What is Conversion Coating:Types and Application?
Conversion coatings are a significant factor in improving the performance and longevity of metal surfaces. These coatings are tightly adherent, protective layers of non-soluble compounds produced on the surfaces of materials through chemical or electrochemical reactions. These coatings are usually applied to protect a metal substrate from corrosion, but they may also serve as a base layer for additional coating layers, enhance surface lubrication, or be applied to achieve a desired appearance.
The four most common conversion coatings include chromate, phosphate, black oxide, and bluing. Black oxide and bluing apply only to steels. They enhance appearance but offer little corrosion protection.
This article will discuss the significance of conversion coatings in surface treatment and metal protection by exploring their functions, workings, and types.
What Is Conversion Coating?
“Conversion coating” is a comprehensive term describing coatings formed through chemical or electrochemical treatment of metallic surfaces, resulting in a superficial layer containing a metal compound. This procedure extends the life and functionality of metal materials by improving qualities like lubrication and corrosion resistance. They can also improve the adhesion of subsequently applied coating layers.
Is Conversion Coating the Same as Barrier Coating?
No. Chemical conversion coatings, or conversion coatings, are a type of surface passivation used to modify the surface of a metal, creating a protective metal oxide layer against corrosion and wear. On the other hand, barrier coatings serve as effective moisture or physical barriers, often applied on metals and ceramics to withstand harsh conditions. While conversion coatings enhance metal surfaces, barrier coatings provide a protective layer against environmental factors, sometimes applied over conversion coatings for added protection.
Is Conversion Coating the Same as Anodizing?
No, anodizing is a specific type of conversion coating. Conversion coating refers to various processes in which a metal surface undergoes a chemical reaction to enhance its properties. One particular technique in this category is anodizing, which involves the electrolytic oxidation of metals, usually aluminum, to produce a layer that is resistant to corrosion.
What Is the Purpose of Conversion Coating?
The purpose of a conversion coating is to create a protective layer on a metal substrate that contributes to the overall durability and performance of the metal. It provides a more suitable surface for painting and bondings.
How Is Conversion Coating Used in Manufacturing?
Conversion coatings are used by manufacturers of metal products requiring protection from environmental degradation to increase their corrosion resistance, and therefore their durability. Conversion-coated products are immersed in or sprayed with a coating solution appropriate to the alloy, which reacts with the metal to produce a layer that inhibits corrosion and aids in the adhesion of additional coating layers, such as paint. This treated surface is essential in several industries. In automotive applications, conversion coatings are used on car bodies and parts to protect against rust and corrosion, prolonging the lifespan of vehicles. In aerospace, conversion coatings are essential for aircraft components to withstand harsh environmental conditions and maintain structural integrity.
How Do Conversion Coatings Work?
Conversion coatings work by chemically reacting with the metal surface to create a protective layer. This process varies among the different coating types, which include: phosphates, chromates, and black oxide coatings. The specific chemical reactions for each type contribute to enhanced corrosion resistance, improved adhesion for subsequent coatings, and increased overall durability in diverse industrial applications.
How Does Conversion Coating Differ From Other Types of Coating?
Conversion coatings differ from other types of coating primarily because they chemically react with the metal surface, forming a protective layer. In contrast, physical coatings like paints merely create a barrier between the metal and the environment. Protective coatings can be categorized into barrier coatings, inhibitive coatings, sacrificial coatings, and combinations thereof. While non-metallic coatings often utilize polymers, epoxies, and polyurethanes, metallic coatings commonly employ zinc, aluminum, and chromium for corrosion protection. Conversion coatings provide unique chemical modification of the metal surface, enhancing its resistance to corrosion and improving adhesion for subsequent coatings.
What Are the Different Types of Conversion Coatings?
The different types of conversion coatings include:
1. Chromate Conversion Coating
Chromate conversion coating, also known as alodine coating, can produce versatile protective layers on metals such as: steel, aluminum, zinc, and others. This coating, applied by immersion or spraying, is produced by a chemical reaction that occurs on the metal surface. The spray or bath contains chromates and dichromates. The resulting gel forms a thin, corrosion-resistant layer that not only inhibits rust but also enhances paint and adhesive adherence. Commonly applied to items hardware, such as screws, hinges, brackets, and washers, chromate conversion coatings often impart a distinct, easily recognizable greenish-yellow hue to metal surfaces. Figure 1 is an example of a chromate conversion coating on a part:
conversion coating
Chromate conversion coating. Image Credit: Shutterstock.com/KPixMining
2. Molybdate
Molybdate conversion coatings provide corrosion protection for aluminum and its alloys, magnesium and its alloys, as well as zinc and zinc-nickel alloys. They are considered a less harmful alternative to chromate coatings, possessing similar anti-corrosion qualities without the associated environmental risks.
3. Phosphate Conversion Coating
Phosphate conversion coating, a common chemical treatment for steel parts, enhances corrosion resistance and provides a foundation for subsequent coatings. The process involves applying a solution of phosphoric acid and soluble salts such as zinc, iron, and manganese phosphate by sponging or spraying it onto the metal surface. These salts increase corrosion resistance by reacting with the metal surface to create a protective coating. The temperature and bath concentration are two examples of parameters that affect coating characteristics in different ways. Greater corrosion protection can be achieved by thicker coatings produced by higher bath concentrations. The speed of chemical reactions is another way that temperature influences coating adherence and thickness. In industrial applications, optimal management of these parameters ensures.
4. Titanate
AZ31 magnesium alloys are coated with titanate to provide protection. Organotitanates, such as tetra isopropyl titanate (TIPT) and tetra butyl titanate (TNBT), are commonly used for this purpose due to their reactivity. This coating creates a thin barrier that prevents corrosion and other forms of wear and tear. The formation process involves the precipitation of silicon hydroxide, titanium hydroxide, magnesium hydroxide, and fluoride, resulting in a porous layer that enhances corrosion resistance.
5. Anodizing
Anodizing, an electrolytic passivation process widely applied to aluminum alloys, titanium, and a couple of other metals, works by passing a direct current through an electrolytic solution, forming an oxide layer on the metal surface. Anodized coatings enhance resistance to corrosion and wear, and provide better adhesion for paint primers and glues. This process finds diverse applications in consumer products like smartphones and kitchen appliances, as well as in aerospace components like aircraft parts.
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Certain chemicals used in conversion coatings may present health risks to those involved in the application process.
Compliance with environmental and safety regulations can be complex and demanding.
Depending on the type, conversion coatings may have limited durability, requiring periodic maintenance.
The application process can be complex, requiring careful control and expertise.
Initial and maintenance expenses can be a factor in the overall cost-effectiveness compared to the long-term benefits or total lifecycle costs of the product or service.
Is Conversion Coating Expensive?
Conversion coating processes can vary in cost depending on factors such as the type of metal being treated, the desired properties of the coating, and the scale of production. While conversion coating can offer significant benefits in terms of corrosion resistance and paint adhesion, it may not always be the most cost-effective solution.
