Decorative Chromium PlatingDECORATIVE CHROMIUM PLATING is different from hard chromium plating in terms of thickness and the type of
undercoating used. The average thickness of decorative plating is actually very thin, usually not more than 1.25 μm (50
μin.). A decorative chromium deposit is used primarily for its pleasing blue-white color. Its highly reflective appearance
is maintained in service because chromium can resist tarnish, chemicals, scratches, and wear. If the deposit is defect-free,
then a level of corrosion resistance also is provided, because the deposit acts as a physical barrier to the environment.
Decorative chromium is applied over undercoatings, such as nickel or copper and nickel, which give the chromium bright,
semibright, or satin cosmetic appearances. Corrosion protection depends on the choice of undercoating, as well as the
type of chromium being applied. Parts made from steel, copper and its alloys, zinc, stainless steel, and aluminum are
typically plated with nickel-chromium or copper-nickel-chromium.
Most decorative chromium coatings have been applied using hexavalent chromium processes that are based on chromic
anhydride. However, since 1975, trivalent chromium processes have become available commercially. They are increasing
in importance because of their increased throwing and covering powers and because they offer environmental advantages.
Both systems are considered in detail in this article.
Chromium Electrodeposits
Decorative chromium plating baths generally produce deposits that range from 0.13 to 1.25 μm (5 to 50 μin.) in thickness.
These deposits generally reproduce the finish of the substrate, or, in a multilayer system, the undercoating that is applied
prior to the chromium layer. Optimum luster of the final chromium deposit is obtained by plating the substrate coating to
a uniformly bright condition. If the substrate is nonuniform, grainy, hazy, or dull, then it should be polished and buffed to
a uniformly high luster before being plated with chromium. When a final chromium coating over a uniformly bright
substrate is hazy in certain areas, these areas can be buffed on a wheel or the coating can be stripped and the substrate
replated. Buffing of chromium is not allowed when corrosive service conditions will be encountered.
In addition to being lustrous, the final chromium deposit should cover all significant areas. When there is not adequate
coverage, because of an improperly operated chromium bath, the chromium should be stripped, the substrate reactivated,
if necessary, and the part replated.
Decorative chromium that has been applied over nickel, the typical undercoating, is readily stripped by immersion in a
1:1 solution of hydrochloric acid. An alternate method involves treating the part anodically in an alkaline cleaning
solution. However, this method requires reactivation of the nickel surface prior to replating, which is typically
accomplished by immersion in dilute sulfuric or hydrochloric acid. Cathodic, but never anodic, alkaline cleaning can also
be used for activation.
Excessively high current densities, improper temperatures, and passivated substrates can produce hazy, nonuniform
chromium deposits. Operating conditions for chromium plating should be in the specified ranges. Properly operated
nickel baths and other similar precautions also are necessary to ensure uniformly lustrous chromium deposits.
The adhesion of chromium to an active or properly prepared substrate is usually not a problem. However, if chromium
is plated on an undercoating that has been improperly applied and has questionable adhesion, then blistering or exfoliation
can occur, either immediately after chromium plating or during storage or service. Organizations that generate standards, such as ASTM, can provide procedures for checking adhesion if a related method has not been specified in the purchase
agreement for the part being plated.
Microporosity and Microcracking. The key to the corrosion durability offered by decorative chromium deposits lies
in controlling the type, size, and distribution of microdiscontinuities that form in the deposit. These can occur as either
pores or cracks. In an outdoor corrosive environment, as well as in accelerated corrosion tests, corrosion has been
observed to proceed by galvanic cell action between the nickel and the chromium, with the nickel acting anodically.
Microcracks or micropores in the chromium expose the underlying nickel through a uniform, diffuse network of
discontinuities. Because the rate of corrosion penetration through the nickel layer is a function of the anodic current
density of the corrosion cell, the reduction of current density that is obtained by the increase in exposed nickel area
prolongs the time required to penetrate a given thickness of nickel. The advantage of such a system lies in its ability to
provide long-term corrosion protection without developing easily visible fine surface pits in the nickel, which eventually
become corrosion sites. The use of microdiscontinuous chromium makes the surface pits much smaller, which means that
the substrate will be protected from corrosion for a longer time. However, after excessive corrosion, these fine pits will
become visible as a haze on the corroded surface.
Chromium deposits, up to a thickness of 0.13 μm (5 μin.), that are obtained from hexavalent processes are somewhat
porous. Because porosity decreases with increasing thickness, at approximately 0.5 μm (20 μin.), the deposits become
nearly pore-free when plated (Fig. 1). However, because of the hard, brittle nature of the highly stressed chromium
deposits, they quickly become cracked during storage or service. These cracks do not improve the corrosion resistance, as
do deposits with intentionally developed micropores or microcracks.
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