Metals are used in almost every industry, and in every country in the world, their workability, strength, versatility and relatively low cost make them suitable for almost every application, in some form or another.
Part of the reason for the prevalence, however, is down to the wide range of applications regular steel can be used in, from long-term outdoor applications to highly aesthetic products, steel in particular, can be coated to suit any application.
For this to be achieved, there are many different types of metal coating processes that work and are applied in different ways.
What is Metal Coating?
At its core, metal coating refers to the application of a thin, protective layer to a metal component. These coatings can serve several purposes, from enhancing aesthetic appeal to improving functional properties such as electrical conductivity and wear resistance.
In essence, metal coatings, often referred to as finishes, are a complex mix of materials designed to augment the utility of the underlying metal.
They can be organic, like paint coatings, which are primarily composed of a resin that dries to form a hard protective layer. Alternatively, they can be inorganic, like powder coatings, which are often made from thermoplastic or a thermoset polymer and provide superior protection against chipping, scratching, and other forms of wear.
These coatings, whether organic or inorganic, play vital roles in extending the life and functionality of the metal component. In essence, they serve as the first line of defense, a protective barrier against the environment.
They shield the metal from harmful interactions with aggressive elements such as oxygen and carbon dioxide, which can trigger oxidation reactions leading to metal corrosion.
This prevention of metal degradation not only preserves the mechanical integrity of the metal but also maintains its visual appeal, thereby ensuring that the metal continues to serve its purpose effectively and efficiently.
Purpose of Metal Coating
Metal coatings help provide a protective layer for a variety of products, protecting them from the elements or other causes of wear and tear.
Increases Durability
When the coating is applied to the outer surface of an item, it, in turn, changes the surface properties of the item. If the item didn’t have the coating, it wouldn’t be nearly as durable or long-lasting.
Prevents Rust Formation
Specifically, metal coatings help to prevent oxidation and rust from occurring on the metal object. Liquids and chemicals can quickly take a toll on the durability and longevity of the product.
Increases Load-Bearing Capabilities
Not only do metal coatings help to provide a durable and corrosion-resistant layer to the product, but they can also increase the load-bearing capability of the product. This means that metal coatings can help increase the load-bearing capabilities in the products they’re on, allowing them to be used in a huge range of applications.
Minimizes Friction
The extra layer of coating provides a protective layer that prevents the amount of friction created between two moving parts. This is especially beneficial in protecting parts that require a ton of constant movement.
Eases Assembly and Disassembly
With the reduction in friction comes an easier time assembling and disassembling products from one another. This helps to simplify the cleanup process and eliminate any non-stick characteristics of the original product.
What are the Different Types of Metal Coatings?
1. Powder Coating
Powder coating is a popular metal coating option due to its durability, environmental friendliness, and wide range of colors and finishes. This dry finishing process involves applying a powdered paint to the metal surface, which is then heated and cured to form a protective layer.
Pros:
- Highly durable and resistant to scratches, chipping, and fading
- Environmentally friendly, with minimal waste and no harmful solvents
- Available in a wide range of colors and finishes to suit any design aesthetic
Cons:
- Can be more expensive than other coating options
- Not suitable for all metal types, such as galvanized steel
Common Applications: Architectural metalwork, outdoor furniture, appliances, automotive parts
2. Galvanizing
Galvanizing is a process that involves coating metal with a layer of zinc to protect it from corrosion. This is typically achieved through hot-dip galvanizing, where the metal is dipped into a bath of molten zinc, or through electro-galvanizing, which uses an electric current to apply the zinc layer.
Pros:
- Provides excellent corrosion protection, especially for steel
- Cost-effective and long-lasting
- Can be painted or powder coated for additional protection and aesthetics
Cons:
- Appearance can be inconsistent, with a rough or spangled finish
- Not suitable for metals other than steel
Common Applications: Structural steel, outdoor metalwork, fencing, roofing
3. Anodizing
Anodizing is an electrochemical process used primarily for aluminum coatings. It involves immersing the aluminum in an electrolytic solution and applying an electric current, which causes a protective oxide layer to form on the metal surface. This layer can be dyed in various colors for a decorative finish.
Pros:
- Enhances the natural corrosion resistance of aluminum
- Provides a decorative finish in a variety of colors
- Resistant to UV radiation and fading
Cons:
- Only suitable for aluminum
- Can be more expensive than other coating options
Common Applications: Architectural aluminum, window and door frames, automotive parts
4. Electroplating
Electroplating is a process that involves depositing a thin layer of metal, such as chrome, nickel, or gold, onto a metal surface using an electric current. This can provide a decorative finish, as well as enhance the metal’s corrosion resistance and wear properties.
