behaves like a metallic electrode (e.g., Pt, which is frequently used in CV), a comparative experiment was carried out Cyclic voltammograms of quinhydrone were obtained using either Pt or TCNQ-containing BLM under very similar conditions. The cathodic portions of the voltammograms were found to be quite similar,... 

The corrosion rate r of metals that degenerate according to Equations 10.32 to 10.35 depends on the rate of supply of oxygen to the cathodic portions of the surface r, i.e. 

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. 

This mechanism is also supported by the kinetic measurements [42,47]. The kinetic measurements were made by measuring either the charge under the cathodic portion of the CV [42] or the peak current arising from the oxidation or reduction of the polymer [47]. Stilwell and Park [42] obtained a rate expression. 

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. 

The success of this method depends on the rapid diffusion of the impurities in the rare earth metal in a strong electric field. Most of the non-metallic elements (carbon, nitrogen and oxygen) and the small interstitial-like transition metals (iron, cobalt, nickel and copper) migrate from the cathode towards the anode, purifying the cathode portion of the rod. After 100 to 1000 hours a steady-state condition is built-up after which the forward diffusion from the cathode to the anode due to the electric field is equal to the backward diffusion due to the chemical concentration difference, and no further purification is realized. 

Linear Polarization (LP) as schematically shown in Figure 3.4 covers both anodic and cathodic portions of the potential E versus current daisity curve for determining Rp. 

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). 

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