Anode auxiliary

A crack count of 30-80 cracks/mm is desirable to maintain good corrosion resistance. Crack counts of less than 30 cracks/mm should be avoided, since they can penetrate into the nickel layer as a result of mechanical stress, whilst large cracks may also have a notch effect. Measurements made on chromium deposits from baths which produce microcracked coatings indicate that the stress decreases with time from the appearance of the first cracks . It is more difficult to produce the required microcracked pattern on matt or semi-bright nickel than on fully bright deposits. The crack network does not form very well in low-current-density areas, so that the auxiliary anodes may be necessary. 

The arc is ignited by means of an ignition needle which carries auxiliary anode H at first this is electromagnetically drawn into the mercury-filled dish then the contact is interrupted which gives rise to an arc which spreads quickly to anodes A. The necessary vacuum in the tubes, some 0.001 millimeters of mercury column (apart from the partial pressure of mercury vapour) is maintained by a diffusion vacuum pump compounded with an oil type rotary vacuum pump. 

The principle of an impressed current system involves the supply of a protective current from some direct current power source, e.g. storage batteries, rectifiers or d.c. generators, through an auxiliary anode. Fig. 15.4 illustrates the arrangement. 

It is possible with impressed currents to "overprotect". Overprotection may give rise to problems comparable to the corrosion which the system seeks to prevent. Moderate overprotection of steel may not represent too much of a problem except the use of unnecessary current and the consumption of the auxiliary anodes. Higher levels of overprotection however, may give rise to excessive hydrogen at the cathodic surface with consequent deleterious effects. With metals such as aluminium, zinc, lead and tin excess alkali generated at the surface by overprotection may cause damage to the structure. 

The electrical current is delivered to the structure in this technique from a dc power source through an auxiliary anode (i.e., the current is forced in, or impressed). The structure will act as a cathode in the cell thus formed, and will sustain the hydrogen evolution reaction in the presence of an anaerobic aqueous environment. If the solution is aerated, the reduction of oxygen becomes possible and may occur in place of or together with the hydrogen evolution reaction depending on the potential to which the structure is depressed. The auxiliary electrode will necessarily become an anode in the cell. If this electrode is a base metal, then the anodic reaction will be... 

Cathodic protection (CP) is an electrical method of mitigating corrosion on metallic structures that are exposed to electrolytes such as soils and waters. Corrosion control is achieved by forcing a defined quantity of direct current to flow from auxiliary anodes through the electrolyte and onto the metal structure to be protected. Theoretically, corrosion of the structure is completely eliminated when the open-circuit potentials of the cathodic sites are polarized to the open-circuit potentials of the anodic sites. The entire protected structure becomes cathodic relative to the auxiliary anodes. Therefore, corrosion of the metal structure will cease when the applied cathodic current equals the corrosion current. There are two basic methods of corrosion control by cathodic protection. One involves the use of current that is produced when two electrochemically dissimilar metals or alloys (Table 19.1) are metallically connected and exposed to the electrolyte. This is commonly referred to as a sacrificial or galvanic cathodic protection system. The other method of cathodic protection involves the use of a direct current power source and auxiliary anodes, which is commonly referred to as an impressed-current cathodic protection system. Then cathodic protection is a technique to reduce the corrosion rate of a metal surface by making it the cathode of an electrochemical cell [3]. 

Rectifiers are used more than any other source of impressed-current power. Areas discussed include rectifier types, rectifier selection, specification requirements, and typical installation details. Various types of impressed-current anodes and components that make up an impressed-current system are also presented. Impressed-current-type cathodic protection systems provide cathodic current from an external power source. A direct current (DC) power source forces current to discharge from expendable anodes through the electrolyte and onto the structure to be protected. Although the current is not generated by the corrosion of a sacrificial metal/alloy, the energized materials used for the auxiliary anodes do corrode. 

