Surface finishing in the manufacturing industry improves the functionality and aesthetics of a product and improves other properties. For example, it is suitable for protecting products against corrosion, abrasion, etc. Among the countless processes used by different manufacturers, the Alodine finish has unique properties, as the surface finishing option offers incredible productivity, ease of use, durability, etc.
Here we present everything you need to know about the Alodine finish. This includes the principle, applications, design considerations, and other things that make it one of the best surface finish options you can imagine for your product.
What is Alodina?
Alodine is a chemical dissolved in aqueous solutions to create a gel that is applied to metals such as aluminum alloys. It forms a coating that protects metals and improves their aesthetic appeal. Other functions of the Alodine coating are:
-Serves as a base for paint and primer
-Helps maintain electrical conductivity.
However, the name “Alodine” is a registered trademark of Henkel Surface Technology. The common name for this type of chemical film is “chromate conversion coating”.
How does Alodine Finish work?
Allodine or chromate conversion coating is a simple process that results in passivation. The process is simple. However, it depends on the Alodine you are working with.
Type 1 chromating process
If you are using the MIL-DTL-5541 Type 1 standard, follow the steps below to apply Alodine chemical conversion coatings to your metal parts.
Step #1: Clean the metal product
Degrease the first part to remove small impurities such as oxides, fats, oils and heavy metal impurities. After degreasing, rinse and dry the piece. Degreasing the product ensures that the chemical conversion coating adheres well and is uniform.
Step 2: Record the product
Etch the part of the metal product that should not have a chemical conversion coating. Engraving covers these areas so that you can properly and accurately coat the metal product. Rinse and dry the product again.
Step 3: Deoxidize
Oxygen is another pollutant you should avoid with Alodine coatings or other chemical conversion coatings. Remove oxygen by subjecting the piece to a deoxidation process. This removes oxygen, oxides and other associated chemicals. Rinse the item to remove the deoxidizer.
Step 4: Apply Chem Film Coat
Immerse the piece in a chemical bath containing the Alodine compound dissolved in an aqueous solution for a specified time. The time chosen depends on the piece and the desired thickness. You can also use a brush or spray the piece with Alodine.
Step 5: Final Wash
After applying Alodine chemical conversion coatings, rinse with water to remove excess coating. Then rinse with warm water to obtain a perfect surface finish. Let the piece dry.
Type 2 chromation process
The Type 2 MIL-DTL-5541 standard is slightly different and has two variants: acid cleaning process and alkaline cleaning process. Follow the steps below to apply the Alodine finish to the piece.
Acid cleaning process
- Clean the product with acid
- Rinse with water
- Apply Alodin Type 2
Alkaline cleaning process
- Clean the product with an alkaline solution
- Rinse with water
- Deoxidize to remove oxygen contaminants
- Rinse with water
- Apply Alodin Type 2
Types of Alodine Chromate Conversion Coatings
There are several types of Alodin, each with a unique chemical composition. Common types are Type 1 and Type 2 of MIL-DTL-5541, AMS-C-5541, MIL-C-81706, and AMS-2473 and 2474.
Of the five above, the two most typical in the manufacturing world are:
MIL-DTL-5541 Standard (TYPE I)
The MIL-DTL-5541 Type 1 standard contains hexavalent chromium, which produces a brown or gold or sometimes clear film. Also known as hexachrome due to the presence of hexavalent chromium, it was the traditional alode for many years until Type 2 and other safer chemicals came onto the market.
Hexavalent chromium is a dangerous chemical classified by OSHA as a carcinogen. Therefore, its use is not permitted in some countries. In countries where its use is permitted, processing licenses, adequate ventilation and disposal methods are required.
MIL-DTL-5541 Standard (TYPE II)
MIL-DTL-5541 Type 2 is the Alodin worldwide standard. Unlike Type 1, it contains trivalent chromium, titanium or zirconium. Therefore, it is called hex-free chrome. Type 1 is also not colored because the coated parts are transparent. Type 2 has the following advantages over Type 1
- Has a better security profile
- Occurs at a lower temperature than Type 1
- The registration process is simpler, faster and less complicated
Alodine vs. Anodizing: What's the difference
Both anodizing and anodizing are conversion processes and are sometimes confused with each other due to their similar manufacturing process. The part is dipped in chemicals, which causes coatings to form. However, both surface treatments are different. Below are some differences between the two methods.
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The difference between anodizing and anodizing becomes clearer when you understand the processes themselves. Anodizing is an electrolytic process in which a metal is coated with oxide. For example, anodizing aluminum makes the part more resistant to corrosion and wear. This is different from chrome plating, a simple chemical process that does not use electrical current.
