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Can Aluminum be Phosphatized?

The answer is no. And you don’t really need it to phosphatize. The purpose of an iron phosphate is to increase the surface profile on steel, so it offers corrosion protection and more surface area for paint to adhere. Let’s look closer at the process.

The Science Behind Phosphatizing

The reason you can’t phosphatize aluminum is that most of Superior’s conversion coatings are iron phosphate products—such as Superclean 353, Superclean 4353 and Superclean 4355. These products are often referred to as “iron phosphates,” but that name is not entirely correct. And this is true for similar products from other manufacturers because there is actually no iron in these products.

So where does the iron and phosphate come from? The iron comes from the steel or ferrous metals to which they are applied. The phosphate portion comes from the products in the form of phosphoric acid.  When the phosphoric acid comes into contact with the ferrous (iron) substrate, an iron phosphate is formed and is an integral part of the substrate. This is called a TRUE conversion coating because the conversion (iron phosphate) includes entities from both the chemical and the substrate in the form of a crystal lattice structure acting similarly to Velcro®.

No Iron, No Phosphatizing

This photo shows the crystal lattice structure visible by a 500x magnification. 

Aluminum contains no iron. So, the above reaction will not occur, and no phosphate is provided.

Other Ways to Improve Paint Adhesion

There are occasions when we recommend pretreating aluminum with Superior products. Typically, if a system runs a majority of steel with the occasional aluminum substrates, it is acceptable and beneficial to do so. The iron phosphate products do contain acid that will slightly etch the aluminum and provide a light surface profile to which paint will adhere. In systems running a majority of aluminum, we would recommend a product such as Superclean 4353 to provide even more etch and enhanced paint adhesion. 

So, while you can’t phosphatize aluminum, you can utilize acidic products to etch it, increase the surface profile and surface area for the paint to adhere.