Custom Sheet Metal Parts: Adding a Finishing Touch

Finishes for Protection: Chemical Conversion Coatings

When iron and water mix to form the dreaded compound iron oxide (FeO2, for you chemists out there), you get rust. Because iron is present in most sheet metal, given the right conditions your part will slowly dissolve into powder. Not good. But treated with the right chemical coatings, sheet metals can be serviceable for years without showing extensive signs of corrosion.

“In the end, correct finishing specifications are the key component of the plating process,” said Michael Garner, Protolabs Materials Supervisor. “You have to think about the chemistry of the part because coatings don’t necessarily work on all metal surfaces. Choosing carefully will help you avoid delays in manufacture.”

Here are some of the most common types of chemical conversions that can keep your parts intact for years to come.

Zinc

This element is key to the coating that protects galvanized and galvannealed metals. Zinc protects the metal beneath, limiting—and even preventing—rust. In this process, zinc chemically bonds with steel, sacrificing itself when exposed to the elements, especially water, saving the core of the part. Even if the coating is mildly damaged, the presence of zinc will limit corrosive effects on your part. The visual end-result of this chemical treatment is that galvanized metals have a spangled quality of silver tones and grays. Galvannealed metals have a smooth, matte-like finish that can be painted later. One thing to be aware of is that these types of metals cannot be welded because they release hazardous toxins. Instead, we can weld cold rolled steel (CRS) and then have it plated with zinc afterward.

If you need zinc plating applied after a part has been created, that can be done either through an acid or alkaline process. Acid zinc is quickly applied and covers the part well, but it doesn’t always spread uniformly. On the other hand, the alkaline zinc process takes longer because it requires electroplating. An electrical current is run through a zinc bath, causing zinc ions to flow and attach to your core material. The advantage of this process is that metals treated in an alkaline zinc bath have a coating that is more uniform and more accepting of stretching and forming.

Chromate Conversion

Zinc isn’t the only chemical that helps avoid corrosion. Chromate conversion, adds the bonus of electrical conductivity to aluminum, zinc, tin, silver, magnesium, or cadmium parts. The finish ends up clear to yellow in quality. There are different treatments used to coat different metals, but ultimately chromate can act as a protective primer of sorts that allows you to paint a surface. Note that steel and iron-based metals cannot be chromated directly—you would first need to pretreat with zinc.

Anodizing

This method works by placing a sheet of aluminum in an acid bath filled with electrolytes. When you run electricity through the tank, the surface of the aluminum will hook up with oxygen atoms in the electrolyte to form aluminum oxide ( Al2O3). This would happen normally in the real world, but doing it in a controlled environment quickly yields an attractive, smooth, corrosion-resistant surface. You may like the finish just like that, but because the surface is porous, you can also add color and an additional sealant. One thing to remember is that while anodizing works well with aluminum, it does not with CRS.

Passivation

Another way to protect the surface integrity of your part is to passivate the surface. One of the most interesting things about this process is that no one really knows how it works. What is known for certain is that passivation adds an extremely thin layer that protects the surface of stainless steel from corrosion. Doing the process wrong, though, can create more opportunities for corrosion than not doing it at all, so cleaning the metal well before treatment is critical to successful passivation.