Metallic corrosion cathodic electron transfer

A second related issue is the asymmetry in the E-i response near Ecelectron transfer reaction that is different from the metal oxidation reaction. Therefore there is no fundamental reason why pa and pc should be equal, and they should be expected to differ. The extent of their difference defines the degree of asymmetry. Asymmetry matters because the extent of the region where Eq. (2) is a good approximation of Eq. (1) then differs for anodic and cathodic polarization (29). The errors in assuming 10 mV linearity using both the tangent to the E-i data at Econ and for +10 or -10 mV potentiostatic polarizations have been defined for different Tafel slopes (30). 

Let us consider the corrosion of metallic iron in acid solution. As mentioned in the foregoing text, metallic corrosion occurs as a coupled reaction of an anodic metal ion transfer and a cathodic electron transfer from the metal into the solution ... 

For more than a century, a number of different aluminum alloys have been commonly used in the aircraft industry These substrates mainly contain several alloying elements, such as copper, chromium, iron, nickel, cobalt, magnesium, manganese, silicon, titanium and zinc. It is known that these metals and alloys can be dissolved as oxides or other compounds in an aqueous medium due to the chemical or electrochemical reactions between their metal surfaces and the environment (solution). The rate of the dissolution from anode to cathode phases at the metal surfaces can be influenced by the electrical conductivity of electrolytic solutions. Thus, anodic and cathodic electron transfer reactions readily exist with bulk electrolytes in water and, hence, produce corrosive products and ions. It is known that pure water has poor electrical conductivity, which in turn lowers the corrosion rate of materials however, natural environmental solutions (e g. sea water, acid rains, emissions or pollutants, chemical products and industrial waste) are highly corrosive and the environment s temperature, humidity, UV light and pressure continuously vary depending on time and the type of process involved. ... 

Fig. 11-1. Mixed electrode model (local cell model) for corrosion of metals i = anodic current for transfer of iron ions i = cathodic current of electron transfer for reduction of hydrogen ions.

Equation 11-6 corresponds to the affinity for the reaction of metallic corrosion. As described in Chaps. 8 and 9, the anodic transfer current i of metal ions and the cathodic transfer current i of electrons across the interface of corroding metallic electrodes are, respectively, given in Eqns. 11-7 and 11-8 ... 

A very important aspect of the corrosion of metal has been only touched upon so far. This aspect concerns the electronation (cathodic) reaction required to complete the corrosion circuit by consuming the electrons transferred to the metal from the metal-dissolution reaction. The question is What is the electronation (cathodic) reaction ... 

Fig. 4 shows a simple phase diagram for a metal (1) covered with a passivating oxide layer (2) contacting the electrolyte (3) with the reactions at the interfaces and the transfer processes across the film. This model is oversimplified. Most passive layers have a multilayer structure, but usually at least one of these partial layers has barrier character for the transfer of cations and anions. Three main reactions have to be distinguished. The corrosion in the passive state involves the transfer of cations from the metal to the oxide, across the oxide and to the electrolyte (reaction 1). It is a matter of a detailed kinetic investigation as to which part of this sequence of reactions is the rate-determining step. The transfer of O2 or OH- from the electrolyte to the film corresponds to film growth or film dissolution if it occurs in the opposite direction (reaction 2). These anions will combine with cations to new oxide at the metal/oxide and the oxide/electrolyte interface. Finally, one has to discuss electron transfer across the layer which is involved especially when cathodic redox processes have to occur to compensate the anodic metal dissolution and film formation (reaction 3). In addition, one has to discuss the formation of complexes of cations at the surface of the passive layer, which may increase their transfer into the electrolyte and thus the corrosion current density (reaction 4). The scheme of Fig. 4 explains the interaction of the partial electrode processes that are linked to each other by the elec-... 

In the case of metallic corrosion, the local cell model assumes that corrosion occurs as a combination of anodic metal oxidation and cathodic oxidant reduction. The anodic metal oxidation (dissolution) is a process of metal ion transfer across the metal-solution interface, in which the metal ions transfer from the metallic bonding state into the hydrated state in solution. We note that, before they transfer into the solution, the metal ions are ionized forming surface metal ions free from the metallic bonding electrons. The metal ion transfer is written as follows ... 

When the corrosion potential of a metal is made by some means more positive than the passivation potential, the metal will passivate into almost no corrosion because of the formation of a passive oxide him on the metal surface. As shown in Figure 22.17, the passivation of a metal will occur, if the cathodic polarization curve for the redox electron transfer of oxidant reduction goes beyond the anodic polarization curve for the metal ion transfer in the active state of metal dissolution. As far as the anodic polarization curve of metal dissolution exceeds the cathodic polarization curve of oxidant reduction, however, the corrosion potential remains in the active potential range and the metal corrosion progresses in the active state. An unstable passive state will arise if the cathodic polarization curve crosses the anodic polarization curve at two points, one in the passive state and the other in the active sate. In this unstable state, a passivated metal, once its passivity is broken down, can never be repassivated again because of its active dissolution current greater than the cathodic current of oxidant reduction. 

When two metals or alloys are joined such that electron transfer can occur between them and they are placed in an electrolyte, the electrochemical system so produced is called a galvanic couple. Coupling causes the corrosion potentials and corrosion current densities to change, frequently significantly, from the values for the two metals in the uncoupled condition. The magnitude of the shift in these values depends on the electrode kinetics parameters, i0 and (3, of the cathodic and anodic reactions and the relative magnitude of the areas of the two metals. The effect also depends on the resistance of the electrochemical cir-... 

Electrochemical Corrosion. In a classical model of metallic corrosion, reduclants are reduced by electron transfer front the metal at cathodic sites while the metal undergoes oxidative dissolution at anodic sites (Figure 7.1). 

The corrosion reaction between the anode and the cathode proceeds, if there is a solution present enabling the transfer of metal ions of the corrosion reaction. The transfer of electrons between the anodic and cathodic areas occurs in the metal. The reaction can, however, be stopped or inhibited, if either the anode or the cathode is passivated or the solution, enabling the corrosive reaction, is removed [3]. 

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