Anodizing vs Alodine: Key Differences and Processes

Anodizing and Alodine are both surface finishing techniques for aluminum parts and their alloys. They are used to protect the aluminum parts from corrosion and abrasion.

Additionally, the appearance of these parts can also be improved by these two methods. However, they are different in many aspects. It is essential to distinguish between them and apply them correctly to your products.

Key Takeaways:

• Anodizing and Alodine both protect aluminum from corrosion, but they work very differently—anodizing is an electrochemical oxidation process, while Alodine (chromate conversion coating) is a pure chemical reaction.
• Anodizing provides a much thicker, harder, and more wear-resistant coating, making it ideal for decorative finishes, outdoor use, and high-wear applications. Alodine is thin, conductive, and dimensionally stable, suitable for tight-tolerance or electrically grounded components.
• Alodine is faster, cheaper, and better for conductivity, but offers limited aesthetics and lower wear resistance. Anodizing is more durable and visually appealing, but slightly increases part dimensions and may have color-matching challenges.
• Select Anodizing for hardness, durability, and color. Select Alodine for conductivity, low cost, and minimal dimensional change.

Kind Note:

Chromate conversion coating is the formal technical term for the protective layer formed when chromate solutions chemically react with aluminum or other metals.

“Chem Film” is simply an industry shorthand for this same process, commonly used in everyday engineering or shop-floor communication.

“Alodine,” on the other hand, is a well-known brand of chromate conversion coating products—and many customers use “Alodine” as a general name for the finish itself.

In practical CNC surface-finish discussions, including in this blog, most clients refer to chromate conversion coating directly as “Alodine,” so this article follows that convention.

1. What is Anodize?

Anodize is a kind of electrochemical surface finishing technique used to improve the hardness, wear resistance, corrosion resistance, and aesthetics of aluminum parts and its alloys.

It can also be applied to other valve metals such as magnesium and titanium.

Through the effects of electro dissolution, a chemically stable and hard oxidation film forms on the metal surface, providing good protection for the parts against corrosion and abrasion.

During the anodizing process, the part would be immersed into the acid electrolyte bath as the anode and then an oxidation reaction would occur on the metal surface after the application of direct current.

Finally, an oxide film would form on the surface.

What’ more, the oxide layer formed on the surface is not externally plated or deposited like that of electroplating.

It is formed in-situ on the surface. In other words, it is formed by the conversion of the base metal itself.

As a result, it bonds extremely well to the substrate, with no risk of interfacial delamination.

With a monolithic structure, there is no distinct boundary between the film and the substrate.

Relative to the original metal surface, one-third part of the film grows inward into the metal, while the other two-thirds part extends outward.

2. What is Alodine?

Similar to anodizing, Alodine is also a surface finishing technique primarily used to protect the aluminum parts and their alloys from corrosion.

However, there are also many distinctions between them.

Compared with the anodizing process, the Alodine process is a pure chemical process that does not need any current.

Through the effects of chemical reaction between the aluminum and the chromium bath, a chromate conversion coating would form on the surface to protect the part.

When the aluminum part is immersed in the acid bath containing hexavalent or trivalent chromium, the metal surface would be slightly corroded in the acidic condition, releasing metal ions.

With the enhancement of local pH, the trivalent chromium ions would precipitate together with metal ions and hydroxides.

Finally, a layer of amorphous and hydrated composite film forms on the surface, mainly composed of Cr(OH)₃, CrOOH, and metal hydroxides.

And if the chemical solution contains hexavalent chromium, a small amount of unreduced hexavalent chromium ions would be left within the conversion film to provide self-healing capability.

The chem film is also chemically stable and dense, shielding the surface from oxidation and corrosion. And since the chem film is typically thin and not insulating, the part’s electrical conductivity can be maintained well.

What’s more, similar to the anodizing film, the Alodine film is also formed in situ on the surface. The chromate conversion film is formed by the conversion of the metal surface itself.

3. What is the Process of Anodizing?

3.1. Pre-process

Degreasing:

Remove oils, waxes, and other organic contaminants typically by alkaline cleaners or solvents.

Etching:

Etch the part in a hot sodium hydroxide (NaOH) solution to remove the natural oxide layer and minor surface defects to achieve a uniform matte finish.

