Pretreatment of Aluminum Extrusions: What You Need to Know
These finishes can give you a range of bright, vibrant colors, and they’ll make your project more visually appealing.
The last thing you want, though, is for that beautiful finish to not adhere properly. And you need to be aware that as soon as your profiles come off the extrusion press, they’re exposed to oxygen.
That oxygen causes some initial oxidation on their surface, and this greyish colored smut needs to be removed. If it isn’t, your paint or powder coat is going to be applied on top of it, and that will cause your coating to come off prematurely.
Enter pretreatment to save the day.
“Pretreatment is an important step before powder coating or painting.
A good manufacturer will take the steps necessary to properly pretreat your profiles. That way, they can ensure paint or powder coat adhesion.”
General Manager, Gabrian International
What in the World is Pretreatment?
Pretreatment is a process that your manufacturer goes through to remove all grease, oils and dirt from the aluminum surface. They need to do this before painting or powder coating to strengthen adhesion.
This process has multiple steps, and each one of them is important. So, doing a thorough job is what will ensure pretreatment success.
How many steps are there? Well, that depends on the type of pretreatment you choose. The process also varies between manufacturers. We’ll get to that in a minute.
The 2 Types of Pretreatment Coatings
There are two main types of pretreatment coatings. They are:
- Chrome Conversion Coatings, and
- Chrome-Free Coatings.
Chrome conversion coatings have been around for many decades. And chrome-free coatings are more recent. The difference between them is pretty important. So, let’s take a look at each one.
Chrome Conversion Coatings
This type of coating chemically converts the surface of the aluminum to an inert form. It also contains hexavalent chromium (uh-oh!).
According to OSHA, it’s known to cause cancer. And it also targets the respiratory system, kidneys, liver, skin and eyes. Meanwhile, the EPA is concerned about discharged chromium waste getting into your drinking water.
That being said, responsible manufacturers can take steps to limit the hazards to employees, other people and the environment.
These coatings were developed in response to the hazards associated with traditional coatings.
It took some time to develop coatings that would have comparable effectiveness to chrome conversion. But, by the early 2000’s, they came more into prevalence. It was then that the chrome-free coatings were shown to meet industry standards for effectiveness.
Chrome-free coatings do not contain hexavalent chromium. And, these days, they can offer similar performance to chrome conversion coatings.
How Does the Pretreatment Process Work?
The pretreatment process will be different depending on whether the manufacturer is doing chrome conversion coating or chrome-free coating.
Different manufacturers will handle each of these in different ways, but pretreatment usually has between 5-10 steps. There are certain basic steps that are always there, though.
Essentially, your profiles will be dipped into a series of tanks. Inside each tank is a different solution. You will generally find tanks of cleaner, etchant, rinse, and the appropriate pretreatment coating.
Here’s an example of the basic process for each coating type. Again, additional steps may be added.
Chrome Conversion Coating
- Chrome Conversion Coating
- Acidulated Rinse
- Metal Cleaner/Deoxidizer
- Chrome-Free Coating
Once the profiles have gone through their succession of tanks, they need to be dried. It is very important that they be fully dried. That’s because any moisture on the surface will negatively impact paint or powder coat application.
Pretreatment is Super Important
Pretreating is an important step before painting or powder coating. It helps to get rid of surface contaminants that would cause your paint or powder coat to not adhere properly.
Traditionally, chrome conversion coatings have been the standard. But, in recent years chrome-free coatings have become prevalent. This is due to worker safety and environmental concerns.
The steps vary between processes, manufacturers and project requirements. But, they involve dipping your extrusions into a series of tanks.
So, keep in mind the importance of pretreatment and remember the difference between pretreatment types when talking to manufacturers. Good luck with your aluminum extrusion project!
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