Anodic dissolution activated state

In the polarization curve for anodic dissolution of iron in a phosphoric acid solution without CP ions, as shown in Fig. 3, we can see three different states of metal dissolution. The first is the active state at the potential region of the less noble metal where the metal dissolves actively, and the second is the passive state at the more noble region where metal dissolution barely proceeds. In the passive state, an extremely thin oxide film called a passive film is formed on the metal surface, so that metal dissolution is restricted. In the active state, on the contrary, the absence of the passive film leads to the dissolution from the bare metal surface. The difference of the dissolution current between the active and passive states is quite large for a system of an iron electrode in 1 mol m"3 sulfuric acid, the latter value is about 1/10,000 of the former value.6... 

Figure 19. Effects of chloride ion and proton on anodic dissolution current of passive metal.20 pCl - og[CT], pH -log[H+]. p.s. pit and a.s. pit indicate polishing-state pit and active-state pit, respectively.

Passivation of an electrode with respect to a certain electrochemical reaction is the term used for the strong hindrance experienced under certain conditions by a reaction which under other conditions (in the electrode s active state) will occur without hindrance at this electrode. Passivation of metals imphes the hindrance frequently observed with respect to anodic metal dissolution. 

Let us assume that the total surface of an electrode is in an active state, which supports dissolution, prior to anodization. The application of a constant anodic current density may now lead to formation of a passive film at certain spots of the surface. This increases the local current density across the remaining unpassivated regions. If a certain value of current density or bias exists at which dissolution occurs continuously without passivation the passivated regions will grow until this value is reached at the unpassivated spots. These remaining spots now become pore tips. This is a hypothetical scenario that illustrates how the initial, homogeneously unpassivated electrode develops pore nucleation sites. Passive film formation is crucial for pore nucleation and pore growth in metal electrodes like aluminum [Wi3, He7], but it is not relevant for the formation of PS. 

The state in which the anodic dissolution of metals proceeds from the bare metal stirface at relatively low electrode potentials is called the active state the state in which metal dissolution is inhibited substantially by a superficial oxide film at higher electrode potentials is called the passive state-, the state in which the anodic dissolution of metals increases again at stiU higher (more anodic) potentials is called the transpassive state. 

In the active state, the dissolution of metals proceeds through the anodic transfer of metal ions across the compact electric double layer at the interface between the bare metal and the aqueous solution. In the passive state, the formation of a thin passive oxide film causes the interfadal structure to change from a simple metal/solution interface to a three-phase structure composed of the metal/fUm interface, a thin film layer, and the film/solution interface [Sato, 1976, 1990]. The rate of metal dissolution in the passive state, then, is controlled by the transfer rate of metal ions across the film/solution interface (the dissolution rate of a passive semiconductor oxide film) this rate is a function of the potential across the film/solution interface. Since the potential across the film/solution interface is constant in the stationary state of the passive oxide film (in the state of band edge level pinning), the rate of the film dissolution is independent of the electrode potential in the range of potential of the passive state. In the transpassive state, however, the potential across the film/solution interface becomes dependent on the electrode potential (in the state of Fermi level pinning), and the dissolution of the thin transpassive film depends on the electrode potential as described in Sec. 11.4.2. 

Anodic passivation can be observed easily and clearly with iron group metals and alloys as shown in Fig. 11-10. In principal, anodic passivation occurs with most metals. For instance, even with noble metals such as platinum, which is resistant to anodic dissolution in sulfuric acid solutions, a bare metal surface is realized in the active state and a superficial thin oxide film is formed in the passive state. For less noble metals of which the affinity for the oxide formation is high, the active state is not observed because the metal surface is alwa covered with an oxide film. 

The intersection of the anodic polarization curve of iron dissolution with the cathodic polarization ctuve of nitric add reduction occurs in the range of potential of the active state in dilute nitric acid, but it occurs in the range of potential of... 

Figure 11-15 shows the corrosion rate observed for a metallic nickel electrode in aerated aqueous sulfate solutions as a function of pH. In addic solutions, nickel corrodes in the active state at a rate which is controlled by the diffusion of hydrated oi en molecules (oxidants). In solutions more basic than pH 6, however, nickel spontaneously passivates by hydrated oiQ n molecules and corrosion is negligible. As shown in the inserted sub-figures in Fig. 11-15, the maximum current of anodic nickel dissolution in the active state is greater in the range of addic pH however, the Tnaximnm current of anodic nickel dissolution is smaller in the range of basic pH than the current of cathodic reduction of os en molecules (dashed curve) which is controlled by the diffusion of hydrated oiQ gen molecules. Consequently, metallic nickel remains in the active state in addic solutions but is spontaneously passivated by hydrated ojQ n molecules in basic solutions. It... 

