From Wikipedia, the free encyclopedia
Decorative chrome plating on a motorcycle.
Chrome plating, often referred to simply as chrome, is a technique of electroplating a thin layer of chromium onto a metal object. The chromed layer can be decorative, provide corrosion resistance, ease cleaning procedures, or increase surface hardness.
Process
The component will generally go through these different stages.
- Degreasing to remove heavy soiling.
- Manual cleaning to remove all residual traces of dirt and surface impurities.
- Various pretreatments depending on the substrate.
- Placed into the chrome plating vat and allowed to warm to solution temperature.
- Plating current applied and component is left for the required time to attain thickness.
There are many variations to this process depending on the type of substrate being plated upon. Different etching solutions are used for different substrates. Hydrochloric, hydrofluoric, and sulfuric acids can be used. Ferric chloride is also popular for the etching of Nimonic alloys. Sometimes the component will enter the chrome plating vat electrically live. Sometimes the component will have a conforming anode either made from lead/tin or platinized titanium. A typical hard chrome vat will plate at about 25 micrometres (0.00098 in) per hour.
The chrome plating chemicals are very toxic. Disposal of chemicals is regulated in most countries.
Process Example
Information provided below is an example of an aluminum metal part being processed. These steps are specific to a number of metal parts, but actual steps will vary according to the design and condition of the workpiece.
- Chrome Stripping
- Rack part for stripping process
- Dip in electrically activated sodium hydroxide
- Dip in hot water
- Strip off nickel with activated sulfuric acid (taking care that the nickel is not eroded)
- Place in Media Blaster for coating preparation
- Grinding
- Grind off any and all pits/ protrusions/ unwanted metal material
- Smooth unwanted edges
- As necessary, grind points of contact for future soldering
- Cyanide Bath
- Rack part with copper wire (some need weights attached to underside to prevent floating giving special attention avoiding the wire to touch the part)
- Dip into Electro-cleaner wash
- Rinse off soup lather
- Dip in non-electrified sulfuric acid
- Spray rinse with water
- Place in cyanide
- Place in cyanide rinse tank
- Touching Up
- Solder pits and fill in undercuts (with attention to avoiding a detrimental cut through part)
- If applicable, attach broken metal pieces and filler metal with solder
- Grind off extra solder to smooth finish
- Acid Copper Bath
- Place in cyanide to cover solder
- Again dip in cyanide rinse
- Spray wash with water
- Cycle following steps for (1) hour intervals until acceptable appearance is seen:
- Copper Bath
- Sanding
- Buffing
- Chome Plating
- Re-rack part with wire giving attention to sensitive areas (points and sharp curves are corroded without care)
- Clean with kerosene and soft-bristled brush
- Hand wash with soap and water
- Spray rinse with water
- Dip in sulfuric acid
- Dip in sterile DI (deionized) water
- Nickel plated
- Dip in DI water
- Dip in chrome tank with settings specific to the part
- Spray rinse with water
- Buff to smooth finish
Industrial chrome
Industrial chromium plating, also known as hard chrome or engineered chrome, is used to reduce friction, add wear resistance, or increase corrosion resistance. It is very hard, measuring between 66 to 70 HRC. Hard chrome tends to be thicker than the decorative treatment, typically ranging from 0.075 to 0.25 millimetres (0.0030 to 0.0098 in), but thinner and thicker layers are not uncommon. Surface defects and roughness are amplified, because hard chrome does not have a leveling effect.[1] Hard chromium plating is subject to different types of quality requirements depending on the application, for instance, the plating on hydraulic piston rods are tested for corrosion resistance with a salt spray test.
There are two types of industrial chrome plating solutions:
- Hexavalent chromium baths whose main ingredient is chromic anhydride.
- Trivalent chromium baths whose main ingredient is chromium sulfate or chromium chloride. Trivalent chromium baths are not yet common, due to restrictions concerning color, brittleness, and plating thickness.[citation needed]
A typical bath composition of a hexavalent chromium bath is as follows:
Automotive use
Formerly most decorative items affixed to cars were referred to as "chrome", by which phrase was actually meant steel that had undergone several plating processes to endure the temperature changes and weather that a car was subject to outdoors. The most expensive and durable process involved plating the steel first with copper, and then nickel, before the chromium plating was applied.
Prior to the application of chrome in the 1920s, nickel was used. In the US for the short production run prior to the entry into the Second World War, plating was banned to save chromium and the decorative pieces were painted in a complementary color. In the last years of the Korean War, the banning of chrome was contemplated and several cheaper processes (such as plating with zinc and then coating with shiny plastic) were considered.
In 2007, a Restriction of Hazardous Substances Directive (RoHS) was issued banning several toxic substances for use in the automotive industry in Europe, including hexavalent chromium, which is used in chrome plating.
See also
References
- ^ Degarmo, E. Paul; Black, J T.; Kohser, Ronald A. (2003), Materials and Processes in Manufacturing (9th ed.), Wiley, p. 793, ISBN 0-471-65653-4 .
Further reading
- SAE AMS 2406
- SAE AMS 2438
- SAE AMS 2460 - Plating, Chromium
