Plating ....... Zinc Plating, Gold Plating, Tin Plating, etc etc etc

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Sorry sayang, abg takde idea nak tulis……Okkeh,sekarang gak abg tulis……..

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Article courtesy of wikipedia.org

Plating

Plating is a surface covering in which a metal is deposited on a conductive surface. Plating has been done for hundreds of years, but it is also critical for modern technology. Plating is used to decorate objects, for corrosion inhibition, to improve solderability, to harden, to improve wearability, to reduce friction, to improve paint adhesion, to alter conductivity, for radiation shielding, and for other purposes. Jewelry typically uses plating to give a silver or gold finish. Thin-film deposition has plated objects as small as an atom, therefore some plating is nanotechnology.

There are several plating methods, and many variations. In one method, a solid surface is covered with a metal sheet, and then heat and pressure are applied to fuse them (a version of this is Sheffield plate). Other plating techniques include vapor deposition under vacuum and sputter deposition. Recently, plating often refers to using liquids. Metallizing refers to coating metal on non-metallic objects.

Electroplating

Main article: Electroplating

In electroplating, an ionic metal is supplied with electrons to form a non-ionic coating on a substrate. A common system involves a chemical solution with the ionic form of the metal, an anode (positively charged) which may consist of the metal being plated (a soluble anode) or an insoluble anode (usually carbon, platinum, titanium, lead, or steel), and finally, a cathode (negatively charged) where electrons are supplied to produce a film of non-ionic metal.

Electroless plating

Main article: Electroless nickel plating

Electroless plating, also known as chemical or auto-catalytic plating, is a non-galvanic type of plating method that involves several simultaneous reactions in an aqueous solution, which occur without the use of external electrical power. The reaction is accomplished when hydrogen is released by a reducing agent, normally sodium hypophosphite (Note: the hydrogen leaves as a hydride ion), and oxidized thus producing a negative charge on the surface of the part. The most common electroless plating method is electroless nickel plating.

Specific cases

Gold plating

Main article: Gold plating

Gold plating is a method of depositing a thin layer of gold on the surface of other metal, most often copper or silver.

Gold plating is often used in electronics, to provide a corrosion-resistant electrically conductive layer on copper, typically in electrical connectors and printed circuit boards. With direct gold-on-copper plating, the copper atoms have the tendency to diffuse through the gold layer, causing tarnishing of its surface and formation of an oxide/sulfide layer. A layer of a suitable barrier metal, usually nickel, has therefore to be deposited on the copper substrate, forming a copper-nickel-gold sandwich.

Metals may also be coated with gold for ornamental purposes, using a number of different processes usually referred to as gilding.

Silver plating

For less demanding applications in electronics, silver is often used as a cheaper replacement for gold. (Although silver is a better conductor than gold it does oxidize and so gold is better for contacts. However, variable capacitors are considered of the highest quality when they have silver plated plates. In this application there is no make and break contact so gold would not offer any advantage over silver).

Care should be used for parts exposed to high humidity environments. When the silver layer is porous or contains cracks, the underlying copper undergoes rapid galvanic corrosion, flaking off the plating and exposing the copper itself; a process known as red plague.

Historically, silver plate was used to provide a cheaper version of items that might otherwise be made of silver, including cutlery and candlesticks. The earliest kind was Old Sheffield Plate, but in the 19th century new methods of production (including electroplating) were introduced: see Sheffield Plate.

Another method that can be used to apply a thin layer of silver to several objects, such as glass, is the Tollen's Test method, which usually is prepared as follows. Using this method the final reaction can occur by placing Tollen's Reagent in a glass and then adding Glucose/Dextrose and shaking the bottle to perform the reaction.

