Laboratory scale electrodepositon: Practice and applications - Journal

--

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, asoeds of electrodeoo. . . some of the ~ractical 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~lexmixtures 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 concentration of platable ions in the solution: thus,. onlv . the most soluble s d 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 increased degree of hazard. Nitrates are usually not acceptable, since the NO; ion tends to he reduced easily. This renders some metals unolatable from nitrate solutions. since at higher potentials the kvo~utionof ammonia a t 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 complex 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 potassium 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~lexsolutions. 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 concentrations; 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 impurities. Other, less-common anionic complexes are formed using hydroxy and pyrophosphate ligands. The most notable example 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 conductors available. Thus, in s i i p l e silt 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 OHions 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 conductivity, 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 increased conductivitv. added acid or base will serve to stahilize a solution against hydrolysis. Hydrolysis is always undesirable since it will often cause the precipitation of the primary 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 a t a much lower rate than would be expected &om purely electrochemical considerations. 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

jo aamos +uarm[s!ualdar e pin? aporlaala w jo uoqaunj p n p aqq amas sapow alqnlos 'alqnlosu! pue alqulos :pasn are sapow jo sadKl p a u a 2 om& 'paleld aq q ap!pe aql 'asmoa JO 's! apoqpa aql !q%eqau!pe[d aq? u! ?mar!a a!rxaala aql salaldmoa qa!qm apoqaala aq? se par!nbar s! apoue aq& epouw a

I

u

'qpq U! h p o a1qepae h p n s n are pue hpsoa an? suog -emdard aTqepae h1le!arammoa aau!s 's2u!aes Ie!auw!j alqe -rap!suoa E u! pallnsal seq qaeordde s!q& 'rqel paluasard M m ~ ~ o q e ~ s , o qaq? l n u! e asn r!aqqjo saldmexa amos aq I ~ K % p a q!nb pasn p w pamdard aq w a pus 'asn hroleroqe~ roj anpA %ear2jo are suo!lnlos n?lna!%md asaq& y q a m mnu!pId aq? pue 'laya!u 'plo2 ‘rasp 'raddoa roj pap!noad am suolp1nmroj uognlos papuammoaaa amos a l q q aql u~ 'uo! hmmpd a w ueql aIqou arom am p q 1 sa!l!mdm! roj aAgaajja p o m s! uo!+ea!pnd a!liCloqaal3 'apoqlea hmmnp E se alqeaapaas al!nb s! (8u!u!ol poom roj pasn adhl aql 30) l raualsej pala2nrroa e 'suo!pmj!md aleas h r o p ~ o q e [[ems rod .aaejms,sl! uo seam 7 e pue ~ s y o d dmqs q%oqap!aord q sa os 'adeqs mIn%arr!us30 aq plnoqs uogerado s!qlmj apo -qlsa aq& .salnu!m maj e 103 (,m/v 001-0$)hqsuap luarma mol e l a uo!ylos aql moq 4u!yqd hq pamrojrad s! (palpa hpommoa s! 1! se 'Bu!hmmnp 10) uo!lea!j!md a!?i(lor%aal3 v!sh1or?aala 2u!sn leqmamos pag!md aq uea 'pamdard aauo 'uo!$nlos uo!l!sodap v ' p o d a p e 0%Ieluamplap hlaraaas aq 01 say!mdm! u!qraa jo sa!?!lwnb aaeq roj 'raaamoq 'alq!ssod s! 11'sa!?!mdm! Zu!rajralu! morj aar3 hlaA!plar aq p~noqss p a p a q a aper8-lua2sar moq paredard suoqnlos '(Z

' I ) q e j p IhmEl mn!pos p w ap!xorad ua2orphq are suoynl -0s 2uqeld u! pasn Kpomuroa slwlaejms 'ranamoq 'uo!sn[a -a0 ua8orphq ?su!e2e aagaajjau! h[aa!qelar are haq& '8ug?!d luaaard 03 dlaq hqaraql p w 'apoqlea aq? JO aaejms aq? mog paaloaa aq ham qaym salqqnq ua8orphq $0 Bu!Zpols!p p!dar arom qourord hay& .aSeqoaraao ua4orphq mol jo suo!qnlos ol pappe am (uo!suq aaejms ram01 qqqm) q u q a e j m g .paq?ooms aq ol p o d a p aql asnea p w aaejms aq? uo s h a p a pue saqaleras qdoasora!m