Pros:
- Provides a decorative and high-quality finish
- Enhances the metal’s corrosion resistance and wear properties
- Can be used on a variety of metal types
Cons:
- Can be expensive, especially for precious metal coatings
- Requires regular maintenance and polishing to maintain appearance
Common Applications: Jewelry, bathroom fixtures, automotive parts
5. E-Coating
E-coating, or electrophoretic coating, is a process that involves immersing the metal in a water-based paint solution and applying an electric current to deposit the paint onto the surface. This provides a uniform and corrosion-resistant finish.
Pros:
- Uniform and corrosion-resistant finish
- Environmentally friendly, with low VOC emissions
- Cost-effective and efficient process
Cons:
- Limited color options compared to powder coating
- Not as durable as powder coating or anodizing
Common Applications: Automotive parts, appliances, metal furniture
What is the Best Way to Coat Metal?
Determining the best way to coat metal is a multifaceted decision that hinges on a variety of factors, such as the application’s demands, the type of metal being used, and the environment in which the component will operate.
Factors like expected wear and tear, exposure to harsh weather conditions, or specific regulatory standards can significantly influence the choice of coating and the application method.
However, irrespective of the type of coating or application technique chosen, one fundamental principle remains paramount the need for proper surface preparation.
This step is critical to the success of the metal coating process. It involves removing any contaminants such as grease, rust, and old paint from the metal surface, ensuring it is immaculately clean and ready for the coating.
Techniques for surface preparation can range from abrasive blasting, which uses a high-pressure stream of abrasive material to clean and profile the surface, to chemical methods like pickling, which involves using acid to remove rust and scale.
Cleaning with solvents is also a commonly used method, especially for removing oil and grease. A thorough surface preparation promotes better adhesion of the coating and enhances its durability, ensuring the coated metal component can deliver reliable performance over a prolonged period.
So, while the best way to coat metal can vary depending on specific circumstances, the importance of diligent surface preparation is a constant that underpins the success of all metal coating applications.
Metal Coating Process
There are four main ways in which metal coatings are applied to different surfaces, all of which we will cover below. The metal coating process may consist of hot-dip galvanizing, thermal spraying, electroplating, and sherardizing.
The type of coating process chosen oftentimes depends on the required thickness of the coating.
Hot-Dip Galvanizing
You can think of hot-dip galvanizing as a bath for the product being coated to ultimately form a corrosion-resistant layer of zinc-iron alloy and zinc metal.
During hot-dip galvanizing, the steel component that is being coated is dipped into a pool of molten zinc that is at approximately 450 degrees Celsius.
During the immersion process, there is a metallurgical reaction that takes place with zinc alloy and zinc layers.
This reaction that takes place to coat the steel component is a diffusion process, helping to create a smooth, uniform layer of metal coating. The thickness of the metal coating is uniform throughout the piece as well.
Start with Surface Preparation
The first step in hot-dip galvanizing is to prepare the surface of the object or material being coated. Ideally, all oxides and contaminating residues will be removed from the surface beforehand to help create this metallurgical reaction. Without cleaning the surface beforehand, the reaction may not take place.
Preparing the surface isn’t a simple task, as the steel pieces being dipped can be quite large. They move through a series of cleaning steps using chains and wires.
- Degreasing is the first step. The steel is immersed in a degreasing bath to help remove contaminants such as oil, dirt, and grease that may be on the steel’s surface. Next, the steel is rinsed with water and ready for step two in the surface preparation process.
- Pickling comes next. This process consists of placing the steel in a dilute solution of other hydrochloric or sulfuric acid. During this process, all oxidation is removed from the steel and it is rinsed again for the final step.
- Fluxing is the last step. The flux layer helps ensure the steel is free from oxidation. A certain type of flux is housed in a separate tank, and it is slightly acidic. It is made from zinc chloride and ammonium chloride. Another flux variety may be used instead, and it is referred to as top flux. This variety serves the same purpose, but it floats in the galvanizing kettle on top of the liquid zinc.
As soon as all three steps in the surface preparation process are completed, the steel color will be close to a white color. It won’t have any oxides or contaminants and is ready to be galvanized.
Galvanizing the Steel
The zinc bath comes next, and the steel will be submerged in a galvanizing kettle. If there is a specific look in the coating that needs to be achieved, other metals are occasionally added during this step.
The kettle itself is heated to incredibly high temperatures, anywhere from 820-860 degrees Fahrenheit is ideal for ensuring the zinc is liquified. Next, the steel being coated is carefully dipped into the galvanizing kettle until it heats to the temperature of the bath.
As soon as the iron and zinc react with one another, the steel product being coated is withdrawn. The process as a whole takes 10 minutes or less, with exact timing dependent on how thick the steel piece is.
Post-Treatment Steps
After the piece has been removed from the kettle, the coating can further be enhanced through a process called quenching.
