Page 674 - Corrosion Engineering Principles and Practice

628 C h a p t e r 1 4 P r o t e c t i v e C o a t i n g s 629

Disbonding
A B

Pit

C

FIGURE 14.11 Shows how a holiday (hole) in a coating cathodic to a base
may cause a pit in the base metal. (A) electrolyte; (B) cathodic layer; and
(C) base metal. The indicated disbonding may or may not take place.

metal objects are plated with a variety of metals, including aluminum,
brass, bronze, cadmium, copper, chromium, iron, lead, nickel, tin, and
zinc, as well as precious metals, such as gold, platinum, and silver.
The process is regulated by controlling a variety of parameters,
including the voltage and amperage, temperature, residence times,
and the purity of bath solutions. Plating baths are almost always
aqueous solutions; therefore, only those metals that can be reduced
from aqueous solutions of their salts can be electrodeposited. The
only major exception is aluminum that may only be plated from
nonaqueous organic electrolytes. The sequence of unit operations in
an electroplating operation typically involves various cleaning steps,
stripping of old plating or paint, electroplating steps, and rinsing
between and after each of these operations. Electroless plating uses
similar steps but involves the deposition of metal on a substrate
without the use of external electrical energy.
For small, intricate shapes such as bolts and screws, plating metal
may be applied by barrel plating. A rotating barrel containing the
parts to be plated is turned slowly in the electrolyte, while current is
discharged to the tumbling parts inside.
Brush plating is also possible, particularly for touch-up of small
surfaces. In this case, an absorbent pad holding the electrolyte is
wrapped around a metal anode and the brush is moved over the
cathodic surface.
Chrome plate, which was extensively used on automotive parts
until the introduction of plastics, is actually a three-ply coating.
A flash of copper is first laid down, followed by a nickel coating
which comprises most of the finished thickness. A thin, hard coating
of bright chromium is then applied as the topcoat. The total thickness
is normally 25 to 50 m m.