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Silver plating
SILVER PLATING by Alan Blair Lucent Technologies, Murray HJ1I, N.J.
The majority of silver-plating solutions in use today are remarkably similar to those patented by the Elkington brothers in 1840. Even in these environmentally aware days, cyanide-based silver-plating solutions offer the most consistent deposit quality at the lowest cost. Silveranodes dissolve readily inelectrolytes containing freecyanide andtheconsumption of brighteners or grain refiners is generally low, making these processes very economical to operate in spite of waste treatment costs. High-speed silver-plating solutions that employ insoluble anodes are well established, and, even though these contain no free cyanide, potassium silver cyanide remains the source of the metal. Truly cyanide-free silver-plating solutions have beensoughtafter for manyyears. Several fonnulations are workable and are described below. CYANIDE SYSTEMS
A typical, traditional silver-plating solution suitable for rackworkwould be as follows: Silveras KAg(CN), 15-40 gIL (2.0-5.5 ozlgal) Potassium cyanide (free),12-120 gIL (1.6-16 ozlgal) Potassium carbonate (min), 15 gIL (2 ozlgal) Temperature, 20-30°C (70-85°F) Current density, O.~.O Aldm2 (5-40 Alft2 ) Barrel plating usually results in muchgreaterdrag-out losses and lowercurrent density during operation so lowermetal concentrations are desirable. A typical formula would be: Silveras KAg(CN), 5-20 gIL (0.7-2.5 ozlgal) Potassium cyanide (free), 25-75 gIL (3.3-10.0ozlgal) Potassium carbonate (min), 15 gIL (2 ozlgal) Temperature, 15-25°C(6O-S0°F) Current density, 0.1-0.7 Aldm2 (1-7.5 Alft2) The formulas above will produce dull, chalk-white deposits that are very soft (less than 100 Knoop). Additions of grain refiners or brighteners will modify deposits causing them to become lustrous to fully bright. Examples of theseadditives are certainorganic compounds, which usually contain sulfur in their molecule, and complexed forms of a group VA or VIA element such as selenium, bismuth, or antimony. Deposits become harderas brightness increases; the usual hardness range will be between 100 and 200 Knoop. Antimony and selenium will produce harder deposits than most organic compounds, although the latter generally have better electrical properties. Carbonate is an oxidation product of cyanide, so additions are not needed afterthe initial solution makeup. Thisoxidation occursslowlyevenwhenthesolution is not in use,and when the potassium carbonate concentration has reached 120 gIL (16 ozlgal)deposits can become dullor rough. Removal of carbonate can be accomplished by freezing-out or precipitation with calcium or bariumsalts. Silver is a relatively noble metal, and as such will fonn immersion deposits on the surfaces ofless noblemetalsthatare immersed in its solution. Thistendsto happen evenwhen the base metal enters the silver solution "hot" or "live," that is, with a voltage already
303
applied. The inevitable resultof this phenomenon is poor adhesion of subsequent deposits. To minimize this effect, it is essential to employa silver strike coatingprior to the main deposit. A typicalsilverstrike wouldbe as follows: Silver as KAg(CN), 3.5-5 gIL (0.5-0.7 ozlgal) Potassium cyanide(free),80-100 giL (10-13 ozlgal) Potassium carbonate (min), 15 gIL (2 ozlgal) Temperature, 15-25°C (6~0°F) Currentdensity,0.5-1.0 Aldm1 (5-10 Alft1) It is not necessary to rinse between such a strike and a cyanide-based silver-plating solution.Silver strike thickness is typically 0.05-0.25 urn (0.000002-0.000010 in.). Anode purity is of paramount importance since typical impurities, such as copper,iron, bismuth, lead, antimony, sulfur, selenium, tellurium, and platinum-group metals will cause solutioncontamination and maylead to anodefilming, which inhibitsproperdissolution of the silver. Silver anodesare produced by roIling, casting,or extrudingthe metal. Care should be takento ensure adequateannealing has taken placeafter fabrication. The object of annealing is to obtain correct grain size so that the anodes do not shed during dissolution. (Shedding meansthat small particles break away from the anode, and these can cause roughness in the silver deposit.) Insufficient concentration of freecyanideand insufficient anodearea willcauseanodesto shedor dissolve improperly. Cyanideconcentration shouldbe analyzed regularly andadditions of potassium cyanidemadeas needed. Optimum anodeto cathodearearatiois 2:I; a maximum anodecurrent densityof 1.25 Aldm1 (13.5 AJftl)is recommended.
