environmentally concious design - electroplating · 2018. 6. 13. · electroplating, the process of...

Electroplating

1997

Prepared by

Dan Hodgkins Matthew Bananno

Peter H o h

with Professor James C OrsquoShaughnessy Worcester Polytechnic Institute

INTRODUCTION This site is one module in a series of many describing processes in the industrial

world and their impacts on the environment Electroplating the process of coating one product with a thin layer of some metal through the use of electricity is the major concern of this pariicular module Not oniy will there be an explanation of electroplating but there will be corresponding pages for the various cleaning and rinsing steps imtolved as well as d o n s of posslble alternatives where appropriate A flow chart outlining the module cnn take you anywhere in the module at anytime Alternatively as you read about dectroplatinamp you can f o b w the links to areas of specific interest and work your way around the module that way For your convdence and understanding a glossary has been addedthat can bejumped to on any page Also a list ofthe daencesused is provided at the end and links to other www sites are placed throughout the module where they might be appropriate

5

SPECIAL THANKS TO JOE POLCHECK (sorry about the spellinamp Joe) and MIKE ELLIOT ampomTEXAS INSTRUMENTS

httpwwwticom - Texas Instruments

INTRODUCTION

1 1 1

I I

I I

1- I - 1

UEANING SOLUTIONS

s o L v J 3 m I AQUEOUS

ACID I -j ALKALINES I

ULTRASONIC- 1 1 VAKIRDEGREASER I

I ELECTROLYTIC 4 DIALYSIS

REFERENCES DeMis JK and Such TE Nickel and Chraniurn Plating Butterworths Boston 1986

Gabe D-R Principals ofMetal Surface Treatment and Protection P-on Press New Yo 1978

Thomas E ed Pollution Prwention Handbook CRC Press Inc London 1995

Kampner Joseph B Water and Waste Control for the Plating Shop Gardner Publications b C m c h d Ohio 1976

Murphy Ja~nes A ed Surface Preparation and Finishes for Metals McGfaw-Hill Book Caqp New Yoamp NY 1971

Kenneth A ed Electroplating Handbook 3rd Ed Van Nostrand Reinhold Corn NewYoriqNY 1971

United States Department of the Interior Bureau of Reclamation Paint Ml US bvemment Printing Oamp Washington 1953

Thomas F ed T h d Spray Research and application^ ASM a t i o d MateriatsPark Ohio 1991

h q ed Electropiationg Handbook 4th ed Van Nostrand Reinhold com YOrkNY 1984

ELECTROPLATING DESCRIPTION In many industries it is desirable to have a component have the strength or other material ampI cs of one metal while having the looks corrosion resistance or other surfice property of another Sewerai examples of p h h g use are listed below Etectroplating is a process by which one met81 is coated with a layer of another by meam ofan eiecttic current The part to be d i s submerged ina tank wntahinga solution of the other metal in the form of ions (PHOTO Copper Cyanide Bath - trackltif) An electric aarent is thenpdthroughthe submerged part through the bath

charges the part to be plated attracting the ions of the metal in solution These ions then attach themsehres to the part forming a thin layer This could be the final objective or this may be the base coat before another layer of the same or a Wment metal to be applied Many ather steps are required to plate a part howewer both befbre and after the actual p k b g bath All these steps taka together can produce qpite a pollution problem ifdone

fact reduce the amount of raw materials both benign and hazardous they require Bdore plating can begin the part to be plated must be properly prepared This

an consist of the use of abrasives to remove excess metal or scale acid and alkaline baths or a Variety of other cleaning processes designed to remove dirt oil and metal h m the part This is considered pq and 85 would be expected there is a form of

treatment usually in the form of some kind of rinse to remove excess plating liquid f i ~ m the part These rinses wentudy become wntat ed with the metal salts fiom the plating bath and must be r e c o v e r e d or disposed oeuro Themareseveralprocessesthat can be implemented that will reduce the amom of rinsing required as well as processes that will allow recovery of those lost metal salts (PHOTO 3 Stage Rinse Tank - tracld7tif)

