Conversion Coatings for Metal and Plastic Parts

Jack Lie CNC machining expert

Specialize in CNC milling , CNC turning, 3D printing, urethane casting, rapid tooling, injection molding, metal casting,

No matter who it is, they don’t want to find that their products have uneven and dirty corrosion layer when designing and creating products.
But we all know that metals will rust when espoused to the natural environment. Even though this is a long-standing problem, it is not without a solution. One of the best solutions to avoid corrosion and wear is to coat the part to protect it from the natural factors that cause degradation.
Coating comes in many forms, each of which had its own unique attribute. This blog describes some of the most common conversion coatings.
Different from passive coating (such as paint and powder coat), conversion coating will react chemically on the surface of your metal parts. And it is suitable for almost all metals. In this blog post, we will elaborate on several conversion coatings and their advantages and disadvantages. There are passivation and black oxide commonly used for coating steel and stainless steel; there also are alodine and anodizing commonly used for aluminum parts.

The meaning of conversion coating

Conversion coatings are applied to metal parts by spraying. Conversion coatings are acidic chemical products. They always are used to convert the metal substrate to an iron or zinc phosphate surface before spraying. The chemical reaction modifies the metal surface to enhance paint adhesion and field performance, as well as protect metal parts from corrosion, rust and other wear. A chemical or an electro-chemical process is a necessary process for conversion coating.

How is the conversion coating different from electroplating?

Electroplating and conversion coating seem to be similar. They will build a protective layer on the surface of metal parts through the use of baths and electrolysis. But there is no chemical reaction on the part surface during the electroplating process, but the metal is covered by another metal. Although electroplating coating is quite strong, it wears in a manner similar to paint.

Anodizing & Alodine for Aluminum Part


One of the common forms of aluminum electrolytic passivation is anodizing. After the parts are stripped of any pre-formed corrosion in a sulfuric acid bath, they will be positively charged by the current. Then the parts will directly attract the negatively charged and highly reactive oxygen atoms to its surface. The metal oxide crystal nanopores formed in this process can be filled with dyes before being sealed in another chemical bath, thereby the parts will exhibit multiple colors.
Since a wide range of colors can be obtained by anodizing process, customers generally like to use this kind of coating on their products.

Holes in the parts need to be covered before anodizing since anodizing requires adding a little bit material to the surface of the part to cause a reaction and then the parts will grow outward. The holes will become smaller if without cover during anodizing. Different layer thicknesses caused by different anodizing process.


Alodine, also known as chemical film, is similar to anodizing. But alodine uses very active chromium atoms to drive the purely chemical reaction instead of using electricity in the creation of its oxidative layer. As we well know, the main catalyst of Alodine conversion coating in history is hexavalent chromium compound. However, after this atom was found to be carcinogenic, it was replaced by trivalent chromium atom and was strictly regulated.

The parts are rinsed in a chrome bath and then dried. The coating is sticky at first, but as the chemical reaction progresses, the coating hardens into a protective layer and becomes conductive. As alodine does not produce significant size differences, not only is it often used for screws, bolts, brackets and other fasteners, but it can be used in many other industries also. Compared with anodizing, alodine is cheaper, but it is also easier to wear and scratch.

Can Anodizing and Alodine Be Used Together?

Yes, both coating are quite common to use, especially for the parts whose material properties need to be change across the surface of the part. For example, if the drawing requires specific tolerance on certain edges or holes, you can first apply Alodine finish to protect the entire part.
Then you can use anodize where there is no special tolerance requirement to provide extra durability

Passivation & Black Oxide for Steels/Stainless Steels


Although the term “passivation” is often applied as a general term for conversion coatings, coating steel parts and stainless steel parts are still need to be used some passivation processes. Nowadays, the passivation process is very simple for stainless steel parts because it has high chromium content after optimizing the corrosion resistance of stainless steel. After cleaning the part, the chromium near the surface of the part will react with oxygen and form a protective layer.

Phosphate conversion coating is another common passivation technique for coating steel and stainless steel. It is driven by phosphoric acid and phosphate applied to the mental part and reacts to form stable metal phosphate nanopores.

Black Oxide

Black oxide, a conversion coating similar to the alodine process for aluminum, is often applied to steel and stainless steel, which called browning or bluing in history. In the past few thousand years, various procedures for black oxide have been developed. Ultimately, this is a purely chemical process that produces a conductive iron oxide layer called magnetite.

The reflection of the metal surface can be greatly reduced by forming a black oxide layer, while the dimension of the coating part hardly changes. The lubricity of metal parts will be increased if oil or wax is used to seal the black oxide coating. Therefore, this method is usually used to prevent wear on locks or gears.

Can Passivation and Black Oxide Be Used Together?

Yes! In fact, this is usually done. Black oxide finishes can be applied well with other passivation finishes, which can greatly improve the service life and corrosion resistance of metal products.

Finally, using a conversion coating on the parts can help them last longer and can increase the required properties, such as color, hardness, lubricity or increased conductivity. Thus, when designing and optimizing your next product, be sure to remember these common conversion coatings!