What Is Alodine, Chem Film, Or Chromate Conversion Coating?

In our shop, we’ve put a chromate coating on too many parts to count, and shipped them to companies across industries and applications. This is one of the most common coatings you can apply to a part, but we realized that a lot of engineers and designers don’t know the difference between Alodine, Chem Film, and Chromate Conversion Coatings.

Our machining experts are here to clear things up and make everything as simple as possible. We’ll take a deep dive into the concept of chromate coating, explain the differences, explain how the coating works, get into the process, and answer some common questions.

Spoiler alert: they’re all essentially the same thing.

What Is Chromate Conversion Coating?

Chromate Conversion Coating (CCC) is a surface treatment for aluminum and its alloys. CCC is the generic name for the process, and it involves immersing the parts in a chromic acid solution, which reacts with the aluminum to form a protective layer. Chromate coatings are typically used to enhance corrosion resistance and improve the adhesion of subsequent coatings or paints.

What Is Chem Film?

Chem Film, also known as Chemical Film, is the industry slang or shorthand for Chromate Conversion Coating. In our experience, it typically refers to aluminum coatings specifically. You’ll hear this term a lot in military projects, aerospace companies, and machine shops.

What Is Alodine?

Alodine is a brand name for a type of chromate conversion coating which comes from Henkel Surface Technology. It’s the same thing as calling POM-C “Delrin” or calling polycarbonate “Lexan”. It doesn’t mean the coating specifically came from Henkel, since there’s no way to tell from just visually inspecting a part — it’s just another way to say Chromate Conversion Coating.

Key Differences Between Alodine, Chem Film, and Chromate Conversion Coatings

In reality, there is no difference between Alodine, Chem Film, and Chromate Conversion Coatings at all. If you want to get nit-picky, Alodine is a brand name, Chem Film is a slang term, and Chromate Conversion Coating is the generic name for the process.

Technically, the correct way to refer to the coating is to call it a “Chromate Conversion Coating”, but you won’t confuse our team if you use any of these three terms.

What Does a Chromate Conversion Coating Do?

First off, you should know that chromate conversion coating is a general mechanical process. This form of technology adds a coating to everyday metals like:

• Aluminum
• Tin
• Zinc
• Cadmium
• Steel
• Copper
• Titanium
• Silver
• Magnesium

In our experience, aluminum is the most common material that gets a Chem Film coating on it. This film has a few different functions.

For starters, it improves the corrosion resistance of the part. The coating protects the base metal by creating a protective oxide layer over the part. It prevents moisture from reaching the raw material and prevents pitting which can commonly happen on aluminum over time.

Other times, you’ll use a Chem Film before painting or powder coating. We do this because the chromate coating creates a microscopically roughened surface that is great for paints, primers, and powder coatings to adhere to. Without this first step, your powder coat might start to chip and fall off too soon.

In addition, chromate coatings will maintain electrical conductivity, so it will work on grounding surfaces as well. If you need EMI or RFI shielding, this coating won’t get in the way.

One of the best parts of Alodine coatings is that they have a minimal dimensional impact. The coating process that we use generates a coating so thin that it won’t significantly alter the part’s dimensions. In other words, we can still deliver precision machined parts within your tolerance specifications, even with a Chromate Conversion Coating applied to the part.

How to Tell if Something Has a Chromate Conversion Coating

The coating itself has a pretty distinct coloring. It has an iridescent green or yellow color when applied. It’s really easy to spot a chromate-coated bolt in a box of non-coated bolts.

Unless you have specialty equipment, the best bet is to just look at the part. The part might have been painted or powder coated a similar yellowish color, but chromate coatings are typically shinier than the matte finish you might see in different paint jobs.

Types of Chem Film

In general, a Chem Film can be broken into one of two categories: MIL-DTL-5541 Type 1 or Type 2. While the two work similarly, they have some pretty major differences in their chemistry. As you’ll see, Type 1 can be incredibly dangerous to use, so it’s important to understand the difference between the two.

MIL-DTL-5541 Type 1

Type 1 Chem Films have been around for a while. Since the early 2000s, any Chem Film you found was more than likely MIL-DTL-5541 Type 1. This coating uses hexavalent chromium and is exceptionally corrosion-resistant and long-lasting.

The major downside to Type 1 is that there were some environmental and health concerns — namely, OSHA and the EPA started cracking down on it since it was found to use a carcinogenic chemical.

For that reason alone, you need to be really careful if you’re calling for MIL-DTL-5541 Type 1 on your parts. You’ll likely need to reach out to your county or state to make sure you have the right permits to use this type of Chem Film.

MIL-DTL-5541 Type 2

Once it was found out that MIL-DTL-5541 Type 1 was so dangerous, most fabricators swapped to Type 2. Type 2 is a “hex-free” coating that uses trivalent chromium instead. This change significantly reduces the environmental and health risks associated with this coating process.

