Mechanism of Passivation

Residual radioactivity accounts for 3 x 10 Cr atoms/cm (1.5 x 10 eq or 0.015 C passive-film substance/cm ). The equation assumes an adsorbed passive-film structure, but the same reasoning applies whatever the structure. 

Reduction of passivator continues at a low rate after passivity is achieved, equivalent in the absence of dissolved oxygen to the value of ipasave [ 0.3 pA/cm ( 3mA/m )] based on observed corrosion rates of iron in chromate solutions. Iron oxide and chromate reduction products slowly accumulate. The rate of reduction increases with factors that increase /passive, such as higher activity, higher temperatures, and the presence of CF. It is found, in practice, that less chromate is consumed as exposure time continues, consistent with /passive that also decreases with time. 

For optimum inhibition, the concentration of passivator must exceed a certain critical value. Below this concentration, passivators behave as active depolarizers and increase the corrosion rate at localized areas, such as pits. Lower concentrations of passivator correspond to more active values of the oxidation-reduction potential, and eventually the cathodic polarization curve intersects the anodic curve in the active region instead of in the passive region alone (Fig. 17.1). 

Passivation of iron by molybdates and tungstates, both of which inhibit in the near-neutral pH range, requires dissolved oxygen [6], contrary to the situation for chromates and nitrites. In this case, dissolved oxygen may help create just enough additional cathodic area to ensure anodic passivation of the remaining restricted anode surface at the prevailing rate of reduction of MoOi or of WOi , whereas in the absence of O2, icnticai is not achieved. 

Sodium benzoate [6, 7] (CeHsCOONa), sodium cinnamate [8] (CeHs-CH-CH COONa), and sodium polyphosphate [9,10] (NaPOsjn (Fig. 17.2) are further examples of nonoxidizing compounds that effectively passivate iron in the nearneutral range, apparently through facilitating the adsorption of dissolved oxygen. As little as 5 X sodium benzoate (0.007%) effectively inhibits steel in 

---

The basic mechanism of passivation is easy to understand. When the metal atoms of a fresh metal surface are oxidised (under a suitable driving force) two alternative processes occur. They may enter the solution phase as solvated metal ions, passing across the electrical double layer, or they may remain on the surface to form a new solid phase, the passivating film. The former case is active corrosion, with metal ions passing freely into solution via adsorbed intermediates. In many real corrosion cases, the metal ions, despite dissolving, are in fact not very soluble, or are not transported away from the vicinity of the surface very quickly, and may consequently still... 

In view of the fact that there are two opposing views on the mechanism of passivity it is not surprising that a similar situation prevails concerning the mechanism of breakdown of passivity. The solid film theory of passivity and breakdown of passivity is dealt with in some detail in Section 1.5, so that it is appropriate here to discuss briefly the views based on the adsorption theory. 

In many aqueous solutions nickel has the ability to become passive over a wide range of pH values. The mechanism of passivation of nickel and the properties of passive nickel have been studied extensively—perhaps more widely than for any other element, except possibly iron. In recent years the use of optical and surface analytical techniques has done much to clarify the situation . Early studies on the passivation of nickel were stimulated by the use of nickel anodes in alkaline batteries and in consequence were conducted in the main in alkaline media. More recently, however, attention has been directed to the passivation of nickel in acidic and neutral as well as alkaline solutions. 

Apart from the work toward practical lithium batteries, two new areas of theoretical electrochemistry research were initiated in this context. The first is the mechanism of passivation of highly active metals (such as lithium) in solutions involving organic solvents and strong inorganic oxidizers (such as thionyl chloride). The creation of lithium power sources has only been possible because of the specific character of lithium passivation. The second area is the thermodynamics, mechanism, and kinetics of electrochemical incorporation (intercalation and deintercalation) of various ions into matrix structures of various solid compounds. In most lithium power sources, such processes occur at the positive electrode, but in some of them they occur at the negative electrode as well. 

Sysoeva et a/.114 made a systematic potentiostatic investigation of anodization in KOH solutions in the concentration range between 0.1 and 12.5 M and in the potential range between -1.5 and 0.5 V vs. SCE. They found a maximum in the aluminum dissolution rate at a KOH concentration of 3-5 M This is interpreted in terms of a change in the mechanism of passivation At low KOH concentra-... 

