Alodine Finish(Chem Film): A Detailed Guide to Chromate Conversion Coating

Alodine finish is a type of aluminum alloy surface finishing, which is mainly used to improve the corrosion resistance of aluminum parts, as well as to enhance the adhesion of the metal, and is a good substrate for painting. We will introduce the definition, working principle, main characteristics, types, advantages and disadvantages as well as applications of Alodine finish in detail in this article. So, let's read on!

What is Alodine Finishing?

Alodine finishing is also known as chem film and its corpses of coating, which are composed of chromate conversion. This coating can generate an anodic oxide film of 0.5~4 microns on the surface of aluminum alloy, thereby improving the metal's corrosion resistance and providing better adhesion for subsequent painting.

Alodine coating is usually light gold, brown or transparent, making it more beautiful and durable while maintaining the mechanical properties of metal parts. In addition, it also retains the conductivity of metal components.

How Does Alodining Work?

The process of Alodining is featured in this section, which provides a step-by-step guide.

Chromating Process Type 1

When it comes to applying the type 1 chromating process, the following steps are undertaken:

• Clean the metal surface that is to be treated; a metal cleaner can be used for this. The surface must be free from all types of impurities and dirt and must ensure degreasing.
• You have to then etch the component by marking the areas that won't be covered by the coating.
• Subjecting the metal component to be coated to the deoxidation process will eliminate the oxygen and related chemicals.
• Now apply the coat of chem film by submerging the component in the chemical bath.
• Make sure to rinse the component with water once you have applied the coating and remove the excess to attain perfect surface finish.

Chromating Process Type 2

The type 2 chromating process has two types one is acid clean and the other is alkaline clean process.

Acid Clean Process

• Rinse the metal component first and clean it using an acid solution.
• After this, rinse it once more and immerse it in the trivalent chromium solution.
• Using deionized water, rinse the metal component gently and allow the chem film to form. You can use compressed air or leave it to air dry.

Alkaline Clean Process

• Wash the metal component and apply some alkaline solution so that a layer of oxide is formed.
• Now rinse the metal part once more and immerse it in a trivalent chromium solution, allowing the chem film to form.
• Gently rinse the component using deionized water and let it dry.

Main Characteristics of Alodine Finish

The main characteristics of Alodine finish are as follows:

• Alodine finish creates a very thin coating that offers excellent resistance against corrosion without impacting the dimensions of the components.
• The chromate conversion coating is applied at room temperature, offering to save energy costs.
• Alodine, which is hex chrome-based, is very toxic, and hex-free chrome is not toxic; however, both should be handled with care.
• The process of applying the chem film is a very easy and quick one it takes just a few minutes.
• Alodine finish improves the conductivity of the metal and gives a seamless base for painting and priming.

When to Choose Alodine Finishing

Some of the reasons that may compel you to choose Alodine finishing are as follows:

• Metal components manufactured using steel and aluminum may end up corroding when exposed to harsh conditions. Such components may need enhanced corrosion resistance, which is achieved by an Alodine finish.
• Alodine coating creates a better base for priming and painting the CNC machined components. The coating enhances the aesthetics and performance of the components.
• Alodine finishes help to improve conductivity in metal components.

Types of Alodine Finishes

There are different types of Alodine finishes, and these are as follows:

5.1 MIL-DTL-5541 Type 1

MIL-DTL-5541 comprises hexavalent chromium, which creates a gold or brown film on the components. It was the most chosen and safest type of Alodine coating until type 2 was invented. However, it is hazardous, is not permitted in many countries, and requires proper ventilation and disposal methods.

5.2 MIL-DTL-5541 Type 2

MIL-DTL-5541 type  2 has trivalent titanium, chromium, or zirconium. It is a hex-free chrome and features a clear coating. It offers enhanced safety and takes place at a temperature lower than type 1; the application process of this type is simple and quick.

5.3 ISO 8081:2021 Aerospace Process

The ISO 8081:2021 standard applies to all types of chemical conversion coatings carried out on alloys of magnesium and aluminum. It is opted for applications in aerospace and has specific corrosion resistance, thickness, and application process.

Pros and Cons of Alodine Finish

The Alodine finish has some pros and cons, which are as follows:

Pros

• It is low cost.
• It can be carried out in less time.
• Alodine finish offers improved resistance against corrosion.
• This coating enhances the electrical conductivity of the components.
• It creates a very thin layer of coating on components.

Cons

• The film created by Alodining is weak and cannot be self-repaired.
• When compared to anodizing, it offers moderate corrosion resistance.
• The materials used in the alodining process are toxic.

Applications of Alodine Finish

The applications of Alodine finishing are as follows:

• It is used to coat the landing gear of aircraft.
• It helps enhance the IR emission property of heat sinks.
• Airframe components of military and civil applications are coated using an Alodine finish.
• The automobile sector uses it for coating wheels.
• CNC-machined components made using light metal are equipped with Alodine coating when exposed to harsh environments.

Alodine vs. Anodizing: What's the Difference

Alodining and anodizing are both processes that help to create the metal components as surface finishing options. They can protect the metals from corrosion and even help improve their aesthetic looks. However, these differ in terms of application processes and the chemicals involved.

Alodining creates a layer of chromate film on the surface of the metal component and offers high electrical conductivity followed by moderate resistance against corrosion. It is a low-cost process and takes less time than anodizing.

Anodizing is a process that creates oxidation on the surface, creating a thick layer that is durable. It offers abrasion resistance, toughness, and corrosion resistance to metal components. It is mostly opted for automobile components and architectural structures.

Choosing the Right Alodine Finish for Your Project - DEK

When your machined parts need to be Alodine coated, you can contact DEK. we have a lot of expertise in surface treatment of aluminum parts. in addition to Alodine surfacing, anodizing, we can also provide sandblasting, powder coating, plating, and other surface treatments for parts with different applications.

Conclusion

As you have read the guide above you must have understood what is Alodine and how it can be applied. The difference between Alodine finish and anodizing must also be clear and now you can make an informed decision for coating your metal components.

FAQs

How to Make Alodine Finish Look Good?

You can make the Alodine finish look good by deburring the surface of the metal component.

How Thick is Chromate Conversion Coating?

Alodine coatings are chemical film coatings that are applied at relatively thin thicknesses, usually in the following ranges:

0.5 to 4 microns (approximately 0.00002 to 0.00016 inches).

More specifically, the thickness of the alodine coating is typically in the range of 0.25 to 1 micron (approximately 0.00001 to 0.00004 inches)

How Do you Apply Alodine Chromate Coating?

Alodine coating is applied using a brush, or you can even dip the metal part in the solution.

Is Alodine Finish Important for CNC machined parts?

CNC-machined components need to have a smooth surface. Any surface finishing process can be used, among which Alodine finish is also a common one.

How Long Does Alodine Last?

It can last for years if subjected to low abrasion and is kept under favorable conditions.

Is Alodine Toxic?

Yes, Alodine coating is toxic but hex-free coating is less toxic.