The item first undergoes a copper, then nickel plating prior to the chromium. Aluminum objects are first zinc coated, then copper, nickel & chrome. Gold plated parts get copper & nickel, (no chrome) then gold. Silver goes right over the copper.
A component to be chrome plated 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
- Placement into the chrome plating vat, where it is allowed to warm to solution temperature
- Application of plating current, under which the 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 1 mil (25 µm) per hour.
Various linishing and buffing processes are used in preparing components for decorative chrome plating. The overall appearance of decorative chrome plating is only as good as the preparation of the component.
The chrome plating chemicals are very toxic. Disposal of chemicals is regulated in most countries.
Trivalent chromium (Tri Plating)
Trivalent chromium plating, also known as tri-chrome, Cr+3, and chrome (III) plating, uses chromium sulfate or chromium chloride as the main ingredient. Trivalent chromium plating is an alternative to hexavalent chromium in certain applications and thicknesses (e.g. decorative plating).
A trivalent chromium plating process is similar to the hexavalent chromium plating process except for the bath chemistry and anode composition. There are three main types of trivalent chromium bath configurations:
- A chloride- or sulfate-based electrolyte bath using graphite or composite anodes, plus additives to prevent the oxidation of trivalent chromium to the anodes.
- A sulfate-based bath that uses lead anodes that are surrounded by boxes filled with sulfuric acid (known as shielded anodes), which keeps the trivalent chromium from oxidizing at the anodes.
- A sulfate-based bath that uses insoluble catalytic anodes, which maintains an electrode potential that prevents oxidation.
The trivalent chromium plating process plates workpieces at a similar temperature, rate and hardness, as compared to hexavalent chromium. Plating thickness range from 0.005 to 0.05 mils (0.13 to 1.3 µm).
Advantages and Disadvantages
The functional advantages of trivalent chromium are higher cathode efficiency and better throwing power. The better throwing power means production rates are greater. Less energy is required because of the lower current densities required. The process is more robust than hexavalent chromium because it can withstand current interruptions.
From a health standpoint trivalent chromium is intrinsically less toxic than hexavalent chromium. Because of the lower toxicity it is not regulated as strictly, which reduces overhead costs. There are other secondary health advantages:
- Higher cathode efficiencies lead to less chromium emitted into the air
- Lower concentration levels result in less chromium waste
- The anodes do not discompose
One of the disadvantages when the process was first introduced was that decorative customers disapproved of the color differences, however additives are now used to adjust the color. In hard coating applications, the corrosion resistance of thicker coatings is not quite as good as hexavalent chromium. The cost of the chemicals is greater, however this is usually offset by greater production rates and lower overhead costs. In general, the process must be controlled more closely than in hexavalent chromium plating, especially with respect to metallic impurities. This means processes that are hard to control, such as barrel plating, are much more difficult using a trivalent chromium bath.
Decorative Chrome Plating (What We Do)
Decorative chrome is designed to be aesthetically pleasing and durable. Thicknesses range from 0.002 to 0.02 mils (0.05 to 0.5 µm), however they are usually between 0.005 and 0.01 mils (0.13 and 0.25 µm). The chromium plating is usually applied over bright nickel plating. Typical base materials include steel, aluminum, plastic, copper alloys, and zinc alloys.
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 electroplating 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. However, chrome plating is metal and contains no hexavalent chromium after it is rinsed, so chrome plating is not banned.