Passivity oxide-film theory

There are various theories on how passive films are formed however, there are two commonly accepted theories. One theory is called the oxide film theory and states that the passive film is a diffusion-barrier layer of reaction products (i.e., metal oxides or other compounds). The barriers separate the metal from the hostile environment and thereby slow the rate of reaction. Another theory is the adsorption theory of passivity. This states that the film is simply adsorbed gas that forms a barrier to diffusion of metal ions from the substrata. 

There are two commonly expressed points of view regarding the composition and structure of the passive film. The first holds that the passive film (Definition 1 or 2) is always a diffusion-barrier layer of reaction products—for example, metal oxide or other compound that separates metal from its environment and that decreases the reaction rate. This theory is sometimes referred to as the oxide-film theory. 

The structure of the passive film on alloys, as with passive films in general, has been described both by the oxide-film theory and by the adsorption theory. It has been suggested that protective oxide films form above the critical alloy composition for passivity, but nonprotective oxide films form below the critical composition. The preferential oxidation of passive constituents (e.g., chromium) may form protective oxides (e.g., Cr203) above a specific alloy content, but not below. No quantitative predictions have been offered based on this point of view, and the fact that the passive film on stainless steels can be reduced cathodically, but not stoichiometric Cr203 itself, remains unexplained. 

Ryan MP, Toney ME, Oblonsky LJ, Davenport AJ (2000) An x-ray diffraction study of the passive oxide film on irom In Interfadal Processes Theory and Experiment. Halley JW (ed), American Chemical Society Proceedings, in press... 

The passive film on nickel can be formed quite readily in contrast to the formation of the passive film on iron. Differences in the nature of the oxide film on iron and nickel are responsible for this phenomenom. The film thickness on nickel is between 0.9 and 1.2 mm, whereas the iron oxide film is between 1 and 4 mm. There are two theories as to what the passive film on nickel is. It is entirely NiO with a small amoimt of nonstoichiometry, giving rise to Ni cation vacancies, or it consists of an inner layer of NiO and an outer layer of anhydrous Ni(OH)2. The passive oxide film on nickel, once formed, cannot be easily removed by either cathodic treatment or chemical dissolution. 

Films, anodic oxide Films, passivating Films, plastic Film theory Film wrappers Filter Filter aid Filter aids Filter fabrics Filtering centrifuges Filter media Filters... 

Refs. [i] Strehblow HH (2003) Passivity of metals. In Alkire RC, Kolb DM (eds) Advances in electrochemical science and engineering. Wiley-VCH, Weinheim, pp 271-374 [ii] Vetter KJ, Gorn F (1973) Electrochim Acta 18 321 [Hi] Strehblow HH (2002) Mechanisms of pitting corrosion. In Marcus P (ed) Corrosion mechanisms in theory and practice. Marcel Dekker, New York, pp 243-285 [iv] Strehblow HH (2003) Pitting corrosion. In Bard AJ, Stratmann M, Frankel GS (eds) Corrosion and oxide films. Encyclopedia of electrochemistry, vol. 4. Wiley, Weinheim, 337... 

Illumination with light having a wavelength larger than the band gap of silicon generates a photocurrent under an anodic potential on an n-Si electrode but has essm-tially no effect on p-Si, as would be expected from the basic theories of semiconductor electrochemistry. However, the photocurrent may not be sustained because of the formation of an oxide film, which passivates the silicon surface to various degrees depending on the electrolyte composition. In solutions without fluoride species, the photocurrent is only a transient phenomenon before the formation of the oxide film. In fluoride solutions, in which oxide film is dissolved, a sustained photocurrent can be obtained. 

In the case of the nickel alloys, the stability of the passive layer is a problem. The alloys depend on the oxide films or the passive layers for corrosion resistance and are susceptible to crevice corrosion. The conventional mechanism for crevice corrosion assumes that the sole cause for the localized attack is related to compositional aspects such as the acidification or the migration of the aggressive ions into the crevice solution [146]. These solution composition changes can cause the breakdown of the passive film and promote the acceleration and the autocatalysis of the crevice corrosion. In some cases, the classic theory does not explain the crevice corrosion where no acidification or chloride ion build up occurs [147]. 

In this respect and regarding the discussion on the initial passivations, as pointed out by an increasing number of writers, the oxide-film and adsorption theories do not contradict but rather supplement one another. In looking at the primary act of passivation as the formation of a tightly held monolayer containing oxide or hydroxide anions and metal... 

