Aluminum Anodizing, Chemical Film and Chromate Conversion Coating for Aluminum Machined Parts

Surface treatment during manufacturing enhances the functionality and aesthetics of a product and enhances other properties suitable for protecting it from corrosion, wear, etc. What is Alodine®? Alodine®, also known as chemical film or chemical film, is a chromate conversion coating that protects aluminum and other metals from corrosion. The process of applying a chromate conversion coating is called chromating. Here we will introduce you to the Alodine treatment of aluminum parts, including the definition of Alodine, common types, differences from anodizing, application areas, etc.

1.What is Alodine?

Alodine is a chemical that dissolves in an aqueous solution to form a gel for metals such as aluminum alloys. It forms a coating that protects the metal and enhances its beauty. However, the name “Alodine” is a registered trademark by Henkel Surface Technologies. The common name for this chemical film is “chromate conversion coating”.

Chromate conversion coating is a chemical conversion coating. In a chemical conversion coating, a chemical reaction occurs on the metal surface. This chemical reaction converts the metal surface into a protective layer. While providing aluminum corrosion protection, the chemical film or Alodine coating also provides a basis for surface treatment of organic coatings, thereby preventing the loss of electrical conductivity.

The chemical film is applied by dipping, brushing or spraying, and the thickness of the chromate conversion coating does not change the dimensions of the part. If the conversion coating is applied to the aluminum surface before primer, it can also improve the adhesion of paint and primer.

2.Types of Chromate Conversion Coatings

The chemical composition and form of different types of chromate conversion coatings usually vary. Although there are many different types, the most typical are Type 1 and Type 2 of the MIL-DTL-5541 standard. Other standards currently available include AMS-C-5541, MIL-C-81706, and AMS-2473 and 2474. Of the five factors mentioned above, the two most typical in manufacturing are:

1) MIL-DTL-5541 Standard (Type I)

The Class 1 MIL-DTL-5541 standard contains hexavalent chromium, which can produce a brown or gold film, and sometimes even a transparent film. Hexavalent chromium, also known as hexavalent chromium due to the presence of hexavalent chromium, was the traditional alotene for many years before Type 2 and other safer chemicals became available. Hexavalent chromium is a hazardous chemical and is listed as a carcinogen by OSHA. As a result, some countries do not allow its use. Substances that are allowed require a processing permit, proper ventilation, and disposal methods. The films are usually brown or gold in color, but may also be colorless, in which case they are “transparent.”

2) MIL-DTL-5541 Standard (Type II)

MIL-DTL-5541 Type 2 is the world standard plating. Unlike Type 1, it contains trivalent chromium, titanium, or zirconium. Therefore, it is called hexavalent chromium-free. The coating is usually colorless and is therefore described as transparent. Soon after the advent of alotene coatings, the Type 2 standard replaced Type 1 as the most widely used standard for chromate conversion coatings.

Type 2 has some particularly desirable characteristics compared to Type 1, including:

1. Better safety, more favored by the precision machined parts manufacturing industry
2. Alodine aluminum coating application temperature is lower than Type 1
3. Simpler, faster, and more straightforward application process

3.Aluminum Oxidation vs. Anodizing: What’s the Difference

Both anodizing and aluminum oxidation are conversion coating processes that are sometimes confused with each other due to similar production processes. They involve immersing parts in chemicals and forming a coating. However, the two surface treatments are different. Here are some differences between the two processes.

1) Process

By understanding the process itself, we can more clearly understand the difference between aluminum oxidation and anodizing. Anodizing is an electrolytic process that involves coating a metal with an oxide. For example, anodizing aluminum can make parts more resistant to corrosion and wear. This is different from chromate conversion coating, which is a simple chemical process that does not use an electric current.

2) Impact

Anodizing has little effect on the dimensions of metal parts, ranging from 0.00001 to 0.00004 inches in thickness. This is different from anodizing, which creates a much thicker layer (ranging from 0.00001 for Type 1 anodizing to 0.001 for Type III anodizing).

3) Purpose

Both conversion coating methods are suitable for achieving corrosion resistance. However, anodizing has a better aesthetic. Therefore, it is more suitable for decorative purposes.

4) Cost

Aluminum oxide treatments cost less than anodizing. However, you should consider the different types of aluminum oxide treatments, the size of the metal, and other effects as they also affect the price.

4.Applications of Anodized Aluminum Surface Treatments

Aluminum oxide polishing is widely used in many manufacturing industries. Here are some applications of polishing options.

1) CNC Precision Machined Parts

One of the most common aluminate coating applications is the surface treatment of CNC precision machined parts. CNC manufactured metal parts are protected from corrosion and surface acting contaminants by aluminate coatings. Aluminate coatings are ideal because the final dimensions of the metal part do not change significantly. Other benefits for CNC machined parts include improved adhesion when applying primers and changing the conductivity of the coating.

These, coupled with the fact that the entire process is simple and straightforward, make Alodine aluminum finishes an indispensable part of precision machined parts.

2) Aerospace Industry

Many parts in the aerospace industry contain aluminum, probably because aluminum is lightweight and strong. Therefore, aluminum anodizing coating is essential as a surface treatment for these parts. These parts include aircraft fuselages, landing gear, shock absorbers, etc.

3) Military and Defense Industry

Parts used to manufacture products for the military and defense industries must be corrosion-resistant. For example, the interiors of ships and other mechanical parts of the navy are often penetrated by salt water, making them susceptible to corrosion. Therefore, aluminum anodizing coatings can protect aluminum and other important materials.

5.Conclusion

Aluminum anodizing coating is a surface treatment for aluminum metal that helps prevent metal corrosion. Since the use of aluminum in industrial parts is so common today, it is necessary to have basic knowledge of aluminum anodizing, especially when dealing with precision parts.