HIGH-SPEED SELECTIVE PLATING Electronic components such as lead frames are usuallyplated withsilver usingselective methods. Silicon chips and aluminum wires can be bonded to the silver by employing ultrasonic or thermosonic bonding techniques. Silver thickness ranges 1.875 to 5.0 um (0.000075-0.000200 in.), with deposition times between 1 and 4 sec. The small areas to be plated demand the use of insoluble anodes. Platinized-titanium meshand platinumwireare examplesof anode materials in common use.Traditional cyanide silver electrolytes sufferrapid degradation underthese conditions, oxidation and polymerization of the cyanide at the inert anodes being the principal cause. Special solutions were developed to overcome this situation; thesecontainessentially no freecyanidebut still depend on potassium silver cyanideas the sourceof silver. A typical formula is as follows: Silver as KAg(CN), 40-75 gIL (5-10 ozlgal) Conductinglbuffering salts, 60-120 gIL (&-16 ozlgal) pH,8.0-9.5 Temperature, 6O-70°C (140-160°F) Currentdensity,30-380 Aldm1 000-3,500 Alft1) Agitation, Jet plating Anodes, Pt or PtlTi Conducting salts can be orthophosphates, which are self-buffering, or nitrates, which requireadditional bufferingfromborates or similareompounds, Buffering is importantin these solutions since there is a significant drop in pH at the inert anode during plating due to destruction of hydroxide ions. Insoluble silver cyanideforms on the anode surfaceas a result of cyanide depletion in this locally low pH. Plating current drops off rapidly due to polarization. The following equationssummarize the reactions involved. 4OH- -t 2H20 + O~ + 4eAg(CN}1 -t AgCN.,J. + CN-
304
From Metal Finishing
SURFACE FINISHING CD-ROM Now, bigger than ever and better value New release Version 5.0 holds almost 43,000 data entries, 25% more than previously offered. Older and newer material has been added including publications up to early 1997. This new CD operates with all Windows platforms from 3.0 up, requiring just 5m of free disk space. Some of the subWets cot'ffed: Cleaning & Degreasing, Electroplating, Anodizing, Electropolisbing, Conversion Coatings, Electroless Plating, Printed Circuit Boards, Electroforming, Waste Treatment, Testing & Analysis
$395.00 Send Orders to: METAL FINISHING, 660 White Plains Rd. Tarrytown, NY 10591-5153 For faster service, call (914) 333-2578 or FAX your order to (914) 333-2570 All orders must be prepaid. Please include $5.00 shipping and handling for delivery of each book via UPS in the U.S., $10.00 for each book shipped express to canada; and $20.00 for each book shipped express to aU other COuntries.
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Silver 1006 Silver Mirror-bright cyanide silver, tolerant of carbonate contamination. 1025 Silver Semi-bright, ductile silver for electronic & decorative applications. Techni-Silver CV(-)Less Non-Cyanide bright silver. Techni-Silver EHS-3 Ultra high-speed, non-immersion, cyanide silver. Electronic Grade Potassium Silver Cyanide
Silver-like Finishes Pallaspeed High-speed process that produces a crack free, low stress, ductile deposit. Pallaspeed VHS High-speed process for reel to reel applications. Platinum AP Neutral plating process that deposits a dark hard fimsh directly over nickel. Platinum TP Hard pore-free platinum for electronic applications. Ruthenium V Mlrrol-bright deposit for decorative and industrial applications.
