Passivating film, characterization

It is probably worth noting that the passivity breakdown is a process different from the pitting dissolution that follows the passivity breakdown the former is associated with the passive film itself, whereas the metal that underlies the passive film characterizes the latter. 

The heat-transfer quaUties of titanium are characterized by the coefficient of thermal conductivity. Even though the coefficient is low, heat transfer in service approaches that of admiralty brass (thermal conductivity seven times greater) because titanium s greater strength permits thinner-walled equipment, relative absence of corrosion scale, erosion—corrosion resistance that allows higher operating velocities, and the inherently passive film. 

Ham D, Mishra KK, Rajeshwar K (1991) Anodic electrosynthesis of cadmium selenide thin films. Characterization and comparison with the passive/transpassive behavior of the CdX (X = S, Te) counterparts. J Electrochem Soc 138 100-108 Stimming U (1985) Photoelectrochemical studies of passive films (Review Article). Electrochim Acta 31 ... 

Some of the passive films have been characterized as semiconductors in this case, corrosion of these oxides may imply transfer of holes (h+) from the valence band to the reductant and of electrons (e ) from the conduction band, in the case of iron(III) oxides as Fe(II). 

The passive film is composed of metal oxides which can be semiconductors or insulators. Then, the electron levels in the passive film are characterized by the conduction and valence bands. Here, we need to examine whether the band model can apply to a thin passive oxide film whose thickness is in the range of nanometers. The passive film has a two-dimensional periodic lattice structure on... 

Although CVD and plasma deposited films offer excellent properties as a passivation layer, the inability to reproduce chemical and physical properties has been a problem. Depending on gas flow rates and deposition conditions, free Si, H, C and 0 may be Incorporated into the films. Characterization of these films has been restricted almost exclusively to surface analytical techniques and ellipsometry. AES and XPS have been used to determine the C, N, 0, and Si content of CVD silicon nitride. 

The specific goals of the experiment were to characterize the passive film and determine if it was primarily protective with chromium in the +3 state or if some chromium was in the +6 state indicating the possibility of a self-healing film. The effects of various impurities on chromate film growth were also examined. 

The self-sustaining nature of the process leads to a drastic reduction in the stability of the passive film. Stage HI is characterized by the breakdown of the passive film due to the attainment of what has become known as the critical crevice solution. This solution has a low pH [typically 1 or less (23)] and a high Cl"... 

Chemical passivity corresponds to the state where the metal surface is stable or substantially unchanged in a solution with which it has a thermodynamic tendency to react. The surface of a metal or alloy in aqueous or organic solvent is protected from corrosion by a thin film (1—4 nm), compact, and adherent oxide or oxyhydroxide. The metallic surface is characterized by a low corrosion rate and a more noble potential. Aluminum, magnesium, chromium and stainless steels passivate on exposure to natural or certain corrosive media and are used because of their active-passive behavior. Stainless steels are excellent examples and are widely used because of their stable passive films in numerous natural and industrial media.6... 

Figure C.l. K-K transforms of real and imaginary components of the electrochemical impedance for passive zinc in a borate solution of pH = 10.5, at 1.2 V (SCE) [13]. (Reproduced by permission of ECS—The Electrochemical Society, from Ismail KM, Macdonald DD. Characterization of the barrier passive film on zinc.)...

Passive films formed on a silicon surface in aqueous solutions are in general oxide films. There is rather limited systematic information on the structure and properties of thin silicon anodic oxide films, particularly those formed in solutions of high silicon solubilities. On the other hand, the thicker oxide films formed at large potentials have been better characterized (see Chapter 3) and the information associated can be used for understanding the thin oxide films formed at relatively low potentials. 

STM offers the possibility of performing local spectroscopic measurements (/ vs. V curves). These measurements can be performed in-situ and ex-situ. Ex-situ UHV conditions are however more appropriate to ensure the nonconductivity of the turmeling barrier between surface and tip. Such measurements on passive films formed on Ni and Cr should provide valuable information on the conductivity of the films. This is a promising perspective for the local characterization with high resolution of the electronic properties of passive films. On the subject of the relation between chemistry at the atomic scale and atomic structure, the STM results on the passive film formed on Ni also show promising perspectives for further characterization accurate bias-dependent measurements of the terraces of the NiO oxide should provide... 

Figure 8. Characterization of passivating films (a) adsorbed layer, (b) monomo-lecularfor less) oxide, and (c) three-dimensional oxide...

The structure of the passive oxide film formed on iron has been the subject of much controversy dating back to the discovery of the phenomenon in the 1700s because of the difficulty in characterizing the thin film in the aqueous environment, and it is only recently that SXS has been able to resolve some of the issues. M. F. Toney and co-workers [92] used SXS to study the passive oxide films formed on single-crystal Fe(OOl) and Fe(llO) substrates at a high anodic formation potential. Fig. 19 a and b show the measured diffraction patterns from the passive film on Fe(OOl) and Fe(llO) respectively. For growth on Fe(OOl), the oxide (001) planes... 

Initiation of pitting corrosion takes place when the chloride content at the surface of the reinforcement reaches a threshold value (or critical chloride content). A certain time is required from the breakdown of the passive film and the formation of the first pit, according to the mechanism of corrosion described above. From a practical point of view, the initiation time can be considered as the time when the reinforcement, in concrete that contains substantial moisture and oxygen, is characterized by an averaged sustained corrosion rate higher than 2 mA/m [8], The chloride threshold of a specific structure can be defined as the chloride content required to reach this condition of corrosion. 

Lopez et al. [160] applied the technique of Fermi level shift monitoring to characterize the add base properties of passive films on aluminum. The decreasing trend of relative basicity was found to be boehmite > thermal oxide > NaOH-degreased surface > silicate containing detergent-degreased surface > phosphoric add anodic film. The... 

D.D. Macdonald, S.I. Smedley, Characterization of vacancy transport in passive films using low-frequency electrochemical impedance spectroscopy, Corros. Sci. 31 (1990) 667-672. 

For some material-environment combinations it has been shown that accelerated anodic dissolution of yielding metal is the significant mechanism. This is the case for austenitic stainless steels in acidic chloride solutions. In these steels, plastic deformation is characterized by a dislocation pattern giving wide slip steps on the surface. For such systems, Scully [7.50] has proposed a model for initiation and development of stress corrosion cracks, which has been supported by other scientists [7.51]. The model in its simplest form is illustrated in Figure 7.52. A necessary condition is that flie surface from the beginning is covered by a passivating film (A). 

The characterization of the resultant film by VC was carried out by various methods. The CH = CHOCO Li was detected by infrared reflection absorption spectroscopy (IRRAS). It also was speculated that the formation of polymers having -OCOjLi group affords good adhesiveness and flexibility to the passivation film. The deposition process by VC reduction, which started from 1.3 V vs. LP/Li, was monitored by in-situ atomic force microscopy (AFM) and the film thickness down to 0.8 V was about 10 nm. Ex-situ AFM also revealed an ultra-thin film (less than 1 nm) on the terrace of the basal plane of a highly oriented pyrolytic graphite (HOPG)... 

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