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U. S. Department of Commerce Research Paper RP1835 National Bureau of Standards Volume 39, November 1947 Part of the Journal of Research of the National Bureau of Standards ( Deposition of and by Chemical Reduction By Abner Brenner and Grace Riddell

A process has been developed for dep ositing nickel and cobalt from hot solut ions of hypophosphite wi thout the use of curren t . T he reduction of occurs only on certain ca talytic metal surfaces. By specific pretreatments, t he a dhesion of t he deposits can be imp roved and by other pretreatments, t he reduction can be induced on noncatalytic surfaces. The r ed uction can be i nhi bited completely by the presence of cer tain ions in t he solu tion or by the ca talytic metal surface becoming in active. TIle possible mechani sm of the reaction and t he factors affecting t he rate of deposit ion arc discussed . T he deposit p roduced by this proces are sound, hard, a nd of good quali ty.

I. Introduction solution, and that depos its arc not usually ob tained on the vessels. No electric curren t is in­ Since the publication of a report by the au thors 1 volved in the depo ition . It is suggested that this of a process for depositing nickel by chemical re­ process be designated as " clecLroless nickel duction, additional information has been obtained plating" and "electroless co balt plating". that broadens the op erating condi tions and the applications of this process. The first paper de­ II. ·Composition and Operating Charac­ scribed the deposi tion of nickel from an ammoniacal teristics of Solutions solution containing hypophosphite. The process has been extended to include the deposition of 1. Nickel nickel from acid solutions and of cobalt and cobalt­ nickel alloys from ammoniacal solutions. (a) Alkaline Solutions The process involve the reducing action of This subject, treated in the previous paper, will hypophosphites in a solution of nickel or cobalt at be reviewed briefly for completeness. The m ost 90 0 C or above in the presence of cer tain catalytic satisfactory alkaline solutions are ammoniacal, . The reactions can be expressed by the and contain citrates, ammonium salts, hypo­ following equations: phosphites, and nickel salts. A typical bath is composed of: NiCI 2 + N aH2P02+ H 20 --? gjiiter Ni+ 2I-ICl+ NaH2P03 (1) Nickel chloride______30 Sodium hypophosphite______10 or (2) Ammonium ______50 Sodiu m citrate ______100 The hypophosphite undergoes oxidation, and the Ammonium h ydrox ide to a pH of 8 to 10 nickel is reduced. Both of these componen ts must Such a solution deposits ni ckel at the rate of be added at intervals to make th e deposition 0.0002 to 0.0003 in. (0.005 to 0.008 mm) pC'!' hour. con tinuous. As previously indicated, the rate of deposition An interes ting feature of the process is that the depends upon the composition of the solution and deposition of nickel or cobalt OCC Llrs only on certain is approximately proportional to the concentration metallic surfaces that are immersed in the hot of the hypophosphite, as shown in table l. Vary­ ing the concen tration of nic.kel salts within the 1 A bner 13ren ll cr and Grace E. R idocll , J . Research NBS 3 7 ,31 (1946) RP1725: AES A nnual P roceedings, p . 20 (1946). limits shown produces no appreciable change in

i Deposition of Nickel and Cobalt 385 r -..------I

the rate of deposition. Other factors, such as the Earlier attempts to prepare an acid-nickel solu­ concentration of organic salt, effect the rate of tion for electroless plating were unsuccessful. deposition to a smaller degree. More recently an acid-nickel solution was formu­ lated that overcomes the disadvantages of the al1m­ TABLE 1. Effect of bath composition on rate oj deposition line solution. When the acid solution is used at elevated , no fumes or loss of constit­ ConccnLralion I R aw of dcposiLion per hour uents by vaporization occurs. The solu tion is SODICM H¥POPHOSPIII'I'E more easily controlled and the deposits are pro­ ------c------.------duced at a faster rate than from alkaline solutions. gll;te, il1. X JO-· mmX I O-3 The proposed acid-nickel solutions differ from 2 0.3 8 4 .6 15 the alkaline-nickel solution in composition as well 10 2.4 51 as in pH. Typical bath compositions are given 50 7.5 165 m table 3. NICKEL TABLE 3. Composition of the acid-nickel soi1ttions

