An Introduction To Chrome Plating
Chrome plating is a kind of finishing treatment that utilizes
the electrolytic deposition of chromium. The thin, decorative
bright chrome is the most common form of chrome plating.
Chrome plating imparts a mirror-like finish to items such as
metal furniture frames and automotive trim. Thicker deposits,
called hard chrome, are used in industrial equipment to reduce
friction and wear and to restore the dimensions of equipment
that has experienced wear.
There are two types of chromium plating: industrial and
decorative. Industrial chromium plating is also called Hard
Chrome or Engineered Chrome.
There are two types of Industrial chromium plating solutions:
hexavalent chromium baths whose main ingredient is chromic
anhydride and 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.
For decorative purposes, the best combination would be chrome
and nickel which offers the most protection against corrosion.
It will have a mirror finish that will only be as good as the
finish you put on the surface before you put on the chrome.
Hard chrome is plated in thickness as required to take advantage
of the extremely low chrome coefficient of friction, or for wear
build-up for functional purposes.
Micro-finished chrome will provide a coefficient of friction
lower than any other metal when used against steel, iron, brass,
bronze, babbitt, or aluminium alloys. This is done when chrome
is used as a bearing surface.
Chrome is a perfect set-up for longwearing working surfaces
because it is much harder than casehardened steel. It is not
advisable to use chrome on chrome. Chrome will resist mostly all
organic and in organic compounds and acids, except hydrochloric
acid.
Chrome plate shall be uniform in thickness on all surfaces.
Plate shall be smooth, homogeneous and free from frosty areas,
pin holes, pits, nodules, and other defects.
Chrome plating is not a difficult process provided that the part
has been properly cleansed and the following requirements met:
Preparation of the chromic acid (CrO3) solution, Temperature
control of the bath (plating solution),Preparation of lead
anodes (peroxide), Agitation method of the bath (bubbles),
Plating current density control and duration (controller),
Ventilation (for safety) Black chrome can also be plated in the
same way and still have similar characteristics to the bright
chrome. For aesthetic or anti-reflective applications, it may be
preferable in some cases. I have not yet used it, but the
formula is as follows:
Having the chrome plating facility right there when needed is
very valuable to the home machinist. Of course there is a
learning curve and some basic investment in dollars but this
investment is mostly non-recurring and will give you the benefit
of this process for quite a long time.
In industrial chrome plating the process is electrolysis. In the
process chromium metal is deposited on metallic surfaces
submerged in a chromic acid plating bath. The part to be plated
is made cathodic by connection to the negative terminal of the
rectifier.
Inert lead anodes are made electrically positive. When voltage
is applied across the two, current flows through the solution
and metal is deposited. Proper control of various plating
parameters result in bright, hard, adherent deposits.
Coupled with other surface finishes such as anodizing aluminum
and the plating of other prime metals such as copper, nickel,
silver, or gold which we may yet cover, we can really dress up
and protect our creations.