Another problem asi with plating is the ampthat as more parts are platedin the same bath the wat ion of the salts in the bath are steadily reduced until the bath reaches the point where it becomes unusable This happens for several reasons As the parts =plated metal isbeing applied to the part and isremovedthatway(ofwurse this is what is desired) Also as the parts are removed fiom the tank to be canid to the rinsing tank a certain amom ofthe phting bath is carried out with the part This loss is ed to as bagout and there are a couple ofways to k t h i s rekredto as -out reduction Beca~w the dragout reduces the liquid level of the tank wirter is usulilly added to bring the levd up to an operational rxhi This loss of solution and - b e n t with plain water only works to Mer dilute the solution Eventually there are not enough ions in solution to efficiently plate any more parts In the past the baths ~ ~ ~ ~ ~ ~ Q c i u m p e d aswaste or diluted still further to meet city sewer stamp then bqxd Today there are s e v d methods for reconcentrating the bath making it d fix makeup in a new batch

into amamp plate or series ofplates d o n g t h e d ofthe tank This

but done responsiblyy there are many areas platers can reduce pollution ad in

EXAMPLES of ELECTROPLATING

4 PRINCIPALUSES + WearResistance

+ Friction Reduction Between Parts + Corrosion Resistance + OentaI

+ TYPICAL METALS AND THEIR USES Chro Corrosion Resistance Decoration Nickel Comsion Resistance Decoration Zinc Corrosion ResistanCe

Gold Silver Conductor Decorative cad WearResiSmq corrosion

+ EXAMPLES + Automotive Parts Rebuilding Worn Parts

Valve stems piston rings shock rods struts Diesel engine cylinders hydraulic shafts B~mirrors tnhubcaps

+ Tooling Applications Plastic molding dies Drawing tools steel gauges

+ Computers Circuit boards electrical connectors component leads

+ Jewehy Necklaces rings etc

+ ornaments Automobile hood ornaments Statues figurines

+ Tools Wrenches Socket Tools Scrapers

ENvlR0NAJLIMPACIS Electroplating is a prime candidate for pollution prevention reduction because of

the tlllwlulls ofwater required duringthe pIating process as well as the amount ofuse of toxic metals and chemicals in the plating baths

conservatnn use of rinsing and bath water greatly reduces the volume of water that must be sent to atreatment plant Also usiagless water meansthat the resulting

and possible recovery much easier Many plating companies use to decide when a bath or was spuxt by a set schedule of bath replacement orjust by deciding that the bath

didnt look good Computer controlled plating lines and better bath testing provides a more dcient plating system reducing rinse and bath waste and often improving h e duency (PHOTO Automated Zinc Chloride Plating - trackl5tif)

duents have a higher wnceatrdon of the contamumt inquestionmabgtreatment

- men the baths that give the best results are also the most toxic Many contain heJravalent chromium cad and cyanide By redwing the amount of waste fi-om a bath and r e c o n m g depleted baths for reuse much less of these toxic chemicals m h tregtment plants and the environment Then is also a signiscant mount of m c h in the area of developing new bath compositions that give equal or better results as these baths without the use of such toxic chemicals cad a materid good for providhg protection against atmospheric and marine corrosion of steel is being replaced with zinc- Pickel and tin-Zinc alloys HeKavaient which provides corrosion and a bright hiamp as well as being highly toxic is receiving competition fiom zinc and nickel Tri- deal chromium a less toxic form is also being researched as an atternatiVe for the hexavalentform hrtp~atsdrlatsdrcdcgov808O~oxProfilesphs88 l 0 W - Chromium Public Health

httphtsdrl atsdrcdcgov8080ToxPro061esphs8808html- Cad Public Health

httpatsdrlatsdrcdcg0~8080t0~html- Toxic Facts Index http~enotinelgovstudiescs564html- A process for recovering c h r o httpm~-~scomproductsmarchcalerthtm - Chrome Defense Fund

Statemerrt

statement

There am also dternative methods of achieving the same finish as electroplating without all the harsh chemicals Cladding hot dipping electroless prllting painting thermal sprays electrophoresis vacuum deposition vapour decomposition oxide reduction and cementation are examples The use of alternative and less toxic methods Wiu become more important in the future as environmental controls become more stringent

S 0 M E E M E N T A L SITES httpwwwepagov - Environmerrtal Protection Agency Home Page http-nbenorg - Northeast Business Environmd Network httpwwwnbeaorgturihtml - Toxic Use Reduction Institute Umass Lowell btpllwasteaetiaelgev~studed - Euvirosense case studies (lot of them) bpcctseasuciaeduccthauhtml- UCLA center For Clean Technology

A COUPLE OF PLATING PAGES httpwwwi-tradecomcatalogltpusa000101 - Plating Resources Inc httpwwwmetal-finiampingcom

