Metals electrochemical series

Metallic gold, which is found free in nature, has always been valued for its nobility, i.e. its resistance to chemical attack. This property is to be expected from its position in the electrochemical series. It... 

Almost all common metals and structural steels are liable to corrode in seawater. Regulations have to be followed in the proper choice of materials [16], In addition, there is a greater risk of corrosion in mixed constructions consisting of different metals on account of the good conductivity of seawater. The electrochemical series in seawater (see Table 2-4), the surface area rule [Eq. (2-44)] and the geometrical arrangement of the structural components serve to assess the possibility of bimetallic corrosion (see Section 2.2.4.2 and Ref. 17). Moreover the polarization resistances have considerable influence [see Eq. (2-43)]. The standards on bimetallic corrosion provide a survey [16,17]. 

The ease with which an atom gains or loses electrons is termed die electronegativity of die element. Tabulation of die elements in order of ease hy which diey lose electrons is called die electrochemical series and is shown in Table 6.10. Chapter 4 explains die importance of diis to die formation and control of coiTosion, and Chapter 6 discusses die relevance to predicting reactivity of metals towards water and their potential to become pyrophoric. 

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 ... 

Zinc is relatively low in the electrochemical series and is widely regarded as an active metal. However, when high-purity zinc is placed in hydrochloric acid it will dissolve extremely slowly, if at all. It may be encouraged to... 

When metals are arranged in the order of their standard electrode potentials, the so-called electrochemical series of the metals is obtained. The greater the negative value of the potential, the greater is the tendency of the metal to pass into the ionic state. A metal will normally displace any other metal below it in the series from solutions of its salts. Thus magnesium, aluminium, zinc, or iron will displace copper from solutions of its salts lead will displace copper, mercury, or silver copper will displace silver. 

Concentrated hydrochloric acid will dissolve many metals (generally those situated above hydrogen in the electrochemical series), as well as many metallic oxides. Hot concentrated nitric acid dissolves most metals, but antimony, tin and tungsten are converted to slightly soluble acids thus providing a separation of these elements from other components of alloys. Hot concentrated sulphuric acid dissolves many substances and many organic materials are charred and then oxidised by this treatment. 

The question arises as to which metal is dissolved, and which one is deposited, when combined in an electrochemical cell. The electrochemical series indicates how easily a metal is oxidized or its ions are reduced, i.e., converted into positively charged ions or metal atoms respectively. The standard potential serves for the comparison of different metals. 

Figure 5. Electrochemical series of metals and their standard potentials in volt (measured against NHE).

Each metal or metal area will develop an electrode with a measurable electrical potential. This potential can be referenced to that of a standard hydrogen electrode, which by convention is set at zero. Thus, all metals have either a higher or lower potential compared to hydrogen, and a comparative list of metals can be produced indicating their relative nobility. This list is the galvanic or electrochemical series and measured as an electromotive force (EMF). 

For transmetallations with a metal (metallo-de-metallations, Scheme 10-95) arylmercury compounds are particularly suitable due to the position of mercury as a noble metal in the electrochemical series of standard potentials (for examples see Makarova, 1970). 

Corrosion is the unwanted oxidation of a metal. It cuts short the lifetimes of steel products such as bridges and automobiles, and replacing corroded metal parts costs billions of dollars a year. Corrosion is an electrochemical process, and the electrochemical series is a source of insight into why corrosion occurs and how to prevent it. 

The range of chemical reactivity of metals is wide, from the inertness of the platinum group to the extreme reactivity of some alkali metals. The order of metal reactivity follows essentially the order of the electrochemical series which is shown in Table 17.4 for the metals commonly deposited by CVD. 

In the anion electrochemical series, sulfur, being the less noble element compared to its heavier congeners, occupies an intermediate position between iodine and selenium [(+)F, Cl, Br, I, S, Se, Te(-)]. Selenium, regarded as a metalloid, is a relatively noble element. Tellurium is rather an amphoteric element it can enter into solution in the form of both cations and anions. Regarded as a metal, i.e., with respect to its cations, tellurium occupies a position between copper and mercury. Regarded as a metalloid, i.e., with respect to its anions, it is located on the extreme right of the above series. 

Table 6.3 Variation in the nature of hydroxides of metals according to their positions in the electrochemical series.

The principle of the displacement of one metal by another, or in other words of the displacement of nobler by base or not so noble metals, as described earlier, must be applied with due caution, without neglecting other effects that may not be immediately obvious from consideration of the electrochemical series. Some of these effects are illustrated in the following. Although the position of lithium is above that of sodium in the series, lithium cannot displace sodium from common salt solutions since both of these metals occupy positions higher up than hydrogen and will displace this element from the solution. It must be borne in mind, therefore, that the series applies to aqueous solutions, and the hydrogen ion, which is present in these solutions, can also take part in the displacement reactions. 

The various possible electrode reactions at the cathode and at the anode in electrolytic cells have been shown in Table 6.2. It has been pointed before that the outcome of an electrolytic process can be made on the basis of knowledge of electrode potentials and of overvoltages. The selection of the ion discharged depends on the following factors (i) the position of the metal or group in the electrochemical series (ii) the concentration and (iii) the nature of the electrode. Examples provided hereunder deliberate on these aspects. 

An important area of application of electrolysis is separation and co-deposition. If several ions exist together in an electrolytic solution in a cell, and the voltage is gradually raised from zero, the first metal to be plated is the lowest in the electrochemical series, provided that the ionic concentrations of the different metals are equivalent. As the voltage is increased, the metals which become plated move progressively towards the top of the series. 

This explains why it is possible to plate metals below hydrogen in the electrochemical series, from acid solutions, without hydrogen evolution, whereas care must be exercised in plating metals that are above hydrogen in the electrochemical series, from such solutions. 

The aforementioned fact is also the basis for the separation of co-occurring metals from each other. Whenever feasible, such electrochemical separation is an interesting and effective technique, in principle. In practice, however, such selective deposition is not considered very feasible, particularly for elements which are close neighbors in the electrochemical series. For example, the decomposition potentials of nickel and of cobalt are -0.25 V and -0.27 V, respectively. This small 0.02 V difference makes the selective deposition of nickel, leaving cobalt in the solution, or vice versa, rather difficult to achieve in practice. On the other hand, it is quite easy to co-deposit nickel and cobalt and to obtain an alloy. 

It has been pointed out that metals residing below the position held by manganese (and, therefore, much below hydrogen) in the electrochemical series (Table 6.11) cannot be electrodeposited from aqueous solutions of their salts. These metals are called base metals or reactive metals and can be electrodeposited only from nonaqueous electrolytes such as solutions in organic solvents and molten salts. As with an aqueous electrolyte, there is a minimum voltage which is required to bring about the electrolysis of a molten salt. 

If dissimilar metals are placed in contact, in an electrolyte, the corrosion rate of the anodic metal will be increased, as the metal lower in the electrochemical series will readily act as a cathode. The galvanic series in sea water for some of the more commonly used metals is shown in Table 7.4. Some metals under certain conditions form a natural protective film for example, stainless steel in oxidising environments. This state is denoted by passive in the series shown in Table 7.4 active indicates the absence of the protective film. Minor... 

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