~,~ ~t~ TECHNIC INC eo. Box 9650, Providence, RI 02940
(401) 781·6100 Fax: (401) 781-2890 EuropeICanadalAsia
www.technlc.com
Other additives include grain -refiners. for example. selenium. and anti-immersion agents. The latter inhibit chemical deposition onto unplated areas of the lead frames. They are usually basedon a mercaptan or similarcompound. which will attachitself to the active base metalsurface. NONCYANIDE SYSTEMS
Many compounds of silver have been investigated as potential metal sources for a noncyanide plating process. Several authors havesubdivided thesestudies intothreegroups by compound type. These groups are (I) simple salts. e.g.• nitrate. fluoborate, fluosilicate; (2) inorganic complexes. e.g.• iodide. thiocyanate. thiosulfate. pyrophosphate. trimetaphosphate; and (3) organic complexes. e.g.•succinimide, lactate. thiourea. Thesimple salts all appearto suffer from the same problem: light sensitivity of the materials. Although some smooth deposits have beenobtained from suchsystems. they are not viable undernormal production conditions. Of the inorganic complexes considered. threeare worth discussing further. Theseare the iodide. trimetaphosphate, and thiosulfate solutions. Iodide Solutions Several authors report some success withbathsthat are quite similar. A typical solution mightbe as follows:
Silveriodide. 20-45 gIL (2.H.0 oz/gal) Potassium iodide. 3Q0-600 gIL (40-80 ozJgal) HI or HCI. 5-15 gIL (0.7-2 oz/gal) Gelatin (optional). 1-4 gIL (0.15-0.55 ozJgal) Temperature. 2HO"C (80-140°F) Currentdensity. 0.1-15 AJdm2 (1.0-150 AJft2) Without exception theseauthors found iodinein deposits from their particular formula. Thisfact,andthe relatively highpriceof the iodide salts.hasprevented further useof this type of solution. Trlmetaphosphate Solution A process was developed for silver plating magnesium and its alloys; its use on other metals is not reported.
Silvertrimetaphosphate (monobasic). Ag2HP30 9 • 3-45 gIL (0.40-0.60ozlgal) Sodiumtrimetaphosphate (trimer). Na6P60 18• 100-160giL (13.5-21.5 ozJgal) Tetrasodium pyrophosphate. Na4P20 7• 50-175 gIL (6.7-23.5ozJgal) Tetrasodium EOTA. 35-45 gIL (4.7-6.0 ozlgal) Sodiumfluoride. 3-5 gIL (OAO-O.700zJgal) pH (adjustwith triethanolamine or sodium bicarbonate). 7.9-9.5 Temperature. 50-60°C(120-140°F) Current density. 0.5-2.5 AJdm2 (5-25 ~/ft2)
ThIosulfate Solutions Thiosulfate-based formulas have proven to be the most successful of any inorganic complex investigated. Early attempts to plate silver from such a solution resulted in rapid oxidation of the complex andprecipitation of insoluble silvercompounds. Additions of sodium metabisulfite were found to minimize this tendency. and all thiosulfate-based processes now
306
containthis ingredient. Solutioncomposition can be expressed: Silver as thiosulfate, 30 gIL (4.0 o1lgal) Sodiumthiosulfate. 300-500 gIL (40-70 o1lgal) Sodiummetabisulfite, 30-50 gIL (4.0-6.7 ozlgal) pH (adjustwith sodiumbisulfiteor hydroxide). 8-10 Temperature. 15-30"C (60-85°F) Currentdensity.0.4-1.0 Ndm 2 (4-10 Nf(2) Theseelectrolytes can be operated withstainlesssteelor silveranodes;however. the latter should be bagged. Problems of pooradhesion can be overcome by using a conventional silverstrike or one in which there is no free cyanide. In either case. rinsing before entry into the thiosulfate solutionis a good practice. A smallamountof cyanidedrag-inwill reactwith thiosulfate in the solutionto form thiocyanate: eN- + 5 203- 2 ..... CNS- + 503- 2 One reported advantage of thiosulfate over cyanidesystemsis that thickness distribution is better on complex-shaped objects. However. depositsseem to tarnish in air much quicker than cyanide-produced ones. Postplating passivation is recommended.
Succlnlmlde Solutions Severalelectrolytes based on this organic complexof silver have been patented. two of which are described below: Silver as potassium silver disuccinimide, 30 gIL (4.0 o1lgal) Succinimide, 11.5-55 gIL (1.5-7.4 oz/gal) Potassium sulfate,45 gIL (6.0 o1lgal) pH. 8.5 Temperature, 25°C (77°F) Currentdensity. I Ndm2 (10 Nft 2) Potassium nitrite or nitratecan be substituted for the sulfate and the additionof ammes, such as ethylenediamineor diethylenetriamine, and wettingagents produce bright,stress-free deposits. Silver as potassiumsilver dlsuccinimide, 24 gIL (3.3 o1lgal) Succinimide, 25 gIL (3.4 ozlgal) Potassium citrate, 50 gIL (6.7 o1lgal) pH. 7.5-9.0 Temperature, 20-70°C (70-160°F) Current density,0.54 Ndm 2 (5.5 Nft2) Potassium borate may be used in placeof potassium citrate. Tarnish resistance of depositsobtainedfrom theseprocessesis inferiorto that of deposits produced from cyanideelectrolytes.