8

CLEANING PROCESSES DESCRIPTION Before piating or many other types of metal working for that matter the part in question must be properly cleaned to remove it of impdes or build up that may affect the quality of the finished product There are quite B few methods of preparation each ditking in ease merials needed and waste prortuced and the effect produced on the part

toeachmethodthemechanicalactionimro~andthetypeofchemicalcleaneaused These two arenot iadependent ofeach other bur aremixed a n d d e d dependingon the desired mdt The typical md means may consist of one or more of the following electropolishing ultrssonics vapor degreasing abrasive d b m g spraying or immersion with agitation Wi the exception of abrasive cleaninamp each of these generally uses some kind of liquid cleansing agemi to assist the loosening moval and tianspOrting away of the SOL These cleausing agents are in the form of any one of a bef of solvents or an aqueous solution of an acid or alkaline substance An example of how thesemaybecombimedisthevapordegreaser Amoresophisticatedvapordepasermay use a solvent as its liquid medium and may also employ dtrasonicq immersion and spray cleaning to aid particulate removal

whenit comes to cleaning there are two aspects that must be looked at that apply

ACID BATHS DESCRIPTION An acid bath is an aqueow solution usu811y coampg dfki~ hydrochtoric or phosphoric acid It operates similar to an alkaline bath in that the parts m id in the bath The bath is heated either by heater Coiis or by theventing of sttampouthe bath adding agitation to the process By iocreaslng thetemperahaeor tank c o n d o n the process time can be redud The boiling point and the e~~nomics ofrunring at high temp are the d limiting factors onthe pmes time

pmesses or to remove a t h a t may have fbrmcdwhiie the part was idle This single acid process is d e d pickhg and can be used on ferrous and copperaUoys

right dippis -the same m u s i n g a e ofthe acids to get varying fcsult~ Bright dipping can be used on a variety of metals and alloys amoq than ferrous

titaniUm uranium and thorium

This process is used to get rid oftarnish aad d e g e n d by other

copper nickel cad zinc dver lead berytlium Zircanium

PHOTO Hydrochloric Acid with Cold Water Rinse (trdc13tif)

I

ALKALINE CLEANERS DESCRIPTION An alkaline cleaner is an aqueous solution containing an alkaline salr such as sodium compounds coxmining carbonates phosphates silicates or hydroxides A synthetic detergent is also d y added at about a twenty to one salt to detergent ratio The specific ratio is varied according to the type of so2 being removed andthe suTf8ce being cleaned In addition to the salts and detergents wetting and inhibiting agents are added to help ampe oils h m t h e metals and protect the metals fiom the akahity of the soiution Ivqeaidy

These cleaners clean the parts through three basic mews -do disphm and saponiiication The first emdsification is the process by which the ts invade the soil and break it down iato small globules that Win disperse in the Soiution Displacement occurs when the ts have a greater auracampion to the part surfbe displacing the soils and forcing them ofeuro These soils are then skimmed off as they ampat on the surfkx The last method saponification is a chemical process in which the deanhg agent reacts with some of the soils such as ampty oils and mnverts them both into awster soluble compound which may be rinsed oE

Alkaline sohxtions are usually applied hot ( 1 4 0 O to 200 F) and ofken as one step in a Series of operations The most popular way of applying the solution is in a bath The part is brought in on a conveyor or some other means and immersed in the bath often these is some other form of agitation to assist the solution This can be in the form of a simple metal shield in h n t of the heater coils angled to make use of the convection currents or may be a set of spray nozzles either stationaxy or moving The bath is then fbllowed by a rinse cycle

APPLICATIONS RESTRICTIONS Materials that can be removed include oc 0 such as mineral animal vegetable and lubricating oils inorganic smuts and metal chips mixed soils containing buffing compound residues shop dirt and other cleaning or pickling process residues These matefials can be removed from a variety of metals fbou copper zinc th d u and lead In general the fixmus metals can handle a

pH without inhibitors The other metals and their alloys however require a lower pH and some kind of inhibitor to protect the metal fbm the bath

BENEFITS + Can be bought preformulated + Canbe firmulated to suit specific clemingneeds + Reusable with a certain amount of maintenance

TRADEOFFS + Maycausefoamixlg + Continuous skimming of the surface of the solution is g e n d y required + DIyingisgenerallyrequired

PHOTO Cleaning Tank with Alkaline and Detergent (track1 1m

SOLVENTS DESCRIPTION A solvent is defined as ldquoa liquid that dissolves another substancerdquo In tams of industrial cleaning thae are five diamprent categories of these substances aiiphatic petraleums chlorinated hydrocarbons alcohols fluorinated hydrocarbons and a miscellaneous group (esters ketones and aromatics)

Aliphatic Petroleums Also known as mineral spirits Cheap but flammable Commonly repiaced by chlorinated hydrocarbons

chlorinated Hydrocarbons (PCE TCE TCA e) Vefy good sohemy Non- fbumablq but more scpensive than the aliphatics Until recently had been the most used in industry The best of these are typically more toxic or as rwent discovered are responsible for depletion of ozone in the atmosphere These ozone depleting substances (ODs) ate scheduled to stop being produced and heavy fines for present use forcing inampstry to find alternate solvents or cleaning methods

