Passivity, electrochemical

Highly protective layers can also fonn in gaseous environments at ambient temperatures by a redox reaction similar to that in an aqueous electrolyte, i.e. by oxygen reduction combined with metal oxidation. The thickness of spontaneously fonned oxide films is typically in the range of 1-3 nm, i.e., of similar thickness to electrochemical passive films. Substantially thicker anodic films can be fonned on so-called valve metals (Ti, Ta, Zr,. ..), which allow the application of anodizing potentials (high electric fields) without dielectric breakdown. 

APPLICATION OF SURFACE ELECTROCHEMICAL PASSIVATION OF LEAD-ANTIMONY ALLOY FOR A SIMPLE AND RAPID ELECTROCHEMICAL DETERMINATION OF ANTIMONY CONTENT OF ITS ALLOYS... 

The anticorrosive action of the chromate pigments is based both on chemical and electrochemical reactions [5.66], [5.108]—[5.113]. Electrochemical passivation and chemical reaction are illustrated in Figure 70 [5.114], [5.115], Passivation is based on electrochemical processes in the cathodic region. In addition, a protective film is also... 

Cadmium telluride — A II—IV compound -> semiconductor frequently employed in infrared systems (active component in infrared detectors) and -> photovoltaic devices. Electrochemical - passivation has been employed to improve surface recombination behavior. 

Fig. 9.5. Acid cooler, courtesy Chemetics www.chemetics.com Cool water flows through 1610 internal 2 cm diameter tubes while warm acid flows counter currently (and turbulently) between the tubes. The tubes are 316L stainless steel. They are resistant to water-side corrosion. They are electrochemically passivated against acid-side corrosion by continuously applying an electrical potential between the tubes and several electrically isolated metal rods. Details shell diameter 1.65 m shell material 304L stainless steel acid flow 2000 m3/hour water flow 2900 m3/hour acid temperature drop 40 K. (Green pipes = water metallic pipes = acid.) Fig. 24.6 gives an internal view.

Electrochemical passivation is in many ways similar to chemical passivation. As the potential of an iron sample is increased in the anodic direction, the rate of dissolution increases, reaches a maximum, and then decreases almost to zero. Further increase of the potential has little effect on the current in the passive region until passivity... 

The passivation can be achieved in several manners. The most frequently used one is a thermal passivation [33-36], by the simple exposure of the Si surface to an 02 ambience at elevated temperature. In contrast, an electrochemical passivation can be performed at room temperature, which is beneficial when a low thermal budget is desired. Both the thermal and the electrochemical treatment result in a consumption of Si from the surface, that is the Si02 grows into the surface (about 1/3). This can be critical for very large scale integration (VLSI) structures. If consumption of Si is not acceptable, a deposition of Si02 by chemical or physical vapor deposition (PVD/CVD) techniques is necessary. 

A good overview on the various passivation and deposition processes can be found in Refs. [267-269]. In Table 1.5 the resulting Dit trap densities for the various possible passivation techniques are shown. Thermal passivation yields the highest interface quality, that is the lowest Dit can be achieved. Quality wise the electrochemical passivation is next. However, electrochemical reactions at a semiconductor surface are only possible in the accumulation mode. Therefore, anodic reactions only take place at p-type doped Si electrodes (accumulation of majority charge carriers, i.e. holes), whereas on n-Si only reduction reactions are possible. Consequently, only p-type doped Si can be anodically passivated. This can be changed by an illumination... 

German-American Coll, on Electrochemical Passivation 1989 Corros. Sci. 29... 

Other techniques may be applied. Future high-tech applications can use the advantages of locaKzed passivation and localized breakdown. Electrochemical passivation can be localized by mask techniques, for... 

Zirconium and zircaloy-4 in 1 M NaCl, 1 M KBr, and 1 M aqueous KI solutions were found susceptible to SCC only above the pitting potential (zone 1) [168]. Zirconium alloy SCC in aqueous halide solutions occurs as a result of electrochemical passive film breakdown followed by intergranular attack due to anodic dissolution (dealloying assisted by stress). The final step was a fast transgranular propagation. A surface-mobility SCC mechanism was suggested to explain experimental results. Figure 9.47 shows... 

Albery et al. [39, 49] prepared poly(3-thiopheneacetic acid) and its copolymer with thiophene by electrochemical polymerization. Bartlett et al. [50] electrochemically synthesized conducting poly(3-thiophene-acetic acid) films in dry acetonitrile containing tetraethyl ammonium tetrafluoroborate. These films are redox active in acetonitrile, however, stability was reportedly poor in comparison with poly(3-methylthio-phene) and poly(methyl 3-thiopheneacetate) due to traces of water. In dry acetonitrile, the polymer can be electrochemically oxidized and reduced. Upon oxidation in water and methanol, poly(3-thiopheneacetic acid) film converted into a passive film. Based on the electrochemistry and an FT-IR study, Bartlett et al. postulate the mechanism for the electrochemical passivation shown in the Figure 4.33. In the mechanism, passivation of the polymer involves the formation of an intermediate cyclic lactone and subsequent breakdown by reaction with solvent. This process does not destroy the conductivity of the polymer so the process can continue until all the monomer units within the film are converted to a lactone form (Figure 4.33, IV). The electrochemical passivation is not observed... 

Figure 4.33 Proposed mechanism of the electrochemical passivation of poly(3-thiopheneacetic acid) in water and methanol. Journal of Material Chemistry, 1994, 4, 1805, P. N. Bartlett, D. H. Dawson. Reproduced by permission of The Royal Society of Chemistry.)...

Electrochemical Passive Properties of AlxCoCrFeNi (x = 0, 0.25, 0.50,1.00) High-Entropy Alloys in Sulfuric Acids... 

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