Among metals there are differences in composition and stoichiometry of the oxide films. Halides such as chlorides play an important role in the growth and breakdown of passive films. Borates help stabilize the oxide film. Chloride ions cause severe localized corrosion such as pitting. Well-developed pits have high chloride ion concentration and low pH. Pitting can be random and amenable to stochastic (statistical) theory and very sensitive to experimental parameters such as induction time and electrochemical properties, which are difficult to reproduce. Electrochemical noise (EN) can clarify the initial conditions for pit initiation (24). 

Chapter 4 presents the fundamentals of passivity the film and adsorption theories of passivity criterion for passivation methods for spontaneous passivation factors affecting passivation, such as the effect of solution velocity and acid concentration alloy evaluation anodic protection systems and design requirements. A fuU discussion on stainless steel composition and crystalline structure, oxidizer concentration, and alloy evaluation is included. The chapter also considers anodic protection to establish a basis for anodic... 

Chloride ions—and, to a lesser extent, other halogen ions—break down passivity or prevent its formation in iron, chromium, nickel, cobalt, and the stainless steels. From the perspective of the oxide-fihn theory, Cl penetrates the oxide film through pores or defects easier than do other ions, such as SOi". Alternatively, Cl may colloidally disperse the oxide film and increase its permeability. 

While these six generalizations are not all encompassing, in that exceptions may exist, they are sufQcient to differentiate between various theories that have been proposed for the growth of barrier oxide layers on metals and alloys. A number of models that have been developed to describe the growth of anodic oxide films on metals are listed in Table 4.4.2, together with some of their important features and predictions. Of the models listed, which were chosen because they make analytical predictions that can be tested and because they introduced new concepts into the theory of passivity, only the point defect model (PDM) in its latest form (D. Macdonald [1999], Pensado-Rodriguez et al. [1999a,b]) accounts for all of the observations summarized above. 

The generally accepted model for passive film growth, illustrated in Fig. 3-14, is of field-assisted film formation, which is essentially a modified Cabrera-Mott model originally established for gaseous oxidation and the formation of thin oxide films in a gas at low temperature (Cabrera and Mott, 1948-1949 Fehlner and Mott, 1970). This classical theory describes the growth, in the direction perpendicular to the surface, of an oxide layer completely covering the substrate surface, by a hopping mechanism. The... 

Based on his experiments, Mormarty postulated that the passivity in stainless steels was caused by an invisible oxide film. This theory was not universally accepted. It was not until 1930 that his theory was proven electrochemically by H.H. Uhlig at the Massachusetts Institute of Technology. 

At ip, the metal dissolution occurs at a constant rate and the oxide film begins to thicken. According to elertric field theory, dissolution proceeds by transport of ionic species through the film under the influence of an electric field [F = (AF)/X]. As the potential is increased in the noble direction, the film starts to get thicker. Hence, the electric field within it may remain constant. For instance, if the potential is increased from i to E2, the electric field [F = AE)/X would change. In order to maintain the electric field constant the film must get thicker. The thickness of the film proceeds by transport of cations M " " outwards and combination of these cations with 0 or OH ions at the film/solution interface. The dissolution rate in the passive region, therefore, remains constant. The process of dissolution is a chemical process, and it is not dependent on potential. The film which is dissolved is immediately replaced by a new film and a net balance is maintained between dissolution... 

Some investigators believe that an oxide film on a metal surface is responsible for passivity and, thus, protection against corrosion. This theory postulates that chloride ions penetrate the oxide film on steel through pores or defects in the film easier than do other ions (e.g., SO/ ). Alternatively, the chloride ions may colloidally disperse the oxide film, thereby making it easier to penetrate. 

The development of acidity within an occluded cell is by no means a new concept, and it was used by Hoar s as early as 1947 in his Acid Theory of Pitting to explain the pitting of passive metals in solutions containing Cl ions. According to Hoar the Cl ions migrate to the anodic sites and the metal ions at these sites hydrolyse with the formation of HCl, a strong acid that inhibits the formation of a protective film of oxide or hydroxide. Edeleanu and Evans followed the pH changes when aluminium was made anodic in Cl solutions and found that the pH decreased from 8-8 to 5-3. 

Many theories on the formation mechanisms of PS emerged since then. Beale et al.12 proposed that the material in the PS is depleted of carriers and the presence of a depletion layer is responsible for current localization at pore tips where the field is intensified. Smith et al.13-15 described the morphology of PS based on the hypothesis that the rate of pore growth is limited by diffusion of holes to the growing pore tip. Unagami16 postulated that the formation of PS is promoted by the deposition of a passive silicic acid on the pore walls resulting in the preferential dissolution at the pore tips. Alternatively, Parkhutik et al.17 suggested that a passive film composed of silicon fluoride and silicon oxide is between PS and silicon substrate and that the formation of PS is similar to that of porous alumina. 

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