Chemical Conversion Coating

Chemical Conversion Coating: Chromate, Phosphate, And Black Oxide

“Conversion coating” is the process by which an insoluble compound coating is produced on the metal surface by reacting the outer atoms of the metal with the anion of the selected medium. This is a chemical coating-forming treatment method, and the covering coating produced by chemical treatment is usually called “conversion coating” or “chemical conversion coating”.

As an anti-corrosion and wear-resistant surface coating, chemical conversion coating can be widely used in machinery, electronics, aviation, and other industrial sectors. Additionally, chemical conversion coating also plays a decorative role through its color and is often used in building materials and daily necessities decoration.

What is Chemical Conversion Coating?

Chemical conversion coating is a chemical or electrochemical treatment applied to metals. It superficially changes the metals into a thin adherent coating of an insoluble compound. These coatings are frequently used to protect metals from corrosion, increase the adhesion of other coatings, provide lubrication, or for aesthetic purposes.

Chemical Conversion Coating Tank

Chemical conversion coatings can be applied to various metal surfaces, including aluminum, steel, and brass. They are often used in aerospace, automotive, and construction industries, where corrosion resistance is important. In addition to corrosion resistance, chemical conversion coatings can also improve the appearance of the metal surface and make it easier to clean and maintain.

The Importance of Chemical Conversion Coating

One of the leading causes of damage and failure of precision machined parts is corrosion and surface damage. The above problems can be avoided by using the chemical conversion coating process. Let us now understand the importance of chemical conversion coatings.

1. Anti-rust and Corrosion-resistant. Chemical conversion coatings can improve the rust and corrosion resistance of metal surfaces to a certain extent and are usually used in conjunction with other protective layers.

2. Paint the Substrate. Its function mainly has two aspects: as an intermediate layer, it improves the adhesion between the coating and the substrate; it prevents the corrosive medium from penetrating through the partially damaged part of the coating to erode the substrate metal.

3. Wear-resistant. Primarily a phosphate coating reduces wear due to its low coefficient of friction and good oil absorption cushioning.

4. For Cold Working Processes. Phosphate coating is formed on workpieces such as steel pipes and steel wires before cold extrusion, deep drawing, etc., which can reduce the drawing force during processing, prolong the life of the drawing die and reduce the number of drawing times.

5. For Certain Special Applications. Some chemical conversion coatings have functions such as electrical insulation, light absorption or reflection, and heat insulation. For example, the phosphating coating is used as the silicon steel sheet insulating layer.

6. Surface Decoration. Relying on the beautiful appearance or good coloring performance of the chemical conversion coating, it is widely used in construction, machinery, instrumentation, and arts and crafts.

Chromate Conversion Coating Parts

 

The Common Types of Chemical Conversion Coating

Chemical conversion coatings can play a dual role in lubrication and friction reduction in cold metal working, which benefits the processing under high load. There are several chemical conversion coatings, including chromate conversion coating, phosphate conversion coating, and black oxide. 

Chromate Conversion Coating

Chromate conversion coating, also known as alodine coating, involves applying a solution of chromic acid or chromate salt to the metal surface and then rinsing and drying it. The acid or salt reacts with the metal to form a thin, protective layer on the surface. Chromate conversion coating can passivate steel, aluminum, zinc, cadmium, copper, silver, titanium, magnesium, and tin alloys.

Chromate conversion coatings serve many purposes. It functions as a corrosion inhibitor, a primer to promote the adhesion of paints and adhesives or a decorative finish. In addition, it offers modest protection against abrasion and minor chemical assault on soft metals.

Typical applications of chromate conversion coatings include screws, hardware, and tools. They frequently provide a particularly iridescent, greenish-yellow color to white or gray metals.

Features of Chromate Conversion Coatings:

  • Corrosion protection.
  • Offers good electrical properties.
  • Good base for paint applications.
  • Easily applied.

Applications of Chromate Conversion Coatings:

  • On aluminum or mixed metal assemblies that can’t be anodized.
  • Paint or other organic coating adhesion.
  • Cabinetry of electronic power supply.
  • Aluminum immersion protective coating at a low cost.
  • Applied by brush in the field as a touch-up for damaged anodize or paint.

Types of Chromate Conversion Coatings

The various types of chromate conversion coating vary in chemical composition and shape. Type 1 and Type 2 of the MIL-DTL-5541 standard are the most common. 

MIL-DTL-5541 Type 1

Type 1 comprises a chemical compound called hexavalent chromium. The compound forms a gold, brown, or clear coating. However, hexavalent chromium is a dangerous substance known to cause cancer. Consequently, its use is extremely hazardous to workers. Some businesses allow its usage, but only with suitable ventilation and elimination method.

