Ask the Expert Question-and-Answer Archive
(Hard Chrome Plating)
by Larry Zitko, ChromeTech, Inc.
Hard Chromium-Molybdenum Plating
In a nutshell, can you tell me if there is any difference between decorative chrome plating vs. hard? I heard someone call decorative chrome plating,
"black chrome plating." Is there such a thing?
You actually referred to three different plating processes.
1) The terms Hard Chromium Plating, Industrial Chrome Plating, Engineering Chrome Plating and Functional Chrome Plating generally refer to the process of electrodepositing a thick layer (0.2 mils to 30 mils or more) of chromium. Usually, the coating is applied directly to ferrous substrates, like steel, although it can also be applied to non-ferrous substrates. The thick chrome is almost always deposited from a hexavalent chromium plating bath. The chromium deposit is usually selected to take advantage of its desirable properties:
- Corrosion resistance
- Low coefficient of friction
You can find all kinds of parts that are hard chrome plated. Some of them are:
- Hydraulic rods and cylinders
- Aircraft jet engine components
- Diesel cylinder liners
- Pneumatic struts for automobile hatchbacks
- Shock absorbers
- Aircraft landing gear
- Railroad wheel bearings and couplers
- Tool and die parts
- Molds for the plastic and rubber industry
2) The terms Decorative Chromium Plating and Bright Chrome Plating generally refer to a different process, whereby a thin layer (sometimes 50 millionths of an inch) of chromium is deposited for appearance or non-tarnishing purposes. It is usually deposited over a layer of nickel, or copper + nickel. I'm sure you are familiar with the kinds of parts that are bright chrome plated:
- Car and truck bumpers
- Zinc die-cast emblems (remember the "Chevy/GMC K5 Blazer"?)
- Motorcycle parts
- Kitchen appliances
There has been a strong movement away from hexavalent decorative chromium baths, to the newer trivalent chromium baths. The trivalent form of chromium is considered to be less toxic.
3) Black Chromium is a specialty chromium coating, which has a black color instead of the usual silver color. It is often used in solar applications, where it enjoys a high solar absorbance and low thermal emittance. It may also be specified for aesthetic reasons.
I hope this information has been helpful.