HOW 'OdHzW

'07W N3W 'IN3)nVW HOW '03'H N3H N36V 'OZH cOEEH qZ13!N 'OS!N 'OSzH 'OSW 'O'H'3~NH =03"N N3H N3n3

aql uo uoqsodap alomord snql haq& .paz!m!xem s! pla!3 a!lqsorlaaIa aql araqm 'apoqlea aql jo a a e p s aql uo ,,sleds qZ!q,, oluo paqrospe 8 q a q hq uoqaunj srauqq2!rq pus sralahal q?oa 'spodap alllpq pue passaqs h@!q lpsal ualjo m a haql 'aauemadde Bu!seald p w ralsnl ampord saay -!ppa asaql al!qM '(asaq%JO suo!lemqmoa 10) sdnor2 aI!q -tu p w 'alu(y1e 'auayp '~huoqreaaql %utu!eluoasarnaalom a!u&ro hIpnan am 'siria% ~u!,aaa~q$m u b ~ j e ~ ~ ~ rpasn no~u! uayo 'slua2e Bu!ualqB!ra .qua% Bu![ana1 jo hl!luenb pue ralaereqa aq$ uo Bu!puadap 'ssaualll!.rq uo raproq samgamos w a ssaupmq pasearau! s!q& y!sodap paaa~ar-ssaqs'Iapreq e u! qnsar ualjo n!m pue (sa!repunoq u!sr2 aql $ehlpnsn) 11 -6odap aql oqu! paprodroau! aq shemp n!m asaq& 'sa$euojps p w 'sauonns 'sap!meuoj~ns '(sauoqyaeq u!eqa pqemlesun p w aqemore qloq uo) sppe a!uojlns se qans spunodmoa mxns a!ueSro are slua2e Bu!laaal isom 'suoyaayadm! aa!lleI jo raqmnu raBre1e s n q p w az!s u!er2 rallems a u! $ p a r pue sramjar u p 2 aq q parap!suoa are saa!q!ppe jo sad&$qqog y@!rq aq q readde lou op sqsodap TaaaI uago al!nb q2noql -1s p q q .a r hlasola are sraua~q8trq . '. . . . .p . w slua2e Buqana7 .(!y'a.4 .nj'ux 'p> ~ i 'US v .qd :lapro aqi u! i I q ~ n J& sasaarau! m a l a papodap aql .jo ssaupleq pue lu!od Bu!llaru aql se sa8iearau!~h~l~nsn bsauaa!laajja a q i - y e l a & luaijj!p roj p~arajj!p s! saag!ppe asaq? $0 hTq!7n aq& 'sauo Bu!lstxa JO qynod a q ?!q!quj o l pw!apnu 181shr3p~aujo qW& a 9 roAej q Qensn s! spunodmoa asaq? jo aauangu! aq& 'raaamoq 'mqshs aaq?anrelodap p w laajja a p o d d o aql aaeq uea 'srauqq2pq pua srataaal se hl[ed!au!rd pasn 'spunod hq) apoqpa aql -moa Bu!rqquoa-rn~ns amos .(Am 091)~ 78 uo!pz!qod pa!maqa asearau! n!m pue a!w&o are saaq -!ppe asaqq jo isom ysodap aqq qu! paprodroau! %u!aq -pn%aehq ro aaejms apoqlea a q uo paqrospe 2u!aq hq raqqa aa!laajja am san!+!ppe asaq& ' s $ u o m I p x pue 's.!analla.! ssa.!$s's.!aua$y+q 's$uaSo 3 q a o a l hllensn are slua!par%u! ,.aasq,, asaq.1, ysodap. aql .TOralaereqa ~.aql uo laa.j.ra .~ T .m* -uqsqns e aaeq q y m lnq s u o ! ~ e ~ ~ u a ~M uOo~u! a pappe sJag ~~~~