MIL-DTL-5541 Type 2

Type 2 is a safer option than type 1. It contains trivalent chromium, zirconium, or titanium. As a result, it is a hex-free chrome. This type of coating is often colorless. Thus, it is described as clear.

Some of the advantages of type 2 over type 1 are:

  • Type 2 occurs at lower temperatures than type 1.
  • Type 2 has a profile that is safer and non-harmful, making it more of a favorite in the precision machining parts manufacturing industry.
  • The application process for type 2 is easier, faster, and more straightforward.

Phosphate Conversion Coating

Phosphate Conversion Coating parts

Phosphate conversion coating is also called phosphate coating, phosphatization, phosphatizing, or phosphating. It is a chemical treatment applied to steel parts that form a thin adherent layer of iron, zinc, or manganese phosphates. Phosphate conversion coating can offer corrosion resistance and lubrication or be a base for later coatings or painting. Phosphate conversion coatings can also be used on aluminum, zinc, cadmium, silver, and tin.

Features of Phosphate Conversion Coatings:

  • Excellent paint and oil base.
  • Protection against corrosion (when oiled).
  • Protects base steel for applications of topcoats.
  • Holds lubricants in place for a smooth break-in.

Applications of Phosphate Conversion Coatings:

  • Paint or dry lube adhesion on steel parts.
  • Lubrication while drawing or shaping.
  • Keep the oil on parts like gun triggers and barrels.
  • Gears and camshafts during break-in periods.

Types of Phosphate Conversion Coatings 

The main types of phosphate coatings are manganese, iron, and zinc.

Manganese Phosphate Coatings

Manganese phosphate coatings are exclusively applied by immersion and are utilized for corrosion resistance and lubricity.

Iron Phosphate Coatings 

Iron phosphate coatings are often applied by immersion or spraying and are typically used as a base for subsequent coatings or painting.

Zinc Phosphate Coatings 

Zinc phosphate coatings are applied by immersion or spraying. They can be used for corrosion resistance, as a lubricant-holding layer, and as a paint/coating base. Additionally, zinc phosphate coatings can also be applied on galvanized steel.

Black Oxide

Black oxide, often known as blackening, is a conversion coating used on ferrous metals, stainless steel, copper and copper-based alloys, zinc, powdered metals, and silver solder. It can improve aesthetics, offer modest corrosion resistance, and reduce light reflection. Black oxide must be treated with oil or wax for maximum corrosion resistance.

Black Oxide Coated Aluminum Parts
Black Oxide Coated Aluminum Parts

Black oxide also has a low coefficient of friction, which can be useful in applications involving sliding or movement. Additionally, black oxide has a pleasing, decorative appearance often used in applications where aesthetics are important, such as in the automotive and firearms industries.

Features of Black Oxide:

  • Cost Competitive.
  • Resistance to corrosion (when oiled/waxed)
  • Improve appearance.
  • Dimensional stability.

Applications of Black Oxide:

  • Precision bearings.
  • Tooling.
  • Model railroad track.
  • Firearm components.

Ways of Black Oxide 

Blackening can be achieved in many different ways. There are three main ways to do this, hot black oxide, mid-temperature black oxide, and cold black oxide.

Hot Black Oxide

Hot black oxide is the process of treating the surface of the metal with chemicals and heat to create a layer of oxide on the surface, typically in the form of magnetite. The process finished by dipping the metal in different tanks and occasionally placed in water.

After the dipping process is completed, the surface of the metal is typically porous, and a layer of oil is applied to help protect the metal from rust and corrosion. The oil seeps into the pores of the metal, creating an additional layer of protection against corrosion. The black oxide process is usually completed by various industry standards, such as MIL-DTL-13924, AMS 2485, ASTM D769, and ISO 11408, to ensure that the finished product meets certain quality and performance criteria.

Mid-Temperature Black Oxide

Like the hot black oxide treatment process,  mid-temperature black oxide changes the surface of the metal to magnetite. But it does not produce caustic fumes. This method can also meet the military specifications MIL-DTL-13924 and AMS 2485.

Cold Black Oxide

The cold black oxide applies a compound (copper selenium) onto the metal at room temperature. The process is generally less hazardous and more convenient than hot black oxide, as it does not require using chemicals at high temperatures.

This coating generates a color comparable to the oxide conversion but is more prone to rubbing off and provides less abrasion resistance. Applying oil, wax, or lacquer raises the corrosion resistance to a comparable level to hot and mid-temperature black oxide.

Factors Affecting the Protective Effect of Chemical Conversion Coatings

Alodine finish Parts

Chemical conversion coatings are used to improve the corrosion resistance, appearance of the metal, and adherence to other coatings. Several factors can affect the protective effect of chemical conversion coatings, including:

Substrate Material: The type of metal substrate can affect the effectiveness of the coating. Some metals are more prone to corrosion than others, and the coating may not be as effective on these metals.

