External cathodic current

Now, with reference to Fig. 6.7(b), consider a cathodic external current, Iex c. In the solution, this current flows from the higher solution po-... 

Fig. 6.7 The IR connection, (a) Anodic external current, (b) Cathodic external current...

This definition covers all the behavior of magnesium - start of the NDE behavior at a cathodic external current density, or at a potential negative to the magnesium corrosion potential. This new definition describes the main features of NDE. 

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). 

The current I is called the total current. In free corrosion, i.e., without the contribution of external currents (see Fig. 2-1), it is always zero, as given by Eq. (2-8). and are known as the anodic and cathodic partial currents. According to Eq. (2-10), generally in electrolytic corrosion anodic total currents and/or cathodic redox reactions are responsible. 

Even with the superposition of the ac with a cathodic protection current, a large part of the anodic half wave persists for anodic corrosion. This process cannot be detected by the normal method (Section 3.3.2.1) of measuring the pipe/soil potential. The IR-free measurable voltage between an external probe and the reference electrode can be used as evidence of more positive potentials than the protection potential during the anodic phase. Investigations have shown, however, that the corrosion danger is considerably reduced, since only about 0.1 to 0.2% contributes to corrosion. 

It had been shown in Section 2.2 that at the equilibrium otential, the net (external) current density i is zero, but partial cimen densities i and i of the anodic and cathodic reaction exist for which the relation i =i = f holds where i° is the exchange current density. The value of i increases, that of i decreases, when the potential is made more positive but i decreases and i increases when the potential is made more negative. The net current density i is the difference of the partial current densities ... 

If the cathodic partial current is larger than the anodic partial current, a total cathodic or reduction current will flow through the electrochemical interface, and vice versa. If both anodic and cathodic partial processes at an electrode are balanced, that is both partial currents are equal, no net reaction will take place at the electrode and no total net current will be observed through the external circuit. However, both... 

Due to the radial symmetry of the three configurations, a 2D model is employed. Figure 4.20 depicts the geometry considered for case 1, i.e. current collectors at the tube ends (not in scale). In cases 2, an additional 16 pieces of conductive material (nickel), with a square section of 1.5 x 1.5 mm, are considered on the cathode (external surface of the tube). In case 3 the conductive materials are present both inside and outside the tube. It should be noted that in case 2 and 3, cylindrical symmetry is not obvious anymore. However, it is reasonable to assume that voltage losses are mainly proportional to ohmic in-plane losses, i.e. to the length of the current path along the tube. In case 2 and 3, this distance is constant in each longitudinal tube cross-section, therefore a cylindrical symmetry is assumed. 

An electrolytic cell is essentially composed of a pair of electrodes submerged into an electrolyte for conduction of ions and connected to a direct current (DC) generator via an external conductor to provide for continuity of the circuit. The electrode connected to the positive pole of the DC generator is called anode, while that linked to the negative one, cathode. The current flow in an electrolyte results from the movement of positive and negative ions and is assumed as positive when directed as the positive charges or opposite to the electrons in the external circuit. When the cell is not operating under conditions of standard concentration, the thermodynamic electrode (or cell) potential (ET) can be estimated from the Nernst equation ... 

Fig. 12.38. Cathodic protection can also be accomplished by using an inert auxiliary electrode and sending current into the circuit from an external current source.

Let i, be the cathodic current of the species to be reduced, e.g. oxygen, iz the anodic current for oxidation of, for example, oxygen evolution, im the cathodic current for metal ions plating out, im the anodic current for metal oxidation, ix a measurable external current, -/ , the Tafel slope for iz, and [i the Tafel slope for fm. 

If for any reason, such as the application of an external current, the electrode potential of the metal is changed from the equilibrium value, or cannot assume this value, there will be a net anodic or cathodic current density according to the value of E. A change in the noble direction (more positive) will cause a net ionic current in the anodic direction, (ia) > equal to the difference between the total anodic and cathodic current. According to Eqs. (8) and (9) this change will be given by... 

The separate anodic and cathodic processes will occur simultaneously but statistically independent of one another. The rate of each reaction will be governed by the electrical potential difference which exists across the metal-solution interface and the appropriate values of i0, a and 0 for each system. In the absence of an external disturbance, for instance an external current, a steady state will usually be reached where the sum of the rates of the cathodic reactions will equal the sum of the rates of the anodic reactions, viz., Sia =21. . The electrode potential will assume some value, Ej p, which is designated the mixed potential, and the electrode is considered as a poly-eieccrode (11, 16,17,18). 

During battery discharge, as shown in Figure 1 with the Daniell cell as an example, the electrode (a zinc rod immersed in a zinc sulfate solution) at which the oxidation reaction takes place is called the anode, and is the negative electrode. The other electrode (a copper rod immersed in a copper sulfate solution) at which the reduction reaction takes place is called the cathode and is the positive electrode. The electron flow in the external circuit is from anode to cathode (the current, /, conventionally flows in the opposite direction to that of the electrons), and in the electrolyte phase the ionic flow closes the circuit. The net result of the charge flow round the circuit is the cell reaetion, which is made up of the two half-reactions of charge transfer that describe the chemical changes at the two electrodes. 

