Active dissolution current density

It will be shown later that the values of icrit, Epp, and ip, which are the important parameters defining the shape of the active-passive type of polarization curve, are important in understanding the corrosion behavior of the alloy. In particular, low values of icrit enhance the ability to place the alloy in the passive state in many environments. For this reason, the maximum that occurs in the curve at B (Fig. 5.4) is frequently referred to as the active peak current density or, in general discussion, as the active peak. It is the limit of the active dissolution current density occurring along the A region of the polarization curve. 

Polarization curves are very instructive for examining the combined thermodynamic and kinetic effects of metal passivation. Figure 5.4 presents the examples of Fe, Cr, and Ni in 0.5 M H2SO4 [17]. The active dissolution current density of all three metals gets up to... 

An increase in the K factor may come from a local increase in the active dissolution current density and should generate a local pH decrease, due to cation hydrolysis, and a local Cb concentration increase, as a consequence of electro-migration to support the current. The combination of the pH decrease and the Cl concentration increase leads then to a dissolution current increase, and so on. 

The Tafel constant was b = 0.20 V decade-1 for iron electrodes [55] and b = 0.20 V decade-1 for austenitic stainless steels [54] in acid solution. It is noticed that these Tafel constants are greater than those (0.03-0.1 V) usually observed with general dissolution of metals in acid solution. The other mode of localized corrosion is the active mode of corrosion that prevails in the potential range less positive (more cathodic) than the passivation potential, EP, in which potential range the localized corrosion is mainly controlled by the acidity of the occluded pit solution. In the potential range of active metal dissolution, the anodic dissolution current density is also an exponential function of the electrode potential, except for diffusion-controlled dissolution. 

A somewhat alternative analysis of pitting attributes pit initiation to the activation of defects in the passive film, defects such as those induced during film growth or those induced mechanically due to scratching or stress. The pit behavior is analyzed in terms of the product, xi, a parameter in which x is the pit or crevice depth (cm), and i is the corrosion current density (A/cm2) at the bottom of the pit (Ref 21). Experimental measurements confirm that, for many metal/environment systems, the active corrosion current density in a pit is of the order of 1 A/cm2. Therefore, numerical values for xi may be visualized as a pit depth in centimeters. A defect becomes a pit if the pH in the pit becomes sufficiently low to prevent maintaining the protective oxide film. Establishing the critical pH, for a specific oxide, will depend on the depth (metal ions trapped by diffiisional constraints), the current density (rate of generation of metal ions) and the external pH. In turn, the current density will be determined by the local electrochemical potential established by corrosion currents to the passive external cathodic surface or by a potentiostat. Once the critical condition for dissolution of the oxide has been reached, the pit becomes deeper and develops a still lower pH by further hydrolysis. 

In de-aerated 10sulphuric acid (Fig. 3.45) the active dissolution of the austenitic irons occurs at more noble potentials than that of the ferritic irons due to the ennobling effect of nickel in the matrix. This indicates that the austenitic irons should show lower rates of attack when corroding in the active state such as in dilute mineral acids. The current density maximum in the active region, i.e. the critical current density (/ ii) for the austenitic irons tends to decrease with increasing chromium and silicon content. Also the current densities in the passive region are lower for the austenitic irons... 

Similar curves determined in 50 Vo sodium hydroxide solution at 60°C show (Fig. 3.46) that the austenitic irons exhibit more noble active dissolution and also lower current densities in the active and passive regions than the ferritic irons the current densities in both regions decrease markedly with increasing nickel content (Fig. 3.47). 

The anodic dissolution of nickel is also dependent on the amount of cold work in the metal and in the active region the anodic current density of cold worked material at a given potential is up to one order of magnitude greater than that of annealed material. 

Corrosion (spontaneous dissolution) of the catalyticaUy active material, and hence a decrease in the quantity present. Experience shows that contrary to widespread belief, marked corrosion occurs even with the platinum metals. For smooth platinum in sulfuric acid solutions at potentials of 0.9 to 1.0 V (RHE), the steady rate of self-dissolution corresponds to a current density of about 10 A/cm. Also, because of enhanced dissolution of ruthenium from the surface layer of platinum-ruthenium catalysts, their exceptional properties are gradually lost, and they are converted to ordinary, less active platinum catalysts. 

In solutions containing different anions, as seen in Fig. 17, the sudden rise in the anodic current density mentioned earlier [see Section 111(2)] and characteristic of initiation of active dissolution occurs at different potentials. It was shown108 that, at least with halides, this potential is a linear function of the crystalline radius of the ion. 

The depth profiling technique used on samples with a barrier film before and after the addition of chloride to the buffering borate electrolyte showed no indication of either chloride penetration or significant reduction of the average oxide layer thickness.123 This, of course, does not rule out the possibility of the formation, by any of the mechanisms suggested above, of pinholes with radii much smaller than that of the ion-gun beam, through which the entire active dissolution could take place, or the possibility that the beam missed pits formed sporadically across the surface. If pinholes which are not visible were formed, the dissolution should proceed in them with extremely high true current densities. 

This phenomenon, however, is not difficult to understand in view of the mechanism of dissolution under such conditions. Since the number of active sites increases linearly with current density and these sites are characterized by a film structure (or thickness or both) different from that at the OCP, one could expect corresponding increases in the corrosion rate. However, as was mentioned earlier, the active surface area in the pits increases with time, and hence one should expect the corrosion rate to increase correspondingly. Therefore, since the effect is not time dependent, one... 

The fact that impurities do not affect the active dissolution in chloride solutions at current densities larger than 0.01mA/ cm2 shows that the inhomogeneity resulting in a pitting mechanism of dissolution is unrelated to impurities and is an inherent property of the metal. 

For the case of Si02 etching, HF, (HF)2 and HF2- are assumed to be the active species [Vel, Jul]. If HC1 is added to the solution the concentration of the HF2-ion becomes negligible, which leaves HF and its polymers to be the active species [Ve3]. Because for high current densities the electrochemical dissolution of silicon occurs via a thin anodic oxide layer it can be concluded that, at least for this regime, the same species are active. This is supported by the observation that F- is... 

In acidic electrolytes with fluoride, silicon is stable at OCP, while electrochemical dissolution takes place for anodic potentials. For anodic current densities below the critical current density JPS PS is formed and the electrolyte-electrode interface is found to be Si-H covered. Species active in the dissolution process are HF, (HF)2 and HF2. A dissolution reaction proposed for this regime is ... 

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. 

In Fig. 16.9, the operationally significant parts of qualitative polarization curves for a typical steel and a stainless steel are superimposed. It is seen that, for a given Eh value E in the active range of the stainless steel, the current density will be higher for dissolution of the stainless steel than for corrosion of the iron. It is therefore very important that stainless steels be prevented from becoming active in service, because, if they do, they corrode rapidly, more than ordinary iron would. 

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