~~

~

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 ratio and should-he made from higg-purity rolled metal. Castings 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 replenishment or replacement. Insoluble anodes are nsed when solnhle anodes are unavailable or too costly. For example, chromium is too hard amaterial t o fabricate in acrack-free rolled form, and gold is simdv too expensive to use as an anode. The most common n~aterialsfokuse as insoluble anodes are graphite, titanium, lead, and sometin~esstainless steels. Titanium and stninless steels are usable because of their natural thin oxide films, while a red oxide film must be produced on lead before use. Insoluble anodes areoften used incornbination with soluble anodes. This allows the use of high current densities without an excess of metal ions (and sludge) being introduced into the bath. In this case, the potential a ~ n l i e dto 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 ronsideratioi 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 cnrrent 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 another of current. Thus, two cathodic pieces can he arranged

close to one another to distribute current more evenly. Another consideration is the inability of current t o penetrate into deep recesses, such as the inside of a tube. I t may therefore be necessary to position an anode inside a workpiece to achieve a useful coating. T o overcome most of the difficulties of bath arrangement for laboratory-scale operations, we have designed and fabricated 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 electrolytic barstock) and nickel (high-purity alloy 270). These materials are readily available, and short lengths can usually he obtained as free sam~lesfrommost snn~liers.The CUD for silver deposition (Fig. i b ) 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 insoluble type have been machined from sections of graphite (obtained as blocks from a bearing manufacturer) and titanium. These anodes are not affected bv the passivation problems mentioned earlier and have given-better performance than our soluble cup anodes for most applications. In addition to the cup anodes, we have made an insoluble nencil anode. shown in Finnre lc. In this aonlication. the plating solution saturates a ball of cotton which is securely w r a ~ ~ around ed a small eraohite rod. Platine is then simnlv brushed on the preparei metal surface (8) i f t e r curreni applied. T o aid in snch "brush datine" a~nlications. we .. L G e fuund the trough electrode tobeof value (Fig.'ld). This issimply ngraphire block with J 3-mm t r o u ~ 'milled in the top surface. A copper rod inserted through the cencer of the block supplies uniform current distribution. A workpiece such &;flat electronic contact is placed in the trough and brush plated using the anode pencil. These simple, homemade electrodes perform better than commercial small scale plating tools, and are ideally suited to laboratory use. In addition, they can be easily constructed for a fraction of the cost of commercial devices and kits. Most of the materials nsed for the anodes were, in fact, obtained just for the asking. The Dower s u n d v for laboratom scale electrodenosition is most convenienifi direct current source capable 6f delivering a current of up to 1 A at a maximum of 6 V. On the industrial scale, the power supplies are much more sophisticated. Commercial power supplies provide for the selection of complex waveforms, as wellas pulse operation and periodic current reversal. The purpose of these different profiles is to disrupt and reform the cathode film and to electropolish the deposit in sitn. The relative benefits of these techniques are of& not clear, and i t would seem that the industry is influenced by fads. Bothan ammeter (0 t o 1A) a n d a voltmeter (0 to 6 V) are needed for the laboratory unit, as well as

-

..

a

Graphite block

Copper rod IA.125V

Graphite

Copper rod

Solid

state

P~CILamp

t

Source

Nylon holder (C) 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. Figure I.

Power

C0"tlDl

Voltage Control

Figure 2. An easily constructed but serviceable power supply for laboratory electrodeposition. The resistance and power ratings required for the variable resl~torsare dependent upon the particular solid state power supply chosan. scale

Volume 83 Number 10 October 1986

885

provisions to adjust hoth the current and voltage. A workable power supply can he easily built using a commercial solid state power regulator (obtained as a packaged "black box" unit) and appropriate circuit components that are readily available ih-recail outlets. The unit built for use in 2, the author's laboratow is shown schematically in Figure . and the total materials cost was under $120. In practice, the appropriate solution is warmed in a water bath to the optimum temperature. Whena cup anode is to he used, it is warmed as well. Just prior to use, the anode is removed and dried and connected to the positive terminal of the power supply. The solution is introduced into the cup usine a pinet. The workpiece. connected to the neeative terminal; then dipped into the solution, and the