Metal Surface Finish: A Comprehensive Overview of Metal Finishing Processes

There are several surface finishing types available, each with its unique advantages. The kind of project and your material choice will determine the type of finishing you will adopt. Below are some of the common metal surface finishes available:

Plating or Conversion Coatings

Metal plating involves altering the surfaces of substrates by coating them with thin layers of other metals like zinc, nickel, chromium, or cadmium. Metal plating works to improve a component’s durability, surface friction, corrosion resistance, and aesthetic appearance. However, plating machines may not be well suited for removing surface defects.

There are two major plating types:

• Electroplating. This process works by submerging the component in a bath containing metal ions for coating. A direct current is then applied to the metal to cause the ions to be deposited on the metal part, leaving a new layer over the surfaces.
• Electroless Plating. This method involves no electric current as it is an autocatalytic plating with no external power supplied. Rather, the metal part is placed in solutions filled with copper or nickel to create a reaction that breaks up the metal ions.

These processes leave a last-longing finish on metal and can also be a means of repairing and maintaining worn-out components. They impart hardness to metal parts and increase their corrosion resistance, making them useful for several applications.

Plating is often a large-scale and chemically intensive process. Therefore, it is carried out by skilled and experienced technicians.

Anodizing

This electrochemical process helps create a durable, decorative, and corrosion-resistant anodic oxide finish. Manufacturers carry out this finish by immersing the metal in an acid electrolyte bath before making an electric current pass through the medium. The aluminum acts as the anode while a cathode is mounted inside the anodizing tank.

Consequently, the oxygen ions released from the electrolyte combine with the aluminum atoms to form an anodic oxide on the surface of the workpiece. Therefore, anodizing is a highly controlled oxidation of the metal substrate. It is mostly useful for finishing aluminum parts but is also effective for nonferrous metals like magnesium and titanium.

Metal Grinding

Manufacturers use grinding machines to smoothen out metal surfaces with the aid of adhesives. It is one of the final steps in machining, helping to reduce the surface roughness left on the metal from machining. There are several grinding machines available to deliver various levels of smoothness.

Surface grinders are the most common machines available. However, many other specialty grinders like Blanchard grinders and centerless grinders exist. A precision Blanchard grinder is widely used for large parts. On the other hand, die grinders help with details works, using special-shaped abrasive wheels.

Polishing/Buffing

In these processes, abrasives help reduce the surface roughness of metals after machining. These abrasive powders are combined with felt or leather wheels to polish and buff metal surfaces as required.

Manufacturers can carry out metal polishing either manually or automatically (with robotic polishing). Asides from reducing the material’s surface roughness, polishing also increases the metal’s luster. Thus, the main aim of polishing and buffing is to improve the aesthetics of a metal-machined component.

Electropolishing

For electropolishing, it is the exact opposite of the electroplating process. Instead of depositing metal ions on surfaces of metal components, electropolishing removes metal ions from the surface. During the process, the substrate is immersed in an electrolyte bath before applying an electrical current.

The substrate becomes the anode, with ions flowing from it to remove defects, rusts, debris, etc. The result is a polished and smoothened surface void of bumps and pocks. Electropolishing removes peaks and valleys from metal surfaces down to the microscopic level. This is one of the best surface finishing options for metals that require a clean and pristine appearance.

Coating

The coating is a broad umbrella that covers several subcategories of surface finishes. The most common and most inexpensive option is the application of commercial paints. Some paints can add color to the product to make it look more aesthetically pleasing. Others can help actively protect the component from corrosion.

There is also powder coating which is a modern version of the painting. It attracts powder particles to metal parts using an electrostatic charge. The powder particles cover the material surface evenly before it is cured through heat treatment or the use of ultraviolet rays. This method is quick and efficient for finishing metal parts like bike frames, vehicle body panels, and several other metallic components.

Blasting

Abrasive blasting is typically used for products that require uniform matte texture. It is a cost-effective process that combines surface cleaning and finishing into a single operation. During abrasive blasting, a high-pressure abrasive flow sprays the metal surface to change its texture, remove debris, and provide a smooth finish.

It can also serve as a surface preparation plating and coating to increase the durability of metal parts. The typical blasting machinery consists of:

• An air compressor to adjust the pressure and volume
• Water-assisted system to suppress dust development
• Moisture separator for humidity reduction
• Air supply line
• Blast machine
• Hose and nozzle
• Blast media, including sand, metal pellets, glass beads, steel grit, steel shots, silicon carbide, etc.

The common abrasive blasting methods are sand blasting and bead blasting. Another unique blasting method is shot peening. This process helps impart compressive stress on metal surfaces to improve fatigue and corrosion cracking resistance. Abrasive blasting is generally cost- and time-saving, allowing product managers to get products to market faster.

Brushing

Brushing finish effectively creates a uniform surface texture and smoothes out the exterior of metal machined products. Unlike plating, this metal finishing technique is ideal for removing imperfections on metal surfaces. It usually employs wire brushes or abrasive belts to achieve these purposes.

Depending on the manufacturer’s application of the process, the results may differ. For instance, moving the brush or belt in a singular direction can help create slightly rounded edges on the surface. On the other hand, wire brushing aims to remove slags produced during welding operations. It also removes grit and scale from the surface of metals before cleaning, plating, or coating.

Painting

Painting and powder coating are quite similar. It involves altering a metal part with substances like paint, pigment, or color. This surface finishing technique adds a colored protective layer to metal surfaces.

The process begins with the atomization of the paint to transform it into small droplets with high compression and other external forces. Then the machinist delivers the paint to the metal component with the most suitable method. The different methods available include spray painting, powder coating, silk screening, etc.

The method used will depend on the desired finish according to the material size, shape, and quality. Painting aims to create a protective coating on the part’s surface and give it an improved appearance, reaching a high gloss degree.