It is clearly desirable that the deposit has an even thickness over the whole of the surface to be electroplated. This requires the potential to be the same at all points over the surface of the cathode and this is impossible to attain when the object to be plated has a complex shape. To some extent the evenness of the deposit can be improved by introducing auxiliary anodes (usually platinized titanium electrodes where the reaction is oxygen evolution) at various positions in the electrolyte, the objective being to increase the cathode current density at points where it would otherwise be very low, i.e. at points on the cathode furthest from the normal anodes (e.g. in holes or recesses in the object being plated). The problem with this approach, however, is that a totally new cell geometry is necessary for each new plating job and in any case its success is limited. Hence, in general, we are dependent... 

In addition to the main anodes, auxiliary anodes of Pt/Ti are sometimes also used to improve the current distribution at the cathode. 

Figure 7.9 Jig for nickel plating the interior and exterior of kettle bodies. Note the PtjTi auxiliary anodes inside the kettles. Photograph supplied by W. Canning Materials Ltd.

Observing the polarization diagram for the copper-zinc cell in Fig. 5.2, it is clear that, if polarization of the cathode is continued, using external current, beyond the corrosion potential to the thermodynamic potential of the anode, both electrodes attain the same potential and no corrosion of zinc can occur. This is the basis for cathodic protection of metals. Cathodic protection, discussed further in Chapter 13, is one of the most effective engineering means for reducing the corrosion rate to zero. Cathodic protection is accomplished by supplying an external current to the corroding metal that is to be protected, as shown in Fig. 5.14. Current leaves the auxiliary anode (composed of any metallic or nonmetaUic... 

Figure 13.1. Sketch of cathodically protected pipe, auxiliary anode, and rectifier.

Whereas auxiliary anodes need not be consumed in order to fulfill their purpose, sacrificial anodes are consumed not less than is required by Faraday s law in order to supply an equivalent electric current. In general, the observed rate of consumption is greater than the theoretical. For zinc the difference is not large, but for magnesium it is appreciable, with the cause being ascribed to local-action currents on the metal surface, to formation of colloidal metal particles [13, 14] or, perhaps more important, to initial formation of univalent magnesium ions [15], The latter ions are unstable and react in part with water in accord with... 

Fig. 3.3 The auxiliary anode adjusted to the cathode shape in order to improve the current density distribution (Reprinted from Ref. [1] with permission from the Serbian Chemical Society)...

Chromium passivates strongly in acid sulphate media. Hence, an inert anode is always employed in chromium plating. It is generally a lead alloy which immediately covers with lead dioxide on positive polarization in the electrolyte. The alloying elements are tin, antimony and silver which are added to the lead to improve its mechanical properties and to reduce the overpotential for oxygen evolution. In addition to the main anodes, auxiliary anodes of platinized Ti are sometimes also used to improve the current distribution at the cathode. 

J8). The jig is either a general-purpose or tailor made assembly, often with appropriate spring contacts to mount the workpieces. Deposition on the jig surface is minimized by a plastisol coating. Specialized jigs may be provided with essential auxiliary anodes (Fig. 8.14(a)). A compact manual plating line based on a modular concept is shown in Fig. 8,15 ... 

Fi s. IM4 Jig rack mourning of electroplated articles, (a) A jig for nickel-plating the interiors and exteriors of kettle bodies. Note the use of platinized titanium auxiliary anodes inside the kettle to provide improved current drsiribution. (Courtesy W, Canning Materials Ltd.) (b) A transported-scrviccd electroplating system, which may be semi- or fully automatic in operation and which utilizes rack- or jig-mounting of components. (Courteiy Electroloid Ltd.) ... 

The main elements of a polarizing circuit are as follows low voltage source of dc current, protected stmcture and auxiliary anode placed in the same electrolytic environment. The protected structure is connected to the negative pole of the power source, and the anode to the positive pole. The control circuit contains a potential meter (voltmeter) of high internal resistance, a reference electrode (permanent or portable), and potential terminal, i.e., an electric conductor connecting the structure with the terminal of the potential meter. Modern reference electrodes can be used for many years. The effectiveness of a protective installation depends fundamentally on the correct functioning of the dc power source and the reliable work of anodes. 

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