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3 ELECTRODEPOSITION OF NICKEL GEORGE A. DI BARI Nickel electroplating is a commercially important and ver- and wear resistance or modify magnetic and other properties. satile surface-finishing process. Its commercial importance The properties of nickel electrodeposits produced under may be judged from the amount of nickel in the form of metal different conditions of operation are of particular interest in and salts consumed annually for electroplatin
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  3 ELECTRODEPOSITION OF NICKEL G EORGE  A. D I  B ARI Nickel electroplating is a commercially important and ver-satile surface-finishing process. Its commercial importancemaybejudgedfromtheamountofnickelintheformofmetaland salts consumed annually for electroplating, now roughly100,000metrictonsworldwide,aswellasitsversatilityfromits many current applications [1]. The applications of nickelelectroplating fall into three main categories: decorative,functional, and electroforming.In decorative applications, electroplated nickel is mostoften applied in combination with electrodeposited chromi-um.Thethinlayerofchromiumwas firstspecifiedtopreventthe nickel from tarnishing. It was srcinally deposited on topof a relatively thick, single layer of nickel that had beenpolished and buffed to a mirror-bright finish. Today decora-tivenickelcoatings are mirror bright  as deposited   and do notrequire polishing prior to chromium plating. Multilayerednickel coatings outperform single-layer ones of equal thick-ness and are widely specified to protect materials exposed toseverely corrosive conditions. The corrosion performance of decorative, electroplated nickel plus chromium coatings hasbeen further improved by the development of processes bywhichtheporosityofchromiumcanbevariedandcontrolledon a microscopic scale (microdiscontinuous chromium).Modern multilayered nickel coatings in combination withmicrodiscontinuous chromium are capable of protectingsteel, zinc, copper, aluminum, and many other materialsfromcorrosionforextended periods oftime.Thecomplexityof modern-day nickel plus chromium coatings is more thanoffset by the greatly improved corrosion resistance that hasbeenachievedwithoutsignificantlyincreasingcoatingthick-ness and costs.There are many functional applications where decorationisnottheissue.Instead,nickelandnickelalloyswithmatteordull finishes are deposited on surfaces to improve corrosionandwearresistanceormodifymagneticandotherproperties.The properties of nickel electrodeposits produced underdifferent conditions of operation are of particular interest inthis connection.Electroformingiselectroplatingappliedtothefabricationof products of various kinds. Nickel is deposited onto amandrel and then removed from it to create a part madeentirely of nickel. A variation of this is electrofabricationwhere the deposit is not separated from the substrate andwhere fabrication may involve electrodeposition throughmasks rather than the use of traditional mandrels.Themanycurrentapplicationsofnickelelectroplatingarethe result of developments and improvements that have beenmade almost since the day the process was discovered. Thisis evident in the following retrospective on the developmentof nickel electroplating solutions as well as in subsequentsections that deal with basics, decorative electroplating,functional applications and deposit properties, nickel elec-troforming, nickel anode materials, quality control, andpollution prevention. 3.1 RETROSPECTIVE ON NICKELELECTROPLATING SOLUTIONS Bottger developed the first practical formulation for nickelplating, an aqueous solution of nickel and ammonium sul-fates in 1843, but earlier references to nickel plating can befound. Bird apparently deposited nickel on a platinum elec-trode in 1837 from a solution of nickel chloride or sulfate,and Shore patented a nickel nitrate solution in 1840 [2, 3].The solution developed by Bottger remained in commercialuse for 70 years, however, and he is acknowledged to be thesrcinator of nickel plating [4].  