Finishing Friday: Conversion Coating

Finishing Friday: Conversion Coating

Need to give your stamped, formed, or machined metal parts a little defense against Mother Nature? Or maybe you just want to make them more attractive or colorful? Either way, there are a number of great options. Paint and powder coating are two of the more common of these, as are nickel, chrome, and zinc plating. All provide years of corrosion resistance as well as an aesthetic yet durable surface coating.

Depending on the material and application, however, there’s an alternative. It’s known as conversion coating. You can think of it as controlled rust. And while it can and often does stand alone, it is typically applied either before or after the plating, painting, and powder coating processes just mentioned, increasing their toughness, adherence, corrosion resistance, and/or electrical conductivity.

Conversion coating works by dunking metal parts into a chemical-filled tank. This might be a type of acid—citric and nitric acid are common—although caustic soda (lye) or electrolyte are also used. Electrical current is sometimes applied, in which case it’s referred to as electrochemical conversion coating. In either case, the end result is an extremely thin protective layer on the workpiece surface.

The same process occurs naturally with stainless steel (hence the name). The relatively high amounts of chromium in stainless steel alloys—between 10 to 30 percent—react with the oxygen in air to form chromium oxide, thus protecting the surface beneath. Aluminum is similarly corrosion resistant—due to its affinity for oxygen, it generates a thin layer of aluminum oxide, the stuff of sandpaper and ceramic. Compare each of these with cast iron and carbon steels, which gradually form that reddish, flaky nuisance known as iron oxide (Fe2O3), or rust. It never sleeps.

Conversion coating mimics these processes. It chemically or electrochemically “converts” the surface of ferrous and non-ferrous metals alike, leaving behind a hard, corrosion-resistant, and sometimes lubricious surface. Numerous types exist, among them:

• Black oxide conversion coating forms a thin layer of magnetite (Fe3O4), a black oxide of iron, on the surface of steel, copper, and brass.
• Phosphate conversion coating creates protective, non-metallic crystalline structures made of manganese phosphate on the surface of steel, iron, and zinc parts.
• Passivation is one of the most common types of conversion coating. It is used to remove the free iron from stainless steel surfaces that, if left untreated, could otherwise lead to rust.
• Chromate conversion coating (also known as Alodine or yellow Iridite) is typically applied to aluminum and magnesium parts, although it’s also used for added protection on zinc-plated components.
• As with chromate conversion coating, anodizing is primarily associated with aluminum parts, but can also be applied to titanium, magnesium, and a few other less common metals. Multiple types are available, each with its own distinct benefits.

As mentioned, conversion coating is robust enough to stand on its own, at least for some applications. However, it’s more likely to be used as a primer for paint and other coatings, or as a secondary finish over previously plated surfaces, enhancing their existing properties. In future installments of Finishing Friday, we’ll dig into the details of the various conversion coatings, among them the bluing and blackening of steels, chromate and “chem-film,” and the big Kahuna of all conversion coatings, anodizing. Stay tuned.