Page 41 - CFCM Nov-Dec-2020 Final_Neat

plating and anodizing: BRUSH PLATING
BRUSH PLATING:

A Variation of (Tank) Electroplating

BY PETER J PAINE

ELECTROPLATING is a general name
for well-established processes that cre-
ate a coating (i.e. plating) of a given
metal (e.g. zinc, nickel, copper,
chromium, etc.) on a solid substrate
(such as steel) through the reduction
of cations of that metal in an electrolyte
(i.e. the plating solution) by means of
a current source.
Electroplating processes typically
take place in plating tanks. Plating Figure 1: P-8 Poseidon showing areas with bare aluminum subject to corrosion.
tanks can range in size from aquar-
ium-sized tanks to very large tanks, in some cases 30 feet directed toward hexavalent chromium plating).
in length and as much as 20 feet (or more) deep. The parts Electroplating is widely used in industrial applications
which are to be plated and which form the basis of a com- to improve the surface qualities of objects such as resistance
pany’s representative work usually determine the dimen- to abrasion and corrosion, lubricity, reflectivity, electrical
sions of their plating tanks. The larger or longer the part, conductivity, or appearance. More importantly, electroplat-
the larger and longer (and/or deeper) the tank. ing (and especially chromium plating) may also be used to
Plating tanks are made of steel and in the case of build up thickness on undersized or worn-out parts thereby
chromium plating, will use a PVC liner. Other plating restoring these parts to their original specifications. It is
processes can use either PE or PP tanks. The tank not only this aspect of chromium plating which makes hard
holds the plating solution but also supports the busbars chromium plating an important part of the industrial world.
and (where necessary) hoods for ventilation requirements. Chromium plating allows a part to be used many times
For chromium plating in Canada and the USA, Cr6+ emis- due to its ease of removal (stripping) from the part and rel-
sions are controlled by either surface tension reduction or ative ease of replating. Chromium also brings several engi-
by use of a control device such as a scrubber. neering benefits to the part – such as hardness, lubricity
In electroplating, the part to be coated is the cathode and improved corrosion protection.
(negative electrode); the plating solution is a solution of
a salt of the metal to be plated; the anode (positive elec- bRuSH ELECTROPLATInG
trode) is usually either a block of that metal, or of some A closely related process to conventional “tank electroplat-
inert conductive material. The plating solution should con- ing” is Brush Electroplating, also called Selective Electro-
tain positive ions (cations) of the metal to be deposited. plating. It is typically used to repair relatively small areas
These cations are then reduced at the cathode to the metal on large objects, and especially to repair components in-
in the zero-valence state. The direct current is provided by place on vehicles, ships, aircraft and machinery to avoid
an external power supply. the cost and down-time of disassembling the equipment.
For example: the plating solution for conventional hexa- Brush electroplating was first developed in France in
valent chromium plating is a solution of chromic acid 1938 by Georges Icxi. In 1945, Icxi and Daloze created the
(H2CrO4) which dissociates into Cr6+. At the cathode, the first commercial brush plating process and were awarded
Cr6+ is reduced to metallic chromium by gaining six elec- a French patent for their process in 1948. Through the
trons. At the anode, oxygen anions (from the dissociation 1950s and 60s this process was picked up and used by
of CrO3) are oxidized and released from solution as oxygen many companies, which resulted in further refinement of
gas. The release of both hydrogen (at the cathode) and the equipment and chemistry.
oxygen (at the anode) and the generally not very efficient The mechanics of brush plating are fairly straightfor-
chromium plating process (about 12 to 15 percent effi- ward:
ciency) accounts for the release of plating mists from these • An AC power pack converts voltage into DC current.
solutions. (This, of course, has resulted in environmental • A ground cable (negative charge) is connected to the
and OSH legislation which has been and continues to be part making it the cathode.

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