Laboratory Scale Electrodeposition Practice and Applications Thomas J. Bnnn, Thermophysics Divlslon, Center For Chemical Engineering, National Bureau of Standards, Boulder, CO 80303
In this paper, some of the ~ractical asoeds of electrodeoo- . . .
sition or electroplating are-discussed. Special emphasis is given to the techniques required to make electrodeposition work reliahlv in the lahoratorv. We will then discuss some of the prohlek-solving applications that have been of value in the author's laboratory.
The Deposltlon Solutions Solutions from which electrodeposition is conducted are
usuallv com~lex mixtures containine comoonents that serve - -
multiple purposes (1, 2). The principal component of the deposition solution is the source of primary ion(s) (the ion or ions which are to be deposited). Simple salts are used most often as the source of the platable ion, even in the case of the complex anion systems. I t is desirable to have a high concen- tration of platable ions in the solution: thus, onlv the most
soluble sd t s should be used. For example, as a source of copper ions, both the sulfate and the fluohorate salts have been used, but the fluoborate is the more soluble of the two. Perchlorate salts are also quite soluble but involve an in- creased degree of hazard. Nitrates are usually not accept- able, since the NO; ion tends to he reduced easily. This renders some metals unolatable from nitrate solutions. since at higher potentials the kvo~ution of ammonia at the cathode presents difficulty. The effect of the anion (of the primary ion source) must also be considered in the formulation of a solution. Adsorption of the anion on the cathode will effect the deposit and will also influence the activity of the metal ion. Metal chlorides are sometimes used as a source of metal ions, but their primary purpose is to furnish chloride anions, which aid in anode corrosion (or dissolution). This will be discussed in more detail later. There has been limited use of organic anion salts as the source of primary metal ion. These include sulfonic acid and aryl acid salts. These solutions tend to he weakly ionized and expensive and are never used commerciallv.
In the complex anionic solutions, a source of liganda must be vrovided rto form the corn~lexl in addition to the metal
ions. The most popular complexing ligand is the cyanide ion, which is used for the deposition of copper, gold, silver, zinc, cadmium, and indium (3.8). These solutions are usually maintained in a strongly alkaline condition since the com- plex is decomposed b&id (with the evolution of poisonous HCN). The only exception to this rule is the gold complex (cyanoaurate), which is stable a t a pH as low as 3. The most common sources of the CN- ion are either sodium or potassi- um cyanide. The potassium salt is preferred due to its higher solubility, although it is more costly. The cyanide ion is maintained in laree excess in the com~lex solutions. The quantity of cyan& which is not neededin the formation of the complex is termed free cyanide. Free cyanide has a profound effect on the depoait because of its tendency to adsorb on the cathode surface. I t increases throwing power
This work was done at the National Bureau of Standards, not subject to copyright.
and total polarization, hut a t a sacrifice in current efficiency and olatine rate. Some lone-term reactions of the cyanide ion ihould-be borne in mind. I t can slowly hydroiize to produce ammonia and formate,
CN- + 2H,O - NH, + OOCH- I t can also be oxidized to cyanate, especially a t an anode which is evolving oxyEen. Carbonate ion is usually present in the cyanide sol&io&, due to the reaction with atmospheric carbon dioxide. This is not detrimental a t low concentra- tions; carbonate isoften added tocyanide solutions when the possibility of iron contamination exists (3). This is done when large scale steel tanks are used for plating and storage, since the carbonate addition will precipitate the iron impuri- ties.
Other, less-common anionic complexes are formed using hydroxy and pyrophosphate ligands. The most notable ex- ample of a noncyanide anionic complex is Sn(OH)i used in the electrodeposition of tin.
An electrodeposition solution must have a high electrical conductivity to allow optimum current densities to be used without excessively high voltages. Pny ionic solu;.;un will be electrically conducting, hut often the conductivity will be low because of low ionic mobilities. For this reason, most plating solutions require the use of conduction additives. The hvdronium and the hvdroxv ions are the best conduc- tors available. Thus, in s i iple sil t solutions, sulfuric acid is added. while in the alkaline (com~lexed) solutions, the most
common conduction additive is either sodium or potassium hydroxide. In the latter case, the presence of excess OH- ions in a cyanide complex solution will help prevent the decomposition of CN- and the evolution of HCN. Simple sodium and potassium salts are also used to increase conduc- tivity, with the potassium salts being generally more soluble. In the case of cyanide solutions, the presence of KCN or NaCN will aid in the dissolution of the metal ion salt. For example, AgCN is poorly soluble in water, hutwilldissolve in an aqueous KCN solution. In addition to providing in- creased conductivitv. added acid or base will serve to stahi-
lize a solution against hydrolysis. Hydrolysis is always unde- sirable since it will often cause the precipitation of the pri- mary metal ion as an insoluble hydroxide.
