Alodine VS Anodize: Complete Guide To Differences

Alodine and anodize are both protective finishes for aluminum material. Alodining for aluminum protects it from corrosion and offers additional benefits for applications in aerospace and automotive. 

Alodining is an entirely different process from anodizing, and unlike anodized aluminum, alodining maintains the electrical conductivity of aluminum, making it useful for electrical applications. 

There are many surface finishing options for aluminum, and some have multiple names. This blog will help you differentiate between alodine vs anodize and how both processes compare.

Meaning of Alodine

Alodining is a surface finishing option for a few selective metals, but primarily aluminum. Alodine is a registered product trademark of Henkel Adhesives and is a name used for their chromate conversion coating. 

Henkel has updated their product’s name to Bonderite, but alodine has been popularized in the industry, leading to names like alodining and alodined parts, chromate conversion coating, chem film, and Type I and Type II coatings. 

Alodining Process

Alodine tank

The alodining process gives the aluminum part a protective oxide finish similar to anodizing. However, the alodining process has different steps than anodizing. 

The aluminum part passes between different tanks during the alodining process. 

Step 1: Cleaning Tank

The first part of the alodining process involves cleaning the aluminum part in an alkaline solution. The solution removes oils, grease, and cutting fluids stuck to the part from the aluminum machining processes. 

After each step, the aluminum part is submerged in a rinsing tank, often multiple times. 

Step 2: Deoxidizing Tank

Over time, the aluminum surface can build a deep oxide layer. Sometimes, this oxide layer must be removed so that the alodine coating sticks to the aluminum surface. 

Aluminum is further cleaned in deoxidizing tanks, where a oxidation-reduction chemical reaction removes the oxidized layer. For small parts, you can use any aluminum deoxidizer, like California custom aluminum deoxidizer. 

Step 3: Rinsing Tank

The final rinsing tank, has clean water entering through and is the starting point of the reverse cascade system (water recycling). Before alodine coating, the part is rinsed in this rinsing tank. 

The aluminum is cleaned here to remove any chemicals before submerging it in the chromate conversion coating. 

Step 4: Alodining Tank

There are many types of alodine coatings, but the most popular are Type I and Type II chromate conversion coatings. Both coatings use a different solution composition, and the main difference is the presence of either hexavalent or trivalent chromate. 

The part is submerged in the alodine coating tank, and the dwell time (submerged time) changes the thickness of the coating. Thicker chem-film or alodine coatings require more time. 

The alodine part can be rinsed before drying and curing. 

Step 5: Curing

The new chem film is flimsy and soft. It hardens over time, but the process is catalyzed with heat. The alodine layer is cured at around 60°C before toughening and becoming hydrophobic (water-repellent).

The alodine coating is thin. The coating thickness of the alodine coating is between 0.00001 and 0.00004 inches, which is thin enough to protect the aluminum and does not alter the conductivity of aluminum.

Alodining VS Anodizing: Difference in Process

The main difference between the anodizing and alodining process is in the coating solution. 

• Anodizing uses an electrolytic technique to grow an oxide layer from the aluminum substrate, but the alodine layer is formed by coating the aluminum with chromate. 
• Anodizing is electrolytic, but chromate coating (alodine) is not. It is a conversion coating, so alodine is also called chromate conversion coating. 
• Anodized aluminum has reduced electrical conductivity because the anodized layer is thick, around 5 – 25 micrometers and can grow up to 100 micrometers, but a chem film (alodine) does not alter aluminum’s electrical conductivity. 
• Anodized aluminum vs alodine coating has another difference: alodine requires a curing process to solidify the chem film.

Uses of Alodine Coating

Alodine coating is a useful passive layer that protects the aluminum from oxidation and corrosion. Alodine coating is not as wear-resistant and durable as an anodized layer but a less expensive solution for applications requiring protective coating. 

Aerospace MIL-DTL-5541

Chem film coating meets the aerospace MIL-DTL-5541 standard, a US military specification defining the requirements for chromate conversion coatings on aluminum. 

Alodine coatings are used in military-grade defense equipment and vehicles.

