Chem Seminar

8/8/2019 Chem Seminar

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Chrome plating , often referred tosimply as chrome , is a technique

of electroplating a thin layer of chromium onto a metal object.The chromed layer can bedecorative,provide corrosion resistance, ease cleaning procedures,

or increase surface hardnessOften, chrome plating is appliedto metal objects when there is anincreased opportunityfor corrosion to occur.


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H exavalent chromium plating , alsoknown as hex-chrome , Cr +6 ,

and chrome (VI) plating, uses chromicanhydride, also known as ChromiumTrioxide as the main ingredient.H exavalent chromium plating solution

is used for decorative and hardplating,


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A typical hexavalent chromium plating process is:(1) activation bath,

(2) chromium bath,

1. The activation bath is typically a tank of chromic acid witha reverse current run through it; this etches the workpiecesurface and removes any scale. In some cases theactivation step is done in the chromium bath.

2. The chromium bath is a mixture of chromic acid (CrO 3)and sulfate (SO 4); the ratio of which varies greatly between

75:1 to 250:1 by weight. This results in an extremely acidicbath (p H 0). The temperature and current density in thebath affect the brightness and final coverage. Finally, thewhole bath is agitated to keep temperature steady andincrease a uniform deposition.


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H exavalent chromium plating is low cathode efficiency, which results inbad throwing power. This means it leaves a non-uniform coating, with moreon edges and less in inside corners and holes.

Hexavalent Chromium is an hazardous air pollutant because it is ahuman carcinogen

Due to the low cathodic efficiency and high solution viscosity a mist of water and hexavalent chromium is released from the bath, which is toxic.

A dditional toxic waste that is created from hexavalent chromium bathsinclude lead chromates which form in the bath because lead anodes areused. Barium is also used to control the sulfate concentration, which leads tothe formation of barium sulfate, a hazardous waste.


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T rivalent chromium plating , also known as tri-chrome , Cr +3 , and chrome (III) plating, uses chromiumsulfate or chromium chloride as the main ingredient.Trivalent chromium plating is an alternative tohexavalent chromium in certain applications andthicknesses (e.g. decorative plating). [2]


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A trivalent chromium plating process is similar to the hexavalent chromiumplating process except for the bath chemistry and anode composition. Thereare three main types of trivalent chromium bath configurations

A chloride- or sulfate-based electrolyte bath using graphite or compositeanodes, plus additives to prevent the oxidation of trivalent chromium to theanodes.

A sulfate-based bath that uses lead anodes that are surrounded by boxes

filled with sulfuric acid (known as shielded anodes), which keeps the trivalentchromium from oxidizing at the anodes.

A sulfate-based bath that uses insoluble catalytic anodes, which maintainsan electrode potential that prevents oxidation.

The trivalent chromium plating process plates workpieces at a similar temperature, rate and hardness, as compared to hexavalent chromium.Plating thickness range from 0.005 to 0.050 mil (0.00013 to 0.0013 mm)


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H igher Cathode Efficiency nbetter throwing power

Less EnergyDensity req

bcoz of lowcurrentdensities req.

Process morerobust than hex-chrome coz it canwithstand currentinterruptions.

H igher cathodeefficiencies lead toless Cr emitted inair

A nodes do notdecompose


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Disadvantages

The corrosion resistance of thicker coatings is not quite as good as hexavalentchromium

The cost of thechemicals is greater,

Process must be controlled more closelythan in hexavalent chromium plating.Soprocess are very hard to conrtrol


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Uses of Chrome Plating

Chrome plating allows non-conductiveobjects, such as plastic, the ability to beformed into lightweight conductive

components, reducing the cost of production.

Decorative chrome. The inclusion of thenickel plating gives the chrome finishincreased luster, creating the shiny finishpeople typically attribute to chrome

H ard chrome plating is utilized onmechanical components to reducewear,corrosion and increase oilretention.

Chrome plating parts of thecar such as window trims,wheel trims, hub caps and the

bumpers.


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Tin plating or electro-tinning as the process is also called is a surface treatment process, which produces a metalliccoating of tin on the surface of metal components.

A cid solutions of tin (stannous) sulfate or flouborate can beelectroplated at very high current densities, but the deposit israther coarsely crystalline unless complex organic brightening

additions are used, e.g. cresol sulfonic acid or -naphtol andgelatin.

The tin plating process is carried out on both large objectssuspended from a rack (rack plating) and on small objects

which are processed in barrel installations (barrel plating).


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The coating is produced in an aqueous solution (electrolyte)consisting of tin salts, as tin ions that are reduced to ametallic tin coating on the surface of the components bymeans of an external power source.