Organic Solvent Solutions Nonaqueous solventsenableinvestigation of silverplatingfromsalts thatare insoluble in water.One such system.based on dimethylformamide (OMF). is illustrated below: Silver chloride, 10 gIL (1.3 ozlgal) Thiourea, 30 gIL (4.0 ozlgal) Aluminum chloride. 10 gIL (1.3 o1lgal) Solventdimethylformamide, balance
307
Room temperature Current density, <1.5 Aldm2 «15 Alft2) Milky white silver deposits were obtained from a small volume of this solution over an extended time period: however,some scale-up problemsare inevitablewith such a system.
SUMMARY Aftermorethan 150years,silver platingis still performedusinga cyanideelectrolytethat resembles the electrode in the original 1840 patent. Most of the work directed at replacing cyanide in silver plating has resulted in little more than technical interest. As yet, no production-proven, noncyanide alternative has been found, although systems based on thiosulfate and succinimide appear to offer some promise. Both of these systems are commerciallyavailable.
The most beautiful book ever published in thefleld of metalfinishing
The Coloring, Bronzing and Patination of Metals by R. Hughes and M. Rowe 372 pages 200 color illustrations $100.00 The 9" x12" volume is in six sections: cast bronze, cast yellow brass, wrought yellow brass, gilding metal, copper and copper plate, and silver and silver plate. Beginning with a historical introduction, the authors describe the basic techniques for surface preparation, coloring chemical application, and finishing. Send Orders to:
METAL FINISHING 660 White Plains Rd., Tarrytown, NY 10591-5153 For faster service, call (914) 333-2578 or FAX your order to (914) 333-2570 All book orders must be prepaid. Please include $5.00 shipping and handling for delivery of each book via UPS in the U.S., $10.00 for each book shipped express to Canada; and $20.00 for each book shipped express to all other countries.
The majority of silver-plating solutions in use today are remarkably similar to those patented by the Elkington brothers in 1840. Even in these environmentally aware days, cyanide-based silver-plating solutions offer the most consistent deposit quality at the lowest cost. Silveranodes dissolve readily inelectrolytes containing freecyanide andtheconsumption of brighteners or grain refiners is generally low, making these processes very economical to operate in spite of waste treatment costs. High-speed silver-plating solutions that employ insoluble anodes are well established, and, even though these contain no free cyanide, potassium silver cyanide remains the source of the metal. Truly cyanide-free silver-plating solutions have beensoughtafter for manyyears. Several fonnulations are workable and are described below. CYANIDE SYSTEMS
A typical, traditional silver-plating solution suitable for rackworkwould be as follows: Silveras KAg(CN), 15-40 gIL (2.0-5.5 ozlgal) Potassium cyanide (free),12-120 gIL (1.6-16 ozlgal) Potassium carbonate (min), 15 gIL (2 ozlgal) Temperature, 20-30°C (70-85°F) Current density, O.~.O Aldm2 (5-40 Alft2 ) Barrel plating usually results in muchgreaterdrag-out losses and lowercurrent density during operation so lowermetal concentrations are desirable. A typical formula would be: Silveras KAg(CN), 5-20 gIL (0.7-2.5 ozlgal) Potassium cyanide (free), 25-75 gIL (3.3-10.0ozlgal) Potassium carbonate (min), 15 gIL (2 ozlgal) Temperature, 15-25°C(6O-S0°F) Current density, 0.1-0.7 Aldm2 (1-7.5 Alft2) The formulas above will produce dull, chalk-white deposits that are very soft (less than 100 Knoop). Additions of grain refiners or brighteners will modify deposits causing them to become lustrous to fully bright. Examples of theseadditives are certainorganic compounds, which usually contain sulfur in their molecule, and complexed forms of a group VA or VIA element such as selenium, bismuth, or antimony. Deposits become harderas brightness increases; the usual hardness range will be between 100 and 200 Knoop. Antimony and selenium will produce harder deposits than most organic compounds, although the latter generally have better electrical properties. Carbonate is an oxidation product of cyanide, so additions are not needed afterthe initial solution makeup. Thisoxidation occursslowlyevenwhenthesolution is not in use,and when the potassium carbonate concentration has reached 120 gIL (16 ozlgal)deposits can become dullor rough. Removal of carbonate can be accomplished by freezing-out or precipitation with calcium or bariumsalts. Silver is a relatively noble metal, and as such will fonn immersion deposits on the surfaces ofless noblemetalsthatare immersed in its solution. Thistendsto happen evenwhen the base metal enters the silver solution "hot" or "live," that is, with a voltage already