3.8 2.2 55 i .5 2.6 66 Bath 15 3. 0 76 II III IV ------.------glliter glUt" glliter glUter Table 2 indicates that the rate of deposition Nickel chloride, NiCl,.6H, O. _____ . __ ._._ 30 30 30 ______increases rapidly with . This effect Nickel , NiSO •.7H,O _. ______._._._ .. _____ . ______. 30 Sodium hypopbosphite, NaH,PO,.H , O __ 10 )0 )0 10 of temperature applies also to the acid-nickel Sodium hydroxyacetate, NaC,H, O,______50 10 __ .. _._. __ . ___ ._ solution and the alkaline cobalt solution, which Sodium acetate, NaC2H , O,.3H,O .. ____ . __ . ___ ._ .. _____ . ____ ._.__ 10 will be discussed later. Sodium Citrate, Na'C . H 5 0 ,. 5~' H 20 ____ . _. ___ ._ .. ___ ._._ )0 ._. ___ .. Rate of deposition: Millimeters per hOUl' ______._ TABLE 2. Effect of temperature on rate of deposition 0.015 0.013 0.005 0.025 Inches per boW' ______. ___ ._._ .0006 .0005 .0002 .001 Appearance of deposit. ______•. _--_. _ Semi- Semi· Semi· Rough, Temperature I Thickness per 30 minutes bright bright bright dull ----_.------,----_.- 4 to 6 4 to 6 4 to 6 4 to 6 o C. il1.XJO- · 1n'lnX I0- 3 54 0.155 0.39 68 .38 .96 78 . 64 1. 63 At a pH of 5, nickel deposition will occur in a 87 .9,1 2.4 92 1.1 2.8 solution containing only soluble nickel salts and sodium hypophosphite. The reaction is vigorous, and the reduction of nickel is very rapid. Without If additions of hypophosphite are made at any regulation of the pH, the latter drops rapidly, suitable intervals, the useful life of such a solu­ and at a pH of 1 to 2, the rate of deposition js low. tion is relatively long. In a I-liter bath, the In order to prevent the rapid drop in pH, salts equivalent of an area of 40 square decimeters was of organic acids are added as buffers, the most plated for 1 hour before the bath became ineffici­ satisfactory being sodium acetate and salts of cer­ ent. (This is equivalent to about 16 square feet tain hydroxycarbOA.J'lic acids. These buffers main­ for 1 hour in a I-gallon bath). If the nickel con­ tain a high rate of nickel deposition, and the centration had been constant, and the solution hydroxycarboxylic salts prevent precipitation of had been filtered at intervals, the useful life of the basic nickel salts. solu tion would have been still greater. To obtain the best deposits from a solution buffered by sodium acetate, its concentration must (b) Acid Solutions be carefully regulated. The data in table 4 indi­ The alkaline-nickel solutions good, sound cate that large variations in the rate of nickel deposits, but in practiee, their use is inconvenient deposition results from changes in th e aceta..te and uneconomical, because at the high temperature concentration. The rate of deposition reaches a of operation, there is a rapid loss of , and maximum in the nickel-chloride bath with an the fumes are disagreeable. acetate content of 2.5 g/liter; and in the nickel-