Coating Thickness: The thickness of the coating can also affect its protective effect. In general, thicker coatings provide better protection than thin coatings. However, the excessive thickness can reduce the coating’s flexibility and may cause it to crack or peel.

Environmental Factors: The environment in which the coated metal is used can significantly impact the coating’s performance. For example, coatings may be less effective in harsh or corrosive environments, such as high humidity or salt spray.

Surface Preparation: Proper surface preparation is essential for effectively applying chemical conversion coatings. If the surface is not properly cleaned and pretreated, the coating may not adhere well and provide adequate protection.

Coating Composition: The specific formulation of the coating can also affect its protective effect. Different coatings may contain different types and concentrations of active ingredients, which can impact their corrosion resistance and durability.

Chromate Conversion Coating vs. Anodizing: What is the Difference?

Aluminum Anodizing parts

Anodizing and chromate conversion coating are two of the most prevalent aluminum finishes. In CNC machining, they both aim to make the metal more resistant to corrosion and rust. However, there are still many differences between these two processes. Let’s take a look.

Work Principle

Anodizing and chromate conversion coating are used in aluminum finishing to improve corrosion resistance. The main distinction between chromate conversion coating and anodizing is that anodizing is an electrolytic process, whereas chromate conversion coating is not.

Anodizing is an electrolytic finishing process that passes an electric current through a metal submerged in an electrolyte solution. Contrarily, chromate conversion coating does not apply an electrical current to metal. Usually, at room temperature, the metal is submerged in a chemical bath.

Size of Metal Substrate

Effects of chromate conversion coating and anodizing on the ultimate size of the metal substrate vary. The chromate conversion coating finish has little to insignificant influence on the final dimensions, as the coating is extremely thin. Comparatively, anodizing layers are thicker than chromate conversion coating films, and the effect on the final dimensions can be tens of microns.

Function

In addition to enhancing corrosion resistance, aluminum anodizing improves lubrication and permits dyeing (coloring). A chromate conversion coating, unlike anodizing, can allow aluminum to retain its electrical or thermal conductivity. As a result, chromate conversion coating is the preferable option when aluminum parts must be electrically grounded.

Cost

Chromate conversion coating finish is often less expensive than anodizing finish. However, the cost will differ from the CNC machining part. In addition, the types of anodizing and chromate conversion coating can further affect the price.

Black Oxide vs. Black Zinc: What is the Difference?

Black Zinc parts

Black oxide is an extremely thin (millionths of an inch) oxidation product formed on the surface of a steel part. It has almost no corrosion resistance except its wax or oil but is attractive and has no effect on part dimensions. Thus, the black oxide is used on things like sprockets, gears, and couplings, where dimensions are critical, and on things that need to be carefully maintained and regularly oiled, like rifles and shotguns.

Black zinc is zinc plating (0.0002″ or thicker) followed by a black chromate conversion coating. Zinc plating offers the sacrificial corrosion protection of zinc and a good aesthetic, but it affects the dimensions sufficiently to be a concern for particularly tightly fitting parts.

Conclusion

Conversion coatings are produced by the electrochemical or chemical reaction of metals, wherein the selected treatment changes the material superficially into a thin adherent coating of an insoluble compound. It can provide many benefits, including surface corrosion protection even at low electrical resistances, improved paint adherence, and ornamental applications.

According to your ultimate surface finishing goals, conversion coatings treatments can be appropriate as a pre-process, post-process, or finishing process. The expert team of LEADRP can solve several different surface finish and CNC machining problems. Reach out to us today, upload your files, and get an instant quote. We are always ready and prepared for you. 

References

Conversion coating – From Wikipedia

Phosphate conversion coating– From Wikipedia

Chromate conversion coating – From Wikipedia

Black oxide – From Wikipedia

Conversion Coatings – From Anoplate

Alodine vs. Anodize: What’s the difference? – From Best Technology

FAQ

Passivation enhances the corrosion resistance of stainless steel parts by eliminating ferrous contaminants such as free iron from their surface, returning them to their initial corrosion specifications.

Conversion coating is a metal coating in which the covered surface is converted into a coating by electrochemical or chemical processes. Conversion coating not only protects against corrosion but also improves surface hardness. It can also serve as a decorative primer.

Anodizing is a "Conversion Coating" that differs greatly from paints, plating, and other conventional metal coatings. While paints and plating lie on top of the metal's surface, anodizing turns the aluminum's outer layer to aluminum oxide, fully integrating the coating into the aluminum substrate.

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