Chromium is bluish-white and lustrous metal that is resistant to corrosion in most atmospheres. Chromium
plating, therefore, is extensively used as a final finishing operation. There are two principal classes of
chromium plating: "decorative, in which thin coatings serve as a non-tarnishing, durable surface finish; and
industrial or "hard" chromium, where heavy coatings are used to take advantage of the special properties of
chromium, which include resistance to heat, wear, corrosion, erosion, low coefficient of friction and anti
galling.
Decorative chrome is almost always plated on top of either nickel or a copper and nickel-plated layer. These
sub-layers of copper and nickel tend to seal off the substrate so that the micro-cracking chrome deposit does
not present corrosion problems. Decorative chromium deposits are used on such items as automobile bumpers
and trim, household appliances, furniture and many other articles that require a bright and aesthetic
appearance. The normal thickness for decorative chromium is in the range of 0.000020'1 - 0.000070". The
total deposit including the copper and nickel under-layers is typically 0.0005" thick. The Dura catalysts used
in decorative chrome plating promote deposition at higher speeds, greatly increase the throwing power of the
deposit and activate the sometimes passive nickel under-layer. Chromium plating baths in general tend to have
very poor throwing power and the deposits may appear burned on the edges and may show a lack of plating or
discoloration in the very low current density areas (those areas furthest from the anodes). These problems are
overcome with the use of the Dura catalysts, proper racking techniques, anode control and maintenance of
optimum bath composition.
Our concern for the rest of this paper, however, will be with hard chrome plating. Chromium is the hardest of
the most commonly deposited metals. Hard chrome is used as a wear resistant coating not only on steel but
also on a wide variety of other metals. Hard chromium differs from decorative chromium not only because of
its use but also because of the difference in deposit thickness. A typical hard chrome deposit is in the range of
0.0005" - 0.0100” thick.
In some cases Thin-Dense hard chrome is used which is typically 0.0002” thick. Other types of specialty hard
chrome include Crackless and Multi-Phase deposits which alter and control the microcrack structure.
The electro-deposition of hard chrome is a recognized means of prolonging the life of all types of metal parts
subjected to wear, friction, abrasion and corrosion. These parts can be protected when newly manufactured, or
they can be salvaged when they are worn and would otherwise be scrapped. As an example, it is much less
expensive to reclaim worn hydraulic components by rebuilding tolerances with a hard chrome deposit than it
is to buy or fabricate the part from scratch. Hard chromium, because it has a low surface energy, is more often
deposited on sliding or revolving parts and is, therefore, used on things like engines, pumps, compressors,
hydraulic and pneumatic rods, etc. The hard chromium deposit is also highly resistant to corrosion, and a large
number of applications where it is used to protect parts from corrosion are in popular demand. Another
attribute of the process is that it is a relatively cold one and can, therefore, be used to impart a very hard
surface onto delicate parts without fear of distortion or changing the substrates properties. Thus, it can be
regarded as a means of "surface hardening".
Hard chrome deposits have a hardness range from 56 - 74 R.C. depending on the bath type used. Obviously,
the higher the R.C. (Rockwell C) number the longer the service life will be provided to the component.
Deposit hardness is a function of the bath chemistry used. Thus it is best to use a plating process that provides
the highest R.C. value. Standard 100:1 sulfate baths only provide a R.C. value of 56-60, while the Dura-100
and Dura-3000 baths provide a R.C. value of 70-74. A component with a surface hardness exceeding 70 R.C.
will provide the longest possible service life.