Passivity oxide theory

Exception to this oxide theory, however, has been taken by Muller and Konigsberger,5 on the ground that the reflecting power of iron, rendered passive anodically when immersed in alkaline solution, remains undimmed, whereas if a layer of oxide were formed an alteration would be expected. But it is not necessary to postulate the formation of a thick layer of oxide. If of merely molecular dimensions, it would still preserve the underlying metal from attack, whilst a thickness comparable with that of the length of a light wave would be necessary to affect the reflecting power.6... 

This, like the oxide theory, has much to recommend it. An explanation is afforded for the superficial nature of passivity, and the theory also offers an explanation for the passivity induced in certain cases where oxygen cannot be the cause. 

Modern passivation theory views the primary passivation act as a formation of a tightly held layer of monomolecular dimensions, containing oxide or hydroxide anions and metal cations the process involves the formation of a new phase, an oxide, in steps that Include the adsorption as an Important intermediate stage. Thus the long-time controversial aspects of the adsorption and oxide theories are, in one sense, combined. 

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. 

This theory is based on the assumption that aggressive anions which are absorbed at the energetically favored sites on the passive layer cause the nucleation of pits. In order to overcome the repulsive forces between anions, there should be an exchange of OH or of the passive oxide layer... 

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

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. 

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. 

The next major theory of metal ammines was proposed by Carl Ernst Claus (1796—1864). In 1854, Claus rejected the ammonium theory and suggested a return to Berzelius view of complexes as conjugated compounds. He compared the platinum ammines not with ammonium salts nor with ammonium hydroxide but with metal oxides. He designated the coordinated ammonia molecule as passive, in contrast to the active, alkaline state in the ammonium salts, where it can easily be detected and replaced by other bases . 

Over the past several years, Gruen and coworkers have examined the SH response from iron electrodes in alkaline solutions [45, 53, 172]. In their work on polycrystalline iron, they concluded that the potential dependent SH response which was observed during surface oxidation could be attributed to two intermediate phases on the electrode surface between the passive film at oxidative potentials and the reduced metal at hydrogen evolution potentials [53]. They have recently extended this work to Fe(110). In this study [172], they examined the SH rotational anisotropy from this crystal under ambient conditions. They found that the experiments reveal the presence of both twofold and threefold symmetric species at the metal/oxide interface. When their data is fit to the theory of Tom et al. [68], they conclude that the measured three-fold symmetric oxide is found to be tilted by 5° from the Fe(110) plane. The two-fold symmetric structure is aligned with the Fe(110) surface. 

While thick film passivity has been documented and understood for many years, the difficulties in studying thin film passivity were daunting. It took many years to determine that indeed a film was responsible for the effect, as these films are so thin that they are invisible to the eye (i.e., transparent to radiation in the visible region). Two main types of theories were developed in order to explain the phenomena observed theories based upon the idea of adsorption reducing the corrosion rate, and theories based upon the formation of a new phase, an oxide of the base metal, on the surface. In all cases, an increased barrier to dissolution results upon the increase in potential. This increased kinetic barrier upon anodic polarization contrasts with the exponentially decreased barrier which develops during anodic polarization of an active material. 

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... 

Bonhoeffer, Vetter, and others (63) have made extensive studies on iron which indicate that the passive film is composed of one or more oxides of iron. Young (64). Vermilyea (65) and Johansen et at, (57) have shown that the Mott-Cabrera concepts are applicable for the thin films on Ta, Ti, Hf, and Hb. Petrocelli (58) has shown evidence that the dissolution of aluminum In sulfuric acid takes place through a thin film and that the process appears to follow the Motr-Cabret a theory. Stern (66) reports data indicating that the kinetic. for the anodic oxidation of stainless steel are similar to those for aluminum apd tantalum (67). Pryor (68) has recently reviewed the work on passive films on iron and suggests a single passive film of y contains non-uniform defect concentra-... 

Passing on to the discussion of very thin layers of 10 to 100 X, such as those forming during oxidation of metals and alloys and also passive layers such as those formed on iron in nitric acid at low temperatures (0 to 200°C). one must consider additional phenomena which are characteristic of the reaction mechanisms involved. Our discussion of these phenomena are based, with certain limitations, on the theory of Mott and Cabrera. 

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