386 Journal of Research sulfate bath, of 4.3 gjlitel'. The acetate can be trations from 10 to 100 gjliter withou t appreciably added as barium, so dium, nickel, 01' other soluble affecting the rate of the process. This is in co n­ salt. The acetate bath has the disadvantage of trast to the behavior of the alkaline solu Lion, in becoming turbid with use and of producing de­ wh.ich the rate of deposition is nearly proportional posits that are dull and rough. to the concentration of hypophosphi te. The preferred pH range is from 4 to 6.5. A TABLE 4. Effect of acetate concentrat1:on on rate of deposition made up, the solutions have a pH of about 6 and no pH adjustment is required for a new bath. Bath composition Aceta te Thickness of de pos it raciical in 1 h I' However, with long use, the pH drops slowly and, ---- for continued rapid deposition, additions of a g/liler i7l .XlO'" mm 2. 0 7. 5 0. 02 dilute hydroxide solu tion should be made at Nickel su lfate, :~ 5 g/l itel', and sod ium 4.3 JO . O . 025 hHOphosphite, 10 g/liter. regular intervals. Because of the action of the { 6. 5 '1. 0 . 01 buffer, frequent additions of hydroxide are not 1.0 2. 0 . 005 11l'cessary. The alkaline solu tion used for the N ickel chlol'ide, 30 g/liiel', ands od ium 2. 5 6. 0 . 015 hypophosphite, JO g/liter. 6.5 1.0 . 0026 pH regulation should be dilute, so that the 1 20. 0 ll.6 .0016 additions will not cause local precipitation of . Sodium a nd potassium hy­ Certain hydl'oxycarboxylic acid s, such as hy­ droxide are used in preferencc to ammonium droxyacetic and ci tric acids, yield good ratcs of hydroxide, because large concentration of am­ ni ckel depoiLion and likewise preven t precipitation monium salts tend to lower the rate of the of basic ni ckel salts, which cause rough deposits. reaction. Cit f' atcs produce deposits of good physical proper­ The yield or effi ciency of the reaction, calculated ties, but .the rate of deposition is lower than from on the hypophosphi te wa determined by weigh­ hydroxyacetate 01' aeetate solutions. Tartaric, ing the amount of n ickel deposited on (1) a mali c, gluconie, and formic acids are unsatis­ relatively large metal urface ill a small volume factory because negli gible amounts of nickel are of solution (i. e., 0.5. dm2j100 ml ) and (2) a deposited in their presence. Table 5 indieates relatively small metal surfaee in a large volume of that 50 gjlitel' and 100 gjliter of sodium hydroxy­ solution. (i. e., 0.5 dm2/1i ter) Under the first acetate give the highest rates of deposition, conditions, the average e£Iiciency is 37 percent, respectively, for the nickel-sulfate and ni ckel i. e. about 2 g of nickel is reduced by 10 g of sodium hypophosphite. In the econd procedure, TABLE 5. Effect of the concentration of hydroxyacetate I)n the e£Ii ciency is only about 20 per cent. Although mte of deposition both alkaline and acid solutions have about the - same weight e£Iicieneies, the rate of nickel depo­ Na 'I' hickncss of deposit ]33 1 h ('Dill position h y clro \': ~' - sition is larger in the acid solution. acetate in 1 hr ------2. Cobalt g/liler i1l .X IO-' 'm'ln 10 1.0 0.003 (a) Alkaline Solutions Nickel su lfate, 35 g/Ji tCI', and sodium 50 4. 0 . 01 hy pophos phite, 10 gfli ter. 100 3. 3 .003 Some investigators 2 3, by using highly con­ 1 200 2.8 . 007 centrated solutions of hypophosphite, obtained JO 4. 0 . 01 Nickel chloride, 30 g/liiel', sodium cobalt in Hake and powdered form by a reduction 50 6. 0 . 015 hy poph osphite, 10 gjlitcl' . { Joo 7.0 .018 similar to that described for nickel. In the studies by Scholder and Heckel 3, cobalt powder was obtained from a trongly alkaline tartrate chloride solutions. The concentrations of the solution. The black powder eontained 90 to 92 other components may vary considerably. The percent of cobalt and 4 to 6 percent of . nickel content may vary from 3 to 50 gjliter with­ The solution slowly decomposed upon standing at out appreciably altering the rate of deposition, but room temperature and the reaction became with a nickel content of 100 gjliter, the rate of vigorous upon heating. deposition is somewhat de creased. 2 P aal and F riedrici, Oer. deut. chem. Oes. 64, 256 l (1 931). Sod ium bypophosphite can be used in concen- 3 Sholder and H ecke l. Z. anorg. a]J ~e m . Chern. 198. 329 (19311. •

