Time to read: 7 min
The Second Law of Thermodynamics declares that everything tends to move into a state of disorder. We all know what happens when metal is exposed to the elements: deterioration. When designing and creating your products, the last thing you want to discover is a layer of deteriorated metal in the form of oxidation, corrosion, or rust.
Although deterioration is an age old problem, there are ways to circumvent it. One of the best solutions is to apply a protective coating to your part, so it’s unaffected by the natural elements that cause degradation. These coatings take many forms, and each has unique properties. This article covers some of the most common kinds of conversion coatings. Conversion coatings, unlike passive coatings like paint or a powder coat, involve a chemical reaction at the surface of your metal part. Although this process can be applied to almost all metals, this article outlines the pros and cons of a few broad categories of conversion coating. Passivation and black oxide are commonly used for coating steel and stainless steel, while alodine and anodizing are commonly used on aluminum parts. Each coating is a protective surface modification treatment that resists oxidation, corrosion, or rust.
What is Conversion Coating?
Chemical conversion coatings, or conversion coatings, are a type of surface passivation used to modify the surface of a metal. A conversion coating produces a metal oxide layer on the surface of a metal part that protects it from corrosion, rust, and other wear, while providing useful material properties. Conversion coatings may also be used to improve coating adhesion.
Because most metals are naturally reactive to oxygen, metal oxides form at the surface of metal parts even without a coating. A certain amount of metal oxide on some parts acts as a naturally occurring protective layer when interacting with the right element. For example, a naturally passive layer forms on a stainless steel surface with chromium.
Of course this process also creates oxides that degrade the metal — like the corrosion of ferrous metals known as rust, where iron and oxygen react. We are all familiar with the classic red rust that tarnishes old steel fences and steel tractors left out in the field.
However, if these metal oxides are created in a controlled environment, the surface reactions can be harnessed to protect metal parts. Conversion coatings use special techniques and processes containing acidic baths and/or electricity to help form a metal oxide surface coating that shields the underlying metal part from external oxygen and other corrosive elements.
Examples of conversion coatings include chromate conversion coating (CCC), phosphate conversion coating (PCC), and black oxide coating. During the conversion process, part of the metal surface is converted into a protective surface layer, as seen in chromate conversion coating, phosphate conversion coating, and black oxide coating. A chromate conversion coating on an aluminum substrate is a chemical film
Also, nanopores within the oxide’s crystal layer can often be filled with dye or other sealants in order to achieve enhanced properties like color, lubricity, and increased hardness. Most importantly, the metal oxide layer is chemically bonded to the metal that it protects. This makes conversion coatings extremely durable and difficult to penetrate.
How is the conversion coating different from plating?
Plating and conversion coatings may seem similar at first glance. Electroplating is an electrochemical process whereby a metal is deposited on the surface of the part. This thin, protective coating that may also alter the appearance, physical properties, wear resistance, biocompatibility or electrical conductivity of the part. Electroless plating is a similar process where the metal is deposited without using electricity (electroless nickel for example).
Benefits of conversion coating
Both types of plating have excellent adhesion and corrosion resistance due to the electrochemical bond formation. However, the most significant difference between plating and conversion coating is that plating forms a new layer of metal on the surface, while conversion coating alters the existing surface metallic layer. Conversion coating is often a preparation coating for subsequent plating or painting whereas electroplating is typically the final finish.
Finishes for Aluminum
Anodizing
Anodizing is a common form of electrolytic passivation for aluminum (and steel also, but we will focus on aluminum here). First, parts are typically cleaned and/or desmutted to remove any scale or heavy contaminants. Next, a sulfuric or other acid solution bath is used to strip (etch) away any naturally occurring oxidation layer. Then parts are connected to a power supply to give them a positive charge and immersed in the anodizing bath, which involves immersion of a component in an acid solution and passing electricity, specifically electrical current, through the component, with the component being the anode in the electrical circuit. While in the bath, parts attract negatively charged and reactive oxygen atoms directly to their surface.