Cheap but flammable c0rdquoonty replaced by Freons

replacement for the alcohols in d d h g and component cleaning Non-flammable low toxicity and they can be h e amp for specific solvency Recently discovered to be ODsrsquos

Miscellaneous (Acetone Toluol Cellosolve) Used for small precise component cleaning Flammable and typically toxic Have also been often replaced by Freons

Alcohols Typically used fbr defhdng and the cleaning of complex mechanisms

pluorinated Hydrocarbons Also known as Freons Widely used 8s a

APPLICATIONS RESTRICTIONS + Used in vapor degreasing or cold cleaning + The low flash point of many of the solvents requites that a combmtion of

solvents to beused to raisethefhhpoint to a safk temperature

httpwastenotinelgovstudidcs3 14M and httpwastenot indgovstudiescS62 html Two studies documenting solvent use reduction and savings

httpesinelgovssdSrssdshtml - Sdlvent Substitution Data Systems

ELECTROPOLISHING DESCRIPTION ElectropoWg is essentially the reverse of plating in that through the use of an electric ament surface metal atoms are forced off the part leaving a smooth finish To do this the part is immersed in a conducting bath (acidic or W e 7 depending on the material) and hooked up to aDC current as the anode As current is applied atoms of the metal dissolve away leaving an eveq smooth finish

APPLICATIONS I RESTRICTIONS Electrop~lisbing produces a unique guaiity in the metal in that the surfirce is exceptionally smooth with no pits or scratches This makes thematerialeeeuroy ~ t o ~ m s i o r ~ e v e a w i t h o u t p ~ Italsomakesthe suTf8ce amenable to receiving very uniform plating or other coating processe~ Also usekl in some instances is the i n d dectivmess of light and heat the increased emissivity for electronic tubes and lessened suTf8ce wear when used in contact with another metal

In addition to being a preparatory step for plating anodizing or some other OOhg process electropolishing can be looked at as a mechanical process As a machining proces~ it doesnt stress or crack the me$al and works regt~dless of the materials hardness It canalso be used asadebumngprocess for d burrs since this process tends to work quicker on edges and protrusions For this to be efficient however huge chunks of metal must be removed first

steel brass aluminum silver nickel and its alloys copper zinc chromium and gold Some alloys do not receive electropo]ishinn well

Factors affecting the electropolishing process include current density temperature time and voltage Temperature is generally in the 110 to 250 F range Current density can be anywhere fiom 50 to 3000 ampsq ft depending on the applicatios though is generaily between 100 to 500 amp sq ft for general polishing Bath durations are in the range of 1 to 15 min againdepending onthe specrfic processbeingpedormed Voltage can be anywhere from 6 to 25 volts with the norm being between 12 to 17 volts

This process can be performed on a variety of metals including stainless steel

BENEFITS + Smoother sudhces which are better for d o r m coatings + Corrosionresistant + Canbe used to deburr or as a machining process

TRADE0FES + Can not be used on all metals or alloys + Does not perfbxm any ampld work on the metal as may be desirable and

achieved through normal machining + Wtll not cover smears scratches7 etc + Sometimes more costly than the equivalent mechanical process

- I lt-

+ There are a number of methods for reducing solvent loss making this process very material efficient

TRADEOFFS + The most popular solvents (TCA Freon etc) have been found to be ozone

depleting substances and are scheduled to be taken out of production fircing companies to find a substitute sohmt or cleaning method

VAPOR DEGREASERS DESCRIPTION A vapor degreaser is basically just a tank containhg a solvent that has built in heaters to bring the solvent to boiling This causes a cloud of solvent vapor to gteadily rise fiom the tank In the upper ampon of the tank condenser coils run around the inside perimeter the solvent vapor to condense and drip back down to the liquid in the tankbefore it can escape This constant cycle mrrjntAjns a cloud of hot sohem vapor in the tank at a given kvd parts to be cleaned are then suspended in this vapor Since the parts are Coolerthan the surrounding vapor the solveat condenses onto the pars loosening and washing away the uted soils and then drippins off to retuna tothe liquid phase The condensed vapor is replaced by fieamp soii hevapor sincethe soil is too heavy too be carried away by the gas and the process repeats conthuousIyuutil the part heats up Once the part mampes the ofthe vapor densation onto the part stops and the part is removed fiom the tank Sice the part is now hot and no

vapor is condensing onto it the part comes out of the tank dry and is ready to move on to the next step with no additional drying or rinsing

Because some parts are too 911811 and heat up to thevapor temperaatre before they are completely clean or so large that they cool down the bath there are several adaptations of the vapor d- A vapor-spray degmaser is esxntMy the same setup as before with the addition of spray nozzles to aid soil removal through impingement The solvent for the spray comes fiom a separate tank of the condensate off the coolers and is therefore cooler than the vapor This dows a longer cycle time for smail parts as the pray win keep the part cooler than the vapor dowing more condensation to occur