Modern Electroplating, Fifth Edition  Edited by Mordechay Schlesinger and Milan PaunovicCopyright  2010 John Wiley & Sons, Inc. 79  Dr.IsaacAdams,Jr.,amedicaldoctoreducatedatHarvardUniversity and at the E´cole de M  edicine in Paris, was one of the first to commercialize nickel plating in the United States,and his patented process gave his company a virtual monop-oly in commercial nickel plating from 1869 to 1886. Hispatent covered the use of pure nickel ammonium sulfate.Although Adams’s solution was similar to Bottger’s, hisemphasis on operating the bath at neutral pH was undoubt-edly vital for controlling the quality of the nickel deposited,since excessive amounts of ammonia would tend to lowercathode efficiency and embrittle the deposit. Largely as aresult of the publicity generated by Adams, nickel platingbecame known worldwide, and by 1886, the annual con-sumptionofnickelforplatinghadgrowntoabout135metrictons [5, 6].Remington,anAmericanresidinginBoston,attemptedtomarket a nickel ammonium chloride electroplating solutionin 1868, but perhaps of greater significance, in view of subsequent developments, were his attempts to use smallpieces of electrolytic nickel as an anode material in aplatinum anode basket [7]. Weston introduced the use of boric acid and Bancroft was one of the first to realize thatchlorides were essential to ensure efficient dissolution of nickel anode materials [8, 9].Professor Oliver P. Watts at the University of Wisconsin,aware of most of these developments, formulated an elec-trolyte in 1916 that combined nickel sulfate, nickel chloride,and boric acid and optimized the composition of the nickelelectroplating solution [10]. The advantages of his hot,high-speed formula became recognized and eventually ledto the elimination of nickel ammonium sulfate and otherproprietary solutions. Today the Watts solution is widelyapplied, and its impact on the development of modern nickelelectroplating technology cannot be overstated.Decorative nickel plating solutions are variations of thesrcinal Watts formulation, the main difference being thepresence in solution of organic and certain metallic com-pounds to brighten and level the nickel deposit. Because of his use of organic additives like benzene and naphthalenedi- and trisulfonic acids, Schlotter’s decision to market abright nickel plating solution in 1934 is a milestone in thecommercial development of decorative nickel plating [11],The introduction of coumarin-containing, semibrightnickel plating solutions by DuRose in 1945 was a majorcontribution because it subsequently led to the developmentof double- and triple-layer nickel coatings with greatlyimproved corrosion resistance [12]. His introduction of semibright nickel processes came at a critical time in thehistory of decorative nickel electroplating, when confidenceintheabilityofsingle-layer,brightnickelcoatingstopreventcorrosion of automotive components was at a relativelylow point.The dominant position of Watts solutions has been chal-lenged from time to time, but the only ones that have beenadopted on a substantial scale are nickel sulfamate solu-tions[13].TheWattssolutionisthebasisformostdecorativenickel plating solutions, although considerable variation inthe chloride content of different proprietary decorative pro-cesses may be specified by the suppliers of those processes.Sulfamate solutions are rarely, if ever, used for decorativeplating. Watts and nickel sulfamate solutions are used forfunctional plating and for electroforming, but sulfamate ismore popular for the latter. 3.2 BASICS Nickel electroplating is similar to other electrodepositionprocesses that employ soluble metal anodes; that is, directcurrent is made to flow between two electrodes immersed ina conductive, aqueous solution of nickel salts. The flow of direct current causes one of the electrodes (the anode) todissolve and the other electrode (the cathode) to becomecovered with nickel. The nickel in solution is present in theform of divalent, positively charged ions (Ni 2 þ ). Whencurrentflows,thepositiveionsreactwithtwoelectrons(2e  )and are converted to metallic nickel (Ni 0 ) at the cathodesurface. The reverse occurs at the anode where metallicnickel is dissolved to form divalent, positively charged ionswhich enter the solution. The nickel ions discharged at thecathode are thus replenished by those formed at the anode. 3.2.1 Application of Faraday’s Laws to Nickel The amount of nickel deposited at the cathode and theamount dissolved at the anode are directly proportional tothe product of the current and time and may be calculatedfrom the expression m ¼ 1 : 095  aIt  ð 3 : 1 Þ where  m  is the amount of nickel deposited at the cathode (ordissolved at the anode) in grams,  I   is the current that flowsthrough the plating tank in amperes,  t  is the time that thecurrent flows in hours, and  a  is the current efficiency ratio(see Chapter 1 for the definition of current efficiency). Theproportionality constant (1.095) in grams per ampere hourequals  M   /  nF  ,where  M  istheatomicweightofnickel(58.69), n  is the number of electrons in the electrochemical reaction(2), and  F   is Faraday’s constant, equal to 26.799 A-h (morecommonly given as 96,500C).The proportionality constant must be multiplied by theactualelectrodeefficiencyratioifprecisevaluesarerequired.The