AgNO₃ + KOH -> AgOH + KNO₃

AgOH + 2NH₃ -> [Ag(NH₃)₂]¹⁺ + [OH]^1- (Note: See Tollen's Reagent)

[Ag(NH₃)₂]¹⁺ + [OH]^1- + Aldehyde(Usually Glucose/Dextrose) -> Ag + 2NH₃ + H₂O

Rhodium plating

Rhodium plating is occasionally used on white gold, silver or copper and its alloys. A barrier layer of nickel is usually deposited on silver first, though in this case it is not to prevent migration of silver through rhodium, but to prevent contamination of the rhodium bath with silver and copper, which slightly dissolve in the sulfuric acid usually present in the bath composition.^[1]

Chrome plating

Main article: Chrome plating

Chrome plating is a finishing treatment utilizing the electrolytic deposition of chromium. The most common form of chrome plating is the thin, decorative bright chrome, which is typically a 10-µm layer over an underlying nickel plate. When plating on iron or steel, an underlying plating of copper allows the nickel to adhere. The pores (tiny holes) in the nickel and chromium layers also promote corrosion resistance. Bright chrome imparts a mirror-like finish to items such as metal furniture frames and automotive trim. Thicker deposits, up to 1000 µm, are called hard chrome and are used in industrial equipment to reduce friction and wear.

The traditional solution used for industrial hard chrome plating is made up of about 250 g/l of CrO₃ and about 2.5 g/l of SO₄^-. In solution, the chrome exists as chromic acid, known as hexavalent chromium. A high current is used, in part to stabilize a thin layer of chromium(+2) at the surface of the plated work. Acid chrome has poor throwing power, fine details or holes are further away and receive less current resulting in poor plating.

Zinc plating

Main article: Galvanization

Zinc coatings prevent oxidation of the protected metal by forming a barrier and by acting as a sacrificial anode if this barrier is damaged. Zinc oxide is a fine white dust that (in contrast to iron oxide) does not cause a breakdown of the substrate's surface integrity as it is formed. Indeed the zinc oxide, if undisturbed, can act as a barrier to further oxidation, in a way similar to the protection afforded to aluminum and stainless steels by their oxide layers. The majority of hardware parts are zinc plated, rather than cadmium plated.^[2]

Tin plating

See also: Tinplate

The tin-plating process is used extensively to protect both ferrous and nonferrous surfaces. Tin is a useful metal for the food processing industry since it is non-toxic, ductile and corrosion resistant. The excellent ductility of tin allows a tin coated base metal sheet to be formed into a variety of shapes without damage to the surface tin layer. It provides sacrificial protection for copper, nickel and other non-ferrous metals, but not for steel.

Tin is also widely used in the electronics industry because of its ability to protect the base metal from oxidation thus preserving its solderability. In electronic applications, lead may be added to prevent the growth of metallic "whiskers" in compression stressed deposits, which would otherwise cause electrical shorting.

Alloy plating

In some cases, it is desirable to co-deposit two or more metals resulting in an electroplated alloy deposit. Depending on the alloy system, an electroplated alloy may be solid solution strengthened or precipitation hardened by heat treatment to improve the plating's physical and chemical properties. Nickel-Cobalt is a common electroplated alloy.

Composite plating

Metal matrix composite plating can be manufactured when a substrate is plated in a bath containing a suspension of ceramic particles. Careful selection of the size and composition of the particles can fine-tune the deposit for wear resistance, high temperature performance, or mechanical strength. Tungsten carbide, silicon carbide, chromium carbide, and aluminum oxide (alumina) are commonly used in composite electroplating.

Cadmium plating

Cadmium plating is under scrutiny because of the environmental toxicity of the cadmium metal. However, cadmium plating is still widely used in some applications such as aerospace fasteners and it remains in military and aviation specs.^[3] Cadmium plating (or "cad plating") offers a long list of technical advantages such as excellent corrosion resistance even at relatively low thickness and in salt atmospheres, softness and malleability, freedom from sticky and/or bulky corrosion products, galvanic compatibility with aluminum, freedom from stick-slip thus allowing reliable torqueing of plated threads, can be dyed to many colors and clear, has good lubricity and solderability, and works well either as a final finish or as a paint base.^[2][4]

Nickel plating

The chemical reaction for nickel plating is:^{[citation needed]}

At cathode: Ni -> Ni²⁺ + 2e^-

At anode: H₂PO₂ + H₂O -> H₂PO₃ + 2H⁺

Compared to cadmium plating, nickel plating offers a shinier and harder finish, but lower corrosion resistance, lubricity, and malleability, resulting in a tendency to crack or flake if the piece is further processed.^[2]

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Article courtesy of finishing.com

Electroplating -- How It Works

An intro to plating for students, offering demos & explaining scientific principles.