There is a quench tank filled mostly with water, however, there are a few chemicals added to form a passivation layer. This layer protects the newly galvanized steel if it needs to be stored or transferred.
There are other finishing steps used besides quenching. Sometimes, there are small zinc drips or spikes that can form on the metal coating that needs to be ground off.
Thermal Spraying
The second metal coating process is called thermal spraying. It may also be referred to as spray welding, plasma spray, flame spray, metalizing, HVOF, and arc spray. All of these names are referring to the same thermal spraying process.
This process relies on a consistent heat source such as a flame as well as a coating material. The coating material is either in powder or wire form that is heated via the flame, resulting in small droplets that are sprayed onto the steel’s surface.
This type of coating cannot only be applied to metal substrates but some plastic substrates as well, as these coatings help to further improve the performance of the component being covered.
Thermal spraying may be used in place of other surface treatments such as nickel and chrome plating, weld overlay, heat treat processes, and anodizing. The results of thermal spraying are a bit thicker compared to other processes, with coatings anywhere from .002”-.025” thick.
There are four main thermal spray processes that we will outline below.
Electric Wire Arc
This type of thermal spraying process is similar to the process used in wire arc welding systems. There is a wire coating material that is electrically charged.
From there, it is contacted creating an arc while small molten droplets of metal wire are sprayed directly onto the substrate. This is achieved by using a high-velocity air stream to shoot the material out.
Electric wire arc spray coatings are inexpensive and can be used for many different metals. Zinc, aluminum, copper, and even metal alloys such as stainless steel all react well to electric wire arc spray coating. This process also allows for changes to achieve the ideal coating thickness and texture.
Flame Spray
Flame spray is also referred to as acetylene combustion spray, and it is a technique that was developed over a century ago.
It can be in either wire or powder form, and it mimics the process of using a welding torch, but it also can add a high-velocity air stream to further propel the substance onto the substrate.
It’s not uncommon for flame spray coatings to fuse after they have been applied to increase the strength of the bond and to increase the coating density.
Plasma Spray
Plasma spraying is also referred to as a non-transferred arc, and it makes use of inert gas that is fed through an electrode.
This induces the gas’s plasma state, allowing the gases to release from the gun apparatus and revert to their normal state. As you can imagine, a huge amount of heat is present for this to work.
The coating material starts as a powder and as it is injected into the plasma flame, it is shot onto the substrate.
Examples of plasma spray coatings include ceramic coatings as they have high melting temperatures. Ceramic coating examples include titanium oxide, aluminum oxide, chrome oxide, and zirconia.
High-Velocity Oxy-Fuel (HVOF)
The final process is HVOF, and it uses combusted oxygen combined with a group of ignitable gases such as hydrogen, propylene, or propane.
This process uses the basic principles of combustion, however, there is a spray gun with a unique design. The HVOF spray gun is designed to produce both higher flame temperatures and higher velocities to create more kinetic energy.
This added energy helps to produce an even stronger bond and create a stronger metal coating. This process is typically used for metals that have a high melting temperature such as chrome carbide or tungsten carbide.
Electroplating
The third metal coating process is called electroplating, also referred to as electrodeposition. This process revolves around an electric current that deposits the material onto the workpiece’s surface.
The current helps to dissolve metal and put it onto the surface using four main components:
- Anode: This is the name for the positively charged electrode.
- Cathode: This is the electroplating circuit that requires plating. The cathode is also called the substrate, as it will act as a negatively charged electrode while in the circuit.
- Solution: This contains at least one metal salt such as copper sulfate to help facilitate electrical flow.
- Power source: A power source adds the current to the circuit and is integral in adding electricity to the electroplating process.
As soon as the anode and cathode are connected, the power supply will begin adding DC, or direct current, to the anode.
In return, the metal begins to oxidize and the atoms dissolve and turn into positive ions. The current then forces the move to the negatively charged substrate, resulting in a thin coating of metal.
Three factors may impact how high-quality the metal coating is. First is the bath conditions. The temperature of the bath and its chemical makeup must be just right to ensure the electroplating process is effective.
Part placement is the second factor, as the farther the substrate is placed away from the cathode, the less effective the plating will be.
The third factor is how high the electrical current is, as this will have an impact on how the electroplating process plays out.
There is a variety of benefits to using electroplating, including:
- Improved hardness of the substrate material to help extend their lifespan
- A cost-effective method of improving electrical conductivity
- Enhanced appearance for a more attractive look
- Protective barrier against environmental elements
Metals such as zinc, tin, copper, nickel, gold, silver, and palladium all use the electroplating process. The automotive industry, medical industry, electronics industry, aerospace industry, and oil and gas industry all use electroplating in a multitude of ways.