303
applied. The inevitable resultof this phenomenon is poor adhesion of subsequent deposits. To minimize this effect, it is essential to employa silver strike coatingprior to the main deposit. A typicalsilverstrike wouldbe as follows: Silver as KAg(CN), 3.5-5 gIL (0.5-0.7 ozlgal) Potassium cyanide(free),80-100 giL (10-13 ozlgal) Potassium carbonate (min), 15 gIL (2 ozlgal) Temperature, 15-25°C (6~0°F) Currentdensity,0.5-1.0 Aldm1 (5-10 Alft1) It is not necessary to rinse between such a strike and a cyanide-based silver-plating solution.Silver strike thickness is typically 0.05-0.25 urn (0.000002-0.000010 in.). Anode purity is of paramount importance since typical impurities, such as copper,iron, bismuth, lead, antimony, sulfur, selenium, tellurium, and platinum-group metals will cause solutioncontamination and maylead to anodefilming, which inhibitsproperdissolution of the silver. Silver anodesare produced by roIling, casting,or extrudingthe metal. Care should be takento ensure adequateannealing has taken placeafter fabrication. The object of annealing is to obtain correct grain size so that the anodes do not shed during dissolution. (Shedding meansthat small particles break away from the anode, and these can cause roughness in the silver deposit.) Insufficient concentration of freecyanideand insufficient anodearea willcauseanodesto shedor dissolve improperly. Cyanideconcentration shouldbe analyzed regularly andadditions of potassium cyanidemadeas needed. Optimum anodeto cathodearearatiois 2:I; a maximum anodecurrent densityof 1.25 Aldm1 (13.5 AJftl)is recommended.
HIGH-SPEED SELECTIVE PLATING Electronic components such as lead frames are usuallyplated withsilver usingselective methods. Silicon chips and aluminum wires can be bonded to the silver by employing ultrasonic or thermosonic bonding techniques. Silver thickness ranges 1.875 to 5.0 um (0.000075-0.000200 in.), with deposition times between 1 and 4 sec. The small areas to be plated demand the use of insoluble anodes. Platinized-titanium meshand platinumwireare examplesof anode materials in common use.Traditional cyanide silver electrolytes sufferrapid degradation underthese conditions, oxidation and polymerization of the cyanide at the inert anodes being the principal cause. Special solutions were developed to overcome this situation; thesecontainessentially no freecyanidebut still depend on potassium silver cyanideas the sourceof silver. A typical formula is as follows: Silver as KAg(CN), 40-75 gIL (5-10 ozlgal) Conductinglbuffering salts, 60-120 gIL (&-16 ozlgal) pH,8.0-9.5 Temperature, 6O-70°C (140-160°F) Currentdensity,30-380 Aldm1 000-3,500 Alft1) Agitation, Jet plating Anodes, Pt or PtlTi Conducting salts can be orthophosphates, which are self-buffering, or nitrates, which requireadditional bufferingfromborates or similareompounds, Buffering is importantin these solutions since there is a significant drop in pH at the inert anode during plating due to destruction of hydroxide ions. Insoluble silver cyanideforms on the anode surfaceas a result of cyanide depletion in this locally low pH. Plating current drops off rapidly due to polarization. The following equationssummarize the reactions involved. 4OH- -t 2H20 + O~ + 4eAg(CN}1 -t AgCN.,J. + CN-
304
From Metal Finishing
SURFACE FINISHING CD-ROM Now, bigger than ever and better value New release Version 5.0 holds almost 43,000 data entries, 25% more than previously offered. Older and newer material has been added including publications up to early 1997. This new CD operates with all Windows platforms from 3.0 up, requiring just 5m of free disk space. Some of the subWets cot'ffed: Cleaning & Degreasing, Electroplating, Anodizing, Electropolisbing, Conversion Coatings, Electroless Plating, Printed Circuit Boards, Electroforming, Waste Treatment, Testing & Analysis
$395.00 Send Orders to: METAL FINISHING, 660 White Plains Rd. Tarrytown, NY 10591-5153 For faster service, call (914) 333-2578 or FAX your order to (914) 333-2570 All orders must be prepaid. Please include $5.00 shipping and handling for delivery of each book via UPS in the U.S., $10.00 for each book shipped express to canada; and $20.00 for each book shipped express to aU other COuntries.