A liquid-vapor degreaser has two tank compartments open to the same chamber one contabhg boiling solvent and the other a waxm solvent bath The part is dipped into the bohg chamber where the action of the b o i i liquid aids in removal of some of the

solvent is rinsed off and the part is cooled The piece is now d i c i e d y cool for vapors to ae on it and a Iinai cycie of vapor condensation and remoyai h m the tank fbllows When a part is very large placing it in the boiling solvent bath can actd ly reduce the tcmpeTLlhlte of the bath eaough so that the vapor phase cannot be P A In instanceswherethismightbeaprobl~ali~d-liquid-vapordegreasereanbeused This m p has two b o i i ampa one fir inrmecringthe pars the other to in the ck In addition ultrasonic transducers can be incorporated into the immersion tanks to aid in agitation

heavier soils It is then dipped into the warm chamber where some of the contammi - e d

APPIJCATIONS RESTRICTIONS Vapor degreasers have found use in many areas ofidustry mcluding m a plastics and glass The main restridom when using a vapor d e g x are can the parts can handle the solvent and can the part handle the t=P=-=

BENETITS + Bn119e of the high t w parts are cleaned fastet and come out dry + The most popular soheats (d recentty) are nodhmmablq reducing worries

overworker~ety

ULTRASONICS DESCRIPTION The uitrasonic cleaning process uses sound waves to agitate a liquid

to enhance cleaning The agitation is brought about through the process of Transducers mounted on the sides or bottom of the liquid immersion tank

p d ~ c e high frequency sound waves As these waves travel through the fiqid medium pockets of high and low pressure appear In the low pressure zones bubbles form but as the wave0 travel these bubbles are impacted by a following high pressute zone which causes the bubbles to implode resulting in amitation It is this cavitation that provides the dbhgcncrgythztcleanstheproduct hthesesmdlpOcketSofcavitatio4imm~

and 20 O O O O F respectively) which 7

tcnnperatures and ptessures are built up aid in the break up of co- h 4=- a

The transducers comrert electrical energy inti the mcai inergy used to WW the sound wave and come in two s t a d d types piezoelectric and magnetostrictive The piezoelectric type is made of ceramics and is more eEcient for wer loads However the liquid temperclture rrmst be kept below 160 F (71 C) The magnetostriampve type made fiom metal can withstand bigher temperatures and is better

mora fixused work concatxahg on one area These considdons are important because the higher the tempemtwe the more cavitation and the better the cleaning

APPLICATIONS RESTRICTIONS Waxes oils soldering fluxes protehs etc can be removed fiom Varying materials such as ceramics metaIs plastics giass and even dectronics and PCBs The critical factor that decides ifa part can be cleaned by dtmonics is whether or not the part is immersible

BENEFTIS + Use of solvents can be greatly reduced or eliminated through use of an aqueous

+ SmallcrevicesandholesthatconventiOnaimethodsmaymisScanbereached + Inorganics as well as oils can be removed + processingspeeuroicanbevaried 4 Heahhhaprrdsaregreatlyreducsd

or alkaline solution

4 The payback period can be relatively short

TRADEOFFR 4 Partsmustbeimmersiile 4 Dryemmayberequireti 4 Eachdifferent part type or batch must be tested to find the o p t i Settings 4 The power required often limits batch size + Thick greases or oils tend to absorb the ultrasonic energy limiting its

+ Batch size may be limited due to the fact that the outer layers of parts may also diktiveness

absorb much of the energy reducing effectiveness on the inner layers cbv- Ilkamp hL( 2g [aL

IMMERSION DESCRIPTION Immersion or soak cleaning is the process by which the part to be ckmed is immersed into the cleaning solution and scrubbed through some meChaniCal action The solution can be in the form of a solvent aqueous acid or alkaline solUtion or may be just water in the case of a rinsing action Mechanical is 4 Y applied in one of two forms bmel cleaning or tray cleaning

Burel Cleaning In barrel cleaning many smaU parts are placed in awire h e barrel or basket that will allow the cleaning solution in but not let the parts fall out The whole baxrel is then immersed in a tank of the solution being used and rotated by means of a motor The constaut rotation and the subsequeat collision of the parts provides the cleaning actio4 8s well as circutating of the solution thst fixamp solution (or wam) is constantly coming in contact with the parts (PHOTO Barrel - track29tiQ

Trag Cleaning Intray cleaning the parts are set in opentrays tbat are dipped in the tank and moved up and down in the liquid Again this mechanically aids cleaning of the part and brings f k h solution into contact with the parts in the tray