What is electroplating?

Electroplating is the deposition of a metallic coating by putting a negative charge on an object and exposing it to a solution containing a metal salt. The positively charged metal ions in the salt solution are attracted to the object and reduced to metallic form upon it.

How does it work?

Look at the figure above: We have a metallic object we want to plate with a metal. First we fill a cell with a solution of a salt of the metal to be plated. Most of the time the salt (nickel chloride in our example) is simply dissolved in water and a little acid.

The NiCl₂ salt ionizes in water into Ni⁺⁺ ions and two parts of Cl^- ions.

A wire is attached to the object, and the other end of the wire is attached to the negative pole of a battery (with the blue wire in this picture) and the object is immersed in the cell. A rod made of nickel is connected to the positive pole of the battery with the red wire and immersed in the cell.

Because the object to be plated is negatively charged (by being connected to the negative pole of the battery), it attracts the positively charged Ni⁺⁺ ions. These Ni⁺⁺ ions reach the object, and electrons flow from the object to the Ni⁺⁺ ions. For each ion of Ni⁺⁺, 2 electrons are required to neutralize its positive charge and 'reduce' it to a metallic atom of Ni⁰. Thus the amount of metal that electroplates is directly proportional to the number of electrons that the battery provides.

Meanwhile back at the anode, electrons are being removed from the Nickel metal, oxidizing it to the Ni⁺⁺ state. Thus the nickel anode metal dissolves as Ni⁺⁺ into the solution, supplying replacement nickel for that which has been plated out, and we retain a solution of nickel chloride in the cell.
As long as the battery doesn't go dead, nickel continues to dissolve from the anode and plate out onto the cathode.

We used nickel chloride in the example chiefly for simplicity of explanation. First, because nickel always dissolves in the "+2" oxidation state (Ni⁺⁺), whereas many other metals like copper and zinc can dissolve in either the "+1" or "+2" state and add some confusion; secondly because chloride is a simple one-atom anion whereas most anions like sulphate or acetate are far more complex. But we do not recommend that nickel be used for school science demonstrations because -- while the explaining is simple -- the plating is difficult :-)

The first demonstration -- Zinc plating a penny

For the first demonstration, the cathode will be copper (pennies), the anode will be zinc, and the electrolyte (solution) will be zinc dissolved in vinegar and water.

With a hacksaw a teacher or group can cut many slices from one anode.

 A second option is to sand down a modern U.S. penny (1983 or later) until the copper surface is removed and the underlying zinc substrate is exposed.

A third possible source of zinc is the shell of conventional carbon-zinc batteries (make sure not to use alkaline batteries like Duracell or Eveready Energizers, nor rechargeable nickel-cadmium batteries -- just the cheap 1-1/2 volt AA, C, or D plain carbon-zinc batteries). The science teacher can cut up such batteries and remove the black glop, and give the student the 

cleaned zinc. 

For the pennies that you wish to plate onto, any pennies will do, but if you start with a dull brown penny, you'll end up with a dull zinc plated penny. Try to find shiny new pennies for best results! Immediately before plating, clean the penny with toothbrush and toothpaste, or a gentle scouring powder like Bon Ami or Multiscrub, and rinse well after cleaning. Your hands are oily, so wear plastic gloves so you do not get fingerprints or other soils on the penny after cleaning.

A transparent plating container is best, a Pyrex beaker is excellent, but a glass dessert bowl can serve well.