Deposition of Nickel and Cobalt 387 In the present studies, it was found that by Table 6 shows the composition of some typical reducing the concentration of the solution con­ baths. Bath IV produces deposits of superior siderably, the formation of the powder was pre­ physical properties. However, deposition may be vented, and smooth, adherent deposits of cobalt slow to start, and in such cases, it can be initiated were formed on metal surfaces immersed in tbe in bath II before the parts to be plated are Im­ hot solution. The alkaline-cobalt solution is very mersed in bath IV. similar to the alkaline-nickel solution in compo­ sition and operation. It is composed of soluble TABLE 6. Com positions of some of th e alkaline-cobalt salts of cobalt, hydroxycarboxylates, ammonia, solutions and hypophosphite. The bydroxycarbm..:ylic salts Bath are used to prevent the precipitation of cobalt in II III IV the alkaline solution, and to afford deposits of ------good physical properties. The most suitable Cobalt chloride: olliter olliter olUter olliter hydroxycarboxylic salts are the tartrate and CoCh.6R20 ______30 30 30 30 Sodium hypophosphitc: citrate. Other salts, such as sodium hydroxy­ N aH 2P02. R 20 ____ -______20 20 20 ]0 Hochelle salt: acetate, gluconate, and salicylate are unsatisfac­ 200 ______tory because they slow down the production of Sodium citrate: deposits. Deposits produced in the citrate solu­ Na 3 C 6 H , 0 7 _5~i H20 ______35 ]00 100 Ammonium chloride: tions have better physical properties than those NR,CL ______50 50 50 50 from the tartrate solut{ons, which tend to be more Hate of deposition: dull, powdered, and porous. Millimeters per hour ______0_ 0076 0. 016 0. 007 0_ 005 In the cobalt solution, ammonium salts decrease Inches per bOuL ______. 0003 . 0006 . 00027 . 0002 Appearance of deposit ______Dull Dull Dull Dull the rate of deposition to a considerable extent, in Alkali for neutralizing bath ______NH,OH NR,OH NH,OH NH,OH contrast to their effect in the alkaline nickel solu­ pH ______9 to 10 9 to 10 9 to 10 9 to 10 tion. However, this decrease in rate is compen­ sated for by the fact that the deposits are superior The pH of the solutions has a considerable in physical properties to the dull, rougb, and effect upon the rate of deposition. For best re­ porous deposits formed at exceedingly high rates sults, the pH should be kept above 9 for the cobalt, in the solutions free from ammonium salts. The whereas, a pH of 8 to 9 was satisfactory for the optimum concentration of ammonium salts is be­ alkaline nickel solutions. The yield or efficiency tween 25 and 50 g/liter; an excessive amount, for of the deposition of cobalt, based on the decom­ example 100 g/liter, decreases the rate of deposi­ position of hypopbosphite, is 66 percent when a tion to a negligible value. relatively large surface is coated in a small volume The cobalt concentration should be between 2 of solution. ,Vhen the area of metal surface in a and 10 g/liter. A concentration much above this given volume of solution is decreased, the efficiency cause darkening of the deposits and finally is lowered. reduces tbe rate of deposition. (b) Acid Solutions The sodium hypophosphite is used in higher concentrations in the cobalt than in the nickel The reduction of cobalt from acid baths is not solu tions. Very little cobalt reduction occurs satisfactory. A very thin film of cobalt forms on with sodium hypophosphite equal to 10 gfliter, and in solutions of certain composi­ but the rate of deposition is steadily increased as tions, but the deposit does not build up and covers the hypophosphite concentration is increased to irregularly. From experiments in which the com­ 100 g/liter. For optimum efficiency, it is recom­ position, concentration, metal surfaces, and pre­ mended that the sodium hypophosphite content treatments were varied widely, it was concluded be 20 g/liter. that deposition from acid solution is impractical. For most purposes, ammonium hydroxide is 3. Solutions for Depositing Nickel·Cobalt Alloys used to regulate the pH of the solution. Other hydroxides can be used, but, when this is done, A deposit of a nickel-cobalt can be ob­ more care is required in making additions in tained by using an alkaline soluLion containing order to keep within the pH range. soluble salts of both metals. In a solution con-

388 Journal of Research taining equal parts of nickel and cobalt, the re­ in 1 hour. Changes in the composition of the sulting deposit contains about 65 percent of solution cause changes in the rate of deposition nickel. When the metal ratio, Co/Ni, in the so­ and in the physical properties of the deposit that lution is changed to 2, the resulting deposit con­ are similar to those mentioned for the individual tains about 50 percent of each metal. metals. ,iVhen sodium citrate (100 g/liter) is used The conditions for plating, wi th respect to pH, in place of Rochelle salts, the deposits are brighter temperature, and composition, are the same as and smoo ther, but the resistance is for alkaline electroless plating of nickel or co balt. inferior. The pH should be 8 or above, and the temperatUTe In the acid solution, an alloy cannot be ob­ at least 90°C. A typical alloy bath co ntains: tained, as nickel alone is deposited. Only 0.25 ofiiter percent of cobalt was found in a deposit from an Cobalt chloride ______30 acid solution containing equal amounts of each N ickel chloride______30 metal. Sodi urn citrate ______100 III. Effects of Impurities Ammonium chloride ______50 Sodium h ypophosphite______20 Certain metals and radicals in the plating solu­ Ammonium hydroxide to regulate pH. t ions decrease the rate of metal reduction. T able This solution deposits a semibright cobalt-nickel 7 shows the concentrations at which orne metals alloy at the rate of nearly 0.0006 in. (0.01 5 mm) will inhibit plating.

T ABLE 7. Effect of impurities

Alkaline·nickel bath Acid·nickel bath Alkaline·co balt bath Impurity Impu· Impu· Impu· rity EtTecls rity Effects r ity Effccts