Fig. 5. Proposed mechanism of passive cis-displacement of a histone octamer by a transcribing SP6 RNA polymerase or RNA polymerase III. (Adapted with permission from Ref. [109].) A. The transcribing polymerase III approaches a nucleosome. B. The nucleosomal DNA is partially unwrapped, a loop containing the polymerase is formed, and the DNA behind the polymerase binds to the vacated histones. C. The octamer is reformed in a new position behind the polymerase.

Such experiments should reveal the extent and mechanism of passivation in various metal systems. These phenomena, when understood, can help to prevent the problem of nonreactivity of many metals and alloys in applied systems. We have begun experiments of this type. 

Stem (77) has recently discussed the mechanism of passivating-type inhibitors on the basis of the theory presented here. He points out that passivity may be accounted for operationally by anodic polarization phenomena irrespective of whether films are assumed to be responsible (79). This approach was also suggested by Mears (80). Tomasfcov (76) has proposed the use of the ratio of the extent of anodic polarization, - Ej p to the extent of the cathodic polarization,... 

The role of chemisorption in the mechanism of passivity is borne out by the typical patterns of data, having the same shape as adsorption isotherms, which describe concentration of radioactive chromium on the surface of iron passivated by chromates (11), or by potential changes induced by surface concentration of chromates (12), both as a function of chromate concentration in solution (Figure 1). It is also illustrated by the initially rapid rate, followed by a measurably slow rate, with which metals achieve passivity as followed by potential change with time for iron immersed in chromates or by 18-8 immersed in aerated water (IS) (Figure 2), and by... 

The content of iron was used in place of an iron surface in the simplified model and linear relationships were presumed between mineral precipitation and passivation of iron. Only a small iron content was used in each cell to achieve results in acceptable simulation times. The model results in PCE concentration profiles which are typically observed in column experiments (Fig. 13.6), even if the assumptions cannot be verified as the real mechanisms of passivation. Nevertheless, the simulation shows a migration... 

This reaction occurs, for example, with passive iron. The inverse reaction is also well known as an additional mechanism of passivation of iron in solutions containing Fe +-ions [50]. If the solubility product is not reached, chemical dissolution takes place... 

In the presence of chloride ions passivation of steel needs higher pH value. Several hypotheses were proposed to explain the mechanism of passive film destmction in the presence of chlorides [342]. Possibility of chloride ions penetration to passive film, the effect of electric field generated around the adsorbed chloride ions, promoting of Fe " ions diffusion from the surface of metal are listed. Other factors will be discussed farther. 

For metals that are passive by Definition 1, based on marked anodic polarization, the films are usually invisible, about 2 to 3 nm thick. Metals and alloys in this category have been the source of extended debate and discussion on the mechanism of passivity over the past 150 years. K the surface is abraded, local high temperatures generated at the surface produce a detectable oxide, but this is not the passive film. 

Passive mass transfer In the case of passive mass transfer, the membrane liquid phase consists of an organic solvent or a mixture of organic solvents. The transfer of the analyte across both membrane/solu-tion interfaces is governed by its partition coefficient. Figure 2A illustrates schematically the passive transport of an organic acid across a liquid membrane involving suitable protolytic reactions in both the feed and receiver solutions. If the volume of the receiver solution is smaller than the volume of the feed solution the analyte of interest can be concentrated as well. The mechanism of passive liquid membrane separation is analogous to that involved in separation based on solid SP and gas-diffusion membranes. However, unlike these membranes, liquid membranes... 

K.E. Healy and P. Ducheyne, The mechanisms of passive dissolution of titanium in a model physiological environment. Journal of Biomedical Materials Research, 26, 319-338 (1992). 

The penetration of chlorine atoms into the passive films is suggested by close examination of the relaxed structures in Figures 7.10 and 7.11. It seems to occur independently of the O-enriched or O-deficient nature of the films and of the implemented defect site. However, this aspect was not addressed by the authors in their study and thus cannot be further discussed. Detailed studies relevant for testing the penetration-induced voiding mechanism of passivity breakdown would require implementing O vacancies as point defects not only at the surface but also in the bulk of the passive films of appropriate crystalline structure. Implementation of field-assisted transport in the passive film and at its interfaces would also be required. 

It appears that it is the compact oxide film which prevents further oxidation. In that sense, these metals become similar to other metals which undergo passivity, e.g., transition metals. A number of phenomena connected with, and considerations of, the mechanism of passivity are given elsewhere (cf. Volume VIII, Chapters 3 and 4). 

It is possible to classify the mechanisms of passive control of the seismic response in four classes ... 

---