A recipe suggested by Tom Pullizzi, and found to work is:

Fill the container about half way with vinegar (vinegar is mild acetic acid). Put the zinc anode into vinegar and let it sit for several hours, allowing some of the zinc to dissolve.

Add 100 g/l of Epsom Salts (this salt helps make the solution conductive)
and 120 g/l of table sugar (this is called a "brightener")

Connect one flashlight battery (1-1/2 volts) to the penny and the zinc anode, and place them into the solution. 

Don't let them touch each other. With luck, within a few minutes you'll begin to get a bright silvery coating. Ted Mooney didn't have quite that much luck when he tried it, but did find that a reapplication of the toothbrush and toothpaste quickly polished the thin greyish coating he got to a fairly bright shine.

A second demonstration -- Copper plating a key or a quarter

Another slightly harder demonstration is plating a quarter or a brass key with copper. The key on the left was copper plated from a solution of vinegar with a pinch of table salt and a pinch of sugar, again using a 1-1/2 volt flashlight battery for power.

Understanding why this is a little harder to do is a good science lesson: You can't plate a metal out of a solution until you can get that metal dissolved into the solution (this is why we don't do a student demo of nickel, silver, gold, or chrome plating; you won't be able to dissolve these metals in vinegar, you would need a stronger and more dangerous acid).

Copper will not dissolve in vinegar without electricity to help it along, so it's best to get started with a small piece of scrap as your cathode and a large coil of copper wire as the anode. I stripped and crunched together about 2 foot of 14 gauge wire to use as the anode (wire is very pure copper) and used a 1/2 inch length of stripped copper wire as the scrap cathode. After I ran it this way for a couple of hours the solution acquired a faint blue tinge to it -- indicating that a little copper was dissolved in it. Then I cut off the scrap length of cathode wire, attached the key and plated it for several hours. Vinegar is too weak an acid to hold much copper in solution, so there is no rushing it, you have to plate slow and for a long time so copper can slowly dissolve into solution to replace what you plate out. I found that just a pinch of table salt (maybe two shakes) was enough. If you use more, what happens is you make a more conductive solution, so more electricity flows, but since there is not enough copper dissolved in the solution to support that current flow, you generate a lot of hydrogen gas and deposit a lot of black "smut" -- you can't plate copper out of solution faster than it goes into solution!

What happens if your solution is too conductive due to too much table salt? The electricity is flowing through the solution, so electrons are flowing into it from the cathode. But if there are no copper ions there to pull out of solution, the electricity will pull hydrogen ions out of the water per this equation:

2H₂0 --> H₂⁺⁺ + 2OH^-

This will cause bubbles of hydrogen gas to accumulate on your key or quarter, and the OH, the hydroxide, will neutralize your vinegar so you'll have no acid left. What is "smut", or as some students call it, "black glop that coats the coin"? When you have too much current flow, what also happens is the moment an ion of copper gets to the cathode, it is "reduced" instantaneously with no opportunity for proper crystal growth, forming a powder of tiny, non adherent individual specs of metal which appear black. So use 1-1/2 volts maximum, very little salt, and take your time.

Waste Disposal

In the "real world" waste disposal is a major headache for electroplating factories. But here you have only used toothpaste, vinegar, salt and sugar. The very small amount of zinc or copper you have dissolved into it is really not a significant issue. But let's learn environmental responsibility, rather than practicing talking ourselves out of it! With the wires disconnected, put a good size chunk of steel wool in your dish. Any copper, and some of the zinc, will "immersion deposit", plating out onto the steel wool, removing the waste from the solution. Copper metal is not a pollutant, but dissolved copper is. Good luck!

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Photos courtesy of netpoint.com.my 

Example of zinc plating part

  (blue zinc)

(black zinc)

(yellow zinc)

Example of Tin Plating 

 (bright tin)







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To Humaira:

Hi Sayang,

Pengetahuan tentang plating ni adalah antara pengetahuan yg penting, terutamanya untuk jurutera2 dalam industri perkilangan besi ("sheetmetal industry" khususnya).

From,
Gjoy

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