What Is Chromium Plating?
Chromium plating generally has the following characteristics:
- Hard and wear-resistant
- Low coefficient of friction
- Glossy
Chromium plating can be broadly divided into decorative chromium plating and hard chromium plating.
Conventionally, hexavalent chromium was used for chromium plating, but in recent years, trivalent chromium has been increasingly used.
The reason for this is that hexavalent chromium is harmful to the human body, and the RoHs directive, enforced by the European Union in 2006, restricted the use of hazardous substances in electrical and electronic equipment.
Uses of Chromium Plating
The following is a description of the uses of chromium plating for each of the hard chromium plating and decorative chromium plating.
Hard Chromium Plating
Hard chromium plating is used for parts in industrial equipment that require abrasion resistance and low wear. Examples of applications where hard chromium plating is used include automobile engine parts, rollers for metal rolling, molds, and drill blades.
Decorative Chromium Plating
This plating is used for parts that require corrosion resistance and a beautiful luster design. Examples of where decorative chromium plating is used include the exterior parts of automobiles and motorcycles, faucet fittings, and decorative accessories.
Principle of Chromium Plating
The most basic method of chromium plating is to use a solution called a Sargent bath. The typical composition of a Sargent bath is 250 g/L chromic anhydride and 2.5 g/L sulfuric acid.
The product to be plated is the cathode, and the lead alloy is the anode. When a direct current is applied to both poles, a reduction reaction occurs at the cathode, and chromium plating is deposited.
There are two types of chromium plating: hard chromium plating and decorative chromium plating.
Hard Chromium Plating Process
In most cases, steel parts are used as materials for hard chromium plating. The process of hard chromium plating is as follows:
- Surface finishing
- Surface cleaning
- Masking
- Jig mounting
- Anodizing
- Chromium plating
- Finishing
Decorative Chromium Plating
In decorative chromium plating, not only metal materials but also plastic parts are used as materials. Intermediate plating of copper and nickel is done before chromium plating.
- Surface Finishing
- Surface cleaning
- Masking
- Jig mounting
- Anodizing
- Copper plating
- Nickel plating
- Chromium plating
- Finishing
Hazards of Chromium Plating
Chromium is classified into trivalent chromium and hexavalent chromium, depending on the oxidation number. Hexavalent chromium is a substance whose use is prohibited by the RoHS and RoHS2 directives due to its reported environmental pollution and toxicity to the human body. Specific examples of hexavalent chromium toxicity include damage to skin and mucous membranes to which hexavalent chromium adheres, as well as liver and kidney failure.
As for chromium plating treatment, the use of hexavalent chromium has been mainstream in the past. In recent years, however, the toxicity of hexavalent chromium has become widely known, and plating processes using trivalent chromium as an alternative have come into use. Trivalent chromium plating is superior in terms of uniformity, and methods have been developed that provide corrosion resistance equivalent to that of conventional plating. In addition, it is harmless to the human body, making it an easy-to-use plating method in terms of workability.
Rust and Microcracks in Chromium Plating
Chromium plating is known to have cracks called microcracks. These cracks extend from the surface to the interior, allowing moisture and dirt to penetrate through the surface to the interior material, causing corrosion.
On the other hand, this structure is thought to inhibit the progress of corrosion. If we consider the chromium as the anode and the metal inside as the cathode, we can see that a corrosion cell is formed in which different metals are in indirect contact with each other due to moisture and dirt. Since the current flowing in a corrosion cell is proportional to its surface area, in the case of large cracks, the current is concentrated there and corrosion progresses rapidly. On the other hand, in the case of microcracks, where there are many tiny cracks, the current is distributed to each location, thus slowing down the corrosion process. Therefore, it is believed that this type of structure can inhibit overall corrosion. As countermeasures against corrosion itself, there are methods such as increasing the thickness of the chromium plating layer or plating another metal, such as nickel plating, under the chromium plating layer.
It is known that the number and size of microcracks correlate with the plating bath temperature and current density during the chromium plating process. For example, a higher plating bath temperature will reduce the number of cracks. However, if the temperature is too high, the plating layer becomes soft and the original properties cannot be obtained.