anode efficiency for nickel dissolution is almost always100% under practical electroplating conditions; that is, a  ¼  1 when estimating anode weight loss. If the pH of thesolution is too high and/or the chloride ion concentration toolow, hydroxyl ions may be discharged in preference to the 80  ELECTRODEPOSITION OF NICKEL  dissolution of nickel, and oxygen will be evolved. Underthose unusual conditions the nickel anode becomes passive,and the efficiency of anode dissolution is close to zero.The cathode efficiency of different nickel plating solu-tions may vary from 90 to 97% and, accordingly,  a  will varyfrom 0.90 to 0.97. A small percentage of the current isconsumed in the discharge of hydrogen ions from water.That reduces the cathode efficiency for nickel depositionfrom 100% to approximately 96% in an additive-free nickelelectroplatingsolution.Thedischargedhydrogenatomsformbubbles of hydrogen gas at the cathode surface. Cathodeefficiencies as low as 90% are characteristic of some brightnickel plating solutions that are formulated to give highlyleveled, mirrorlike deposits rapidly, that is, at thicknessesbelow12 m m[14].Anaveragecathodeefficiencyof95.5%iscommonly used to make estimates when precise values arenot essential.Becausetheanodeandcathodeefficienciesarenotexactlyequal,thenickelionconcentrationandthepHofthesolutionwill slowly increase as plating proceeds. The rate ofincreasein nickel ion concentration depends on the difference be-tween cathode and anode efficiencies. Because cathodeefficiencies may vary from 90 to 97%, whereas anodeefficiency is almost always 100%, the rate of increase innickel ion concentration depends on the cathode efficiencyand the nature of the plating solution, not on the type of solublenickelanodematerialthatisused.Theuseofaspecialinsolubleanodeincombinationwithsolubleanodeshasbeendevelopedtohelpcontroltherateofincreaseofthenickelionconcentration [15]. 3.2.2 Average Coating Thickness An expression for calculating nickel thickness,  s  in micro-meters,canbederivedbydividingEq.(3.1)bytheproductof thedensityofnickel, d  (8.907gcm  3 ),andthesurfaceareatobe electroplated,  A , and multiplying by 100 to obtain thethickness in micrometers: s ¼ m  100 dA  ¼ 109 : 5  aIt 8 : 097  A  ¼ 12 : 294  aIt A  ð 3 : 2 Þ The ratio  I   /   A  is the current density and thus the aboveexpression shows that the coating thickness depends on the current density  and time, whereas the amount or mass of nickel deposited, Eq. (3.1), depends on the  current  and time.Equation (3.2) is the basis for the electrodeposition datacompiled in Table 3.1, which gives the time  in minutes required to deposit a nickel coating of specified thicknessat different values of current density. The expression aboveand Table 3.1 provide a means of estimating the  average coating thickness. 3.2.3 Current and Metal Distribution The  actual   thickness at any point on the surface of a shapedarticle is dependent on the current density at that point. Thecurrent density at anypoint is determined by how the currentis apportioned over the surface of the article being electro-plated. In nickel plating, the current distribution is largelydetermined by geometric factors, that is, by the shape of thepart, the relative placement of the part with respect tothe anode, how the parts are placed on plating racks, andthe dimensions of the system. Because almost all except thesimplest shapes to be electroplated have prominent surfacesthat are nearer to the anode than recessed areas, a uniformlythick nickel coating is difficult to produce. The currentdensity at prominences is greater due to the shorter anode-to-cathode distance and the lower resistance to current flowthat implies. Conversely, recessed areas, being further awayfrom the anode, will have a lower current density because of increased resistance to current flow. This inevitably means TABLE 3.1 Nickel Electrodeposition Data DepositThicknessWeight perUnit AreaAmpere Hoursper UnitTime (min) to Obtain Deposit at Various Current Densities (A dm  2 )( m m) (gdm  2 ) (Ah dm  2 ) 0.5 1 1.5 2 3 4 5 6 8 102 0.18 0.17 20 10 6.8 5.1 3.4 2.6 2.0 1.7 1.3 14 0.36 0.34 41 20 14 10 6.8 5.1 4.1 3.4 2.6 26 0.53 0.51 61 31 20 15 10 7.7 6.1 5.1 3.8 3.18 0.71 0.68 82 41 27 20 13 10 8.2 6.8 5.1 4.110 0.89 0.85 100 51 34 26 17 13 10 8.5 6.4 S.I12 1.1 1.0 120 61 41 31 20 15 12 10 7.7 6.114 1.2 1.2 140 71 48 36 24 18 14 12 8.9 7.116 1.4 1.4 160 82 54 41 27 20 16 14 10 8.218 1.6 1.5 180 92 61 46 31 23 18 15 11  9.2 20 1.8 1.7 200 100 68 51 34 26 20 17 13 1040 3.6 3.4 410 200 140 100 68 51 41 34 26 20  Note:  Based on 95.5% cathode efficiency.BASICS  81  thatprominentareaswillhavethicker coatings thanrecessedones.Because geometric factors exert the greatest influence oncurrentdensityatlocalizedareasinthe case ofnickelplating,currentdistributionisvirtuallythesameasmetaldistribution.Thus shields and auxiliary anodes can be used effectively toobtain acceptable thickness uniformity. Shields are made of nonconductive materials, and they may be placed on theanode, on the cathode, or between electrodes to block orcontrol current flow. Auxiliary anodes may be either solubleor insoluble, and are placed closer to the cathode thanprincipal anodes so as to direct the current to a recessed orrelativelysmallareaonthecathode.Theanalysisofgeometriceffects by computer modeling has received attention [16]. 