Electroplating
is a method of coating a metal or plastic surface with a metal.
A common example is the brightwork on automobiles -- bumpers, door
handles, emblems, script, etc. Much of this begins as a piece of
zinc, steel or plastic. The electroplater applies an electroplate
of copper, then a nickel electroplate, then chromium three deposits
over one another. The result is a surface brighter and more corrosion
resistant than bare metal or plastic.
Electroplates
are applied by immersing the object to be coated in a tank containing
the proper chemicals dissolved in water. If nickel is being applied,
nickel metal is one of the components of that solution.
Now imagine
that the part to be plated is attached to a negative electrical
lead (like that on your car battery). Once it is attached to the
negative electrical lead it is called a cathode.
The other electrical
lead, the positive (+) is in the solution. When current is turned
on, the negatively charged part to be plated attracts positively
charged metal from the solution (opposites attract). This continues
as long as current is on, and the coating or deposit becomes thicker
and thicker. But most electroplates are not very thick. One one
thousandth of an inch (0.001 inch) is regarded as pretty thick.
Since metal
is being taken from the solution, it must be replenished. Often
this is done by hanging pieces of the metal nickel, if nickel is
being plated, for example, in the solution. The chunks of metal
are called ANODES, and the positive electrical lead is then attached
to them. They dissolve in the solution as metal is taken away by
plating. So at this point we have metal being removed from the anode
and deposited on the cathode, which are the parts to be plated.
Since a car
battery is not a good source of power for this application, platers
use electrical current supplied by their power companies. But they
must have DIRECT current (DC), while the power company supplies
ALTERNATING current (AC). To convert AC to DC platers use a RECTIFIER.
Its function is to convert AC to DC.
Nearly every
metal plated is plated in this same way.
Before the
parts ever enter the line. they are hung on a RACK. This is just
a way of carrying the parts through the various solutions.
Before anything
is plated, the parts to be coated must be CLEANED. Platers use CLEANERS
for this. They are alkaline materials that remove oils, dirt and
rust.
In a typical
plating line the part is first immersed in a cleaning tank, then
in an electrocleaning tank (uses power from a rectifier to aid in
cleaning), and then into the plating tank.
A typical plating
tank has three copper bars suspended over its top, one connected
to the negative lead from the rectifier, two connected to the positive
lead. The racks of parts to be plated hang from the bar that is
connected to the negative lead, the anodes (metal to be plated)
from the positive bars.
The solution
in the tank may have to be heated or cooled. For this, platers use
IMMERSION HEATERS or HEAT EXCHANGERS.
The solution
becomes contaminated with dirt and other particles, which would
cause rough plates. To prevent this, platers use filters.
In some cases
the plated part is chromated. Zinc plated parts, for example, will
become bluish or yellowish if they are chromated. You see these
appearances on nuts and bolts you buy in a hardware store. The chromate
coating is applied by dipping the zinc plated part in a tank containing
chromic acid and other chemicals. The acid reacts with the zinc
plating to form a zinc chromate. This is called a conversion coating,
because the chromic acid solution converts the surface to zinc chromate.
This coating further improves corrosion resistance. There are also
black and olive drab conversion coatings.
Larger parts
are usually plated on racks. But if you have a million nuts and
bolts to plate, you don't want to hang each of them individually
on a plating rack. For this reason a plating barrel is used. The
parts are dumped into a plastic barrel with holes drilled into the
plastic sides. Then the barrel load of parts is immersed into the
plating solution. Inside the barrel is a dangler, a piece of flexible
metal that reaches down into the load of nuts and bolts to carry
current to them. The current is conducted from part to part by their
electrical conductivity and the whole load begins to be plated.
The barrel is rotated while current is applied. The nuts and bolts
become plated with zinc or cadmium or whatever is desired. This
is barrel plating.
If you have
lots of racks or lots of barrels and you don't want to hand carry
them from tank to tank you will attach them to a conveyor that moves
the racks or barrels from tank to tank, immersing them in each solution
for a preset time. This is conveyorized plating, which may be done
from an automatic line or from a hoist line.
