Cathodes, aluminum cadmium

Electrophoresis coating Molybdenum Nickel Aluminum Cadmium Chromium Copper Brass Gold Silver Noble/cathodic Noble/cathodic Anodic Anodic Noble/cathodic Noble/cathodic Noble/cathodic Noble/cathodic Noble/cathodic 1-10 25.4-255 0.25 6.5 0.15-0.5 4-12.5 0.005-20 0.15-510 0.01 -30 0.25-760 0.07-0.1 1.8-2.5 0.03-0.8 0.75-20 0.1-1 2.5-25 Small parts... 

Coating with anodic metals (zinc, aluminum, cadmium), which may be appreciated either as part of surface separation or part of cathodic protection. 

Cadmium also may be recovered from zinc ores and separated from other metals present as impurities by fractional distillation. Alternatively, the cadmium dust obtained from the roasting of zinc ore is mixed with sulfuric acid. Zinc dust is added in small quantities to precipitate out copper and other impurities. The metal impurities are removed by filtration. An excess amount of zinc dust is added to the solution. A spongy cadmium-rich precipitate is formed which may he oxidized and dissolved in dilute sulfuric acid. Cadmium sulfate solution is then electrolyzed using aluminum cathodes and lead anodes. The metal is deposited at the cathode, stripped out regularly, washed and melted in an iron retort in the presence of caustic soda, and drawn into desired shapes. More than half of the world s production of cadmium is obtained by elecrolytic processes. 

Zinc also may be produced by electrolysis of zinc sulfate solution. The zinc oxide in the roasted concentrate is leached with sulfuric acid. The oxide is converted to soluble zinc sulfate. Impurity metals, such as iron, copper, cadmium, arsenic, tin, and cobalt are removed by precipitation, floe formation, and other methods. The purified zinc sulfate solution is electrolyzed using aluminum cathodes and lead anodes. Zinc is deposited on the cathode. 

Electroplating is achieved by passing an electric current through a solution containing dissolved metal ions as well as the metal object to be plated. The metal object acts as a cathode in an electrochemical cell, attracting metal ions from the solution. Ferrous and nonferrous metal objects are typically electroplated with aluminum, brass, bronze, cadmium, chromium, copper, iron, lead, nickel, tin, and zinc, as well as precious metals such as gold, platinum, and silver. Common electroplating bath solutions are listed in Table 7-1. 

Nearly pure cadmium sponge is precipitated by the addition of high-purity. lead-free zinc dust. The cadmium sponge then is redigested in spent cadmium electrolyte, alter which the cadmium is deposited by electrolysis onto aluminum cathodes. The metal is then stopped from the electrodes, melted, and cast into various shapes. Reactions which occur during the electrolytic process are (Roasting) ZnS +1,0 — — ZnO +... 

The second type of cell is a mercury pool type. A mercury cathode is particularly useful for separating easily reduced elements as a preliminary step in an analysis. l or example, copper, nickel, cobalt, silver, and cadmium are readily separated from ions such as aluminum, titanium, the alkali metals, and phosphates. The precipitated elements dissolve in the mercury little hydrogen evolution occurs even at high applied potentials because of large overvoltage effects. A coulomet-ric cell such as that shown in Figure 24-5b is also useful for coulometric determination of metal ions and certain types of organic compounds as well. 

For sacrificial coatings (e.g., zinc, cadmium) and, in certain environments, also aluminum and tin on steel, the direction of galvanic current through the electrolyte is from coating to base metal as a result, the base metal is cathodically protected (Fig. 14.1). As long as adequate current flows and the coating remains in electrical contact, corrosion of the base metal does not occur. The degree of... 

Cathodic control protection protects the substrate by coating with a less noble metal, for which the slopes of the cathodic polarization curves are steep. The cathodic overpotential of the surface is increased by the coating therefore, the corrosion potential becomes more negative than that of the substrate. Coating materials used for this purpose are zinc, aluminum, manganese, cadmium, and their alloys. The electrode potential of these metals are more negative than those of iron and steel. When exposed to the environment, these coatings act as sacrificial anodes for the iron and steel substrates. 

By providing a barrier between the substfate and the environment, or by cathodically protecting the substrate, metallic coatings protect the substrate from corrosion. Coatings of chromium, copper, and nickel provide increased wear resistance and good corrosion resistance. However, these noble metals make the combination of the substrate (mostly steel or an aluminum alloy) with the protective layer sensitive to galvanically induced local corrosion. Nonnoble metallic layers such as zinc or cadmium provide good cathodic protection but show poor wear resistance. 

Impurities such as lead, tin, and cadmium are cathodic to zinc-aluminum alloys and can cause relatively rapid intergranular corrosion under humid conditions. The effects of these impurities are checked by humidity tests such as exposing test samples to saturated steam at 95°C for a period of ten days [I]. 

Zinc is an element of group 12 of the periodic table, of which the outer electrons are 3d 4s and zinc is in the same group of elements, such as Cd and Hg. Zinc exists as 70 ppm in the Earth s crust and exists in similar amounts to Cr (100 ppm) and Ni (80ppm) [1]. Zinc is produced from sphalerite (ZnS) as ores. There are two processes, a dry process and a wet process, for the refinement of zinc. However, zinc is largely produced by the wet process. The wet process is equally to be said as an electrolysis process. The sphalerite contains lead, iron cadmium and copper, etc. besides zinc and, by a flotation separation of the ores, the zinc concentrate (50-55 % Zn) and the lead concentrate are separated. The zinc concentrate is burnt and put into a vessel and dissolved in an electrolysis foul solution. The zinc sulfate solution and concentrated mud (the common name is red mud) are separated by a filter. Cadmium and copper, etc., as impurities dissolve in the zinc sulfate solution. Then zinc dust is added to the solution to precipitate these impurities to form a clean solution of zinc sulfate to be electrolyzed. In the electrolysis, lead containing 1 % of silver is used as a cathode and aluminum is used as an anode, and zinc is produced on the cathode [2—4]. The properties of zinc are shown in Table 5.1. 

Zinc and cadmium have similar properties both are in Group nB. 1331 No. Most aluminum is available in clays and aluminosilicate minerals, which cannot be used economically as ores to obtain aluminum metal. 1333 Boron acts as a dopant (doping agent), which yields positive holes that become current carriers. 13.35 Examples MgCaSisOg and KNaMgSisOg 13.37 The impure lead metal serves as the anode and pure lead serves as the cathode. Impurities either remain at the anode... 

While studying the electrolysis of ethereal solutions of a Grignard reagent, French and Drane [55] observed that anodes of aluminum, zinc, and cadmium dissolved and that these metals were deposited at the cathode. More recently Evans [56] proposed that triethylaluminum is formed when an aluminum anode dissolves in ethylmagnesium bromide ... 

In natural environments, including saline conditions, zinc is anodic to aluminum and corrodes preferentially, giving protection to aluminum. Magnesium is similarly protective, although in severe marine environments it causes cathodic corrosion of aluminum because of an alkaline condition produced on the aluminum surface. Cadmium is neutral to aluminum and can safely be used in contact with it. The other stmc-... 

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