Alodine parts

Automotive Industry

Alodined aluminum parts are used in car chassis, panels, and internal frames to prevent oxidation. Alodine coatings also aid in heat dissipation and electronic components like batteries and housings to avoid oxidation while maintaining the same electrical conductivity. 

Electronics

Alodine anodizing is used on aluminum enclosures for power supplies. Chem film can protect aluminum from weathering elements and maintain conductivity. 

Aldine bus bars are a common product of the alodining process. Many aluminum PCBs use alodine as a protective barrier. 

Paint Adhesion

Alodine is also used as a pre-treatment process for painting aluminum parts. Alodine coatings are thin and improve the paint adhesion to the surface.

Benefits of Aluminum Alodining

Alodine coating on a part

• Tight tolerances: Alodine has a thickness of 0.00004”, which is considerably less than an anodized layer. For precision machined parts, alodine is a better option. 
• Electrical conductivity: Alodine maintains the electrical conductivity of aluminum because of its thickness. 
• Cheaper: Alodine coating is a cost-effective alternative to anodizing for corrosion resistance, but it does not offer the same level of wear and abrasion resistance. 
• Corrosion resistant: Alodine MIL-DTL-5541 Class 1A can withstand more than 336 hours without white corrosion during salt spray tests.
• Paint adhesion: Alodine is a porous coating that allows paint and powder coatings to adhere to the surface more effectively. 

Cons of Aluminum Alodining

• Environmentally toxic: type I alodine uses hexavalent chromium, which can leach into the environment and cause harm. Hexavalent chromium is toxic during coating and a carcinogen (cancer-causing) product. 
• Wear and abrasion resistance: Alodine provides limited wear and abrasion resistance for friction applications and moving aluminum parts. Anodizing is a better protective method for moving and external use of aluminum parts like aluminum profiles. 
• Corrosion resistance effectiveness: Compared to anodizing, alodine offers moderate corrosion resistance. Alodine does not create a thick oxide layer like anodizing. 
• Limited color options

You should avoid alodine coating when using mechanical aluminum assemblies and for prolonged exposure to weather elements. Type I alodining is sometimes unpermissible according to some environmental restrictions as well.

When Can You Use Alodine Coating VS Anodizing?

Anodized aluminum parts

Which one should you choose, alodine or anodize? The answer depends on your application and corrosion resistance requirements. Anodized aluminum has the advantage of a tougher and scratch-resistant thicker oxide layer, which is ideal for mechanical parts. 

Anodizing also offers better aesthetic and colored finishes for aesthetics. You’d use alodine coatings and chromate conversion coatings when thermal and electrical conductivity are a priority. Alodine also offers a thinner coating that does not interfere with tolerances in most applications. 

If you’re trying to coat your aluminum parts at home, alodining is a simpler and easy-to-follow process. You can apply an alodine coat to your bike frame, tennis racket, and other aluminum parts at home.

Enhancing the Life of Your Aluminum Parts With Pro-lean Tech

Whether you anodize or alodine, the right surface treatment can significantly improve the durability and corrosion resistance of your aluminum parts. 

At Prolean-Tech, we offer consultation and guidance with top-tier finishing solutions for your parts.

Conclusion

Both processes offer good protection, and the better choice depends on your requirements. Chromate conversion coatings are cost-effective, quick and an electrically conductive solution for components in aerospace and electronics. 

Anodising, although more expensive, provides durability, a tougher oxide layer, and wear resistance, making your part versatile for any industry and environment.

FAQs

• What is alodine?

Alodine® is also called chem film or chromium conversion coating. Alodine is a protective coating for aluminum that protects it from corrosion without compromising electrical conductivity.

• Can you anodize and alodine?

You can alodine and then anodize aluminum parts. However this requires modification to the anodizing process. First, mask the alodine layer in order to not dissolve it during anodizing. Second, use the anodizing tank to grow the anodized layer from the unmasked surface. This process is not practical in industrial use, but works for few parts. 

• What is the difference between alodine and anodize?

Alodining is a chemical coating process that builds a layer of chromium-based compound on aluminum surface whereas, anodizing is an electrochemical process that grows an oxide layer from the aluminum substrate and it is part of the aluminum.