Impedance oxide film

To further understand and characterise the oxide deactivation process, a.c. impedance studies were carried out, primarily with a 30 at.% Ru/Ti electrode, at various stages during deactivation. These data were compared to those obtained for freshly formed Ru/Ti oxide films, ranging in Ru content from 5 to 40 at.%. Impedance data were collected at the oxide OCP (approximately 0.9 V versus SCE) in fresh NaCI solutions. Under these conditions, no chlorine reactions can occur and the OCP is defined by the equilibria of the redox states on the Ru oxide surface. Deactivation was generally accomplished by square-wave potential cycling, using overpotentials versus the chlorine/chloride potential of 1.59 to — 0.08 V (60 s cycle-1) in 5 M NaCI + 0.1 M HC1 solutions at room temperature. 

Analysis of the impedance parameters/potential sweep rate observations [19], summarized in Table 6, demonstrates significant dependence only in the case of the oxide film resistance where a negative trend is evident. The probability, 1 - P[Z) < 38.5] = 1 -... 

The formation of a rare earth metal oxide on the metal surface, impedes the cathodic reduction of oxygen and thus cathodic inhibition is achieved by the addition of a rare earth metal salt to a system. The surface atom concentration ratio, [Ce/Ce + M], where M is Fe, Al or Zn, is a function of cerium oxide film thickness determined by AES depth profiles as shown in Fig. 12.2. 

It is all but impossible to prepare any semiconductor electrode without some surface film being present. The III/V semiconductors, for example, will normally possess oxide films whose thickness will vary from less than 10 A to more than 40 A after exposure to air and similar observations have been reported for silicon [77], Although the capacitance of these films will normally be considerably larger than that of the depletion layer, the film may affect the a.c. response both by virtue of the analysis leading to eqn. (72) and, if Css becomes sufficiently large, that the impedance of the depletion layer falls to a value comparable with that of the film. If the film has a finite resistivity, which may be ionic in character, then the equivalent circuit takes the form... 

Carpio and coworkers [4] supported this hypothesis via a potentiodynamic study of a set of HN03-containing slurries. The corrosion currents and potentials under both the static and the dynamic conditions were practically the same. This is consistent with the fact that there was no native copper oxide film formed because of the presence of these slurries. As a matter of fact, the corrosion currents decreased slightly upon abrasion of the copper surface. The contact between the metal surface and the abrasive pad may have limited the mass transport of chemicals to and from the copper surface. This was verified via an AC impedance measurement that showed the importance of the systems mass transport. It was also concluded that in a dissolution-controlled process, mechanical abrasion would not enhance the chemical corrosion rate or reduce the mass transport of reactants and/or products in the system. 

Stainless steels offer useful resistance because they tend to exhibit passive corrosion behavior as a result of the formation of protective oxide films on the exposed surfaces. Under normal circumstances, stainless steels will readily form this protective layer immediately on exposure to oxygen. When this protective film is violated or fails to form, active corrosion can occur. Some fabrication processes can impede the reformation of this passive layer, and to insure that it is formed, stainless steels are subjected to passivation treatments. 

L. Yoimg, "Anodic Oxide Films 4. The Interpretation of Impedance Measurements on Oxide Coated Electrodes on Niobium," Transactions of the Faraday Society, 51 (1955) 1250-1260. 

Nielsen and Hackleman ° found that the dissolution of thermal S1O2 in BHF solutions is impeded by the application of a cathodic potential of 2-AV across the oxide film and postulated that a layer of partially reduced oxide is formed on the surface and... 

P. Schmuki, H. Bohni, and J. A. Bardwell, In-situ characterization of anodic silicon oxide films by AC impedance measurements, J. Electrochem. Soc. 142, 1705, 1995. 

Concerning the two-layer model, the thickness and properties of each layer depend on the nature of the electrolyte and the anodisation conditions. For the application, a permanent control of thickness and electrical properties is necessary. In the present chapter, electrochemical impedance spectroscopy (EIS) was used to study the film properties. The EIS measurements can provide accurate information on the dielectric properties and the thickness of the barrier layer [13-14]. The porous layer cannot be studied by impedance measurements because of the high conductivity of the electrolyte in the pores [15]. The total thickness of the aluminium oxide films was determined by scanning electron microscopy. The thickness of the single layers was then calculated. The information on the film properties was confirmed by electrical characterisation performed on metal/insulator/metal (MIM) structures. 

