As you might be well aware, most mechanics’ tools such as ratchets, sockets, and wrenches, are chrome (chromium) plated. This layer of chrome, although extremely thin, greatly enhances the properties of these tools.
Tools may be chrome plated for one or more of the following reasons: to minimize wear, to prevent seizing or galling, to reduce friction and facilitate cleaning, and to minimize or prevent corrosion.
There are two types of chrome plating – hard chromium and decorative chromium plating. The difference between these two options are the thickness of the chromium plating and type of undercoating used.
Hard chromium is usually used in industrial environments and for parts that need exceptional wear resistance, with a typical thickness in the range of 100 to 200 μm (0.004 to 0.008 in).
Decorative chrome plating, on the other hand, is typically only ~1.25 μm (0.00005 in) thick. Decorative chrome plating is applied over an undercoating of either nickel or copper plus nickel which imparts the bright or satin appearance to the chrome layer.
Reference: Metals Handbook Desk Edition, 2nd Ed.
This introduction is part 1 of a four part series that briefly describes the chromium plating process. Part 2 describes the cleaning and preparation process, part 3 will cover finishing, and part 4 will cover the actual plating process.