3.2.4 Throwing Power In addition to the geometric factors, metal distribution isinfluenced by cathode polarization, the cathode efficiency–current density relationship, and the electrical conductivityof the solution [17]. The complex relationship between thefactors that influence current distribution and hence metaldistribution is called  throwing power.  A solution with a highthrowing power is capable of depositing almost equal thick-nesses on both recessed and prominent areas. For example,in copper cyanide solutions, high cathode polarization andthefavorablecathodeefficiency–currentdensityrelationship(cathode efficiency is lower at high than at low currentdensities) result in a solution with excellent throwing power.As already implied, cathode polarization and current effi-ciency do not significantly affect the throwing power of acidnickel plating solutions formulated with simple salts. Thecathodepolarizationislowandthecathode efficiencyishighand relatively constant above 1Adm  2 .Throwing power can be measured to give relative va-lues [18]. Measurements indicate that the throwing power of nickelelectroplatingsolutionscanbesomewhatimprovedbylowering the current density, increasing the electrical con-ductivity of the solution, increasing the distance betweenanode and cathode, and raising the pH and the temperature.Table 3.2 compares the throwing power of various platingsolutions; it is based on the work of Watson, who applied aHull cell to make the measurements. The nickel platingsolution with the best throwing power contained a highconcentration of anhydrous sodium sulfate; its compositionis given in Table 3.3, which also indicates that throwingpower decreases as the current density is increased [19]. 3.2.5 Internal Stress Internal stress refers to forces created within the deposit asa result of the electrocrystallization process and/or the co-deposition of impurities such as hydrogen, sulfur, and otherelements [20]. Internal stress is either tensile (contractile) orcompressive (expansive) in nature. In tensively stresseddeposits, the average distance between nickel atoms in thelattice is greater than the equilibrium value, creating a forcethattendstodrivetheatomsclosertogether.Whenatensivelystresseddepositisdetachedfromitssubstrate,itcontracts.Inaddition, if a thin cathode strip is electroplated on one sideonly (by painting the back and placing the bare side facingthe anode), a deposit stressed in tension will cause the stripto bend or curl toward the anode. In compressively stresseddeposits, the atoms are closer together and the force tends todrive them further apart. When detached from the substrate, TABLE 3.3 Composition and Throwing Power at VariousCurrent Densities of a High-Sulfate Solution Average Current DensityThrowing Power (%) at PrimaryCurrent Density RatiosAdm  2 Aft  2 5:1 12:1 25:10.2 2 63 76 851.0 10 38 50 644.3 40 23 31 40 Source:  Watson [19].  Note:  Nickel sulfate, 30gL  1 ; nickel chloride, 38gL  1 ; boric acid,25gL  1 ; sodium sulfate (anhydrous), 180gL  1 . The 100% represents auniform thickness over the cathode surface;  100 indicates the oppositeextreme where recessed areas are thinly plated and prominent areas arethickly plated. TABLE 3.2 Throwing Power of Various ElectroplatingSolutions Average CurrentDensityThrowing Power (%)at Primary CurrentDensity RatiosSolution Adm  2 Aft  2 5:1 12:1 25:1Watts nickel 4.3 40 8 7 14Sulfamate 4.3 40 11 13 19nickelAll-chloride 4.3 40 18 18 27nickelNa/high sulfate 4.3 40 23 31 40Mg/high sulfate 4.3 40 16 18 32Proprietary 4.3 40 1   12   6bright nickel AProprietary 4.3 40 3   12   6bright nickel BAcid copper 4.3 40 0   29   61Rochelle 4.3 40 86 91 93copperConventional 16 150   42   48   100chromium Source:  Watson [19].  Note:  The 100% represents a uniform thickness over the cathode surface;  100 indicates the opposite extreme, where recessed areas are thinly platedand prominent areas are thickly plated. 82  ELECTRODEPOSITION OF NICKEL
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