Inert anodes

The workpiece to be plated is the cathode (negative terminal). The anode, however, can be one of the two types sacrificial anode (dissolvable anode) and permanent anode (inert anode). The sacrificial anodes are made of the metal that is to be deposited. The permanent anodes can only complete the electrical circuit, but cannot provide a source of fresh metal to replace what has been removed from the solution by deposition at the cathode. Platinum and carbon are usually used as inert anodes. 

E. Renaud, "Modelisation des Equilibres Thermodynamiques Impliquant le Fer dans la Ciyolithe lors de I Electrolyse de rAluminium a I aide d Anodes Inertes ". in Departement de Genie Chimique, Vol. M.Sc.A. Ecole Polytechnique de Montreal, Montreal, 2009. 

Electrode processes are a class of heterogeneous chemical reaction that involves the transfer of charge across the interface between a solid and an adjacent solution phase, either in equilibrium or under partial or total kinetic control. A simple type of electrode reaction involves electron transfer between an inert metal electrode and an ion or molecule in solution. Oxidation of an electroactive species corresponds to the transfer of electrons from the solution phase to the electrode (anodic), whereas electron transfer in the opposite direction results in the reduction of the species (cathodic). Electron transfer is only possible when the electroactive material is within molecular distances of the electrode surface thus for a simple electrode reaction involving solution species of the fonn... 

For practical applicability, several aspects have to be considered such as tire anode material (sacrificial (e.g. zinc) or inert (e.g. Pt/Ti or graphite)), tlie conductivity of tlie medium and tlie current distribution. Catliodic protection is typically used for buried constmctions (e.g. pipelines), off-shore stmctures or ship hulls. 

The most common fused salt baths are complex mixtures of alkah chlorides, rigorously purified and dried. Fused salt plating must be done under an inert atmosphere. Often argon is used because nitrogen can react with some metals. Inert anodes, eg, Pt-coated titanium or graphite, are used and the plating metal is suppHed by additions of an appropriate metal salt. 

The potential of the reaction is given as = (cathodic — anodic reaction) = 0.337 — (—0.440) = +0.777 V. The positive value of the standard cell potential indicates that the reaction is spontaneous as written (see Electrochemical processing). In other words, at thermodynamic equihbrium the concentration of copper ion in the solution is very small. The standard cell potentials are, of course, only guides to be used in practice, as rarely are conditions sufftciendy controlled to be called standard. Other factors may alter the driving force of the reaction, eg, cementation using aluminum metal is usually quite anomalous. Aluminum tends to form a relatively inert oxide coating that can reduce actual cell potential. 

Metalliding. MetaUiding, a General Electric Company process (9), is a high temperature electrolytic technique in which an anode and a cathode are suspended in a molten fluoride salt bath. As a direct current is passed from the anode to the cathode, the anode material diffuses into the surface of the cathode, which produces a uniform, pore-free alloy rather than the typical plate usually associated with electrolytic processes. The process is called metalliding because it encompasses the interaction, mostly in the soHd state, of many metals and metalloids ranging from beryUium to uranium. It is operated at 500—1200°C in an inert atmosphere and a metal vessel the coulombic yields are usually quantitative, and processing times are short controUed... 

RoUs and other relatively simple shapes make use of inert shields and thieves to avoid edge buildup and produce a more even plate thickness. For more compUcated shapes having deeper recesses thicker deposits from cyanide copper baths have been used as an undercoat to the copper sulfate deposit. Acid copper baths operate near 100% efficient over a wide current density range. The cathode efficiency is usuaUy slightly less than the anode efficiency, bringing about a slow increase in copper unless drag-out losses are high. 

Tafel Extrapolation Corrosion is an elec trochemical reac tion of a metal and its environment. When corrosion occurs, the current that flows between individual small anodes and cathodes on the metal surface causes the electrode potential for the system to change. While this current cannot be measured, it can be evaluated indirectly on a metal specimen with an inert electrode and an external electrical circuit. Pmarization is described as the extent of the change in potential of an electrode from its equilibrium potential caused by a net current flow to or from the electrode, galvanic or impressed (Fig. 28-7). 

The main part of the anode slurry, which can contain CaC03, depending on the hardness of the water, is chemically inert from the corrosion point of view, and settles to the bottom of the tank. The slurry must be removed once or twice a week through a stub with a stopcock this takes place in short bursts of opening and closing of the tap at full water pressure. The deslurrying outlet is at the end opposite to the water inlet in a horizontal tank. 

CoiTosion prevention is achieved by correct choice of material of construction, by physical means (e.g. paints or metallic, porcelain, plastic or enamel linings or coatings) or by chemical means (e.g. alloying or coating). Some metals, e.g. aluminium, are rendered passive by the formation of an inert protective film. Alternatively a metal to be protected may be linked electrically to a more easily corroded metal, e.g. magnesium, to serve as a sacrificial anode. 

Clean metallic aluminum is extremely reactive. Even exposure to air at ordinary temperatures is sufficient to promote immediate oxidation. This reactivity is self-inhibiting, however, which determines the general corrosion behavior of aluminum and its alloys due to the formation of a thin, inert, adherent oxide film. In view of the great importance of the surface film, it can be thickened by anodizing in a bath of 15% sulfuric acid (H2SO4) solution or by cladding with a thin layer of an aluminum alloy containing 1 % zinc. 

Alumiojuffl resists corrosion not because of its position in the electrochemical series but because of the ra Hd formation of a coherent, inert, oxide layer. Contact with grafihite, Fe. Ni. Cu, Ag or Pb is disastrous for corrosion resistance, the effect of contact with steel, Zn and Cd depends on pH and exposure conditions. Protection is enhanced by anodizing the metal this involves immersing it in 15-20% H2SO4 and connecting it to the positive terminal so that it becomes coated with alumina ... 

Numerous materials fall into the category of electronic conductors and hence may be utilised as impressed-current anode material. That only a small number of these materials have a practical application is a function of their cost per unit of energy emitted and their electrochemical inertness and mechanical durability. These major factors are interrelated and —as with any held of practical engineering—the choice of a particular material can only be related to total cost. Within this cost must be considered the initial cost of the cathodic protection system and maintenance, operation and refurbishment costs during the required life of both the structure to be protected and the cathodic protection system. 

There are obviously situations which demand considerable over-design of a cathodic protection system, in particular where regular and efficient maintenance of anodes is not practical, or where temporary failure of the system could cause costly damage to plant or product. Furthermore, contamination of potable waters by chromium-containing or lead-based alloy anodes must lead to the choice of the more expensive, but more inert, precious metal-coated anodes. The choice of material is then not unusual in being one of economics coupled with practicability. 

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