Passivity electrochemical studies

Pitting corrosion always remains a worthy subject for study, particularly with reference to mechanism, and the problem conveniently divides into aspects of initiation and growth. For 6061 alloy in synthetic seawater, given sufficient time, pit initiation and growth will occur at potentials at or slightly above the repassivition potential . In an electrochemical study, it was found that chloride ions attack the passive layer as a chemical reaction partner so that the initiation process becomes one of cooperative chemical and electrochemical effects . 

The values of AX for pc Ni and Fe are reported in Table 27 in brackets. Polycrystalline surfaces of these metals are still used both in surface science and in electrochemical studies. The relevance of Eam0 to the potential of initial passivation of metals has been pointed out.862... 

Influence of pH, solvent composition, type of anions and surfactants on zinc electrochemical dissolution, and passivation was studied intensively. The nature... 

Cyclic potentiodynamic polarization method. Electrochemical studies of pitting corrosion usually indicate that pitting occurs only within or above a critical potential or potential range. Therefore the susceptibility of passive metals to pitting corrosion is often investigated by electrochemical methods such as potentiodynamic or potentiostatic... 

Noise analysis obtained from microelectrochemical investigations of stainless steels under potentiostatic conditions revealed that the current noise, expressed as standard deviation a of the passive current, increases linearly with the size of the exposed area, whereas the pitting potential decreases.47 However, to complete the electrochemical studies and distinguish between repassivating superficial pits and penetrating ones, microscopic studies are highly desirable. The scanning reference electrode technique (SRET) should be an appropriate complementary tool.28... 

Under -> open-circuit conditions a possible passivation depends seriously on the environment, i.e., the pH of the solution and the potential of the redox system which is present within the electrolyte and its kinetics. For electrochemical studies redox systems are replaced by a -> potentiostat. Thus one may study the passivating properties of the metal independently of the thermodynamic or kinetic properties of the redox system. However, if a metal is passivated in a solution at open-circuit conditions the cathodic current density of the redox system has to exceed the maximum anodic dissolution current density of the metal to shift the electrode potential into the passive range (Fig. 1 of the next entry (- passivation potential)). In the case of iron, concentrated nitric acid will passivate the metal surface whereas diluted nitric acid does not passivate. However, diluted nitric acid may sustain passivity if the metal has been passivated before by other means. Thus redox systems may induce or only maintain passivity depending on their electrode potential and the kinetics of their reduction. In consequence, it depends on the characteristics of metal disso-... 

L. Tomcsanyi, K. Varga, I. Bartik, G. Horanyi, and E. Maleczki, Electrochemical Study of the Pitting Corrosion of Aluminum and Its Alloys II, Study of the Interaction of Chloride Ions with a Passive Film on Aluminum and Initiation of Corrosion, Electrochim. Acta, Vol 34, 1989, p 855-859... 

Schmucki, P, Virtanen, S, Isaacs, HS, Ryan, MP, Davenport, AJ, Bohni, H, Stenberg, T. 1998. Electrochemical Behavior of Cr203/Fe203 Artificial Passive Films Studied by in situ XANES. J. Electrochem. Soc., 145, 791. 

Birch conducted the first electrochem-icaUy driven reduction process in liquid ammonia and demonstrated that the same type of products are obtained compared with the alkali metal dissolution method [33]. However, a common problem when carrying out electrochemical studies in liquid ammonia is the passivation of the working electrode by trace water present in the system [34]. This problem can be overcome by applying ultrasound. Asymmetric cavitation and collapse at the electrode surface casts microjets of solution towards it, thus depassivating by removing blocking material. In a typical procedure applied to 3-methylanisole in... 

Electrochemical studies on the effect of biofilms on SS 304 were carried out. It was observed that algal dominated biofilms cause ennoblement in stainless steels. Also it was shown that while biofilms enhanced the pitting potential (and thus decrease susceptibility to pitting), they also increased the passive current rendering the sample susceptible to crevice corrosion. 

The earliest electrochemical studies of biological systems conducted with simple metal electrodes were not especially meaningful, yielding only irreversible behavior due to poor electron transfer between the metal center and the electrode, and passivation of the electrode caused by aggregation or denaturation of the biological species. Thus, early studies made use of indirect methods involving mediators to aid in electron transfer between the electrode and redox species. Kuwana and Hawkridge utilized potassium ferricyanide in coulometric titration of heme proteins as illustrated in Equations (28) and (29). ... 

System-specific procedural requirements for electrochemical studies of CMP systems have been discussed in the literature, and are briefly noted later in the context of experimental results for individual cases. In this regard, the ASTM guides for corrosion and polarization measurements can serve as a comprehensive guideline for the analysis of corrosion parameters (ASTM, 2004, 2010). As noted in the context of Figure 3.2, additive induced shifts in the OCP can occur by multiple mechanisms, and a given OCP shift can arise from a number of reasons for instance, an upward shift in Eoc can occur from anodic passivation or from cathodic stimulation of the same system. For this reason, full Tafel plots should be used to adequately interpret the mechanisms of OCP variations. 

Scherer, J., Ocko, B.M. and Magnussen, O.M. (2003) Structure, dissolution, and passivation of Ni l 11 electrodes in sulfuric acid solution an in situ STM, X-ray scattering, and electrochemical study. Electrochimica Acta, 48,1169-1191. 

In contrast to the situation for aluminum, electrochemical studies on the pitting of stainless steels are fi aught with contradictions because of the effect of experimental factors. Several investigations have used breakthrough potential or breakdown potential, as a measure of pitting susceptibility. As pointed out earlier, the current increases rapidly at the breakdown potential (Fig. 4.24), this is considered as an indication of a breakdown of the passive, corrosion-resistant steel film and initiation of pitting. 

M. Bojinov, G. Fabricius, P. Kinnunen, T. Laitinen, K. Makela, T. Saario, G. Sundholm, Electrochemical study of the passive behaviour of Ni-Cr alloys in a borate solution — a mixed conduction model approach, J. Electroanal. Chem. 504 (1) (2001) 29-44. 

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