Metal anodic

Selenium is found in a few rare minerals such as crooksite and clausthalite. In years past it has been obtained from flue dusts remaining from processing copper sulfide ores, but the anode metal from electrolytic copper refineries now provide the source of most of the world s selenium. Selenium is recovered by roasting the muds with soda or sulfuric acid, or by smelting them with soda and niter. 

This is essentially a corrosion reaction involving anodic metal dissolution where the conjugate reaction is the hydrogen (qv) evolution process. Hence, the rate depends on temperature, concentration of acid, inhibiting agents, nature of the surface oxide film, etc. Unless the metal chloride is insoluble in aqueous solution eg, Ag or Hg ", the reaction products are removed from the metal or alloy surface by dissolution. The extent of removal is controUed by the local hydrodynamic conditions. 

Two observations relevant to ECM can be made. (/) Because the anode metal dissolves electrochemicaHy, the rate of dissolution (or machining) depends, by Faraday s laws of electrolysis, only on the atomic weight M and valency of the anode material, the current I which is passed, and the time t for which the current passes. The dissolution rate is not infiuenced by hardness (qv) or any other characteristics of the metal. (2) Because only hydrogen gas is evolved at the cathode, the shape of that electrode remains unaltered during the electrolysis. This feature is perhaps the most relevant in the use of ECM as a metal-shaping process (4). 

Characteristics of ECM. By use of Faraday s laws if is the mass of metal dissolved, and because m = r p where r is the corresponding volume and p the density of the anode metal, the volumetric removal rate of anodic metal Tjdot is given by... 

Table 4 shows a galvanic series for some commercial metals and alloys. When two metals from the series are in contact in solution, the corrosion rate of the more active (anodic) metal increases and the corrosion rate of the more noble (cathodic) metal decreases. 

Most galvanic corrosion processes are sensitive to the relatively exposed areas of the noble (cathode) and active (anode) metals. The corrosion rate of the active metal is proportional to the area of exposed noble metal divided by the area of exposed active metal. A favorable area ratio (large anode, small cathode) can permit the coupling of dissimilar metals. An unfavorable area ratio (large cathode, small anode) of the same two metals in the same environment can be costly. 

Naturally, because the protection depends on the dissolution of the anodes, these require replacement from time to time (hence the term sacrificial anodes). In order to minimise the loss of anode metal, it is important to have as good a barrier layer around the pipe as possible, even though the pipe would still be protected with no barrier layer at all. 

Current content values Q and Q" for the most important anode metals are given in Table 6-1. These data apply only to pure metals and not to alloys. For these the Q values corresponding to the alloy composition can be calculated from ... 

The sum of all the cathodic partial reactions is included in e.g., oxygen reduction according to Eq. (2-17) and hydrogen evolution according to Eq. (2-19). The intermediate formation of anode metal ions of anomalous valence is also possible ... 

Anodic Metallic Coating—a special coating usually comprised, either entirely or in part, of an anodic metal, which is electrically positive to the substrate to which it is applied. 

Species with more positive corrosion potential, located toward the bottom of the series, are called noble or cathodic metals and alloys. Those species with more negative corrosion potential located toward the top of the series are referred to as active or anodic metals and alloys. 

At the anode metal goes into solution as it ionizes to... 

Finally, it is important to point out that although in localised corrosion the anodic and cathodic areas are physically distinguishable, it does not follow that the total geometrical areas available are actually involved in the charge transfer process. Thus in the corrosion of two dissimilar metals in contact (bimetallic corrosion) the metal of more positive potential (the predominantly cathodic area of the bimetallic couple) may have a very much larger area than that of the predominantly anodic metal, but only the area adjacent to the anode may be effective as a cathode. In fact in a solution of high resistivity the effective areas of both metals will not extend appreciably from the interface of contact. Thus the effective areas of the anodic and cathodic sites may be much smaller than their geometrical areas. 

Fig. 1.69 Effect of resistivity of solution on the distribution of corrosion on the more negative metal of a bimetallic couple, (a) Solution of very low resistivity and (b) solution of very high resistivity. Note that when the resisitivity is high the effective areas of the cathodic and anodic metals are confined to the interface between the two metals...

Stimulation of the corrosion of vital parts made of more anodic metals may occur if they are connected to copper. 

Very small amounts of copper taken into solution may cause considerable corrosion of more anodic metals elsewhere in the system, particularly zinc , aluminium , and sometimes steelSmall particles of copper deposited from solution set up local cells that cause rapid pitting. 

Non-consumable anodes sustain an anodic reaction that decomposes the aqueous environment rather than dissolves the anode metal. In aqueous solutions the reaction may be ... 

Selected suitable anode metal calculate total mass of metal for required life. 

The overall process is metal transfer from anode to cathode via the solution. The form of anode corrosion is important, and materials may be added both to the anode metal and to the electrolyte, to influence it. There are important instances where an insoluble anode is used, and the anode reaction becomes the oxidation of water or hydroxyl ions ... 

Similar considerations apply to oxidation. An anion which is considerably more stable than water will be unaffected in the neighbourhood of the anode. With a soluble anode, in principle, an anion only needs be more stable than the dissolution potential of the anode metal, but with an insoluble anode it must be stable at the potential for water oxidation (equation 12.4 or 12.5) plus any margin of polarisation. The metal salts, other than those of the metal being deposited, used for electroplating are chosen to combine solubility, cheapness and stability to anode oxidation and cathode reduction. The anions most widely used are SOj", Cl", F and complex fluorides BF4, SiFj , Br , CN and complex cyanides. The nitrate ion is usually avoided because it is too easily reduced at the cathode. Sulphite,... 

It is at the anode that oxidation takes place, with the anodic metal suffering a loss of negatively charged electrons. The resulting positively charged metal ions dissolve in the water electrolyte and metal wastage occurs. In the corrosion cell, the metal or metal area having the lowest electrical potential becomes the anode. 

A form of corrosion resulting from the presence of two dissimilar metals such as steel and copper in an electrolyte such as water forming a galvanic couple, whereby the less noble anodic metal (in this case steel) corrodes. 

Figure 9. Adsorption of intermediate layer (metal-ion complex) in anodic metal dissolution. A (aq), hydrated anion M2+(M), metal adion MA+(ad), adsorbed metal-ion complex MA (aq), hydrated metal-ion complex.

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