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I eC11n.IC
Silver 1006 Silver Mirror-bright cyanide silver, tolerant of carbonate contamination. 1025 Silver Semi-bright, ductile silver for electronic & decorative applications. Techni-Silver CV(-)Less Non-Cyanide bright silver. Techni-Silver EHS-3 Ultra high-speed, non-immersion, cyanide silver. Electronic Grade Potassium Silver Cyanide
Silver-like Finishes Pallaspeed High-speed process that produces a crack free, low stress, ductile deposit. Pallaspeed VHS High-speed process for reel to reel applications. Platinum AP Neutral plating process that deposits a dark hard fimsh directly over nickel. Platinum TP Hard pore-free platinum for electronic applications. Ruthenium V Mlrrol-bright deposit for decorative and industrial applications.
~,~ ~t~ TECHNIC INC eo. Box 9650, Providence, RI 02940
(401) 781·6100 Fax: (401) 781-2890 EuropeICanadalAsia
www.technlc.com
Other additives include grain -refiners. for example. selenium. and anti-immersion agents. The latter inhibit chemical deposition onto unplated areas of the lead frames. They are usually basedon a mercaptan or similarcompound. which will attachitself to the active base metalsurface. NONCYANIDE SYSTEMS
Many compounds of silver have been investigated as potential metal sources for a noncyanide plating process. Several authors havesubdivided thesestudies intothreegroups by compound type. These groups are (I) simple salts. e.g.• nitrate. fluoborate, fluosilicate; (2) inorganic complexes. e.g.• iodide. thiocyanate. thiosulfate. pyrophosphate. trimetaphosphate; and (3) organic complexes. e.g.•succinimide, lactate. thiourea. Thesimple salts all appearto suffer from the same problem: light sensitivity of the materials. Although some smooth deposits have beenobtained from suchsystems. they are not viable undernormal production conditions. Of the inorganic complexes considered. threeare worth discussing further. Theseare the iodide. trimetaphosphate, and thiosulfate solutions. Iodide Solutions Several authors report some success withbathsthat are quite similar. A typical solution mightbe as follows:
Silveriodide. 20-45 gIL (2.H.0 oz/gal) Potassium iodide. 3Q0-600 gIL (40-80 ozJgal) HI or HCI. 5-15 gIL (0.7-2 oz/gal) Gelatin (optional). 1-4 gIL (0.15-0.55 ozJgal) Temperature. 2HO"C (80-140°F) Currentdensity. 0.1-15 AJdm2 (1.0-150 AJft2) Without exception theseauthors found iodinein deposits from their particular formula. Thisfact,andthe relatively highpriceof the iodide salts.hasprevented further useof this type of solution. Trlmetaphosphate Solution A process was developed for silver plating magnesium and its alloys; its use on other metals is not reported.