Sherardizing
Sherardizing also referred to as vapor galvanizing or dry galvanizing is a process that forms a protective layer of zinc-iron alloy on top of steel or iron. It has been practiced since approximately 1900 and it is named after Sherard O. Cowper-Coles, the inventor.
The object starts by getting heated in an airtight container with extremely high temperatures before it goes into a rotating drum. Next, a thermal diffusion process forces the zinc onto the surface of the metal, forming the zinc-iron alloy. The coating itself has a smooth surface and uniform thickness, providing a barrier against abrasion and corrosion.
The sherardizing process is especially ideal for coating small objects solely based on the type of uniform coating that it provides in addition to the limiting size of the container’s dimensions (2m x 5m x 4m). It is also ideal for geometrically complicated objects.
Examples of objects that commonly undergo sherardizing include rail track fittings, various cable hooks, metal fasteners, universal joints, or rubber bonding.
The adherent surface that results from the sherardizing process is ideal for:
- Painting
- Metal-to-rubber bonding
- Lubricants
- Organic coating applications
- Oils
- Sealants
Benefits of Metal Coating
Protection
Metal coating is great for corrosion protection which is why it is used in industries where equipment is exposed to moisture.
This includes marine environments, offshore rigs, and anywhere else where there is a high risk of corrosion. This process works by applying a coat made of a metal that does not corrode easily.
Durability
The use of metal coating on surfaces and objects often helps to make the objects a lot more durable than they otherwise would be. Heavy machinery that is exposed to a high risk of wear and tear is typically coated with a metal coating.
By creating a barrier between the surface and the elements, the metal coating helps to reduce wear and tear and increases the lifespan of machinery.
Aesthetics
Metal coating can often help to improve the aesthetic appeal of an object. This is because metal coating often creates a glossy or matte finish which improves the look of an object.
This is one of the reasons why metal coating is often used for facades, high-value furniture, and objects where the finish is important. Metal coatings are also used in the arts for precisely this purpose.
It is common to find this process being applied to sculptures in order to enhance their looks. An additional advantage of using metal coating for such an application is that the sculpture is also protected from the effects of rain and wind.
Better Performance
Metal coating is often used to improve the performance of objects. For example, by coating objects with a particular type of metal coating, one can improve electrical conductivity as well as thermal conductivity.
This type of coating is usually found in the automotive industry. Metal coating can help improve engine performance.
Cost-Effective
Metal coating is one of the most cost-effective methods of protecting surfaces. This is part of the reason why it is popular across many different industries.
Not only it is cost-effective to apply but it can also help reduce costs for a company in other ways. For example, by protecting machinery and surfaces from corrosion, metal coating helps to improve the life of such objects and significantly reduces the cost of maintenance.
Using metal coating also leads to less downtime as machines are less likely to break down. This way the productivity of the entire facility is improved.
Eco Friendly
The use of metal coating helps to protect the environment in many different ways. The first is that by applying metal coating on objects, one extends the life of such machinery.
This in turn means that there is less landfill as machinery is used for longer periods than would be typically the case.
The process of applying a metal coat on an object is also relatively easier and more eco-friendly than most other approaches.
Disadvantages of Metal Coating
While the advantages of metal coatings are compelling, it’s crucial to also consider the potential downsides. Every process has its limitations, and metal coating is no exception.
Understanding these drawbacks can help in making informed decisions and implementing measures to mitigate these challenges. Here are some potential disadvantages associated with metal coatings:
Environmental Impact
Certain metal coating processes, particularly those involving volatile organic compounds (VOCs) or heavy metals, can have a negative environmental impact.
These processes can result in the release of harmful substances into the environment, contributing to air and water pollution.
Hence, there is an increasing emphasis on adopting more eco-friendly practices, such as powder coating, which generates negligible VOCs and waste products.
Health Hazards
Some coating materials, especially those used in processes like electroplating or paint spraying, can pose health risks to workers during the application process.
Exposure to these substances can lead to a variety of health issues, from respiratory problems to skin irritation. Therefore, appropriate safety measures and protective gear must be in place to safeguard workers’ health.
Cost Implications
While metal coating can save costs in the long run by enhancing the durability and performance of components, the initial investment for some metal coating processes can be quite high.
This is especially true for complex or large-scale projects requiring specialized equipment or techniques. Furthermore, ongoing maintenance and quality control can also add to the cost.
Limited Material Compatibility
Not all coatings are suitable for every type of metal. For instance, hot-dip galvanizing is primarily used for steel and isn’t compatible with metals like aluminum.
Thus, understanding material compatibility is essential before proceeding with the coating process.
Process Complexity
Some metal coating processes are complex and require highly trained operators. Inconsistent application or mistakes during the process can lead to coating failure, affecting the final product’s quality and performance.