Polarization has various meanings and interpretations depending on the system under study. For an electrochemical reaction, this is the difference between actual electrode potential and reaction equilibrium potential. Anodic polarization is the shift of anode potential to the positive direction, and cathodic polarization is the shift of cathode potential to the negative direction. In an electrochemical production system driven with an external current source, polarization is a harmful phenomenon. It will increase the cell voltage and therefore production costs. A system that polarizes easily will not pass high currents even at high overpotentials. The reaction rates are therefore small. 

While the metal or alloy electroless deposition reactions can be considered as cathodic processes, formation of oxides at the metallic surfaces without an external current source can be analyzed as anodic processes. This type of deposition can be illustrated in the example of chemical oxidation of aluminum in chromic acid solutions.9... 

When an n-type semiconductor which is in contact with a metal ion-containing electrolyte is illuminated, then two equal partial currents occur under open-circuit conditions (Fig. 11.25a). The anodic photocurrent is due to O2 formation in H2O, whereas the cathodic partial current corresponds to the reduction of the metallic ions. Since the holes cannot diffuse very far, most of them collect at the illuminated interface. In the case of an n-type semiconductor, sufficient electrons are availabe everywhere, so that metal deposition should occur at illuminated as well as at dark surface sites (Fig. 11.25b), according to which conclusion, selective deposition would be impossible. Experimentally, however, selective metal deposition has been observed, e.g. at CdS at illuminated surfaces [130] and at Ti02 at the dark sites [131]. In the case of CdS, this phenomenon was interpreted as a downward shift of the energy bands at the illuminated surface which is more favorable to an electron transfer there [130]. The result obtained with Ti02 has been explained by strong internal and external recombination... 

A 120-cm2 piece of Nafion 390 was-installed in an electrochemical cell using a Ti02/Ru02 coated titanium anode and a mild steel cathode. The current density of the cell was varied from 0.1 to 0.5 amps cm 2 over a period of several days the fluxes of the various components were measured at each current density. Figure 2 shows the variation of current efficiency with current density at constant external concentrations of sodium chloride and sodium hydroxide (that is, constant chemical potentials). It is evident that the flux of sodium ions increases relative to that of hydroxyl ions, with increasing current density. Figure 3 shows a dramatic change in the ion fluX o/h -... 

The earlier sections of this chapter discuss the mixed electrode as the interaction of anodic and cathodic reactions at respective anodic and cathodic sites on a metal surface. The mixed electrode is described in terms of the effects of the sizes and distributions of the anodic and cathodic sites on the potential measured as a function of the position of a reference electrode in the adjacent electrolyte and on the distribution of corrosion rates over the surface. For a metal with fine dispersions of anodic and cathodic reactions occurring under Tafel polarization behavior, it is shown (Fig. 4.8) that a single mixed electrode potential, Ecorr, would be measured by a reference electrode at any position in the electrolyte. The counterpart of this mixed electrode potential is the equilibrium potential, E M (or E x), associated with a single half-cell reaction such as Cu in contact with Cu2+ ions under deaerated conditions. The forms of the anodic and cathodic branches of the experimental polarization curves for a single half-cell reaction under charge-transfer control are shown in Fig. 3.11. It is emphasized that the observed experimental curves are curved near i0 and become asymptotic to E M at very low values of the external current. In this section, the experimental polarization of mixed electrodes is interpreted in terms of the polarization parameters of the individual anodic and cathodic reactions establishing the mixed electrode. The interpretation then leads to determination of the corrosion potential, Ecorr, and to determination of the corrosion current density, icorr, from which the corrosion rate can be calculated. 

The foregoing discussion developed individual expressions for the external cathodic and anodic currents, Iex red and Iex ox. Although this approach was instructive, it was not necessary mathematically. Note that the external current, whether reduction or oxidation, was consistently defined as the sum of the individual oxidation currents minus the sum of individual reduction currents (Eq 4.48). In general then, the external current is defined as ... 

At potential ranges where Iex < 0, that is, when E < Ecorr, the external current is cathodic (net reduction), and at potential ranges where Iex > 0, that is, when E > Ecorr, the external current is anodic (net oxidation). Thus, the sign of Iex is sufficient to identify whether it is an external cathodic or anodic current. An expression for the external current is obtained on substitution ofthe individual Tafel relationships in Eq 4.66 ... 

Interpretation of cathodic protection of iron in an environment of PH = 1 may be made by reference to Fig. 4.26. Without an external current, steady-state corrosion occurs under the conditions, Ecorr and icorr. If electrons are supplied to the metal, the potential will decrease, and at any arbitrary reduction of potential (e.g., Ej), a current balance requires that Iex = Iox M - Ired x, or iexA = iox MA - ired xA for a given area A (assuming that Ac = Aa = A), or iex = iox m - bed x- This external current density is represented in Fig. 4.26 as the span between the respective polarization curves at Ej. It is evident that for corrosion to be stopped, E must be reduced to E Fe, and to maintain this protection, the external... 

Fig- 4.25 Components used to impose and monitor conditions providing cathodic protection by an impressed external current. Note Power supply may be either a galvanostat or a potentiostat. In the latter, the electrometer provides feedbackto the potentiostatto control to constant potential. Electrometer provides check to show that the metal is at the protection potential. 

Fig. 4.26 Schematic polarization curves used in the analysis of cathodic protection by an impressed external current. Cathodic reaction is under Tafel control.

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