Over time, Type 2 has continually improved and become a more desirable option, and it’s the Chem Film coating that we focus on at Rapid Axis. In our experience, Type 2 is faster, has fewer steps, and can be done at a lower temperature than Type 1 — even if you ignore the health and environmental impact, Type 2 is still the better option in our opinion.

How Is the Coating Applied?

Since this is an all-purpose coating, it shouldn’t surprise you that the application can be performed in a few different styles. The coating can be applied to the base material by:

• Brushing
• Dipping/ immersion
• Spraying

The Chromate Conversion Coating Process

The process is pretty straightforward. Though there are some variations from one manufacturing facility to the next, they basically follow the same process. In our shop, the Chromate Conversion Coating process looks like this:

Step #1: Clean

The part is first degreased. This gets rid of small impurities on the part. It also removes oxides, grease, heavy metal contaminants, and oil from the part. This helps the chromate conversion stick and creates a more even coating.

With all of the contaminants removed, the part gets rinsed and dried.

Step #2: Etch

If you have sections of your part that you don’t want to coat, the piece will need to be etched. This process covers the areas in question and ensures the part gets correctly coated. It works just like masking pieces of a part you don’t want to paint.

With the part etched, it will go through another round of rinsing and drying.

Step #3: Deoxidize

Another contaminant that can ruin the chem film is oxygen. By putting the part through a deoxidization process, the oxygen, oxides, and chemicals will get removed. It’s really important to correctly prepare your part so the coating is perfect.

Again, the part will be rinsed and dried to remove the deoxidizing agent. At this point, it’s ready for the processing step.

Step #4: Apply Chem Film Coat

Typically, the part will be fully immersed in this step. The chromate conversion coating is applied, and the part will soak in the mixture for a predetermined time. This time will change from part to part and varies between methods.

Step #5: Final Wash

After the chem film is applied, the part will go through a series of final washes. The first is a simple rinse to remove any excess coating. The second round is a warm rinse to get a perfect surface finish.

How Long Does an Alodine Coat Take?

Despite the process being so specific and taking 5 steps and multiple rinses, it doesn’t take that long. Depending on the size, application, and specs, an Alodine coat will take between 10 to 30 minutes altogether.

In addition, you can apply an Alodine coat to multiple different parts at the same time, so you won’t multiply the time it takes to add the coating to a batch of parts.

How Long Does an Alodine Coating Last for?

Once applied, an Alodine coating can last for a while. The total duration depends heavily on the environmental conditions and part usage.

For parts exposed to salt and moisture, you’ll need to maintain the coating to have it last over a year, or just reapply it every year or so.

Industrial settings within mildly corrosive environments can last between 1 and 5 years pretty easily, in our experience.

If the part is used indoors with low humidity and no chemical interactions, then the coating can last over 20 years with no problem.

Industries that Use Chromate Conversion Coated Parts

Since this technology is so universal and versatile, you’ll find a number of industries that use it:

• Automotive. Automotive applications commonly use lightweight aluminum, and the added Alodine coating can protect parts from road salt and thermal cycling. You might find heat shields and valve bodies with this type of coating on them.
• Military and Defense. Hardware needs to survive extreme environments, like the heat of a desert, salty air near the sea, or incredible colds of the Arctic. This is why defense companies use CCC for housings, mounts, and exterior panels.
• Aerospace. Chem Film is used in aerospace applications to prevent corrosion without altering the part’s tight dimensions. You can find these coatings on fuel systems, enclosures, and structural brackets or ribs.
• Electronics. Electronics companies will use Chem Film coatings on their sensitive electronics to maintain conductivity while shielding from electromagnetic interference (EMI). Things like server and network switch enclosures as well as backplanes are commonly coated.
• Marine. To prevent corrosion from sea exposure, maritime vessels might use Chromate Conversion Coating on their radar system housings and mast fittings.
• Medical Devices. Medical devices often use a coating to meet reliability and cleanliness standards. Since CCC creates a non-toxic and uniform surface, you can use this coating for diagnostic machines, mounting plates for scanning systems, and across laboratory equipment.
• Other Industries like Industrial Equipment, Energy, Oil, and Space Systems. Generally speaking, you can find an example of a Chem Film-coated part across every major industry and across multiple different applications. This coating is universal and can be applied to custom parts.

You might even stumble across a box of fasteners that have been coated with chromate conversion.

Conclusion

Hopefully, this cleared up the questions that you had about Alodine, Chem Film, and Chromate Conversion Coatings. They are all the same process, but Alodine is a trademarked version by a massive coating company. If you want to learn more about this process or you have other manufacturing inquiries, reach out to our pros at Rapid Axis . You can reach out at any time to get a free quote and we’ll manufacture the parts that your business needs.