SPRAY CLEANING DESCRIPTION Parts are either hung on a rack or placed in wire baskets suspended in a tank They are subjected to multiple streams of high pressure cleaning solhons from nozzles at several different angles in the tank The mechanical action of the high pressure spray aids the chemical action of the cleaning solution to remove the soils This process g m d y uses a lower temperature and lower concentrations of the cleaning solution than immersion cleaning Automation of this process c8n provide etkctive cleaning with a short cleaning time

~ P U C A T I O N S RESTRICIIONS The spray pattern must be cadblly designed h d e r to maintftin an evem deaniq The pat must be able to withstand the force ofthe Wpressure spray The part can be my size but must have alow complexity level in order to achiewe proper cleaning Spray cleaning is not extremely effective on baked on dirt Soil type is restricted by the dezmiq solution used

DRAGOUT REDUCTION DESCRIPTION Dragout is what platers call the amount of plating solution that removed fiom the bath with the part and is lost to the rinse tank There are several methods for recovering this solution

MTNIMIZATION The easiest way to save dragout is not to have any in the ht place Parts should be tested to find out the position on the plating rack that will cause the least amouat of solution to be removed with them Platers can also make apoint ofdiscussing part designwith eneen Many parts that contain c u ~ e s or hollows carry out extra solution in these locations A redesign for better drainage may be possible at no loss of part guality

IMMEDIATE RECOVERY As the parts is lifted fiom the plating bath and carried to the rinse u s d y be means of a rack and conveyor system excess plating solution will drip off the part leaving a steady trail to the rinse tank It is a le process to place a drain board here A drain board is merely some form ofsloping plank made of a material that wont be dissolved by or contaminate the Solution that has its lower end over the plating bath As parts drip on their way to the rinse these droplets are caught by the board and returned to the bath in a still concentrated form

immediately following the bath where parts can be allowed to drip and still be out of the way of the plating bath Again the recovered solution is still concentrated and the tank can periodidly be emptied directly into the plating bath Udbrhutely this method requires time and floor space neither of which is always available

Improving on the idea of the drip tank is the addition of spray nodes These nodes can be designed to produce a h e spray that will provide rinsing to the part with a ai consumption ofwater What is caught in the tank then iS a more concentrated solution than n o d y found in a rinse tank This solution am thm be used as make-up forthe plating bath or can be recovered at a low expense of magy

Another more ampcient rinsing method would be that of a desalter tank

Siar to this is the idea of a drip tank This would be an empty tank

This operates similar to avapor degreaser inthat a layer ofwater is kept boiling ~ 0 t I y at the bottom ofthe tank As the part is broughr into the tank fbr riming watex condenses on the part and drips back iat0 the tank Carrying the c~ccss plating solution with it This process will continue until the part reaches the b o i g temperaape of the water steam stops co- on it and the part is removed b m the tank Even ifpure water is started with the constant addition of dragout to the tank ~uttually brings the c o m e on of the tank up to or near that of the plating bath This solution can then be returned to the bath as makeup or held unfil later to be used aspart of anew batamp

evaporation either of these rinsing methods may ais0 be employed as a constant Depending on the rate of losses of the plating bath due to dragout and

refill to the plating bath with addition of fiesh water to the rinsing cyde This would keep a higher concentration in the plating bath and eliminate the need to schedule times when the rinse tank shouid be returned to the plating bath

httpwastenotinelgovstudieshml10309htmi - Chromium Dragout Reduction Case Study

Id

METAL RECOVERY DESCRIPTION As the plating bath becomes diluted or the rinsing bath becomes more contaminated with plating solution it becomes desirable to replace these fluids with fksh ones for proper operation Either way this leaves a weak solution of the plating ions that

the past was disposed oc but today can be recovered often into pure water and a r e concentrated solution There are Several methods for accomplishkg this evaporation ion-exchange reverse osmosis and dialysis

EVAPORATION Evaporation is a simple process in which the a large part of the water in the dilute solution is moved leaving behind a more concenttated solution This is accomplished by placing the weak solution in a glass h e d chamber or still The liquid is then heated to the boiling point of water and the steam produced siphoned 0 leaving the metafs and some of the water behind The c0n-d sowon can then be returned to the bath and the now distilled water to a rinse cycle or other process

BENEFTIS +Jhytouse + Both resulting products can be returned to the production line + During off times the still can p tap water ifneeded

