Passivity activation potential

Active region Max. c.d. prior to passivation, i, Potential of passivation, Ep C.d. in passive region, /p Passive region... 

The theoretical aspects of molybdenum s corrosion behaviour are complex and there is as yet no clear cut, generally applicable picture. There are, however, a large number of literature references which include data on polarisation, passivation and potential of molybdenum under widely assorted conditions. The electrode potential of molybdenum depends on its surface condition. For example, some tests showed an of -t-0-66V when the molybdenum was passivated by treatment with concentrated chromic acid and —0-74 V after activation by cathodic treatment in sodium hydroxide. 

Potential measurement This technique has provided valuable information as to the condition of passive/active materials, particularly in the chemical industryAlthough quantitative weight loss measurements are not obtained, measurements can be on-line and more importantly, can be monitored using the actual plant material in situ) as a sensor. 

Introduction of electrOcheiiiicaUy active cathodes that facilitate passivation Raise potential by external e.m.f Additions of Pt. Pd and other noble metals to Ti, Cr and stainless steels Anodic protection of steel, stainless steel and Ti... 

Let us mention some examples, that is, the passivation potential at which a metal surface suddenly changes from an active to a passive state, and the activation potential at which a metal surface that is passivated resumes active dissolution. In these cases, a drastic change in the corrosion rate is observed before and after the characteristic value of electrode potential. We can see such phenomena in thermodynamic phase transitions, e.g., from solid to liquid, from ferromagnetism to paramagnetism, and vice versa.3 All these phenomena are characterized by certain values... 

As mentioned in Section II.3, in the presence of film-destructive anions such as chloride ions, beyond the critical pitting potential Epiti pitting dissolution proceeds, creating semispherical pits (polishing-state pits), which are different in shape from the irregular pits that develop at the active region that is less noble than the activation potential Ea, where the corrosive reaction moves from the passive state to the active state (usually the activation potential Ea is different and less noble than the passivation potential Ep). 

When the polarization curve is recorded in the opposite (cathodic) direction, the electrode will regain its active state at a certain potential The activation potential is sometimes called the Flade potential (Flade, 1911). The potentials of activation and passivation as a rule are slightly different. 

The transition from the active state to the passive state is the passivation, and the transition in the reverse direction is the activation or depassivation. The threshold of potential between the active and the passive states is called the passivation potential or the passivation-depassivation potential. Similarly, the transition from the passive state to the transpassive state is the transpassivation, and the critical potential for the transpassivation is called the transpassivation potential. Further, a superficial thin film formed on metals in the passive state is often called the passive film (or passivation film), the thickness of which is in the order of 1 to 5 nm on transition metals such as iron and nickel. 

Figure 14 Evans diagrams for active-passive metal when coupled to (a) a metal that holds corr in a passive region, (b) a metal that holds Ecm above pitting or transpassive potential, and (c) a metal that causes a passive-active transition.

An improvement on the SL-EPR test is the double loop, or DL-EPR, test, which is shown schematically in Fig. 39. In this test, the potential is first scanned in the anodic direction from Ecoss to a point in the middle of the passive region before the scan is reversed. The ratio of the two peak current densities, L//a, is used as the degree of sensitization indicator. During the anodic sweep, the entire surface is active and contributes to the peak current. During the reactivation sweep, only the sensitized grain boundaries contribute to the passive-active transition. Thus in unsensitized specimens there is a small / and therefore a small ratio, while in heavily sensitized specimens, /r approaches /a, as shown in... 

The mode of transport through a membrane may be passive, active, or facilitated type. In passive transport, the membrane acts as a barrier and permeation of the components is determined by their diffusivity and concentration in the membrane or just by their size. In facilitated transport along with the chemical potential gradient, the mass transport is coupled to specific carrier components in the membrane. In active transport driving force for transport is achieved by a chemical reaction in the membrane phase. 

On n-Si the cathodic current shows similar dependence on potential with positive (initially active surface) and negative (initially passive surface) potential scan directions. However, on p-Si cathodic current is only observed on an active surface in the dark (with a positive scan rate). Also, the cathodic current on illuminatedp-Si on a passivated surface is much smaller than that on an active surface. Furthermore, the cathodic photocurrent on p-Si at low light intensity is double that without H2O2. At high light intensities the photocurrent becomes the same on active and passive surfaces and the current multiplication factor is less than 2. 

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. 

In deaerated 1 N H2SO4 (pH = 0.56), hydrogen-ion reduction is the cathodic reaction with the cathodic polarization curve intersecting the iron, nickel, and chromium curves in the active potential region. Hence, active corrosion occurs with hydrogen evolution, and the corrosion rates would be estimated by the intersections of the curves. The curves predict that the titanium will be passivated. However, the position ofthe cathodic hydrogen curve relative to the anodic curves for titanium and chromium indicates that if the exchange current density for the hydro-... 

The result is a corrosion rate of about 10 mA/m2, icorr (L). In contrast, the alloy with the higher anodic peak would not be passivated. The polarization curves cross in the active potential range of the alloy resulting in an active corrosion rate corresponding to about 250 mA/m2. 

Thus this theory satisfactorily describes the i-V curve in the active potential region, with a qualitative description of possibilities in the peisslve potential region and a semi-quantltatlve explanation of the passivation behavior of a variety of metals. 

The observed current-potential behavior is a function of the simultaneous processes of film formation, its dissolution, and metal dissolution. The latter seems to be mostly responsible for the magnitude of the current at all potentials. In the active potential region dissolution is hindered by a decrease in the free electrode area, and in the passive region dissolution depends entirely on the properties of the passivating film. 

FIGURE 8.4. Space-time plots of the potential distributions on a Pt ring electrode during passive-active transitions. The ring position refers to Fig. 8.3 [44]. (a) Local triggering, (b) Remote triggering. (See color insert.)... 

For the formation of passive layers, at least 10.5% Cr is necessary. The activation potential of iron-chromium alloys in sulfuric acid changes, depending on the chromium content. Alloys with chromium... 

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