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Purpose, Type and Processing Steps of Metal Coating

Metal Coating is a type of surface treatment, forming a thin film of a metal different from the material on the surface of metal or resin. The Chinese character for “gold plating” can be written as “plating”, “electroplating”, and sometimes “plating”. This time, we will introduce the types and methods of gold plating.

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What is Metal Coating?

Plating is a surface treatment that covers the surface of a workpiece that becomes a base with a thin metal film. There are three main purposes of plating.

• Improvement of Decorativeness

Plating is done to improve the aesthetic appearance, such as creating a shiny surface or a high-grade color. Representative examples include gold plating.

• Improvement of Corrosion Resistance

Some materials such as iron commonly used in industrial products are prone to rust. Therefore, the surface is covered with a corrosion-resistant metal, which is plating to avoid rust. Zinc plating is a representative example.

• Improvement of Functionality

In addition to decorativeness and corrosion resistance, plating can also add various functions. For example, in plating to optimize the suitability of solder called solder wettability, nickel plating is performed, or copper and tin alloys are used for plating. In addition, there is plating to improve sliding properties or conductivity.

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Types of Plating

There are many types of plating.
*The values ​​listed are for reference only.

Types

Vickers Hardness (HV)

Layer thicknessμm

Processable materials

Uses

Purpose and Features

Remark

Galvanizing

3~20

Carbon Steel

Thin plate
Wire

· Rust-proof, low price

· Poor appearance

Chrome

1~2

Carbon Steel

Sheet metal parts
Bolts and nuts

· Rust-proof, low price

· Suitable for mass production

· Decreased aesthetics but can replace nickel plating

Bright Galvanized

1~2

Carbon Steel

Trivalent Chromates

1~2

Carbon Steel

Bolts and nuts

· Rust-proof and low-priced

· Does not contain hexavalent chromium

Nickel Plating

Carbon steel
Copper
Brass

· Improve corrosion resistance and decoration

· Chrome plating has better corrosion resistance in the atmosphere

· Copper-based electroplating can be applied as needed

· Deeper depressions cannot be applied

 

No. 1 plating

500

5~20

· The appearance is better than the No. 3 plating

Material → polishing → plating → polishing

No. 3 plating

· Material → Plating

Pear skin plating

· Fatigue resistance

· Small defects are not noticeable

· Material → Pear skin treatment → Plating

Electroless nickel plating

500

Ability to specify

Carbon steel
Stainless steel
Copper
Aluminum alloy
Glass
Plastic

Parts that cannot be nickel-plated
Parts that undergo curing after plating

· Easy to manage film thickness

· Strong corrosion and wear resistance

· Can make non-metallic materials into conductors

· Can be cured by heat treatment after plating

镀铬

Carbon steel
Copper
Brass

· Lustrous appearance

· Good corrosion resistance

· Chrome plating is easy to sinter when sliding against each other

· Nickel-based plating is performed as needed

· Deeper depressions cannot be performed

 

No. 1 plating

500

5~20

· The appearance is better than the No. 3 plating

Material → polishing → plating → polishing

No. 3 plating

· Material → Plating

Pear skin plating

· Fatigue resistance

· Small defects are not noticeable

· Material → Pear skin treatment → Plating

Hard chrome plating

1000

10~30

Shafts

Sliding parts

· Excellent wear resistance

· More expensive than other chrome plating

· Material → Plating (No. 3 plating)

Iron trioxide protective film
(blackening treatment)

Carbon Steel

Bolts
Nuts
Measuring instruments

· Paint base

· Appearance (glossy)

· more prone to rust than diffusion nitriding

· Produce iron(III) oxide (black)

Low temperature black
Chrome

1~2

Carbon steel
Copper
Stainless steel

Parts that require precision

· Long-term anti-rust ability

· Best corrosion resistance among black colors

· Ultra-thin film

· Because it is processed at low temperatures, there is no heat effect on the material, and parts combined with plastics, rubber, etc. can also be processed directly.

