How the Different Aluminum Alloys Affect Chemical Conversion Coatings
Aluminum alloys are widely used throughout manufacturing mainly due to their excellent strength-to-weight ratio and the easy machinability of these metals. Naturally, aluminum reacts with oxygen in the atmosphere to form a thin, protective layer of aluminum oxide, but for harsh environments, especially those found in military and aerospace applications, this natural layer isn’t enough. This is where surface treatments and metal finishes like chem film or chemical conversion coatings come into play–they offer a way to enhance the surface of aluminum, making it far more usable and long-lasting in those harsher environments.
In a previous article, we explored the mechanism behind Chem Film, a chromate chemical conversion coating. Aluminum benefits greatly from this finish–but what some may not realize is that the different aluminum alloys can affect how well the surface treatment performs. Today, we’re taking a deeper dive into the nuances of applying chem film to the common aluminum alloys.
A Brief Overview of Chem Film
Chem film or chromate conversion–both are names for the same process of chemical conversion coating aluminum, forming a protective layer on the surface of aluminum parts to enhance corrosion resistance and adhesion for later coatings. Though there are different types of chem film, this article focuses solely on the more popular Type II TCP (trivalent chromium process) chemical conversion coating. Aluminum parts are cleaned and alkalized or etched, and then the applicator applies the chem film treatment to create that protective layer.
Different Alloys, Different Properties
Aluminum can be alloyed with a variety of different metals, though the more common series are the magnesium (5xxx), magnesium and silicon (6xxx), zinc (7xxx), and nearly pure aluminum (1xxx). Copper is another popular element (2xxx), though not quite as popular compared to newer alloys. And just like each alloying element has different properties, they impart different characteristics onto the resulting aluminum alloy–and affect metal finishing treatments as well.
The more pure the aluminum alloy, no matter what the alloying elements are, the better its resulting corrosion resistance will be after chem film is applied. A recent study also found that certain elements can have positive or negative effects on the corrosion resistance of Type II chem film. Alloys with higher copper content tended to decrease the effectiveness of the chem film treatment, while magnesium and zinc exhibited even better corrosion resistance.
The type of alloy being treated also affects the resulting appearance. Color and color uniformity tend to vary between different alloys, especially in less pure aluminum alloys, where the higher content of alloying elements can cause inconsistencies in the final appearance. This is important to keep in mind, especially in cases where chem film is being used as a primer for colored coatings to be added later.
About Reid Metal Finishing
Reid Metal Finishing is an expert in a wide variety of metal finishing services, including chromate chemical conversion coating, aluminum anodizing, plating, and more. If you’re interested in our services, please contact us today to request a quote and get started.
