Copper-zinc alloy electrodes

Six iron anodes are required for corrosion protection of each condenser, each weighing 13 kg. Every outflow chamber contains 14 titanium rod anodes, with a platinum coating 5 /tm thick and weighing 0.73 g. The mass loss rate for the anodes is 10 kg A a for Fe (see Table 7-1) and 10 mg A a for Pt (see Table 7-3). A protection current density of 0.1 A m is assumed for the coated condenser surfaces and 1 A m for the copper alloy tubes. This corresponds to a protection current of 27 A. An automatic potential-control transformer-rectifier with a capacity of 125 A/10 V is installed for each main condenser. Potential control and monitoring are provided by fixed zinc reference electrodes. Figure 21-2 shows the anode arrangement in the inlet chamber [9]. 

For the electrochemical measurements, the copper-2% zinc alloy was mounted into epoxy (Buehler epoxide resin) by curing at room temperature for about 10 h. The finished metal surface had 1 cm2 exposed. The reference electrode probe and both high-density graphite counter electrodes were also positioned into the beaker. The working electrodes were immersed in the solutions for up to 0.5 h prior to the cyclic voltammetry for monitoring the open circuit potential. 

Watanabe and coworkers have found that alloy electrodes are often more efficient than electrodes made of pure metals. They have studies the reduction of CO on copper alloys with nickel, tin, lead, zinc cadmium and mercury [24]. The alloys were all produced by electroplating from mixtures of the metal ions in solution. In the cases of nickel, tin, lead and zinc alloys, current densities and faradaic efficiencies were all found to be greater than those of the pure components. Also product selectivity was found to be a function of the metal alloyed with copper. 

Sacrificial Anodes Incontrastto the impressed current technique, the use of sacrificial anodes does not depend on the creation of driven electrochemical cell. Rather, a galvanic cell is formed between the structure and the sacrificial anode in which electrons pass spontaneously from the latter to the former (Fig. 9). Thus, the source of the electrons (the sacrificial anode) must have a more negative electrode potential than the structure. It was for this reason that Humphrey Davy chose zinc or iron to protect copper, and it also explains why magnesium, aluminum and zinc alloys are used to protect steel today. 

Tungsten-copper-boron nitride pseudoalloy electrodes have been tested in electric discharge machining of hard alloys [101,102]. Alloys used in the manufacture of electric contacts are frequently composites of metals and a-BN, providing for increased heat durability [103]. Thus, silver cermet electrical contact material for circuit breakers is made by hot-pressing of the constituents [104 to 106]. Again, Ag/BN composite layers can also be prepared by electrodeposition [107]. Zinc alloys as used in coating sheet steel as anticorrosion layers may contain dipersed a-BN for increased weldability and corrosion resistance [108 to 111]. 

Many users do not recognize that aluminum alloys themselves span a range of about 400 mV in their respective corrosion potentials. In aerated sodium chloride solutions, pure aluminum, 3xxx alloys, and many other alloys have a potential of about —740 mV when measured with a saturated calomel reference electrode (SCE). Aluminum alloys with high magnesium or zinc contents will be more anodic by as much as 260 mV, while high copper content alloys will be more cathodic up to about 140 mV. Care must be taken therefore, that all alloys and tempers are compatible, even in an all aluminum structure. 

The galvanic series of aluminum alloys and other metals representative of their electrochemical behavior in seawater and in most natural waters and atmospheres is shown in Table 19.6. The effect of alloying elements in determining the position of aluminum alloys in the series is shown in Figure 19.4. These elements, primarily copper and zinc, affect electrode potential only when they are in solid solution. 

Thus, co-deposition of silver and copper can take place only when the silver concentration in the electrolyte falls to a very low level. This clearly indicates that the electrolytic process can, instead, be used for separating copper from silver. When both silver and copper ions are present, the initial deposition will mainly be of silver and the deposition of copper will take place only when the concentration of silver becomes very low. Another example worth considering here is the co-deposition of copper and zinc. Under normal conditions, the co-deposition of copper and zinc from an electrolyte containing copper and zinc sulfates is not feasible because of the large difference in the electrode potentials. If, however, an excess of alkali cyanides is added to the solution, both the metals form complex cyanides the cuprocyanide complex is much more stable than the zinc cyanide complex and thus the concentration of the free copper ions available for deposition is considerably reduced. As a result of this, the deposition potentials for copper and zinc become very close and their co-deposition can take place to form alloys. 

The values of the standard electrode potentials of copper (Cu+ + e- -> Cu, 0.522 V) and zinc (Zn2+ + 2e - Zn, —0.76 V) make it appear unlikely that the metals could be codeposited as the alloy brass. However, if, for example, solutions which are 0.025 M in [Zn(CN)4]2 and 0.05 M in [Cu(CN)3]2 are mixed, simple calculation can show that the static electrode potentials of the two ions have values which approach each other more closely and codeposition becomes much more probable. This can be understood with the help of equation (5) and the knowledge that the dissociation constants of [Zn(CN)4]2 (1.3 x 10-17) and [Cu(CN)3]- (5.6 x 10-28) will greatly modify the activity term. 

Zinc is also used in making alloys, the most important of which is brass (the alloy with copper), and as a reacting electrode in dry cells and wet cells. 

However, formation of intermetallic compounds can cause problems. When metals such as copper and zinc are present in solution there is a tendency to form a Zn/Cu intermetallic compound when larger amounts are deposited at a mercury electrode. When an intermetallic compound is formed the stripping peaks for the constituent metals may be shifted, severely depressed, or even be absent altogether. When an alloy is formed at a solid electrode its dissolution potentials, in the stripping step, may be quite different to those of the constituent metals. 

Anodizing is an electrolytic passivation process that increases the thickness of natural oxide layers on the surface of metals [13]. It basically forms an anodic oxide finish on a metal s surface to increase corrosion resistance. For the anodizing process, the metal to be treated serves as the anode (positive electrode, where electrons are lost) of an electrical circuit. Anodized films are most often applied to protect aluminum alloys. An aluminum alloy is seen on the front bicycle wheel in Fig. 2 [14]. For these alloys, aluminum is the predominant metal. It typically forms an alloy with the following elements copper, magnesium, manganese, silicon, tin, and zinc [15]. Two main classifications for these alloys are casting alloys and wrought alloys, both of which can be either heat treatable or non-heat treatable. 

The injection charge polarity relationships we have shown for various metal electrodes are consistent with the ordering of the Galvanic series of alloys, proceeding from most negative to positive as zinc, aluminum, active steel, brass, copper, and passivated steel. To verify this ordering for highly purified water (resistivity 26 MQ-cm at 20 C,... 

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