Designing a metal fabrication involves more than selecting material and specifying dimensions. Deciding how it should be finished is just as important. Metal finishing refers to coating or modifying the surface. In this blog, we’ll explain why choosing a finishing method should be part of the design process, the options available, and how to go about choosing the right one.
Don’t Make Metal Finishing an Afterthought
Finishing determines what a metal fabrication will look like and how long it will last. It may also influence how well it performs, and the type of process chosen will inevitably have some impact on what it costs.
Finishing processes should be chosen in concert with material selection. In simplistic terms, an inexpensive base material may need extensive finishing treatment, while choosing a more expensive metal could reduce or even eliminate the need for finishing. (Of course, other factors are at work too, such as the ease of cutting, bending, and welding.)
Finishing has two principal objectives: to protect the metal from corrosion and to improve or optimize its appearance for the application or use. It may also be done to improve wear or abrasion resistance. The extent to which these enhancements are needed depends on the intended application, the expected or required life, and any specific aesthetic requirements.
Options for Finishing Metal Fabrications
Wiley offers metal fabrication customers an extensive range of finishing services. These can be grouped under the headings of coatings, meaning compounds or materials applied to the surface, and treatments that modify the surface.
Metal Coatings
Coatings include paint, powder coating, and plating.
Paint can refer to the traditional brushing or spraying processes, although for many metal fabrications E-coating is the preferred metal finishing method. E-coat, or electrophoretic deposition, involves making the fabrication part of an electrical circuit, then lowering it into a tank of paint. Current draws charged paint particles onto the fabrication.
A major advantage of E-coating is that it gets paint into cavities and confined spaces that other painting processes can’t reach.
Electricity is also used in powder coating. Here, charged particles of a polymer are sprayed at the fabrication, with the electrostatic effect helping attract them onto the surface and ensuring all surfaces are coated. After spraying, the fabrication is oven-baked to cure the powder.
Powder coatings are thicker than paint and generally have better abrasion and scuff resistance.
Plating refers to the deposition of a thin metal layer over the surface of the fabrication. This gives the fabrication the appearance and to a lesser extent, the properties, of the applied metal. (Copper plating, for example, provides electrical conductivity.)
Plating is a way to give an inexpensive substrate material the appearance of a higher-value metal. Metals commonly applied by plating are:
- Chrome – This can be decorative chrome, to create a highly reflective appearance, or hard chrome, to increase the wear resistance of tooling.
- Tin – This is an inexpensive way of providing a basic level of corrosion resistance.
- Copper – Often applied for decorative effect, and less often in the fabrication industry, for electrical conductivity
- Nickel – Forms an attractive silver-gray finish.
While not technically a plating process, galvanization is similar in that it applies an additional layer of material – in this case, zinc – over the surface of the substrate. This provides corrosion resistance and is particularly effective at reducing the impact of galvanic corrosion.
Hot and cold rolled steel fabrications are galvanized by hot-dipping. (The process is not suitable for other metals.) The resulting “spangled” surface limits the process to applications where aesthetics are not the primary concern.
Two points to consider for all these processes are surface preparation prior to coating and whether the parts or fabrication will be welded.
For a coating to adhere properly to the substrate, the metal must be clean and free from impurities or contamination. Blasting followed by degreasing is typical.
Welding will typically remove the corrosion protection afforded by plating. There’s also a risk of the plated metal becoming alloyed with the substrate, potentially weakening the weld. This is avoided by making plating the final manufacturing operation.
Metal Treatments
“Treatments” refers to processes that modify the surface, either mechanically or chemically.
Commonly used mechanical treatments include brushing, polishing, and bead blasting. Brushing gives the material a directional appearance akin to grain in lumber. Applying a clear coating after brushing creates an attractive decorative effect.
Polishing rubs away the microscopic peaks otherwise present on the surface. This reduces light scatter, so making it appear more reflective.
In blasting processes, the surface is bombarded with some form of media propelled by compressed air. The type of media and characteristics like size and shape determine the effect on the surface. Media can range from sand and metal shot to crushed walnut shells.
Two widely used chemical surface treatments are anodizing and passivating.
Anodizing creates an oxide layer on the surface of aluminum. (Aluminum naturally oxidizes rapidly, but anodizing creates a deeper, harder, oxide effect.) Dyes can be used during the anodizing process to create attractive colors.
Passivating is used to remove iron from the surface of stainless steel. Skipping this metal finishing process risks iron particles causing unsightly corrosion. It’s typically done after machining and/or welding and entails dipping the fabrication in a weak acid solution.
Talk to Your Fabricator About Metal Finishing
Finishing should be considered when designing a fabrication, along with geometry and materials. The right metal coating or treatment can extend the life of your design, improve its aesthetic appeal, and even make it function better.
Key considerations in finishing process selection are:
- Objectives for finishing (appearance, durability, etc.)
- Compatibility with the substrate material
- Total cost (remembering that a professionally applied coating or treatment may allow use of a less expensive substrate metal)
As discussed here, many finishing options exist. If you’re uncertain which would best meet your requirements, your fabricator should be able to help.
At Wiley Metal Fabricating we offer an extensive range of metal finishing processes. Ask us what we would recommend for your project.