TRADEOFFS + Requires a lot of heat

ION-EXCHANGE Ion-exchange is a considerably more complicated but has the added benefit that it can be ran as a continuous flow in line with the p h g and rinsing process During ionexchange the solution is passed through a bed of resin usually organic and the metal ions of the bath pass into the resin and stay there exchanging places with whatever ion was imbedded in the resin For Cationic exchangers the Ereed ion is hydrogen while for anionic7 it is a hydro ion The resultant liquid is ionized water which them may need to be neutrafized -re it can be re-amp or discharged (Passing a mixed solution of cations and araionS through two successive beds would produce near n d water ifbalanced correctly) Thebeds must be regenerated when they run out of exchangeable ions This is accomplished by passing either a strong acid (Cationic) or sodium hydroxide (anionic) through the resin The fhshing of the resin produces a strong solution of the metal in question which may be returnamp to the bath In the case of metals however this solution must go on to further procesSing for recovery or disposal (PHOTO Resin Tank for Sdver Recovery - track49tif)

BENEFITS + Can recover the lost metal ions + Can be used as coatinous flow

+ Less energy intensive than evaporation

TRADEOFFS + Resultant water must be treated + The beds must be regenerated periodically + Requires handling of strong acids andlor bases

httpwastenotinelgovstudieshml1032html and httpwastenotinelgovstudiescs278html- Ion-exchmge case studies

REVERSE OSMOSIS Osmosis is the natural flow of water tbrough a semi- permeable membrane h m a solution that has a higher conCentration of water to one that was lower in an attempt to balance the conwmtmtions Reverse osmosis reverses this process through the application of mechanical energy to the solution that already has a lowex concatration of water The principal is that as the mechanical energy is applied the solution will seek to balance the energy with the other side by the only means possile the forcing of water to the other side With enough pressure (around 1000 KPa with a maximum system pressure of 5500 ma) the concentration of the solution can be greatly inmeasxi leaving plain water on the other side This process has gained ground in recent years as newer more durable membranes have been developed

BEITS + Low energy requirements + Both products can be returned directly to the plating process

TRADEOFFS + The membrane wears out and has to be replaced

ELECRO-DIALYSIS Electrdysis also makes use of semi-permeable membmes The tank used has 811 anode at one end and a cathode at the other The tank itselfis divided by s e v d membranes and the whole tank is filled with the dilute solution The membranes dtemate as to whether they will let an anion or Cation pass througA As a charge is applied the anions tiy to move to one end of the taxamp and the cathodes the other Eventually the solution is divided into three sections one of pure water and two of a brine sahrtion Unfortunately this process requires several passes as it tends to only be 30 to 60 percent ampdent Also the electrodes and membranes tend to wear ouf though by reversing which plate is the anode or cathode their life can be extended

BENEFITS + Can separate mixed metals + Eventually produces pure water and d e solution

i

c

TRADEOFFS + Energy intensive + Requires several passes + Not eflicient ifthere are no mixed metals + Membranes and electrodes have to periodically be replaced

4

PLATING ALTERNATIVES

Thermal Spray Coatings DESCRIPTION Thermal spray coating is another method by which a part is coated by a layer of some metal to improve corrosion resistance or some other surfitce characteristic This costing is applied by injecting a powder of the desired coafhg metal into a high speed gas stream This stream forces the powder through a melting device such as an electric ar~ a laser or an oxy-fuel combustion chamber The molten powder is finally sprayed and deposited onto the work piece Once the powder has been applied some type of sealant such as avinyl based coating is often put over it This is because the original coating may be porous and the sealant will enhftnce the efbtiveness of the coating

These metal powders are created in a d e r of ways The most wmmon method k gas or water atomhtion Molten metal is p o d through a series of gas or water jets that disperse the metal into very tiny droplets which cool down quickly to form fine metal powders Another method of powder creation is casting and crusbhg This method is useful when using a very brittle alloy or ceramic Compounds are melted cast into a mold and then cxushed into powder Depending on the method used different properties can be ahlmced For example gas atomized powders are very h e and result in a denser coating when sprayed on the work piece reducing the porosity of the coating

APPLICATIONS Thermal spray waiings can be used as a comsion preventative and also hcrease the wear and thermal resistance of the work piece It can often be used as a substitute for electroplating tuthough it has not been errtensively used in the United states on steel coating much rampch has been done to make it a feaslble option

BENEFITS + There is no part size limitation + Complete control over thickness of coating + M a y possiily be applied under field conditions + Nochdcalbathsnecessaty + Much less waste as metal is being applied

TRADEOFFS + High cost of equipment and materials + High degree of skill is required