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Adv antages

G ud resistance to corrosionin moderately corrosive

environment.Tin coatedsurfaces havegud solderability .

Tin coated surfaceshave gud decorativevalue.

The products formedafter tin plating arenon-toxic.


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DIS A DV AN T AG ES

Tin coatings deposited directlyon zinc rich copper-alloys suchas e.g. brass, have poor solderability coz the tin coatingdiffuses into the zinc rich areasof the brass. This also decreasesthe resistance of the coating tooxidation.

Tin coatings can under certaincircumstances spontaneously formlong, thin (1-3 m) threads of tin theso-called w hiskers w hich can createdifficulties w ith current-division or short-circuiting in electronics.

Fretting is a common problem with Tin Plating. This is surfacedamage created when there is relative motions and strongcontacts between parts, usually during shipping and handling.The tin plating shows black or worn spots caused by rubbed

or scratched by another part.


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I. Tin plating is mostly used for functional applications, but for someapplications the appearance of the surface is also important, and tincoatings have good resistance to oxidation in moderately corrosiveenvironments.

II. Tin coating is applied for surface treatment of equipment and machinesused in the food industry bcoz tin plated materials are corrosion proof non-toxic and ductile.

III. The main application of the tin coating is, however, used for surfaces on

which soldering is necessary as well as electric contacts and outlets.

IV. Tin provides sacrificial safety of copper mineral, nickel combined with other nonferrous metals.


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G old plating is used for decorative,electrical and thermal conductivity.G old plating methods include bathplating in a jar or tank (electrolessand electrical), barrel plating, brushplating, and rack plating.


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Electroless PlatingN o power supply is required for electroless plating. Chemical reactions provide all theelectrical potential needed. The part is immersed into a bath which contains the source

of gold ions plus usually a chemical reducing agent, such as borohydride. Electrolessgold baths may require heating to well above 50 degrees centigrade in order to initiatereaction.

Electroplating Using a Jar or a TankThe laboratory form of electroplating involves either a Pyrex jar or a small tank, usually acyanide or a sulfite gold-based bath, and one or more anode(s) of expanded niobiumclad with a platinum metal. A constant current is applied Parts are immersed into thebath and are plated for the requisite time, which controls the thickness of the resultantgold layer.

Barrel PlatingSome projects call for a large number of small parts that don't require much personalattention, such as minute nuts, bolts, and screws used in eyeglasses and other electricaldevices. The parts are placed inside a rotating cylindrical drum or barrel. The rotationprovides the needed agitation and guarantees an even plating-distribution thickness.


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Brush PlatingBrush plating comes in handy for certain special projects. Perhaps the gold on a largeplated object wears thin in one particular area, but is in good condition over most of its

area. Rather than electroplate the entire piece, a brush-electroplating kit can be used toplate only the needed area by "painting" the area with the electric brush.

Rack PlatingIt is more often useful for plating large objects not requiring precious metals, such asnickel plating. Objects on a rack may be lowered into a large, heavily agitated, openbath.This agitation results in an uniform plating


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G old does not oxidize when exposed to air, so itselectrical conductivity won't diminish over time. Thismakes it ideal for electroplating.

G old plating acts as an anticorrosive coating to thematerial to which it is applied.

When bonded with cobalt, the result is a wear-resistant material.

These advantages make gold plating ideal for electrical switch contacts and connector pins where

electrical contact occurs.


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G old is very costly.

In the direct gold-on-copper or silver plating, the copper atomstend to diffuse through the gold layer, causing tarnishing of itssurface and formation of an oxide and/or sulfide layer.

G old plating may lead to formation of gold whiskers.

Soldering gold-plated parts can be problematic as gold is

soluble in solder. Solder which contains more than 4-5% gold canbecome brittle. The joint surface is dull-looking.


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1. G old plating has many applications, including uses in jewelry, scientificexperiments,electronics, industry and dentistry.

2. G old plating was first used for aesthetic purposes, creating gold jewelry, goldleaflets and plating keepsakes.

3. Since gold does not oxidize in air and its conductivity stays consistent over time, it is especially useful in the electronics industry. It is used in electricalswitch contacts, connector pins and barrels, and other devices that needstrong conductivity.

4. Industries like energy, medicine, electronics, aerospace,communications,automobiles and defense have used gold plating for itsresistance to corrosion and conductivity.

5. G old plating is sometimes used over stainless steel braces for customerswith expensive tastes.


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S.C.AshwinS.C.Ashwin NaraayanNaraayan 10BEC1075 10BEC1075

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Chem Seminar