Passivation spontaneous

Spontaneous Passivation The anodic nose of the first curve describes the primary passive potential Epp and critical anodic current density (the transition from active to passive corrosion), if the initial active/passive transition is 10 lA/cm or less, the alloy will spontaneously passivate in the presence of oxygen or any strong oxidizing agent. 

Amorphous Fe-3Cr-13P-7C alloys containing 2 at% molybdenum, tungsten or other metallic elements are passivated by anodic polarisation in 1 N HCl at ambient temperature". Chromium addition is also effective in improving the corrosion resistance of amorphous cobalt-metalloid and nickel-metalloid alloys (Fig. 3.67). The combined addition of chromium and molybdenum is further effective. Some amorphous Fe-Cr-Mo-metalloid alloys passivate spontaneously even in 12 N HCl at 60° C. Critical concentrations of chromium and molybdenum necessary for spontaneous passivation of amorphous Fe-Cr-Mo-13P-7C and Fe-Cr-Mo-18C alloys in hydrochloric acids of various concentrations and different temperatures are shown in Fig. 3.68 ... 

Fig. 3.68 Critical concentrations of chromium and molybdenum necessary for spontaneous passivation of amorphous Fe-Cr-Mo-13P-7C and Fe-Cr-Mo-18C alloys in hydrochloric acid of various concentrations and temperatures ...

X-ray photoelectron spectroscopic study of the spontaneously passive amorphous Fe-10Cr-13P-7C alloy in 1 N HCl revealed that the passive film consists of Cr, 0 , OH" and HjO, and hence the passive film has been called a passive hydrated chromium oxyhydroxide film (CrO (OH)j Subsequent investigations have revealed that... 

The consequences of shape change are densification and loss of electrode porosity, increased current density caused by loss of zinc surface area, and finally earlier passivation. Two different forms of pasi-vation can stop the discharge of a zinc electrode before the active material is exhausted. "Spontaneous" passivation occurs... 

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... 

Almost all metallic materials in practical environments perform their service in the state of spontaneous passivation, in which hydrated oxygen moleciiles or hydrogen ions act as oxidants to passivate the surfaces. Stainless steel is a good and widely known example of corrosion resistant metals it is spontaneously passivated and remains in the passive state with a thin passive oxide film even in fairly corrosive environments. 

In the presence of oxidizing species (such as dissolved oxygen), some metals and alloys spontaneously passivate and thus exhibit no active region in the polarization curve, as shown in Fig. 6. The oxidizer adds an additional cathodic reaction to the Evans diagram and causes the intersection of the total anodic and total cathodic lines to occur in the passive region (i.e., Ecmi is above Ew). The polarization curve shows none of the characteristics of an active-passive transition. The open circuit dissolution rate under these conditions is the passive current density, which is often on the order of 0.1 j.A/cm2 or less. The increased costs involved in using CRAs can be justified by their low dissolution rate under such oxidizing conditions. A comparison of dissolution rates for a material with the same anodic Tafel slope, E0, and i0 demonstrates a reduction in corrosion rate... 

Armed with an understanding of the underlying physical processes, the electrochemical phenomenology of localized corrosion can be better understood. Figure 23 shows three schematic polarization curves for a metal in an environment in which it spontaneously passivates and (1) can be anodized, (2) transpassively dissolves at higher potentials, and (3) pits upon further anodic polarization. We have discussed cases 1 and 2 in the section on passivity. For case 3, the region of passivity extends from to a potential labeled EM at which point the current increases dramatically at higher potentials. 

Figure 23 Schematic polarization curve for metal that spontaneously passivates but pits upon anodic polarization. A hysteresis loop, which can appear during a reverse scan, is shown ending at Erp. One dotted line shows behavior for anodizing conditions, while the other shows transpassive dissolution.

Austenitic stainless steels are generally regarded as being spontaneously passive in aerated, near-neutral aqueous solutions, but surface treatment has a significant... 

Passive films formed in aqueous solutions consist of an oxide or a mixture of oxides, usually in hydrated form. The oxide formed on some metals (e.g., Al, Ti, Ta, Nb) is an electronic insulator, while on other metals the passivating oxide film behaves like a semiconductor. Nickel, chromium, and their alloys with iron (notably the various kinds of stainless steel) can be readily passivated and, in fact, tend to be spontaneously passivated upon contact with water or moist air. It should be noted that passivation does not occur when chloride ions are introduced into the solution indeed a preexisting passive film may be destroyed. Many other ions are detrimental to passivity, such as Br, I, SO, and CIO, but chloride is the worst offender, because of its... 

The silicon surface in nonfluoride and nonalkaline solutions is spontaneously passivated due to the formation of a thin native oxide film at a rate depending on many factors as discussed in Chapter 2. For n-Si samples in aqueous solutions under illumination the occurrence of passivation causes a decrease of the photocurrent as shown in Fig. 5.11. " In the absence of HF, photocurrent rapidly reduces to near zero due to the formation of an oxide film. The stationary photocurrent increases with increasing HF concentration. For a given light intensity, there is a HF concentration above which the photocurrent does not decrease from the initial value. The surface is free of oxide film at this HF concentration. 

It is in fact the acidification of the occluded crevice solution that triggers the crevice corrosion. The critical acid concentration, < , , for crevice corrosion to occur corresponds to what we call the passivation-depassivation pH, beyond which the metal spontaneously passivates. This critical acidity determines the crevice passivation-depassivation potential, and hence the crevice protection potential Ecrev. The electrode potential actually measured consists of the crevice passivation-depassivation potential, E -ev, and the IR drop, A/iIR, due to the ion migration through the crevice. Assuming the diffusion current from the crevice bottom to the solution outside, we obtain AEm = icmv x h constant, where crcv is the diffusion-controlled metal dissolution current density at the crevice bottom and h is the crevice depth [62], Since anodic metal dissolution at the crevice bottom follows a Tafel relation, we obtain Eciev as a logarithmic function of the crevice depth ... 

Figure 6. Spontaneous passivation of titanium by galvanically coupling to platinum...

Effect of oxidizer concentration on passivity Methods for Spontaneous Passivation of Metals Alloy Evaluation... 

Mixed potential theory and the Levich equation are used to construct the anodic and cathodic polarization curve of the active-passive alloy and to estimate the value of the oxygen limiting diffusion current when a spontaneous passive film is formed on the surface. Figure 4.11 correlates the relationship between potential and current for the... 

The potentiodynamic polarization technique is used to determine the potential region at which the alloy or the metal is passive when exposed to a particular environment. It estimates both the ability of the material to spontaneously passivate as well as the critical current density necessary for its passivation. Potentiodynamic polarization measurements identify the corrosion properties of passivating metals and alloys and are very usefirl in predicting how a material will behave when exposed to a corrosive environment. The method estimates the corrosion active region, the onset of passivation, the critical current density, the primary passive potential, the current in the passive region, and the voltage span of the passive region. 

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