Neutralizing:

Remove the intermetallic residues (smut) left after alkaline etchingtypically by a nitric acid bath or a mixture bath of nitric acid and hydrofluoric

Rinsing and Drying:

Rinse the part completely after every step to clean any residual solution off from the surface and then dry it totally to prevent water spots.

3.2. Anodizing

Setup:

Fix the aluminum part as theanode on a specifically designed fixture and connect it to the power sully.

Electrolyte Immersion:

Submerge the part in an electrolyte solution such as sulfuric acid, chromic acid, phosphoric acid, and oxalic acid.

Power Application:

Apply direct current. Then the aluminum would react with oxygen ions to form an aluminum oxide film (Al₂O₃).

Under the influence of the electric field, the oxide film continues to grow, while the electrolyte slightly dissolves the film, forming a honeycomb-like porous structure (pore size approximately 10–20 nm) with pores oriented perpendicular to the surface.

3.3. Post-process

Rinsing and Drying:

Completely rinse the part with pure water to clean residual electrolyte bath off and then dry the part properly.

Coloring (Optional):

Immerse the workpiece in a solution containing organic dyes or metal salts.

The dye molecules or metal ions would be absorbed or deposited into the porous structure of the oxide film. The color options are various.

Sealing (Optional):

Seal the porous structure of the oxidation film to enhance itscorrosion resistance and improve color robustness. The sealing methods mainly include hydration sealing and chemical sealing.

The former one works by immersing the part in boiling deionized water. This would cause the aluminum oxide to hydrate and expand into hydrated aluminum oxide (Al₂O₃H₂O), thereby sealing the pores.

The later one functions by nickel salts or other chemical agents to close the pores, with lower temperature and higher efficiency.

Rinsing and Drying:

Finally rinse the part thoroughly to clean any remained chemical agents left after sealing process and then dry it naturallyor by hot air.

4. What is the Process of Alodine?

4.1. Pre-process

Cleaning:

Remove all surface contaminants such as oils, grease, cutting fluids, and dustby soaking or spraying the surface with alkaline or acidic cleaners.

Rinsing and Drying:

Remove any residual cleaners to keep the subsequent baths from contamination by clean water rinsing, typically by deionized water. Then dry the part completely by hot air.

Deoxidizing:

Remove the natural oxide layer on the metal surface and residues left by alloying elements by an acidic solution such as nitric acid or a fluoride-containing solution.

Rinsing and Drying:

Rinse any residues from the deoxidizing solution thoroughly with clean water and dry the part totally.

4.2. Alodine Coating:

Apply Alodine solution on the aluminum part by immersion, spraying, or brushing at room temperature.

Then chromate ions in the solution would react with the aluminum metal to form a gel-like mixed protective layer of chromium oxide and chromium hydroxide.

4.3. Post-process

Rinsing:

Remove excess Alodine solution to prevent residue crystallization by rinsing the part gently with deionized water. Note: Avoid excessive rinsing to prevent damage to the forming film.

Dyring:

Fully harden and passivate the gel-like conversion coating by warm air or natural air drying.

5. What are the Pros and Cons of Anodizing?

5.1. Advantages of Anodize

• Excellent Corrosion Resistance:

The anodized film is chemically dense and stable. It can effectively isolate the aluminum substrate from the external corrosive environment.

With such a protective oxidation film, the surface’s corrosion resistance would be significantly enhanced, especially in humid, salt-spray, or acidic/alkaline conditions.

• Excellent Hardness and Wear Resistance:

The aluminum oxide itself has certainly high hardness, providing the surface with good resistance to abrasion.

The hardness of the hard anodizing film can even reach up to 50 µm or over 100 µm. Its abrasion resistance can be comparable to that of hard chrome plating, making it suitable for mechanical components.

• Various Decorative Appearance:

The anodized film has a porous chemical structure that allows it to absorb various organic/inorganic dyes. This enables anodizing film to produce a wide range of colors, from bright to subdued, to meet design requirements.

More importantly, the treated surface can retain the aluminum’s original metallic luster and texture, making it suitable for high-end consumer electronics.

• Good Adhesion:

The oxide film grows in place from the aluminum substrate, eliminating the risk of peeling and providing excellent adhesion.