Nonactive/slightly reactive electrode materials include metals whose reactivity toward the solution components is much lower compared with active metals, and thus there are no spontaneous reactions between them and the solution species. On the other hand, they are not noble, and hence their anodic dissolution may be the positive limit of the electrochemical windows of many nonaqueous solutions. Typical examples are mercury, silver, nickel, copper, etc. It is possible to add to this list both aluminum and iron, which by themselves may react spontaneously with nonaqueous solvent molecules or salt anions containing atoms of high oxidation states. However, they are not reactive due to passivation of the metal which, indeed, results from the formation of stable, thin anodic films that protect the metal at a wide range of potentials, and thus the electrochemical window is determined by the electroreactions of the solution components [51,52],... 

Flade, Friedrich — (Sep. 16, 1880, Arolsen, now Bad Arolsen, Germany - Sep. 5, 1916, near Manancourt, France) After studies of chemistry in Halle and Munich, Flade received his PhD in 1906 from the University of Marburg, Germany. There he qualified as University teacher (habilitation) in 1910 [i], Flade observed that iron shows a sudden potential change when it goes from the passive to the active state. Now, the electrode potential of a metal where the current associated with the anodic metal dissolution drops to very small values bears his name (- potential, subentry -> Flade potential). He also showed that loading of the iron surface with oxygen is essential for its -> passivation [ii—vi]. Flade fell in World War I in the Battle of the Somme, and he was buried in Manancourt, France. 

The expected Tafel slope of 60mV/decade is not always found. There are a number of reasons for this, aside from kinetic effects in the bulk of the semiconductor. The kinetic effects associated with faradaically active surface states is of considerable significance, as shown below, but another common problem is that part of the potential change may appear across the Helmholtz layer rather than across the depletion layer. A well-known case in point is germanium, for which the surface is slowly converted from "hydride to "hydroxylic forms as the potential is ramped anodically. This conversion gives rise to a change in the surface dipole and hence Aij/ AT. In fact, the anodic dissolution of p-germanium is found to follow a law [106]... 

In the equilibrium state, the anodic and cathodic partial reactions of an electrochemical reaction have equal rates. The system is in a dynamic equilibrium state, and no net reaction occurs. For example, when a copper sheet is immersed in copper sulfate solution, in the equilibrium state the anodic dissolution rate of copper from sheet to solution equals the cathodic deposition rate from the solution to the surface of the sheet. Theoretically, one can calculate the equilibrium state of an electrochemical reaction from thermodynamic values. This is the standard electrode potential, E°, or equilibrium potential of the electrochemical reaction. The standard electrode potential corresponds to a determined standard state of 0.1 MPa, 25 °C, activity of reactive species of 1 or ideal solution of 1.0 mol L-1, and equilibrium potential of any other state. 

High-rate anodic dissolution of metals has been studied intensively (Reviews [9, 12, 14, 21]). Several types of anodic metal dissolution are recognized. Sometimes, the dissolution of metals in the active state is used in ECM. Deep, small-diameter holes drilled in acidic solutions are an example. An acid is used in order to avoid sludge formation, which hampers the drilling. However, highly aggressive... 

The anodic metal dissolution current thus increases with the electrode potential up to a certain potential, called the passivation potential. As shown in Figure 22.7 for iron in acid solution [9,10], in the potential region more positive than the passivation potential, the metal passivates into almost no dissolution current due to the formation of a surface oxide film several nanometers thick, which we call the passive film [11], In contrast to the passive state of the metal, the active state refers to the metal undergoing anodic dissolution at significant rates below the passivation potential. 

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. 

The anodic passivation of semiconductors in aqueous solution occurs in much the same way as that of metals and produces a passive oxide film on the semiconductor electrodes. Figure 22.25 shows the anodic dissolution current and the thickness of the passive film as a function of electrode potential for p-type and n-type silicon electrodes in basic sodium hydroxide solution [32,33], As mentioned earlier, silicon dissolves in the active state as divalent silicon ions and in the passive state a film of quadravalent insoluble silicon dioxide is formed on the silicon electrode. The passive film is in the order of 0.2-1.0 nm thick with an electric field of 106 107 V cm 1 in the film within the potential range where water is stable. 

FIGURE 22.29 Schematic polarization diagrams (a) for the repassivation of pitting dissolution of metals and (b) for transformation from the electropolishing mode of pitting to the active mode of localized dissolution EP = passivation potential in the solution bulk, p = passivation potential in the critical pit solution, /sR = pit repassivation potential, /pit = pitting dissolution current, /a = anodic metal dissolution current in the active state in the bulk solution, and / = anodic metal dissolution current in the critical pit solution. 

For metals such as chromium and alloys such as stainless steel, the plot of potential versus corrosion rate above the range is shown in Figure 20.67. Figure 20.68 shows a sudden sharp drop in corrosion above some critical potential. Despite a high level of anode polarization above V, the corrosion rate drops precipitously due to the formation of a thin, protective oxide film as a barrier to the anodic dissolution reaction. Resistance to corrosion above is termed passivity. The drop in corrosion rate above can be as much as 10 to 10 times below the maximum rate in the active state. With increasing corrosion potential, the low corrosion rate remains constant until at a relatively high potential the passive film break down, and the normal increase in corrosion rate resumes in a transpassive region. 

---