Metal Coating

Metal coatings refer to layers of paint applied to metal to protect the metal surface from corrosion in order to increase the product’s lifespan. Coating can consist of many layers of paint and refers to the end result that remains once the work is finished.

Metal coatings range from simpler metal coatings and metal surface treatments, and are used to impart corrosion-resistance, increase durability of the metal, increase chemical-, water-, and fire-resistance as well as to improve the aesthetic appeal of the metal. The metal coatings may be organic, like paint coatings, primarily composed of a resin that dries to form a hard protective layer, or they may 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 defence, a protective barrier against the environment. The coatings may be solvent-born, water-borne, or solids.

The process

Metal coating process involves the following steps:

• Surface preparation: The success of metal surface treatment depends largely on the surface to which it’s applied. The surface needs to be prepared by properly cleaning and roughening to ensure adhesion. Solvent-borne coatings are more forgiving of contaminated surfaces, while water-borne coatings always require a clean environment and surface.
• Application: The metal coatings are applied to the prepared metal surface at this stage. The application method depends on factors including the equipment, the type of paint, production speed, output speed, and on-site or off-site application.
• Drying: With water-borne coatings, the water in the paint can’t evaporate properly in extremely humid conditions, and thus the paint will take longer to dry. Performing the application in a climate controlled room can accelerate drying. The recommended humidity level is 20-70%.
• Maintenance: The treated metal surface will need to be maintained regularly to extend the lifespan of the metal coating and the metal underneath.

Methods of applying metal coatings

• Spray Painting/Coating: Spray coating is a prevalent method which involves spraying the paint onto the metal surface using a specialized spray gun. This technique is often used for automotive coatings and provides a high degree of control over the application process. This method allows for a uniform distribution of the coating, ensuring an even layer that contributes to both aesthetics and durability.
• Electroplating: Electroplating is a sophisticated technique that uses an electric current to deposit a thin layer of metal onto the surface of a component. This is used for the deposition of various metals, such as nickel, copper, gold, and more, on the substrate. This results in coated metal component with improved wear resistance, reduced friction, and higher aesthetic value.
• Powder Coating: This technique has relatively higher environmental friendliness and excellent finish quality. It involves applying a free-flowing, dry powder onto a metal surface, typically using an electrostatic gun. The powder clings to the metal surface due to the electrostatic charge, is then heated in a curing oven. The heat triggers a chemical reaction, causing the powder to melt and form a hard, protective layer. This layer is impervious to many common forms of wear and tear.
• Hot-Dip Galvanizing: This method involves immersing a metal component, usually steel, into a bath of molten zinc. The zinc forms a protective coating on the metal that imparts excellent corrosion resistance, making it ideal for outdoor applications.
• Anodizing: Anodizing is an electrochemical process that converts the metal surface into a decorative, durable, corrosion-resistant, anodic oxide finish. Aluminum is ideally suited to anodizing, although other nonferrous metals, such as magnesium and titanium, also can be anodized. The anodic oxide structure originates from the aluminum substrate and is composed entirely of aluminum oxide. This aluminum oxide is not applied to the surface like paint or plating, but is fully integrated with the underlying aluminum substrate, so it cannot chip or peel. It has a highly ordered, porous structure that allows for secondary processes such as coloring and sealing.

Common materials used as Metal coatings

Different materials are used as coating for metals depending on the application of the metal formation.

• Plastic Coatings: Metal components are dipped into thermoplastic powders to create a protective coating. It is also referred to as fluidized bed dipping and dates back to 1952. For this coating, the metal component is prepared by degreasing and shot blasting. It is heated between 250-400 °C while the exact temperature is dependent on the type of coating that’s required. A thermoplastic coating is ground into a fine powder and stored in a fluid bed on top of a layer of fine gauze. The gauze helps ensure no particles are let through. Next, air flows through the gauze to allow the particles to rise and move around as fluid would. The metal piece is then placed into the fluid bed while the plastic particles melt onto the metal, resulting in a smooth, even plastic coating.
• Paint Coatings: Using paint is one of the most common ways to provide a protective layer to steel. There are three steps involved in applying paint as metal coating: priming, painting, and finishing. Methods of applying paint include roll coating, air-spraying, brushing, and electrostatic spraying. Weather conditions during application also have a significant impact on how well the paint coating lasts. Ideal results come from coats applied between 40-90 °F with a relative humidity between 40 and 70 %. The quality and chemical makeup of paint have evolved over the years to keep up with demands from bridge and building owners, to make the coating durable. The number of coats required has also been decreasing thanks to the increase in paint quality and performance. What once required five or six coats of paint now only requires three coats or less.
• Metal Coatings: Metal coatings are applied to a variety of components to reduce the risk of corrosion and help the material last longer. Following are most common metal coatings used:
  • Zinc Coatings: Metallic zinc is applied to carbon steel through hot-dip galvanizing to create a protective coating.
  • Aluminum Coatings: Both hot-dipped aluminum-silicon and hot-dipped pure aluminum are examples of aluminum coatings. Hot-dipped aluminum-silicon can resist heat up to 1250 °F and it has a heat reflectivity of 900 °F, meaning aluminum coatings are ideal for coating heat panels, shields, and automobiles. Hot-dipped pure aluminum offers longer-lasting atmospheric corrosion resistance. It is ideal for applications in the medical field, automotive industry, and aerospace industry.
  • Ion Plating: Ion plating is achieved through thermal evaporation. Ion plating is achieved through electroplating, thermal evaporation, ion and atom sputtering, and ion implantation. Ion plating is ideal for coatings that fit unique and complex surfaces.
  • Nickel Coating: Nickel coatings are commonly used in industries that require high wear and corrosion resistance, such as the aerospace, marine, and oil and gas industries. Nickel coatings can be applied through electroplating or thermal spraying, and they provide a hard and durable surface that resists abrasion, corrosion, and oxidation. They also provide excellent adhesion, making them suitable for use with various substrate.
  • Titanium Coatings: Titanium coatings are a relatively new type of metal coating with unique advantages over other coatings. They are commonly used in the medical, aerospace, and automotive industries, where high wear and corrosion resistance are essential. Titanium coatings can be applied through physical vapor deposition or ion plating, and they provide excellent adhesion and high hardness.
  • Chrome Coatings: These are used in the automotive and aerospace industries, and offers excellent wear and corrosion resistance, making them ideal for use in harsh environments. Chrome coatings can be applied through electroplating, vacuum deposition, or spraying, and they provide a shiny and reflective finish. They can also improve the appearance of metals and enhance their durability. However, one of the disadvantages of chrome coatings is that they can be toxic and hazardous to apply, requiring careful handling and disposal.
• Ceramic Coatings: Ceramic coatings are preferred in industries that require high-temperature resistance, such as the aerospace, automotive, and energy industries. They provide a hard and durable surface that resists wear, abrasion, and corrosion, making them ideal for use in harsh environments. Ceramic coatings can be applied through thermal spraying or chemical vapor deposition, and they provide excellent thermal insulation and chemical resistance. They can also improve the performance of metals by reducing friction and enhancing their electrical conductivity.

External links

• Metal Coatings | Machine Design