What’s more, anodizing film is also ideal for use as a primer.

The porous structure of the anodized film provides excellent mechanical anchorage sites for subsequent coatings, such as paints, adhesives, or powder coatings.

• Electrical Insulation:

Since aluminum oxide is an insulator, anodized films exhibit high resistivity, making them suitable for applications requiring electrical insulation.

5.2. Disadvantages of Anodize

• Dimensional Change:

The anodized film grows both inward and outward from the surface, increasing the part’s dimensions, especially for those with a hard anodized layer.

In precision machining, allowances for the anodized film thickness must be accounted for during the machining stage.

• Coloring Limitations:

Though there are various options for color in anodized films, the coloring performance would certainly be limited.

Due to slight differences in aluminum alloy grade, film thickness, dyeing time, and other factors, it is difficult to achieve perfect color consistency across different production batches.

Additionally, anodized films dyed with organic dyes may fade under ultraviolet light exposure.

• Poor Toughness:

Although the anodizing coating has high hardness, it is essentially ceramic with poor toughness. When subjected to impact or bending, microcracks or even flaking may occur.

• Material Limitations:

Anodized coating is primarily used on valve metals, including aluminum, magnesium, and titanium. It is ineffective on common metals, such as steel.

Additionally, aluminum alloys with high silicon or high copper content tend to develop uneven films, dull colors, or have difficulty forming during the anodizing process.

6. What are the Pros and Cons of Alodine?

6.1. Advantages of Alodine

• Excellent Corrosion Resistance:

Chrome conversion coating can effectively passivate aluminum surfaces, providing a durable and protective layer. Even very thin layers can provide high corrosion resistance, especially in salt-spray environments.

More importantly, traditional hexavalent chromium coatings have a certain self-healing ability. When the coating is scratched, surrounding Cr⁶⁺ ions can migrate and re-passivate the exposed metal.

• Electrical Conductivity:

Since the conversion coating is extremely thin and conductive itself, it can effectively maintain the conductivity of the aluminum substrate.

Alodine chromate conversion coating is suitable for electronic devices and aerospace components that require grounding or electrical shielding.

• Excellent Adhesion:

The conversion chemical film is microporous and moderately active. It can serve as an excellent foundation for paints, adhesives, and powder coatings.

• Low Cost:

As a chemical process performed by immersion, spraying, or brushing, Alodine does not require complex power supply or temperature control systems.

Additionally, the Alodine process is relatively simple and fast, typically taking only a few minutes to complete.

6.2. Disadvantages of Alodine

• Poor Wear Resistance:

Chromate conversion coating is very thin and soft, lack of hardness and abrasion resistance. The chem film is easily to be scratched or worn, unsuitable for direct exposure to high-wear environments.

• Environmental Concerns:

Hexavalent chromium used for Alodine Type 1 is a n extremely pollutant carcinogen. It has been restricted or banned by many countriesand industries.

• Limited Decoration:

Chromate conversion coatings are typically colorless, transparent, pale yellow, or golden. Compared with anodize process, its decorative feature is worse.

7. What are the Top 10 Differences between Anodize and Alodine?

On Process:

Anodizing is an electrochemical process in which the aluminum part is immersed in an acidic electrolyte, serving as the anode. While Alodine is a pure chemical process that does not need a current supply.

On Thickness:

Anodizing coating is usually much thicker than Alodine. The thickness of anodizing film is often 5–25 μm; that of the hard anodized film can even be up to 50–100 μm. While the chromate conversion coating is extremely thin, only 0.5–4 μm.

On Corrosion Resistance:

Both anodized and Alodine coatings are resistant to corrosion.

But the former one performs better on this aspect. The anodizing layer has excellent corrosion resistance, especially after proper sealing. It is widely used in outdoor or high-humidity environments.

The Alodine coating usually serves as temporary protection or as a primer. In salt spray tests, it generally performs worse than anodized aluminum but better than bare aluminum.

On Hardness/Wear Resistance:

Since the anodizing layer is much thicker and chemically harder than the chromate conversion coating, it performs better in terms of abrasion resistance under mechanical conditions.

On Electrical Conductivity:

This is one of the most important distinctions between anodizing and Alodine.