It is made
The effect of the Alodine coating on the dimensions of the metal part is very small, with a thickness between 0.00001 and 0.00004 inches. This is different from anodizing, which creates a thicker layer (from 0.00001 for Type 1 anodizing to 0.001 for Type III anodizing).
Purpose
Both conversion coating processes are suitable for achieving corrosion resistance. However, anodizing is more aesthetically pleasing. Therefore, it is more suitable for decorative purposes.
Cost
Alodine coating is more economical than anodizing. However, you should consider the different types of Alodine, the size of the metals, and other factors as they can also affect the price.
What are the properties of Alodine Finish
Alodine finishing is a common industrial surface finishing process due to its simplicity without loss of functionality. Below are some features that make it one of the best that you should consider.
Thin film
One of the unique features of Alodine coating is the coating film formed. This is because the coating film is very thin, generally between 0.5 and 4 microns. It can serve as a corrosion-resistant surface, primer or base without changing the dimensions of the part.
Use at room temperature
The application of chromate conversion coating, especially the Type 2 standard, can be done at room temperature. Therefore, manufacturers and designers do not need to worry about coating application. It also reduces energy consumption, which in turn reduces the cost of obtaining that energy.
Alodine solution is toxic
The type of Alodin you use determines its toxicity. Chromium hexagon is very toxic and is classified by OSHA as a carcinogen. However, chromium hexagon is non-toxic. However, handle both with care and dispose of them as per regulations prescribed by government authorities.
Quick application process
Applying the chemical film to a metal part should only take a few minutes. However, this depends on the method used.
Excellent conductive surface
An Alodine coating makes the metal not only corrosion resistant but also conductive. Parts treated in this way are also suitable as a base for primers and paints.
Areas of application of Alodine Finish
Alodine finish is used in many manufacturing industries. Some application areas for surface finishing are listed below.
CNC Precision Machining Parts
One of the most common applications of Alodine coatings is the surface treatment of CNC precision machined parts. CNC-manufactured metal parts are protected against corrosion and surface-active contaminants with Alodine coatings. What makes Alodine ideal is that the final dimensions of the metal parts are not significantly changed.
aerospace industry
Therefore, Alodine coating is an important surface finish as it protects aluminum alloys and other materials used in the aerospace industry. Parts used in the manufacture of aerospace parts, such as aircraft fuselages, landing gears, shock absorbers, etc., are exposed to atmospheric gases at high altitudes. This can lead to corrosion of such parts.
Military and defense industry
Parts used in the manufacture of products for the military and defense industries must be resistant to corrosion. For example, boat interiors and other marine mechanical parts are often affected by salt water and are therefore susceptible to corrosion. Therefore, Alodine coatings protect aluminum and other important materials.
Design Considerations: How to Make Alodine Finish Look Good
Chromate conversion coating is a very simple process. However, to keep your Alodine siding looking good, you need to take into account some design considerations. Below are some design considerations to consider:
– Closing threaded/drilled holes
This may be necessary if tolerance is very tight. However, most manufacturers should not take this into consideration as the change in thickness of chromate conversion coatings is minimal.
-Deburring
You can improve the consistency of the coating by deburring the surface. Deburring removes or hides small machine marks on parts, making the surface smoother and the finish higher quality.
-PH and temperature
Make sure to use Alodine finish at the correct pH and temperature stated in the specifications. Failure to do so would result in non-adhesive coatings.
-Poor Alodine material
Using the wrong Alodine finish can also affect the quality of the product. So make sure you don't have a can of bad stuff. You can determine this by asking manufacturers for their QC sample and seeing if they meet your specifications. You can also request a testing service.
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In summary, among the various surface finishing processes used by different manufacturers, the Alodine finish stands out for its incredible productivity, ease of use and product quality.
The most important thing is that the metal part is coated to protect it from corrosion, serve as a base or primer and improve its electrical conductivity. Due to its thin coating, chromate conversion coating has many possible applications and is ideal for coating precision parts.
Common questions
What is an Alodine finish?
Alodine or chromate conversion coatings are used to protect metals from corrosion. They also serve as a base, primer or to show the electrical conductivity of the metal.
How to apply Alodine chromate coating?
There are different ways to apply Alodine. You can brush, dip, or spray the coating onto the metal piece. Of the different types, immersion is the most common.
Is Alodine finish important for CNC machined parts?
Yes, CNC machined parts must be precise, even if they undergo surface finishing. One of the various surface finishing options available is chrome plating. This surface treatment can help protect against corrosion without changing the dimensions of the metal part. It also helps reduce poor electrical resistance.
How thick is the chromate conversion coating?
The thickness of the chromate coating is between 0.25 and 1.0 µm or 0.00001 and 0.00004 inches.