Anodizing

White

3~5

Aluminum Alloy

· Anti-corrosion and wear resistance

· Non-conductive

· Heat resistance

There is a coloring anodizing treatment, which forms a hard oxide film on the surface and uses the pores of the oxide film for coloring.

Plating is roughly divided into three categories based on the processing method. We will explain each of them starting in the next chapter.

Plating and Type

Electroplating is the most common method in plating and is used to make materials decorative, rust-proof, wear-resistant, and conductive. The metal to be plated and the plating metal are placed in a solution containing metal ions, the metal to be plated is connected to the cathode, the plating metal is connected to the anode, and a direct current is passed. Then, a reduction reaction occurs in the solution, and the plating metal is precipitated on the metal surface of the anode.

The amount of metal precipitated changes with the current density, so the thickness of the plating becomes uneven depending on the shape of the part. Specifically, the plating is thicker in protruding parts and thinner in recessed parts. The plating thickness of electroplating changes with the current and time passed, so it is necessary to find the plating thickness that suits the purpose for each plated part.
There are mainly the following types of electroplating.

Gold Plating

Gold is a metal with a beautiful luster, but it is characterized by its high price. Therefore, gold plating is performed to improve decorativeness by covering only the surface of the product with gold. Gold is not only beautiful in appearance, but also chemically stable, so it has excellent corrosion resistance and can also be used for rust prevention and conductivity.

Silver Plating

Silver is also a metal with excellent decorative properties like gold. However, since it is easily oxidized, it is necessary to perform a treatment to prevent discoloration after silver plating. Since it has high conductivity, it is used for surface treatment of connector parts of precision equipment, etc.

Galvanizing

This is the most commonly used processing method for rust-proof plating. When zinc is plated on carbon steel parts, it prevents corrosion of carbon steel due to the self-sacrificing effect on carbon steel. After zinc plating, gloss zinc plating and trivalent chromate gloss treatment are performed to protect the zinc plating layer.

Chromium Plating

Chromium plating is performed to make decorative plating glossy. In addition, it has strong wear resistance and excellent corrosion resistance, so it is also used for mechanical parts in the industrial field.

Electroless Plating and Types

Electroless plating is a plating method that does not use electricity. The product to be plated is immersed in a solution containing metal ions that form a coating, and the metal ions are reduced to form a coating. Since no electricity is used, non-conductors such as resins can also be plated. At this time, since the metal film formed on the surface is conductive, electroless plating is sometimes performed on non-conductor products to make them conductive as a pretreatment for electroplating.

Nickel plating is a common plating method in electroless plating.

Electroless Nickel Plating

This is a plating method also known as chemical nickel plating. The thickness of the coating is more stable than electrolytic nickel plating, but the temperature of the plating solution is higher, so you need to pay attention. Electroless nickel plating has excellent wear resistance and rust resistance.

Other Plating

In addition to electroplating and electroless plating, there are chemical conversion treatments (black oxide film), anodizing treatment (aluminum oxide), hot-dip galvanizing, etc.

Chemical Conversion Treatment (Black Oxide Film)

When you want to obtain a black surface, chemical conversion treatment is used. It is also called blackening treatment, which forms a tetra-trioxide film. The thickness of the film is thin and stable, and it can also be applied to parts with strict dimensional tolerances.

Anodizing Treatment (Aluminum Oxide)

The method of forcibly oxidizing the surface of aluminum materials to obtain a film is called anodizing treatment. It has high hardness and excellent corrosion resistance.

Hot-Dip Galvanizing

Hot-dip galvanizing is the method of immersing parts in a heated and molten solution to form a film on the surface. It is used for wires, structures, etc. Because it is immersed in molten zinc, the influence of heat is greater.

In Conclusion:

Plating is a method of forming a coating of other types of metal on the surface of metal or resin. Plating is performed for various purposes such as improving decorativeness, achieving rust resistance, and durability. In addition to electroplating, electroless plating, which precipitates metal in a solution, is also used.

Huaxiao Metal can provide instant quotes and processing for plating.

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