©Copyright 1999, 2001 Mario S Pennisi
WHAT IS ELECTROPLATING?
Electroplating was first reported almost 200 years ago.
It may be defined as the process wherein an electric current
is carried across an electrolyte and in which a substance
is deposited at one of the electrodes.
Electrolysis is possible because solvents, water in particular,
have the ability to ionise substances dissolved in them, that
is they split them into components that carry positive and
negative charges. These ions are electrically charged and
are attracted to oppositely charged electrodes where they
are neutralised by the charges on these electrodes. The products
of electrolysis appear on the electrodes. The cathodic product
is the deposition of metal and the anodic product most often
is the dissolution of metal.
Electroplating is a means available to the surface finisher
to apply a metal coating to a metallic or plastic component.
Electroplating is used to deposit a very wide range of pure
metals and alloys for use in decorative, functional and jewellery
Nickel/chromium composites (usually called "chrome" plating),
copper, brass (an alloy), bronze (an alloy) and zinc are used
for decorative applications.
For functional (engineering) applications zinc, tin, heavy
coatings of nickel, "hard" chromium, silver and gold are used.
Various alloys are deposited for bearing surfaces. Functional
uses include building up worn parts and changing the surface
characteristics of a metal such as electrical inductance,
or conductivity, or corrosion resistance.
For jewellery applications, gold, silver and rhodium are
plated. Gold is often plated on 18-carat and 9-carat solid
gold jewellery to obtain colour matches.
The basis of any good coating is preparation.
For electroplating, oils and greases are removed in weak
to strong alkali solutions (depending on the metal being cleaned
and the soils being removed). The surface oxides are removed
in acids (strong pickles are used to remove heavy oxides and
rust from steel). After rinsing the parts are presented for
HOW IS IT DONE?
Electroplating is carried out in an electrolytic cell.
The part to be plated is made the cathode and the metal to
be deposited is often made the anode. The electrolyte contains
salts of the metal being deposited as well as other compounds.
parts to be plated are placed on racks (for still plating)
or barrels (for barrel plating of small components such as
fasteners) and the racks or barrels immersed in the electrolyte.
A DC (direct current) electric current is passed between the
items (negative terminal), the electrolyte and the anode.
The composition of the electrolyte, the temperature and current
are all controlled within close limits to give consistent
results. In most cases, the electrolyte is filtered continuously
to remove solids, which might otherwise stick to the part
being plated and so produce rough deposits.
When the current is applied, metal simultaneously deposits
from the electrolyte onto the part and dissolves from the
anode to replenish the electrolyte. When an insoluble anode
is used such as in chromium plating or gold plating, the metal
ions in the electrolyte are replenished by adding chemicals.
The process for decorative nickel plating after the preparation
treatment is similar to the following.
After plating the items are rinsed, dried, inspected and
When zinc is plated, the surface of the zinc reacts with
the air and becomes stained. To overcome this tendency to
stain and to corrode quickly, the zinc coated parts are dipped
in a chromate solution for a few seconds. This chromate treatment
(called a chromate conversion coating) can be clear or blue,
or iridescent gold, or iridescent gold/green, or black in
colour. The corrosion resistance is least for the clear and
blue chromates and highest for the iridescent chromates.
INSTALLATIONS AND MAINTENANCE
During installations the electroplated coating should
be protected from damage from abrasion and materials of construction
such as mortar and brick cleaning chemicals.
Once installed, maintaining the initial appearance of the
electrodeposit is a simple matter. The soot and grime, which
builds up on surfaces from time to time, contains moisture
and salts which will adversely affect the electroplated coating
and must be removed. Metallic coatings should be washed down
regularly (at least once each 6 months in less severe applications
and more often in marine and industrial environments). The
coating should be washed down with soapy water - use a neutral
detergent - and rinsed off with clean water.
When electroplated parts are installed without damage to
the coating and the coating is maintained regularly, it should
be relatively permanent.
Although the film is not metallurgically bonded to the metal,
it should not crack or peel when applied correctly.