ALLOY GROUP F. Electrodeposition of Alloys of Several Metals More Noble Than Hydrogen

T h e following several chapters deal with the electrodeposition of the alloys of several

metals having standard electrode potentials more noble (more positive) than that of

hydrogen. These metals are gold, platinum and their congeners, and antimony, arsenic,

b ismuth, and rhenium. T h e deposition of alloys of two other metals, copper and silver,

which belong to this group were discussed in earlier chapters.

With the exception of rhenium and some of the platinum metals, the metals of group F

can be deposited readily from acid baths with theoretical efficiency and readily from some

alkaline baths. Rhenium can be deposited from either acid or alkaline solutions, but only

with low cathode current efficiencies and the deposits cannot be built up to a thickness

greater than about 10 (0.4 mil) without exfoliating.

Since the metals of group F are relatively noble, their deposition potentials must be

made more negative by complexing agents in order to bring about their codeposit ion with

more active metals. T h e codeposit ion of the group F metals does not proceed as easily as

one might expect on the basis of their electrode potentials. T h e s e metals are so

m u c h nobler than other metals in acid solutions that (with the exception of rhenium)

they tend to deposit on them readily by chemical replacement in the form of mossy

deposits. T h i s reactivity may be checked by converting the simple ions to complex ions,

but the latter may be difficult to reduce at the cathode. For example, plat inum is readily

deposited from acid solutions, but does not deposit from a cyanide solution.

Gold posesses the most noble (positive) standard electrode potential of all the metals.

In a solution of monovalent gold ions its potential is 1.68 volts and in a solution of trivalent

gold ions, it is 1.50 volts. In comparison, the standard electrode potentials of several other

noble metals are as follows: Pt (divalent salt), 1.2 volts; palladium, 0.99 volt; rhodium,

about 0.8 volt; silver, 0.799 volt; and copper (cupric salt) 0.377 volt.

Since the group F metals are at one end of the electromotive series, their electrode

potentials are usually rather far removed from those of most metals with which they are

codeposited. Consequently, the alloy plating systems of the group F metals are frequently

of the regular type (see Section 5.1.B*); that is, under diffusion control.

Although a large number of alloys containing the metals of group F are probably

capable of deposition, only a small number have been investigated, and of these only the

gold alloys are commercially important. Studies have been made of the codeposit ion of

gold with copper, silver, t in, nickel, and a few other metals. T h e deposit ion of alloys of the

platinum metals has received only cursory attention. A few alloys of bismuth or antimony

with copper, silver, or lead have been electrodeposited.

* See Volume I of this treatise for Chapters 1 to 21 .

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