u/liter ulliter ulUt er Ouso •.. _... ______0. 01 Oopper deposition __ •...... _ 0. 1 Rate decreased...... O. l N ormal del)Os ition. ZnOh ______. __ ... _..... _. _____ .. _. 1. 0 Rate decreased _.. _...... 1 N ormal depositio n ...... 1. 0 Rate decreased. OdCb __ ...... _._ ... . 0.1 N o deposition .. _...... _...... 1 Rate decreased . .. __ ...... 0. 1 Rate greatly decreased. M gOb ...... _...... 1. 0 N ormal depOsition...... 1 Normal deposition ...... _..... 1. 0 Rate decreased. AIOh _..... ___ ._ ... _.... __ .. _... ___ _ 1. 0 .. _.. do ...... _... _...... _. . 1 .... .do ... _...... _...... 1. 0 Normal deposition . F eOh.. _...•... _... _..... __ ..... ___ _ 1.0 .... _do ..... __ ...... •.. . 1 . . ... do ...... _ ._...... 1.0 Rate decreased. K SON ... _.. __ ...... ___ ... . 0. 01 No deposition...... _...... 1 N o deposition...... 0. 01 N o deposition. PbO!,...... _._ ...... 1 .... . do ...... _.. . __ ...... 1 _._ .. do .....•. _...... _.. _._ ..... _...... __ KCN _... _..... _... _.. _. __ . _._. _... . .01 .. _.. do ..... _...... •...... 01 N ormal deposition ..... _...... 01 N o deposition. N aH,PO,._ .... _.... _.. ___ . ____ . __ .. 10 Rate deereased_.... .•. _... _.. _. . . . _...... _...... 10 Rate deereased.

The cobalt solution is the most sensitive, and Frequently, failure to plate is causcd by ­ the acid-nickel solution the least sensitive to ing of the metal surface. An interestino- exampl<' impurities. Palladium does not inhibit the dep­ of this is the of a surface by osition, but it causes a gradual decomposition of cyanide solutions. :Massive does not the hypophosphite when present in the solution. always have the catalytic properties shown by To render the contaminated solutions operable the freshly prepared palladium film . again, the impurities must be removed. The removal can be made by conventional means of IV. Deposition on Various Surfaces separation. For example, the suggested malmer for removing cadmium from the solution is to , nickel, gold, cobalt, aluminum, and pal­ adjust the bath to pH 3.5 to 4.0 and to pass ladium have catalytic properties that initiate the through the solution for about reduction of the nickel and cobalt immediately . 15 minutes. The cadmium is precipitated under upon their introduction into the hot solution. these conditions and it can be separated by filter­ surfaces reduce some nickel, but the deposits ing. If the pH of the solution is above 4, much have very poor physical properties. Some metals, nickel is precipitated, and if the pH is below 3, particularly , occasionally act as catalytic the cadmium precipitation is not complete. surfaces.

Deposition of Nickel and Cobalt 389 D eposition can be obtained on noncatalytic duration to initiate the deposition on the non­ metal surfaces such as , copper, and , catalytic surface. Once the plating is star ted, if certain measures are taken prior to immersing presumably as a result of galvanic action, the the metal into the plating solution. Lead and reduction continues on the nickel that is first cadmium are non catalytic metals that are not deposited. By this procedure, such metals as favorably affected by pretreatments, and deposi­ silver, copper, and brass can be activated so as to tion cannot be induced upon their surfaces. To cau s~ deposition of nickel upon their surfaces. induce deposition on specific non catalytic sur­ Lead cannot be so plated. See table 8. faces and on certain catalytic surfaces that may be temporarily passivated, three methods may be TABLE 8. Pretreatments f01' noncatalytic sU1jaces used: (1) Change the composition of the plating +, surface will plate; -, surface " 'ilI nof; plate solution (applicable to alkaline solutions only), (2) make con tact with a more electronegative C'on tart process Pall adium dip ----,------;------;--- Metal surface metal, and (3) deposit a thin layer of a catalytic Alka- Acid I Alka- Alk a- Acid Alk a- metal on the surface of the noncatalytic metal. ______~ne ~ _~~_ ~~~~ _ l~~~~ _~_ ~~~~ The latter two processes are applicable to both alkaline and acid solut ions. i~:e:· ::::::::::::::::: ! I ! I ! ! ! ! Procedure (1) . Copper and brass can be plated P latinum .______+ . + + Gold a______+ I + + + I + + in alkaline solutions containing no ammonium Sil ver______+ I + + salts, but these solutions are not as stable as the Lead ______.______--- ~- --I ammoniacal solutions that have been recommended previously. As much as 0.0001 in. (0.0025 mm) • Altbough gold is listed as catalytic for this process, it is frequen tly inac· tive. of nickel is deposited on copper m 30 minutes from solutions containing: (3). The third method of initiating deposition y/liter involves depositing minute amounts of catalyti­ N ickel chloride ______30 cally active metals, such as palladium or , Sodium h ypophosphite______20 Rochelle saIL ______200 upon the noncatalytic surface. This can be done Sodium hydroxi de to reg ulate pH to 8 to 10. by dipping the metal into a solution containing palladium, 0.02 gjliter, and HCI (20 mljliter). Because of the instability of this solution, this This dip is made after the conventional cleaning, If procedure is not of practical value. an excess and immediately prior to immersion in the hot of sodium or is used in regulat­ nickel solution. The immersion deposit of palla­ ing the pH, metal compounds will precipitate, and dium is visible as a faint tarnishing, and is sufficient in turn, may act as nuclei for vigorous reduction to catalyze the nickel reduction. This procedure of the metal and thus cause rapid decomposition is particularly satisfactory for obtaining deposits of the solution. It is of interest that, although of nickel on copper and copper alloys. The metal reduction is induced on copper surfaces in time required for immersion is dependent upon this type of solution, it does not consistently the cleaning treatment that th e metal is given occur on steel surfaces. This is opposite to the prior to the palladium dip, and also upon the behavior of these metals in the ammoniacal solu­ temperature of the palladium solu tion. tions. Though no t always necessary, a preliminary (2). It was learned in this research that some bright dip (in H 2S0 4 and HN03 or H 3P04 and non catalytic metals can be plated by momen­ HN0 ) is recommended for copper and brass tarily bringing a more electronegative metal, such 3 surfaces. If copper is given a bright dip, the tim e as aluminum or iron into contact with the surface required for immersion in the palladium solution of the article while it is immersed in the electroless at 25 ° C is only a few seconds; otherwise the im­ solution.4 The con tact needs to be of only short mersion in the palladium solution should be in­ • William Paecht of the Wright Aeronautical Corp., in a subsequent com­ creased to several minutes. With or without a m unication to the authors, suggested t he usc of iron or steel for this purpose. A copy of a note was recently received that was published ill Science and Oul­ brigh t dip, brass should be immersed in a palladium ture (Oalcutta), 12, 503 No. 10 (April 1947), by A. P. Goswami. H e also solution at 60° C for about 1 minute. reported successful deposition of nickel on copper, b rass, and platinum by bringing them into contact with steel. In spi te of palladium being a precious metal,