Some deposition solutions have pH values fairly close to 7, and require careful pH control to insure proper operation. In these solutions, a buffer system is often added. Commonly used buffers are boric acid (enough in excess to exist as ~olvmers) and citric acidlsodium citrate. Formates have also found limited use as huffers in deposition solutions.
As was mentioned earlier, a source of chloride ion is often added to a solution to aid in the corrosion (dissolution) of a soluble anode. We will reserve detailed discussion of the different types of anodes until later but will briefly mention the problem of passivation now. Passivity is a condition in which a metal or allov will corrode at a much lower rate than would be expected &om purely electrochemical consider- ations. This is due to the formation of a thin oxide layer on the surface of the metal and is responsible for the excellent corrosion resistance of aluminum and stainless steels. To prevent passivation and to aid in soluble anode corrosion
Volume 63 Number 10 October 1986 883
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the primary metal ion. Ideally, for each equivalent of metal deposited from the solution, one equivalent will dissolve from the anode and he incorporated into the solution. I t is desirable that these types of anodes should undergo uniform dissolution. under the influence of current onlv. and nroduce metal ions in the proper oxidation state ( u s u k y thk lowest possible). Thev should have a hieh surface-area-to-mass ra- tio and should-he made from higg-purity rolled metal. Cast- ings are unsuitable due to their microscopic ~orositv. A problem with soluble anodes is that they can ofien produce too high a concentration of metal in the anode film, with the resultbeing the formation of a metal sludge.
The second type of anode used is the insoluble anode, the sole purpose of which is to remove the electrons which are introduced a t the cathode. Thus, the metal ions are supplied only by the solution, which will require periodic replenish- ment or replacement. Insoluble anodes are nsed when soln- hle anodes are unavailable or too costly. For example, chro- mium is too hard amaterial to fabricate in acrack-free rolled form, and gold is simdv too expensive to use as an anode. The most common n~aterials fokuse as insoluble anodes are graphite, titanium, lead, and sometin~es stainless steels. Ti- tanium and stninless steels are usable because of their natu- ral thin oxide films, while a red oxide film must be produced on lead before use. Insoluble anodes areoften used incornbi- nation with soluble anodes. This allows the use of high cur- rent densities without an excess of metal ions (and sludge) being introduced into the bath. In this case, the potential a ~ n l i e d to the insoluble anode must be hieher than that applied to the soluble anode. If equal p o t e n t k are used, no current will flow from the insoluble anode. This usuallv requires the uJe of two separate power supplies.
The placement of anodes and cathodes isa critical ronsid- eratioi in setting up a plating bath. Current will always concentrate on the corners and edges of a cathode. Thus, these areas will receive a heavier deposit than the rest of the piece unless the current distribution is changed. Since cnr- rent does not readily flow around a nonconductor in a bath, snch high intensity areas may be shadowed by sheets of insulator such as PTFE. Also, one conductor will rob anoth- er of current. Thus, two cathodic pieces can he arranged
Graphite Copper rod
t Nylon holder
Graphite Copper block rod
Figure I. Sketcnes of (a) cup ancaeo made from copper, nickel, graphm and titanium. Ib) cdpanode used lor si ver: (clpencll anode (the mnon ball. w h m 5 tviSled arouna the grsphlle rod. s not shown): (d) trough slechode.
close to one another to distribute current more evenly. An- other consideration is the inability of current to penetrate into deep recesses, such as the inside of a tube. I t may therefore be necessary to position an anode inside a work- piece to achieve a useful coating.
To overcome most of the difficulties of bath arrangement for laboratory-scale operations, we have designed and fabri- cated cup anodes, which have proven to be extremely useful (see Fig. la). These anodes are easily machined from short lengths (20 cm) of appropriate high-purity barstock. The cvlindrical walls and hemispherical bottoms movide an ideal current distribution for sm& laboratory components. These anodes are connected to the power supply (to be discussed later) using standard banana plugs. Soluble anodes have been fabricated from copper (C11000, tough pitch electro- lytic barstock) and nickel (high-purity alloy 270). These materials are readily available, and short lengths can usually he obtained as free sam~lesfrom most snn~liers. The CUD for silver deposition (Fig. ib ) was made from'a length of P?FE rod, with the soluble anode being a slice of pure silver given to the author by a commercial electrode manufacturer. Cup anodes of the insolu...