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Chrome plating,
often referred to simply as chrome, is a technique of electroplating
a thin layer of chromium onto a metal object. The chromed layer
can be decorative, provide corrosion resistance, or increase surface
hardness.
Chrome Plating Process -
Industrial Chrome -
Motorcycle Use -
The component will generally go through these
different stages.
- Degreasing to remove heavy soiling.
- Manual cleaning to remove all residual traces of dirt and surface
impurities.
- Various pretreatments depending on the substrate.
- Placed into the chrome plating vat and allowed to warm to solution
temperature.
-
Plating current applied and component is
left for the required time to attain thickness.
There are many variations to this process depending on the type
of substrate being plated upon. Different etching solutions
are used for different substrates. Hydrochloric, hydrofluoric,
and sulfuric acids can be used. Ferric chloride is also popular
for the etching of Nimonic alloys. Sometimes the component will
enter the chrome plating vat electrically live. Sometimes the
component will have a conforming anode either made from lead/tin
or platinized titanium. A typical hard chrome vat will plate
at about 25 micrometres (0.00098 in) per hour.
The chrome plating chemicals are very toxic. Disposal of chemicals
is regulated in most countries
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Industrial chromium plating, also known as hard
chrome or engineered chrome, is used to reduce friction, add wear
resistance, or increase corrosion resistance. It is very hard,
measuring between 66 to 70 HRC. Hard chrome tends to be thicker
than the decorative treatment, typically ranging from 0.075 to
0.25 millimetres (0.0030 to 0.0098 in), but thinner and thicker
layers are not uncommon. Surface defects and roughness are amplified,
because hard chrome does not have a leveling effect.[1] Hard chromium
plating is subject to different types of quality requirements
depending on the application, for instance, the plating on hydraulic
piston rods are tested for corrosion resistance with a salt spray
test.
There are two types of industrial
chrome plating solutions:
- Hexavalent chromium baths whose main ingredient is chromic anhydride.
- Trivalent chromium baths whose main ingredient is chromium sulfate
or chromium chloride. Trivalent chromium baths are not yet common,
due to restrictions concerning color, brittleness, and plating
thickness.[citation needed]
A typical bath composition of a hexavalent chromium bath is as
follows:
- Chromic acid (CrO3): 225–300 g/l
- Sulfuric acid: 2.25–3.0 g/l, although it is usually measured
in molarity or percent concentration
- Operating temperature: 45 to 60 °C
- Plating current: 1.55–3.10 kiloamperes per square meter
DC
- Anodes: lead with up to 7% tin or antimony
- Maximum superimposed AC ripple allowed is 18%, preferred ripple
is 5% to 10%
- Chromium may be stripped anodically in an aqueous solution
of sodium hydroxide or inhibited hydrochloric acid.
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Formerly most decorative items
affixed to motorcycles were referred to as "chrome",
by which phrase was actually meant steel that had undergone several
plating processes to endure the temperature changes and weather
that a motorcycle was subject to outdoors. The most expensive
and durable process involved plating the steel first with copper,
and then nickel, before the chromium plating was applied.
Prior to the application of chrome in the 1920s,
nickel was used. In the US for the short production run prior
to the entry into the Second World War, plating was banned to
save chromium and the decorative pieces were painted in a complementary
color. In the last years of the Korean War, the banning of chrome
was contemplated and several cheaper processes (such as plating
with zinc and then coating with shiny plastic) were considered.
The largest plated parts were Wheels and Frames/Swingarms.
These were done by specialty shops, which had a different sequence
of labor relations. Just after the Second World War, some American
cars initally came off the assembly line with wooden bumpers --
not due to economy or rarity of the metal, but due to labor action
at the plating shops. No doubt when the shops got on-line again
these would be replaced.
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