Cladding

DESCRIPTION A clad metal is composed of several dissimilar metals The cladding operation produces a composite metal combing two or three layers that have been bonded together The clad metal can be produced by co-rolling welding or casting The clad metal has mechanical properties which rival those of med in terms of ldquo b i i A co-rolling operation produces a siwcansly smaller amount of waste then a comparative plating operation rsquoQpiCal wastes for a co-rolling clad operation are avap~rated lubricant and dirt fiom the assocaated ionization precleaning opedon Although initially expensive ampe to equipment costs clad metals are more mechanically

and environmentally sound

Hot Dipping DESCRIPTION A process by which the coating metal is applied as a Iiquid forming a hyer of alloy on the part The liquid is applied by means of a tank and can be either a batch or continuous process This method works well for materials with low melting pints the most often used being tin lead zinc and alurdquo Generally this method is Oniy used when a thick layer is desired as it is hard to control layer thickness and thin layers are very difficult

Electroless Plating

DESCRIPTION This process is similar to electroplating in that the part is dipped in a bath that contains the metal to be coated The difference lies in the method that the metal is applied The bath contains a catalyst that causes a reaction between the part and the plating metal that applies that metal to the part No electricity is required There is still the drawback that the baths become depleted overtime and deposition rate is often slow Nickel is the most commonly plated metal by this method and there are others but still Llowfiere near the variety available through electroplating (PHOTO Electroless Nickel - track1 tif)

Electrophoresis

DESCRIPTION Metal particles are statically charged and held in a polar solvent such as alcohol Once the metal is deposited by means of a bath the part must be heat treated to Sinter the applied metal and achieve a good bond Coatings of this sort are g e n d y vciy porous however and are not appropriate for all applications

Painting

DESCRIPTION Paint is broken down into two basic components the pigment and the vehicle The pigment is d y made of some type of metallic compound such as iron or Zinc oxide Pigments impart the desired color onto the work piece and atS0 greatly influence the mechanical properties of the paint The particle size of the pigment is very important It can be a d to give the paint better coating properties The smaller the particle the denser the coafirtg The painting vehicle has two fe9nires It acts 85 the c m k ofthe pigment as well as the bmder As a canier the vehicle needs to regdate the shy so the paint can be sprayed It must also keep the pigment in solution The binder portion of the carrier holds the pigment particles together and adheres them to the work surfrice The bmding agent is largely responsiile for the protective ampties and durability of the paint- For ind- applications a type of resin would be used as the binding agent such as a vinyl or a phenolic resin would be used

Paint is used to enhsnce the appearance of a workpiece as well as for corrosion prevention The thickness of the coating is easily controlled by use of multiple coatings of thin layers Waste caused by overspray can be a problem Also it is d i f l i d to monitor the condition of the metal below the surface of the paint

Cementation DESCRIPTION In this process the coating metal is d i ihed into the part ampom a gas solidgas or liquid This produces a hardened casing on the part but may take several days at high temperatures Chromizing is the most commonly used metal but both zinc (Sherardizing) and alu (Calorizing) are also used

GLOSSARY AQUEOUS Water Based System

ATOMIZATION The formation of a metal powder by forcing a jet of the molten metal I

through a gas or liquid stream breaking the jet into small droplets which quickly cool to form the powder

i

CAVITATION The creation and destruction of smaU bubbles in a liquid medium by alternating waves of high and low pressure mnes produced by a transducer

DRAGOUT The loss of a solution from a bath carried out by the workpiece

EFFLUENTS Liquid by-products or waste streams of s process

ELECTROPLATING The electrodeposition of a metallic ion onto a workpiece through the application of 811 electric current

IMPINGEMENT The impacting of one material on another

PICKLING Removal of oxides ampom a workpiece surfha by means of an acid

REVERSE OSMOSIS Flow of water through a semi-permeable membrane leaving a high concentration of contaminants on one side and water on the other This action is against the normal osmotic flow and is accomplished through the application of mechanical pressure

SCALE An oxide coating thicker than a tamish on a workpiece

TRANSDUCERS Electronic components that convert electric current to mechanical a m g y through the use of a viirating membrane

INTRODUCTION This site is one module in a series of many describing processes in the industrial

world and their impacts on the environment Electroplating the process of coating one product with a thin layer of some metal through the use of electricity is the major concern of this pariicular module Not oniy will there be an explanation of electroplating but there will be corresponding pages for the various cleaning and rinsing steps imtolved as well as d o n s of posslble alternatives where appropriate A flow chart outlining the module cnn take you anywhere in the module at anytime Alternatively as you read about dectroplatinamp you can f o b w the links to areas of specific interest and work your way around the module that way For your convdence and understanding a glossary has been addedthat can bejumped to on any page Also a list ofthe daencesused is provided at the end and links to other www sites are placed throughout the module where they might be appropriate

5

SPECIAL THANKS TO JOE POLCHECK (sorry about the spellinamp Joe) and MIKE ELLIOT ampomTEXAS INSTRUMENTS

httpwwwticom - Texas Instruments

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environmentally concious design - electroplating · 2018. 6. 13. · electroplating, the process of...

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