390 Journal of Research this treatment is relatively inexpensive, because phosphite (eq 2) to produce hydrogen whi ch, in of the very small amoun t of palladium consumed turn, reduces the nickel on the catalytic surface. by the process. At the current of pallad ium There is some evidence to support this mechanism (about $1 per g) , the solu tion will not cost more for the alkaline solutions. Hydrogen evolution than about $0.02 per liter. Only very thin films occurs at the surface of catalytic metals, even in of palladium are required; in fact th e film used is the absence of nickel or cobal t salts, when immersed almost of unimolecular thickness, (about 2 x 10- 8 in the hot alkaline hypophosphite solutions. cm or 8 x 10- 9 in.) as judged by the palladium F urthermorc, the potential of the catalytic metal consumption. One liter of solu tion , containing in this solu tion is the same as the poten tial of the 0.01 g of palladium,is suffi cient to treat nearly 200 metal during the electroless plating process. The dm2 (20 ft.2) of motal surface . potential is - 1.1 to - l.2 v, relative to the satu­ Plastic surfaces are noncatalytic, and none of rated calomel electrode (see table 9). This poten­ the above procedures will initiate the nickel tial is more negative than the reversible potential deposition. In a n endeavor to plate plastics, of either hydrogen or nickel, and hence is sufficient several types were tried in the hot nickel solution. to account for the separation of the free elemen ts. Even when precleaned with organic solven ts and acids, the plastics remained noncatalytic. No TARLE 9. Electro de potentials (against satuTated calomel deposition occulTed on the surface wh en immersed electrode) in an ammoniacal nickel solution in highly concentrated solutions. When a plastic Potenl.ial with 1'('­ SPOZL t 'J saturated was silvered prior to immersing in the nickel solu­ calomel electrode tion, satisfactory deposition faiJ ed to occur. r: lcct rode Before After This failure was due primarily to the silver flaking pIaU g plating from th e plastic in the hot solution. Other methods of apply ing a me.tal film to the plasti c 8ieeL . ______. ______. ______. ______.. __ .... ______- 1.16 A luD1 i nUlTI * ______.______-1. 2 might be more successful. P latinum . ______- 0.44 a_I. 2 Copper __ . ______. ______.. ______-. 60 ' -1. 2:; Sil ver ___ ...... _...... _. __ ...... _ - . 17 '- 1. J5 v. Mechanism of the Reaction Oold ______. ______... ____ .______- . 04 ' -1. 23