Silvertrimetaphosphate (monobasic). Ag2HP30 9 • 3-45 gIL (0.40-0.60ozlgal) Sodiumtrimetaphosphate (trimer). Na6P60 18• 100-160giL (13.5-21.5 ozJgal) Tetrasodium pyrophosphate. Na4P20 7• 50-175 gIL (6.7-23.5ozJgal) Tetrasodium EOTA. 35-45 gIL (4.7-6.0 ozlgal) Sodiumfluoride. 3-5 gIL (OAO-O.700zJgal) pH (adjustwith triethanolamine or sodium bicarbonate). 7.9-9.5 Temperature. 50-60°C(120-140°F) Current density. 0.5-2.5 AJdm2 (5-25 ~/ft2)
ThIosulfate Solutions Thiosulfate-based formulas have proven to be the most successful of any inorganic complex investigated. Early attempts to plate silver from such a solution resulted in rapid oxidation of the complex andprecipitation of insoluble silvercompounds. Additions of sodium metabisulfite were found to minimize this tendency. and all thiosulfate-based processes now
306
containthis ingredient. Solutioncomposition can be expressed: Silver as thiosulfate, 30 gIL (4.0 o1lgal) Sodiumthiosulfate. 300-500 gIL (40-70 o1lgal) Sodiummetabisulfite, 30-50 gIL (4.0-6.7 ozlgal) pH (adjustwith sodiumbisulfiteor hydroxide). 8-10 Temperature. 15-30"C (60-85°F) Currentdensity.0.4-1.0 Ndm 2 (4-10 Nf(2) Theseelectrolytes can be operated withstainlesssteelor silveranodes;however. the latter should be bagged. Problems of pooradhesion can be overcome by using a conventional silverstrike or one in which there is no free cyanide. In either case. rinsing before entry into the thiosulfate solutionis a good practice. A smallamountof cyanidedrag-inwill reactwith thiosulfate in the solutionto form thiocyanate: eN- + 5 203- 2 ..... CNS- + 503- 2 One reported advantage of thiosulfate over cyanidesystemsis that thickness distribution is better on complex-shaped objects. However. depositsseem to tarnish in air much quicker than cyanide-produced ones. Postplating passivation is recommended.
Succlnlmlde Solutions Severalelectrolytes based on this organic complexof silver have been patented. two of which are described below: Silver as potassium silver disuccinimide, 30 gIL (4.0 o1lgal) Succinimide, 11.5-55 gIL (1.5-7.4 oz/gal) Potassium sulfate,45 gIL (6.0 o1lgal) pH. 8.5 Temperature, 25°C (77°F) Currentdensity. I Ndm2 (10 Nft 2) Potassium nitrite or nitratecan be substituted for the sulfate and the additionof ammes, such as ethylenediamineor diethylenetriamine, and wettingagents produce bright,stress-free deposits. Silver as potassiumsilver dlsuccinimide, 24 gIL (3.3 o1lgal) Succinimide, 25 gIL (3.4 ozlgal) Potassium citrate, 50 gIL (6.7 o1lgal) pH. 7.5-9.0 Temperature, 20-70°C (70-160°F) Current density,0.54 Ndm 2 (5.5 Nft2) Potassium borate may be used in placeof potassium citrate. Tarnish resistance of depositsobtainedfrom theseprocessesis inferiorto that of deposits produced from cyanideelectrolytes.
Organic Solvent Solutions Nonaqueous solventsenableinvestigation of silverplatingfromsalts thatare insoluble in water.One such system.based on dimethylformamide (OMF). is illustrated below: Silver chloride, 10 gIL (1.3 ozlgal) Thiourea, 30 gIL (4.0 ozlgal) Aluminum chloride. 10 gIL (1.3 o1lgal) Solventdimethylformamide, balance
307
Room temperature Current density, <1.5 Aldm2 «15 Alft2) Milky white silver deposits were obtained from a small volume of this solution over an extended time period: however,some scale-up problemsare inevitablewith such a system.
SUMMARY Aftermorethan 150years,silver platingis still performedusinga cyanideelectrolytethat resembles the electrode in the original 1840 patent. Most of the work directed at replacing cyanide in silver plating has resulted in little more than technical interest. As yet, no production-proven, noncyanide alternative has been found, although systems based on thiosulfate and succinimide appear to offer some promise. Both of these systems are commerciallyavailable.
The most beautiful book ever published in thefleld of metalfinishing
The Coloring, Bronzing and Patination of Metals by R. Hughes and M. Rowe 372 pages 200 color illustrations $100.00 The 9" x12" volume is in six sections: cast bronze, cast yellow brass, wrought yellow brass, gilding metal, copper and copper plate, and silver and silver plate. Beginning with a historical introduction, the authors describe the basic techniques for surface preparation, coloring chemical application, and finishing. Send Orders to:
METAL FINISHING 660 White Plains Rd., Tarrytown, NY 10591-5153 For faster service, call (914) 333-2578 or FAX your order to (914) 333-2570 All book orders must be prepaid. Please include $5.00 shipping and handling for delivery of each book via UPS in the U.S., $10.00 for each book shipped express to Canada; and $20.00 for each book shipped express to all other countries.