External reduction current density

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... 

Imposing an anodic current density on the iron with an external device results in the generation of the anodic branch of the polarization curve. Increasing the applied anodic current decreases the reduction reaction rate as the surface is polarized in the positive direction. At small anodic current densities, the HER current density is still an appreciable fraction of the anodic current density. Under these conditions the applied current density is less than anodic current density. For example, at a potential of -0.225 V(NHE), 4 is 2 X 10 3 A/cm2, 4pP is 6 X 10 3 A/cm2, and 4 is 8 X 10 3 A/cm2. At sufficiently large anodic current densities (e.g., 10 2 A/cm2 in Fig. 26), the cathodic reaction is insignificant rela-... 

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... 

Upon polarization of either electrode, the cell potential moves along the oxidation and reduction curves as shown in Fig. 1.1. When the current through the cell is f, the potential of the copper and zinc electrodes is Cj cu and e zn > and each of the electrodes have been polarized by (Ceq.cu i.Cu) and (Ceq.zn i,z )- Upon further polarization, the anodic and cathodic curves intersect at a point where the external current is maximized. The measured output potential in a corroding system, often termed the mixed potential or the corrosion potential (Tcorr)> h the potential at the intersection of the anodic and the cathodic polarization curves. The value of the current at the corrosion potential is termed the corrosion current (Icon) and can be used to calculate corrosion rate. The corrosion current and the corrosion potential can be estimated from the kinetics of the individual redox reactions such as standard electrode potentials and exchange current densities for a specific system. Electrochemical kinetics of corrosion and solved case studies are discussed in Chapter 3. 

When a metal, M, is immersed in a solution containing its ions, M, several reactions may occur. The metal atoms may lose electrons (oxidation reaction) to become metaUic ions, or the metal ions in solution may gain electrons (reduction reaction) to become soHd metal atoms. The equihbrium conditions across the metal-solution interface controls which reaction, if any, will take place. When the metal is immersed in the electrolyte, electrons wiU be transferred across the interface until the electrochemical potentials or chemical potentials (Gibbs ffee-energies) on both sides of the interface are balanced, that is, Absolution electrode Until thermodynamic equihbrium is reached. The charge transfer rate at the electrode-electrolyte interface depends on the electric field across the interface and on the chemical potential gradient. At equihbrium, the net current is zero and the rates of the oxidation and reduction reactions become equal. The potential when the electrode is at equilibrium is known as the reversible half-ceU potential or equihbrium potential, Ceq. The net equivalent current that flows across the interface per unit surface area when there is no external current source is known as the exchange current density, f. 

At the value of the limiting (maximum) current density, the species M + are reduced as soon as they reach the electrode. At these conditions, the concentration of the reactant at the electrode is nil and the rate of deposition reaction is controlled by the rate of transport of the reactant to the electrode. If an external current greater that the limiting current ii is forced through the electrode, the double layer is further charged, and the potential of the electrode will change until some process other than the reduction of M + can occur. 

---