Anodizing
is an operation performed mainly on aluminum. The effect is to develop
an oxide coating on the aluminum. Again, this is a conversion coating:
the surface is converted from aluminum to aluminum oxide.
Unlike plating,
the part to be coated is connected to the anode --- not the cathode.
The part is immersed in a dilute solution of sulfuric acid, current
is passed, and the part is anodized an oxide coating forms. The
oxide coating is harder and more corrosion resistant than bare aluminum.
Some aluminum
pots and pans are anodized, and door and window frames are often
anodized. There are ways to make the anodizing pick up the color
of brass or bronze or black, as well and almost any color of the
rainbow.
Electroless
plating
is just what the name implies. It is a way to coat one metal with
another without passing current. Certain chemicals cause this to
happen. The most common example you will see advertised is electroless
nickel. The reason it is applied is that there are some limitations
in electroplating when complex shapes are being coated. Often you
cannot electroplate deep recesses and holes or interior passages.
Electroless plating plates anything that is wetted by the solution.
Electroless
plating is more expensive than electroplating in most cases, and
is slower to build up a given thickness of deposit, which explains
why it is used only for certain applications.
Plating
on plastics.
Plastics cannot be plated in the same way as metals because plastics
are not electrically conductive. Thus one cannot immerse a plastic
part connected to the negative lead and expect it to plate. Instead,
electroless plating is applied first to get a conductive surface,
then the electroless plated parts are electroplated. Many automotive
parts, including grilles and all manner of decorative trim have
been plated plastic.
Printed
circuits.
The simplest printed circuit begins with a plastic board that has
copper foil glued onto its surface. The circuit paths are made by
printing onto the copper foil coatings that resist the etchants
used. These coating or resists are applied in the shape of the circuits
that must remain. Then the exposed copper is etched away and the
circuit paths remain. The resist is removed and you have a rudimentary
printed circuit. Some circuits or portions of them are plated with
tin or tin lead to provide solderability or with gold to provide
contact reliability. So this is another market for plating chemicals
and equipment.
Many small
electronic parts are also plated with tin or tin lead or gold or
other metals, to provide various electrical properties.
Polish/Buffing.
Generally done as a preplate operation, to smooth the surface before
plating.
Vibratory
finishing. Put
parts into a vibrator filled with tiny pebbles or other media and
a cleaning solution, shake the vibrator and you will smooth the
surfaces of the parts.
Vacuum
Coating.
Put a part in a vacuum chamber, exhaust the air, heat aluminum to
vaporize it inside the chamber and the aluminum vapor will travel
through the vacuum and condense on the parts' surfaces. A cheap
way to get a bright surface. Picture frames are made this way, as
are many other bright surfaces, including mirrors.
Electropolishing.
Use
the right chemicals in a tank and you can immerse certain metals
and pass current to smooth the surface. The part is the anode and
tiny high points are deplated, making the surface smoother.
Water
Pollution Control.
After each plating step, the chemicals remaining on the surface
of the plated part must be rinsed off. There are also spent plating
solutions that must be disposed of. The rinse water and the spent
solutions contain metals, cyanides and acids, all of which are damaging
to fish and other things reached eventually by sewer water. So the
EPA requires that platers not dump these chemicals into sewers or
streams
The plater
has two choices: reclaim the chemicals from the rinse water, or
treat the rinse water with other chemicals that remove the toxic
materials and allow disposal of the remaining water.
The cheapest
and easiest way has been to add chemicals that neutralize the offending
chemicals. The problem is that a sludge is left after the water
has been treated. Sludge is becoming difficult to dispose of in
landfills, because the EPA has rules about the sludges, too. Thus
reclamation is becoming a more attractive option.
Recovery/Recycling/Closing
the Loop.
A simple way to reclaim is to evaporate the water from the contaminated
rinse water until the plating chemicals remaining are concentrated
enough to return to the plating tank. There are a number of other
ways to reclaim, but all of them find some way to reconcentrate
the chemicals or to purify the chemicals so that they can be reused.