The metal oxide crystal nanopores formed in this process can be filled with dye revealing a wide range of colors before being sealed in another chemical bath. Because of the brilliant spectrum of color available through the anodizing process, it is used often for consumer products.
Anodizing adds a insignificant amount of material to the surface of the part to cause a reaction, so the parts grow outward — the standard thickness of an anodized coating is up to a few thousandths of an inch. So, if your part requires tight tolerances, you should take this change into account for your designs, and use masking or hole plugs on those areas prior to anodizing.
It’s also important to remember that different anodizing processes (Type 1/1B, Type II, Type III/III w/PTFE) result in different layer thicknesses. These different types of anodizing allow this coating to be useful in various industries and applications, from trusses on the International Space Station to artistic sculptures and sporting goods.
Alodine
Alodine, known as chem film or Iridite, is similar to anodizing because it produces a modified, corrosion-resistant oxide layer on metal parts, but does not use electricity in the creation of its oxide layer. Alodine is the trademarked name for a chemical conversion coating produced by Henkel (the product line is also known as Bonderite). I like to think of alodining as kind of the electroless version of anodizing. Alodine is a proper corrosion resistant aluminum surface treatment for painting, powder coating, adhesive bonding, heat sinks, and automotive wheels.
Instead of an electrochemical reaction, alodine is produced using a reactive chromium atom to drive a purely chemical reaction. Historically, a hexavalent chromium compound served as the main catalyst in alodine conversion coatings. However, given this atom’s carcinogenic nature, hexavalent chromium has become tightly regulated and now trivalent chromium, including type 2, can be used. This type of chromating, known as hex-free chromating, is typically accepted as the standard across many industries.
To get an alodine finish, parts are first cleaned, degreased, and rinsed. Then parts are placed in the chromium bath, removed, rinsed, and dried. Alodine is often applied to screws, bolts, brackets and other fasteners because it doesn’t produce a noticeable dimensional change. Alodine is cheaper than anodizing, but more susceptible to wear and scratches.
Can I Use Both Anodizing and Alodine?
While it likely wouldn’t hurt your parts to apply both an anodize and alodine coating, it probably won’t help either. Both coatings result in a modified surface of the metal and once you apply one, the other won’t have any impact; you won’t get increased paint adhesion or corrosion resistance.
Some CNC machined aluminum parts require only specific sections of their surface to be painted, while other areas are plated or coated with a different process. For such parts, you can opt to anodize certain areas and alodine others.
Finishes for Steel/Stainless Steel
Passivation
Passivation removes free iron from the surface of a stainless steel part to form a much more corrosion resistant version of the material’s naturally occurring oxide layer. Although passivation is often used as a general term for conversion coating, there are a few passivation processes that are commonly used for coating steel and stainless steel parts.
Stainless steel is and alloy optimized for anti-corrosion, and its high chromium content makes passivation simple. Once parts are cleaned, the alloyed chromium near the part surface reacts with oxygen, and a protective barrier is formed.
Another common passivation technique for steels and stainless steels is phosphate conversion. This coating process is driven by phosphoric acid and phosphate salts, which combine to react and create stable, metalophosphate nanopores ready for a manganese or zinc seal (also called parkerization).
Black Oxide
Black oxide, historically called browning or bluing, is a conversion coating most often used for steel and stainless steel that is similar to the alodine process. A variety of procedures have been developed for applying black oxide finish. Ultimately, it is a purely chemical process which creates a conductive iron oxide layer called magnetite that provides corrosion resistance for the base metal.
The black oxide finish greatly decreases reflection from the metal’s surface and barely changes the dimensions of the coated part. When a black oxide finish is sealed with oil or wax, lubricity increases, so it’s used on parts to prevent galling in gears and locks. Black oxide coatings can be applied to both copper and steel.
Ultimately, applying a conversion coating to your parts can help them last longer and can add desired properties like color, hardness, lubricity, or increased corrosion resistance. So, keep in mind these common conversion coatings when designing and optimizing your next product!
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