To interpret the impedance data, a model of the anodic aluminium oxide film must be established. The anodic aluminium oxide film is a sandwich film consisting of two layers, a barrier layer and a porous layer (Figure 23.4A [2]). 

The aluminium oxide films formed for different times (marked in Figure 23.10) were characterised by electrochemical impedance spectroscopy. For the aluminium oxide film anodised for 160 s, the open circuit potential (OCP) is not stable. This can be explained by instability of the film structure. The processes of the film formation were not yet completed. The OCP is more stable and positive for films anodised for more than 700 s. This can be explained by the formation of the compact barrier aluminium oxide layer. 

The impedance response of the films was observed with just one time constant (Ri, CPEi), except for the film formed for 10900 s. This film showed two time constants similar to the films formed at voltages > 30 V. In this case, the aluminium oxide film might be contaminated by anions migrating from the surface into the film. 

Multiwall carbon nanotube (MWCNT)-reinforced hydroxyapatite composite coatings (80% HAp/20% MWCNT) were deposited on austenitic stainless steel AISI 316L by laser surface alloying (LSA) with a 2.5-kW CW Nd YAG laser (Kwok, 2007). EIS of unprotected AISI 316L and HAp/MWCNT-coated steel obtained at open circuit potential are shown in Figure 7.60 after immersion in 0.9% NaCl solution for 2 h. The Bode plot shows that the total impedance Z has noticeably increased for the steel substrate coated with HAp/MWCNT. While the thin passive oxide film on the stainless steel surface was rendered less protective... 

Chemical Treatment. A wide variety of chemicals and water treatments are used for corrosion control. Corrosion inhibitors usually act by forming some type of impervious layer on the metallic surface of either the anode or cathode that impedes the reaction at the electrode and thereby slows or inhibits the corrosion reaction. For example, various alkali metal hydroxides, carbonates, silicates, borates, phosphates, chromates, and nitrites promote the formation of a stable surface oxide on metals. The presence of these chemicals in the electrolyte allows any faults in the metal surface or its oxide film to be repaired. If they are used in too small a quantity as anodic inhibitors, they may promote intense local attack because they can leave a small unprotected area on the anode where the current density will be very high. This is particularly true of chromates and polyphosphates. 

Figure 3. a) Impedance spectra for sol-gel films with titania nanoparticles b) Evolution of oxide film resistance with time for samples doped with cerium. 

To determine numerical values for the different elements of the equivalent circuit they have to be separated, for example, by electrochemical impedance spectroscopy (EIS). Similar to the above-described lock-in measurement a small ac signal of a few mV is superimposed to the electrode potential. The resulting current and its phase shift are then measured as a function of the frequency. Typical impedance spectra of thin oxide films on aluminum are shown in Fig. 17. At high frequencies (10 — lO Hz) the capacitors act as shorts and only the electrolyte resistor determines the impedance, which is typically 10 Ohm for concentrated electrolytes and independent of the electrode. At the lowest frequencies, for example, 10 Hz or below, current flow through the capacitors is impossible and the impedance of the system is given by the sum of the 3 resistors in the current path. The... 

Conductivity is of course closely related to diffusion in a concentration gradient, and impedance spectroscopy has been used to determine diffusion coefficients in a variety of electrochemical systems, including membranes, thin oxide films, and alloys. In materials exhibiting a degree of disorder, perhaps in the hopping distance or in the depths of the potential wells, simple random walk treatments of the statistics are no longer adequate some modem approaches to such problems are introduced in Section 2.1.2.7. 

Practical limitations are imposed at low frequencies, however, where the rectification-smoothing function necessary to transduce the ac voltage magnirnde to a dc level becomes inaccurate. Ac voltmeters typically become seriously in error at frequencies below 20Hz. To obtain an accurate KK transform, it is necessary to extend the measurement frequency range significantly beyond the limits of frequency needed to elucidate the equivalent circuit under test. Thus, the method described here is not appropriate for aqueous electrochenfical systems for which the diffusional impedance is prominent. This method can be useful for systems in which the lowest frequency of interest is greater than 50 Hz or so, as is usually the case for solid ionic conductors, oxide films, and semiconductor surfaces. 

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