Different Types of Metal Finishing: Processes & Techniques

Surface finishing is a critical step in the production of metal components. A quality metal surface treatment will improve any completed component, making the part look better, and crucially, last longer.

Metal finishes come in many different styles. As a result, it is important to understand the specifics of each finish, ensuring optimum performance and reducing waste.

What is metal finishing?

Metal finishing refers to a variety of treatments or processes that range from polishing to processes that affect a metal’s molecular structure. Metal finishing includes a variety of cleaning treatments, polishing methods, and other operations designed to improve and enhance the surface of a given metal product.

Why is it important for metals?

While metals are an extremely versatile material group, individual metals have drawbacks and benefits that cater them towards certain applications. Metal finishing can increase the range of suitability, specifically allowing cheaper metals to be used in place of more expensive ones. Other processes such as polishing can improve electrical conductivity, durability, chemical resistance and electrical resistance.

The vulnerability of all metals to corrosion is always a big concern among producers, but metal finishing is widely used purely to increase the finish or aesthetic quality of a part. Metals, while durable and effective, do not always have a uniform finish after manufacturing, which is often desired in consumer products.

Choosing a metal finishing process or method

Before making a manufacturing decision, it is important to consider all aspects; you need to select a technique that best fulfils the finished product’s required criteria (all while keeping costs as low as possible).

Key things to consider

The method of finishing that you choose to use can have a significant impact on the value of the finished product. The main driver of industrial processes is usually cost. Finishing techniques can vary wildly depending on:

• Level of finish required
• Base metal
• Time taken
• Prep work required

For example, electroplating can be time-consuming, but buffing and polishing can be done instantaneously with electro techniques. This critical stage of the finishing process impacts delivery schedules as well as on the supply chain.

Types of metal finishing

Plating (or conversion) coatings

Metal plating is the process of changing the surface of a substrate by covering it with thin layers of another metal such as zinc, nickel, chromium or cadmium.

Metal plating can improve the durability, surface friction, corrosion resistance and aesthetic look of a component. However, plating equipment may not be ideal for eradicating metal surface imperfections. There are two major types of plating:

Electroplating

This plating process involves immersing the component in a bath containing metal ions for coating. A direct current is then delivered to the metal, depositing ions on the metal and forming a new layer over the surfaces.

Electroless plating

This process uses no electricity because it is an autocatalytic plating that requires no external power. Instead, the metal component is immersed in copper or nickel solutions to initiate a process that breaks up the metal ions and forms a chemical bond.

Anodizing

An electrochemical procedure that contributes to the creation of a long-lasting, attractive, and corrosion-resistant anodic oxide finish. This finish is applied by soaking the metal in an acid electrolyte bath before passing an electric current through the medium. The aluminum serves as the anode, with a cathode housed within the anodizing tank.

The oxygen ions released by the electrolyte mix with the aluminum atoms to form an anodic oxide on the workpiece’s surface. Anodizing, therefore, is a highly controlled oxidation of the metal substrate. It is most often used to finish aluminum parts, but it is also effective on nonferrous metals such as magnesium and titanium.

Metal grinding

Grinding machines are used by manufacturers to smooth out metal surfaces with the use of abrasives. It is one of the final phases in the machining process, and it helps to decrease the surface roughness left on the metal from previous processes.

There are many grinding machines available, each providing varying degrees of smoothness. Surface grinders are the most commonly used machines, but there are many more specialty grinders available too such as Blanchard grinders and centerless grinders.

Polishing/Buffing

With metal polishing, abrasive materials are used to reduce the surface roughness of a metal alloy after it’s been machined. These abrasive powders are used in conjunction with felt or leather wheels to polish and buff metal surfaces.

Aside from reducing surface roughness, polishing can improve the appearance of part — but this is only one purpose of polishing. In certain industries, polishing is used to create hygienic vessels and components.

Electropolishing

The electropolishing process is the inverse of the electroplating process. Electropolishing removes metal ions from the surface of metal components rather than depositing them. Before applying an electrical current, the substrate is immersed in an electrolyte bath. The substrate is transformed into the anode, with ions flowing from it to eliminate flaws, rust, dirt and so forth. As a result, the surface is polished and smooth, with no lumps or surface debris.

Painting

Coating is a wide term that encompasses various surface finish subcategories. The most common and least expensive choice is to use commercial paints. Some paints can add color to a metal product to make it more visually appealing. Others are also used to prevent corrosion.

Powder coating

Powder coating, a modern kind of painting, is also an option. Using an electrostatic charge, it attaches powder particles to metal parts. Before being treated with heat or ultraviolet rays, the powder particles evenly cover the material surface. This procedure is fast and efficient for painting metal items such as bike frames, automobile parts and general fabrications.

Blasting

Abrasive blasting is commonly used for products that require a consistent matte texture. It is a low-cost method for combining surface cleaning and finishing into a single operation.

During the blasting process, a high-pressure abrasive flow sprays the metal surface to modify the texture, remove debris and produce a smooth finish. It can also be used for surface preparation, plating and coating to extend the life of metal items.

Brushing

Brushing is a similar operation to polishing, producing a uniform surface texture and smoothing off a part’s exterior. The process uses abrasive belts and tools to impart a directional grain finish to the surface.

The results may vary depending on how the technique is applied by the manufacturer. Moving the brush or belt in a single direction, for example, might aid in the creation of slightly rounded edges on the surface.

It is only recommended for use on corrosion resistant materials such as stainless steel, aluminum and brass.

What is the best finish for metal?

There are many types of metal finishes, each with their own best-case scenario. The best procedure or method to follow during processing comes down to the end product’s specifications, the material used as well as your budget.

Best finish for corrosion resistance

Material selection can play a big part in limiting finishing cost, for example using a stainless steel over carbon steel. If cost is a factor though, galvanizing is considered the most effective treatment for mild steel to prevent corrosion.

Best looking metal finish

Depending on the use case, polishing is often synonymous with high quality, in part due to the high labor cost associated with it. A high quality sanded and painted finish such as the ones used on automobiles can result in a high-quality product.

Cheapest metal finishing technique

If budget is the primary driver of finishing choice, wet painting or powder coating can be good options. This is mostly due to the highly automated nature of these processes and the low raw material cost.

Applications of metal finishing

Almost every metal component used in industrial settings or sold as part of a consumer product will incorporate some level of surface finishing. There are some exceptions such as non-consumer goods made in non-ferrous materials.

Benefits & drawbacks of metal finishing

Advantages of metal finishing techniques

There are numerous advantages to the aforementioned finishing techniques.

At the basic level they each give corrosion resistance, better aesthetics and improved functionality. On a more nuanced level, metal finishes often enable cheaper, more readily available materials that are easy to work to be used such as mild steel. Other benefits can include improvements to conductivity and wear resistance.

Disadvantages of metal finishing

All manufacturing processes come with a cost — increases in the price of the end product as well as the time and effort taken planning out the manufacture.

Some finishing techniques such as polishing and painting require the components to be handled with extra care after processing. Finishing also takes time and logistics to execute, increasing the lead time.

Other finishing types can limit the operating range of the end product. Paint for instance can only withstand a certain range of temperature, unless specific high temperature paint is used.

Conclusion

Surface finishing is critical in the manufacturing of metal products. It is not only advantageous to end users, but it also benefits the manufacturer if controlled effectively.

However, not utilizing the techniques effectively may have long-term consequences for both stakeholders. As a result, it is vital to understand both the limitations of the metal finishing process as well as the requirements of the product fully.