Covered with an anodizing layer, the aluminum part would not be electrically conductive anymore due to the film’s insulation.

Therefore, it is suitable for applications that require electrical insulation.

However, since the chromate conversion film is very thin, the aluminum’s conductivity would not be reduced significantly. Alodine is widely used for aluminum applications needing electrical protection.

On Dimension:

Covered with anodized film, the part’s dimensions would certainly be changed due to the film’s thickness.

However, the dimensional change caused by Alodine chem film can usually be ignored, making it more suitable for precision components with strict tolerances.

On Appearance:

Since the anodized surface is more porous, it can achieve a more colorful appearance by proper dyeing. But it is a difficulty for Alodine  Therefore, anodize is more popularly applied to applications requiring various decoration.

On Environmental Friendliness:

The anodizing process is relatively more environmentally friendly than Alodine.

The anodizing coating primarily utilizes sulfuric acid systems, with treatable waste solutions that contain no heavy metals.

But the Alodine solution is usually much more difficult to treat. Hexavalent chromium, used for traditional chromate conversion coating, is toxic.

On Cost:

The Alodine process has several advantages over the anodizing process in terms of cost.

Firstly, the coating process of anodizing is more complex than that of Alodine, requiring higher equipment standards.

Secondly, the consumption of the anodizing process is much higher than that of the Alodine coating. It needs a continuous current supply and high temperature.

Finally, the processing time of anodized film is much longer than that of Alodine.

On Application:

Both anodizing and Alodine are widely used in various fields.

However, their primary functions differ, enabling them to serve distinct applications.

Anodized film is typically applied to aerospace parts that require exceptional hardness and wear resistance, or architectural parts that require various decorative effects.

Apart from these, it is also widely used for consumer electronic housings that need no electrical conductivity.

Compared with anodized coating, Alodine coating is more suitable for short-term corrosion resistance and low-duty protection due to its limited resistance to corrosion and abrasion. It is usually used as a primer.

Additionally, Alodine coating is preferred when electrical conductivity is required for electronic components.

More importantly, Alodine is widely used in quick aerospace maintenance.

Anodizing vs. Alodine (Chem Film) Comparison Table

8. Summary

Anodizing and Alodine are two of the most widely used aluminum surface treatments, but they serve very different purposes.

Anodizing provides a thick, hard, and highly corrosion-resistant oxide layer with excellent wear resistance and optional decorative colors, making it ideal for parts that require durability, insulation, or a premium appearance.

Alodine (chem film), by contrast, forms a very thin chromate conversion coating that offers good corrosion resistance, preserves electrical conductivity, and works well as a paint base or for tight-tolerance components.

Choosing between the two depends on your priorities:

Anodizing for hardness, wear resistance, aesthetics, and insulation.

Alodine for conductivity, lower cost, minimal dimensional change, and fast processing.

Understanding these differences helps you specify the right finish for your CNC drawings, ensure compliance with standards like MIL-DTL-5541, and achieve optimal performance for your aluminum parts.

9. FAQs

9.1. Can you paint over anodized or Alodine-treated aluminum?

Yes. They can both serve as substrates for painting. But Alodine is more commonly used as a pretreatment before painting. An anodized layer can also be painted if it is unsealed or specially treated.

9.2. Can you anodize over Alodine?

No. It is generally not recommended to anodize over Alodine.

The Alodine coating would significantly hinder the effectiveness of the anodizing process. It might hinder the even distribution of electrical current or lead to impurities in the acid solution for the anodizing process.

9.3. Which is cheaper and faster: anodizing or Alodine?

Alodine is much cheaper and faster than anodizing. Its equipment is simpler and consumption is lower.

9.4. Is Alodine the same as chromate conversion coating?

Yes. Alodine is actually the trademark of chromate conversion coating. Aldine products are used to produce chromate conversion coating.

9.5. Can anodized parts be repaired or touched up like Alodine?

No. It is very easy to repair damaged Alodine coating on site simply by brushing or wiping, so Alodine is widely used in aerospace maintenance.

But anodizing coating cannot be locally repaired. Once damaged, the entire coating should be removed and anodized again.

9.6. What is the downside of anodized aluminum?

The anodized aluminum is not electrically conductive anymore.