The data given in section II regarding the effe ct a P lating induced by contact wilh aluminulll. of concentration of the bath constituents on the rate of reaction show that the reaction does not This explanation does not apply to the reaction go according to tll e equations. If the reaction in th e acid solution in which, as already empha­ proceeded in the fashion expressed in eq 1, the sized, the rate of deposition is not aff ected by the rate of deposition of nickel would be proportional concentration of the hypophosphite. Furthermore, to the concentration of hypophosphi te and nickel. the catalytic decomposition of hypophosphite with As previously discussed, the rate of deposition in the production of hydrogen does not occur at the acid solution is virtually unaffected by the metallic surfaces in the absence of nickel salts. The concentration of the reactants, and in the alkaline fact that the rate of deposition of nickel is in­ solution is affected only by the concentration of creased considerably by the movement of th e metal the hypophosphite and not by that of the nickel. being plated would seem to indicate that the rate Therefore, th~ reaction docs not follow a simple may be controlled by some type of diffusion proc­ course, but undergoes intermediate steps. ess. Agitation of the metal does not affect the rate Actually, the rate of the reaction is affected as in the ammoniacal solution. Therefore, the much by the change in pH and the concentration mechanism seems to be different for acid and and specific nature of the organic salts as by the alkaline solutions. concentration of the primary reactants. Vi e have Another possible mechani m for electroless plat­ attempted to explain the reaction by one of two ing is that the presence of hydrogen on the cat­ hypothesis, neither of which is applicable to all the alytic metal constitutes a galvanic cell, in which phenomena. hydrogen is the anode and the metal the . One suggested mechanism is that the reaction The process co uld then be considered as electro­ that determines the rate is the reaction of ,hypo- lytic. The potential of the metal dUl'ing the

Deposition of Nickel and Cobalt 391 electroless process is negative enough for such to some cases will detach on flexing. The adhesion be the case in both the acid and alkaline solutions. to various metals can be improved by variations (See table 10.) An objection to this mechanism in the cleaning of the metal prior to immersion in is that from an acidic electroless solution con­ the reducing solution. Excellent adhesion to steel taining equal amounts of cobalt and nickel, can be obtained by an anodic treatment of the only nickel is deposited. If this were an electrolytic steel in a 70-percent , but in order to process, more cobalt would be deposited than make the process independent of current, it is nickel. preferable to use a dip in concentrated (95%) sulfuric acid. TA BLE 10. S ummary of potentials (against saturated calomel electrode) in solutions containing hypophosphite The adhesion to surfaces that have been given a palladium dip may be inferior because of the So lutions Aver­ usually poor adhesion of immersion coatings of age Condition of electrode poten­ palladium to the . The adhesion of the No. Composition tial nickel to aluminum surfaces is unsatisfactory, but can be improved if the aluminum is given a zincate A Alkaline nickel with ammo- During deposition ______- 1. 2 nium salts. dip prior to immersion in the reducing solution. B Alkaline nickel with no am- _____ do ______- 1. 3 monium salts. C Same as B, but without nickel. As hydrogcn evolnlion OCC lll'S _ - 1. 3 VII. Physical Properties of the Deposits D Alkalin e cobalt with ammo- Dnring deposition ______- 1. 2 nium sal ts. E Acid nickel with hypopbos- _____ do ______- 0.9 1. Microstructure pbite and organic salt. F Same as E , but without nickel No observable reaction at - . 8 As shown by figures 1, 2, and 3, the deposits catalytic surfaces have laminations running parallel to the base G Same as E , but without hypo- During electrol YSIS at 0.5 -. 77 phosphite. amp/dm' . metal, and within each layer there is a columnar H _____ do ______Theoretical H, potentiaL __ _ . 59 structure perpendicular to the base metal. Figures 2 and 3 show that upon heating, precipitation In connection with the measurement of the occurs. potentials of electroless plating, it was of interest 2. Hardness to determine the effect of impurities on the poten­ The hardness of the electroless nickel deposits tials. As small concentrations of cadmium inhibit is greater than that of electrodeposited nickel. deposition, this impurity was selected for study. The Knoop hardness number for nickel, electro­ The addition of cadmium to the extent of 100 deposited in a Watts type bath, may vary from mg/liter in the course of an electroless plating 120 to 450. For the electroless nickel from the experiment caused the potential of the work to alkaline citrate solution, the hardness is about become more noble, as shown in the following 425, and from the ammopiacal solution containing data, and both the metallic deposition and hydro­ no organic salts, about 530. Deposits from the gen evolution ceased. (Potentials measured against acid-nickel solution have an average hardness of calomel half-cell.) about 500 . Upon h eating the electroless nickel deposi ts , their hardness is increased; this is in Potential of steel before hypophosphite is added to the bath ______- 1. 15 contrast to the so-called "hard" nickel that is Potential of surface while plating with usually electrodeposited from a bath containing hypophosphite added to the bath ______- 1. 28 ammonium salts and has an initial hardness of Potential of surface after cadmium chloride, about 500, but which softens upon heating. A 0.01 g/liter is added to bath ______- 1. 14 possible explanation of this difference may be the VI. Adhesion of Deposits occurrence of in the elec­ troless deposits. As formed, the electroless de­ The adhesion of the deposits to the base metal posits are brittle. but they become ductile upon is such that the deposits will not flake off, but in hef1ting.

