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How to choose the coatings of auto parts?

Surface treatment plays a big role in the automotive spare parts for increasing the service lifespan of steering and suspension parts, developing wear and corrosion resistance and improving fatigue strength in ball joints, tie rod end, control arms and cross rod, etc.

What are the normal coatings for the car steering and suspension parts in China?

The normal coatings for the car steering and suspension parts in China

  1. Electroplating

Electroplating is the most common form of auto parts plating, which is primarily used to build surface thickness and corrosion resistance. Also known as electrodeposition, electroplating is accomplished by sending an electric current through an electrolyte solution. This is referred to as a bath, which contains the metal substrate along with dissolved ions of the metal used as the coating. While the electric current passes, metal ions from the anode move towards the cathode and deposit on the electroplated material of the cathode, which is called electroplating.

Zinc coating

Reacts with oxygen or carbon to generate oxide and form an alkaline zinc carbonate film to protect the metal from rust and corrosion. It is a low cost, convenient coating method with good effect, and be commonly used.

Chromium coating

As the byproducts of chromate conversion coatings are highly hazardous, now it is strictly limited to use.

2. Electrophoresis

Electrophoresis is one of the most effective methods of coating. Electrophoretic coating is a coating method for immersing a conductive substrate as an anode (or cathode), and set another corresponding cathode (or anode) in the electrolyte bath, after the DC current passes through the bath, the non-metal coating materials deposit on the substrate (for example control arm) to prevent the metal oxidated which is called electrophoresis.

Common points: Both electrophoresis and electroplating use direct current; all of them play an anticorrosion protection and aesthetic role on the metal surface.

The difference is that electrophoresis “coats” the surface of the workpiece with an organic paint film (not metal), while electroplating “coats” the surface of the workpiece with a metal or alloy. Electrophoresis only applies to metals, electroplating can be applied to nonmetals.

3. Blackening

The blackened finish is a conversion coating for various metal materials like stainless steel, copper and copper based alloys, zinc etc by immersing the auto parts after degreasing into an alkaline or acid solutions and heat to 135~155 degrees celsius to form an oxide film. It is used to add mild corrosion resistance for appearance, which is also called “bluing”.

The key processes affected the quality

The quality of the blackening depends on the pre-cleaning processes of “rust removal” and “oil removal” before blackening, and after-processes “oil immersion” after blackening.

The outer layer of the oxide film formed after the “blackening” treatment is mainly Fe3O4 and the inner layer is FeCO3.

4. Phosphoryization

Phosphorization treatment is a chemical reaction of the metal surface in contact with an acid solution containing dihydrogen phosphate and form a stable phospholated film on the metal surface, which is a simple, reliable, inexpensive and convenient process method to greatly improve the corrosion resistance of the metal surface. It can also improve the adhesion and decrease the friction.

Applications of the phosphoryization

Phosphating of metal surface is a common pretreatment technique, it is used as a primer before painting to improve the adhesion and corrosion resistance of the paint film; and used to reduce friction and lubricate in metal cold working process.

Comparing above 4 coatings

CoatingAppearnceCorrosion resisteanceAdhesivenessHardnessEnviromentalCost
1.Electroplating***************
2.electrophoresis **********
3.blackening*****
4.phosphoryization
***** means the best relevant characters, and ***** means the lowest cost

How to Select the Suitable Coating Method

When selecting your steering and suspension parts’ finish, it’s important to consider a variety of factors, including:

  • Material of the substrate
  • Purpose of the finish
  • Budget
  • Relevant standards or regulations

The substrate is one of the most important factors to consider when choosing a finish, since it will influence the choice as well as the type of plating process required. For example, if it is made of aluminum, we suggest the electorphoresis.

Another important factor to consider is the purpose of the finish and their work environment . Will they need to stand up to intense environmental conditions? Consider how the properties of the finish could enhance your product before making a final decision.

Your budget may also influence finishing decisions. Weigh function against cost as you make your decision.

Finally, you may also need to consider environmental regulations and industry standards as you choose a metal finish. The Environmental Protection Agency and other organizations may limit the amount of certain metals released into the air or water.

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