Characterization of passivating films

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

Physicochemical characterization of passive films and corrosion layers by differential admittance and photocurrent spectroscopy. 

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 ... 

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. 

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... 

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... 

Among the ex-situ techniques, surface analysis methods, in particular XPS, AES and SIMS, have been widely used for characterizing the chemical composition of passive films. When these techniques are employed, it is usually assumed that the film undergoes no significant chemical modification during the transfer from the electrolyte to the high-vacuum analysis chamber, an assumption that needs to be critically examined in each case. 

Major advances in materials characterization and in materials modeling and simulation in recent years also have provided new tools with impressive capabilities for addressing the most perplexing problems in corrosion science. For instance, there was once an ongoing debate over the very basic nature of the passivity (films versus adsorbed layer), and various theories were advanced. Development of techniques in surface science such as optical and later photoelectric and synchrotron radiation methods to characterize valence state, thicknesses, and composition produced a leap forward in the understanding of passive films. 

Extensive work has also been performed on characterizing the electronic structure of passive films on various metals in terms of semiconductor models (Gerischer, 1990 Di Paola, 1989 Schultze, 1978). It is well recognized that passive Elms exhibit semiconductor properties, and numerous attempts have been made to interpret these in terms of classical theory for n- or p-doped semiconductors. The application of EIS has proven very successful (Quarto et al., 1981 Schmuki and Bohni, 1992 Stimming 1986) provided that the frequency is sufficiently high that only the electronic properties are being probed (f> 1 kHz), and ion and vacancy relaxation processes are unimportant. 

H2O adsorption from water vapor has been extensively studied, first on powdered samples, more recently on single crystals. Most experiments on powders were carried out at high vapor pressures (> 1 torr). Experiments on single crystals are usually performed at low gas pressures (< 10 torr) because of the limitations of the modem surface science techniques. Powdered samples are highly defective and contain various crystalline orientations and stmctures. They are more representative of the behavior of passive films formed in aqueous solution. Studies on well-characterized surfeces of single crystals make it possible to investigate the role of surface defects and surface orientation on the mechanisms of chemisorption. 

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. 

Corrosion products formed as thin layers on metal surfaces in either aqueous or gaseous environments, and the nature and stability of passive and protective films on metals and alloys, have also been major areas of XPS application. XPS has been used in two ways, one in which materials corroded or passivated in the natural environment are analyzed, and another in which well-characterized, usually pure metal surfaces are studied after exposure to controlled conditions. 

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 the literature on electrodeposited electroactive and passivating polymers is vast, surprisingly few studies exist on the solid-state electrical properties of such films, with a focus on systems derived from phenolic monomers, - and apparently none exist on the use of such films as solid polymer electrolytes. To characterize the nature of ultrathin electrodeposited polymers as dielectrics and electrolytes, solid-state electrical measurements are made by electrodeposition of pofy(phenylene oxide) and related polymers onto planar ITO or Au substrates and then using a two-electrode configuration with a soft ohmic contact as the top electrode (see Figure 27). Both dc and ac measurements are taken to determine the electrical and ionic conductivities and the breakdown voltage of the film. 

The intensities of the integrated signals may be evaluated on the basis of well-characterized standards. Consequently ISS provides qualitative and quantitative information on the composition of the surface. Noble gas ions that penetrate the first layers of the surface are backscattered as neutrals, and thus may not pass the energy analyzer. As a consequence, only ions backscattered at the first atomic layer are detected and the method is sampling the outmost atomic layer. A soft sputter process by noble gas ions yields an ISS depth profile with atomic depth resolution. Therefore ISS has been applied to the study of very thin oxide films, as e.g. of passivated Fe/Cr alloys. This method may be applied in addition to XPS due to its high depth resolution. 

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.)...

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