Anodisation potential

Fig. 13. Growth of a hydrated oxide on aluminium in 0.1 M NaOH at three anodisation potentials.

It is somewhat less corrosion resistant than tantalum, and like tantalum suffers from hydrogen embrittlement if it is made cathodic by a galvanic couple or an external e.m.f., or is exposed to hot hydrogen gas. The metal anodises in acid electrolytes to form an anodic oxide film which has a high dielectric constant, and a high anodic breakdown potential. This latter property coupled with good electrical conductivity has led to the use of niobium as a substrate for platinum-group metals in impressed-current cathodic-protection anodes. 

Because of its good performance in mineral acids, there is little need or incentive to invoke anodic passivation techniques for zirconium. The metal can be anodised in sulphuric acid, but, again in contrast to the behaviour of titanium, it does not form a stable anodic film in chloride solutions, and even in neutral sodium chloride, zirconium rapidly corrodes if an anodic potential of 2 V is applied. 

Those experiments that did involve anodisation at more reasonable potentials, i.e. below oxygen evolution, suffered from an inability to characterise the initial PtOH species formed at coverages below a monolayer. Dickinson et ai (1975) systematically investigated the surface composition of a large number of Pt electrodes, polarised at various potentials in sulphuric acid, using XPS via the emersion approach. Figure 3.24 shows XPS spectra obtained from a Pt electrode after polarisation in sulphuric acid at 1.00 V and 1.5 V vs. SCE. 

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. 

Visible photoluminescence (PL) from porous silicon (PS) observed at room temperature has inspired sustained research into its potential application in Si-based optoelectronic devices and its theoretical basis (Canham 1990). This property is reviewed in the handbook chapter Photoluminescence of Porous Silicon. Most PS layers are prepared by anodic etching on/>-type Si substrates, a technique in which metal is often deposited on the rear surface of the Si substrate in order for it to be used as an ohmic back contact (see handbook chapter Porous Silicon Formation by Anodisation ). However, the requirement for a back contact electrode is a limitation of this method for example, it is difficult to form a PS layer on a sihcon-on-insulator (SOI) structure or on Si integrated circuits. A photoetching method, on the other hand, requires no electrodes and allows the formation of a visible luminescence layer on not only single-crystaUine Si substrates but also SOI structures. 

However, if the film is a poor electronic conductor, then high anodic potentials may be reached along (F H) with a constant, high-current deaisity. This may facilitate anodising , which is commercially used as a process for protecting aluminium. 

Fig. 12 (A) Nynquist plot of pristine graphene before and after anodisation at 2 V for 500 s. (B) Admittance plot for different potentials for pristine graphene before and after anodisation (reprinted with permission from ref 65).

Anodising is the process of forming oxide (and chloride) films or coatings on certain metals by electrolysis in a suitable solution. On the application of an electric potential to a cell in which the metal to be treated is the anode, the oxidising conditions convert the surface of the metal to the oxide. The oxide film is essentially an integral part of the... 

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