392 Journal of Research FIGURE] . Electroless nickel deposit jTom an acid solution showing lami nations and columnar struct1!re, X 500. Knoo p hardness number 500.

FIGURE 2. Electroless nickel deposit, after annealing at 4000 C for Y:i hI' , X 500. Knoop hardness number 800.

Deposition of Nickel and Cobalt 393 FI G URE 3. Electroless m:ckel deposit, after annealing at 8000 C for 7f hl' , showing appal'ent pI'eci pitation, X 500. Knoop hardness number 470.

3 . Magnetic Properties deposits of Co-Ni alloy fail in the salt spray Qualitative measurements of magnetic proper­ sooner than the single deposits of cobalt or nickel. ties were made by measuring the attractive force 6 . Composition of a permanent for the coating. It was found that the electroless nickel is not as magnetic Nickel deposits from the alkaline solutions con­ as electrodeposits from the Watts nickel bath. tain an average of 93.5 percent of nickel and from The magnetic properties of electroless nickel are the acid solutions, an average of 93.1 percent of increased by annealing at 400 0 C for about 30 nickel. The electroless cobalt deposits contain minutes. 94.5 percent of cobalt. Although quantitative 4. Appearance analysis has not been made, qualitative tests indicate that the deposits contain considerable In general, the alkaline nickel solutions produce amounts of . the brightest deposits. Deposits from the acid­ nickel solutions can be brightened somewhat by 7 . Reaction with Ferric Chloride the addition of cobalt or very small amounts of By immersing electroless nickel deposits in an cadmium (0.01 g/liter), but by such additions, the acid solution of ferric chloride, 200 g/liter, the rate of deposition is decreased. The cobalt de­ metallic surface is blackened. It has not been posits are dull (similar to electroplated cobalt in possible to produce a uniform degree of blacken­ appearance) and tend to be dark when deposited ing, and hence this property has little commercial at high rates. value. The elec koless black nickel does not 5 . Corrosion Resistance protect steel in the 20-percent salt spray tests a.s well as does electroplated black nickel of th e In the 20-percent salt-spray test, electroless same thickness. nickel, deposited on steel from either the acid or alkaline solutions, affords protection to the steel VIII. Possible Analytical Applications equal to that of electroplated nickel. The cobalt deposits from the citrate solutions give protection 1. Detection of Palladium to steel superior to those from the tartrate solu­ As previously described, reduction in th e tions, but neither are comparable to electroplated hypophosphite solutions can be initiated by the deposits of the same thickness. The electroless presence of small amounts of palladium. Feigl

394 Journal of Research and Frankel 5 used this reaction to detect palla­ 2. Quantitative Reduction of Nickel dium. They added the unknown to a hot solution Paal and Freiderici 6 reduced nickel compound containing nickel and hyposphite and observed to the metal in an ammoniacal hypophosphite and the tormation of a powder or of nickel on tartrate solution. They were not able to obtain the wall of the test tube. They were able to com plete precipitation of the metal by this method. detect one part of palladium in a billion parts of In the present studies, a quantitative reduction solution. of nickel was obtained by increasing the con­ As a res ul t of our work, an alternative pro­ centration of the sodium hypophosphite to 50 cedure is suggested as being more direct. A glliter and that of ammonium chloride to about cleaned copper is immersed in an acidified 600 g/liter. The reaction occurred spontane~ u sl y solution to be tested; any palladium present will without a catalyst being present. The mckel tend to be reduced on the copper suriJl,ce. Nickel powder was separated by filtration, leaving the will then reduce on the wire upon introducing fil trate free from nickel as indicated by a test the latter into the typical electroless hypophos­ with . The of this separa­ IJhite solution (composition suggested in sec~ ion tion has not been investigated. II, 1, (a). This is not a specific test, as rhodmm will also cause copper to react in a similar manner. W ASHING'l'ON, July 18, 1947.

, Ber. deut. ehem. Oes. G5. [BI, 539 ([932). 6 Br r. deut. chem. Oes. G{ , 1766 to 1776 (1931).

Deposition of Nickel and Cobalt 395