As engineers and industry experts, choosing the correct aluminum finish seems easy.
Most of the time, the requirements and specifications dictate the choice. But what happens if you’re not an expert? What choices do you have when buying aluminum? And is mill-finish aluminum the right choice for your project?
This blog quickly covers the characteristics of mill-finish aluminum, discovers other types of aluminum finishes, and delves into comparing anodized aluminum and mill-finish aluminum.
Characteristics of Mill Finish Aluminum
Mill finish aluminum is used to describe aluminum with no post-processing. The surface of the freshly extruded aluminum profile is a mill finish.
In fact, that’s where the term originates, raw metal coming from mill rollers without surface treatment.
Mill finish is also found in aluminum extrusions. In most extrusion products, in this case, raw aluminum rods are extruded through an extrusion die into desired aluminum extrusions with unchanged surface properties.
When an aluminum surface does not undergo polishing or chemical coating, it is a mill finish.
This will be important when we delve into other types of aluminum finishes.
Properties of Mill Finish Aluminum
Mill-finished aluminum has properties that depend on the type of aluminum being used. Since mill-finished aluminum is in its natural appearance, the mechanical and chemical properties come from particular alloys like aluminum 6061, aluminum 3003, and aluminum 1100.
Here are common mill finish aluminum properties:
Aluminum is one of the few metals with a natural oxide layer. Aluminum gains corrosion resistance as a protective oxide layer forms upon exposure of the aluminum to air on its surface. The new oxide layer is tough and doesn’t break or flake like oxides of other metals.
Mill finish aluminum can however rust under certain pH ranges and constant exposure to moisture. Anodizing or painting is a popular way to enhance corrosion resistance.
Mill finish aluminum has the best electrical conductivity as there is no paint or coating that resists current.
Aluminum is not the best thermal conductor when compared with metals like copper but if your application requires thermal dissipation using aluminum, mill-finish aluminum is better as it has no oxide coating.
Mill-finish aluminum is easier to form, melt and shape. It can undergo an extrusion process, and be turned into a variety of forms, disregarding the complexity that comes with coating intricate shapes.
What Does Mill Finish Look Like?
Mill finish is a factory floor product that is often further processed. So, mill finish typically has a rough texture. It’s dull in color compared to brushed and finished aluminum.
Mill finished aluminum also has light scratches, nudges, spots, and tooling marks.
Mill finish aluminum has visible manufacturing lines because it comes straight from the rolling mill. In the case of extruded raw aluminum, the surface marks and imperfections are similar.
You might be used to a glossy aluminum finish, but natural finish aluminum lacks the shine and reflectivity associated with aluminium metal.
Difference Between Mill Finish Aluminum And Brushed Aluminum
A standard mill finish aluminum is often given a brushed finish.
Brushed finishes are aesthetically sought after in architectural construction such as window panes or door locks, aluminum tubes, angle trims, etc. In the transportation industry with supporting rails in busses and subways, and automotive parts.
But brushed finishes go beyond just the aesthetics! Brushed-finish is given using an abrasive mesh that partly polishes the aluminium surface. Brushed finished aluminium doesn’t give reflections, prevents fingerprint stains, and is good at hiding regular scratches and imperfections.
The difference between brushed-finish aluminum and mill-finish aluminum is the directional grain and slight polish. Imperfections and marks are more easily spotted on mill finishes.
What Is Anodizing?
Anodizing is an oxidation electrolysis process that forms an oxide film on the metal surface to protect it.
In straightforward terms, it rapidly increases the rate of natural oxidation and grows an aluminum oxide layer on the mill finish aluminum.
Anodized aluminum is a popular industrial material. China leads the global production of anodized aluminum and Wellste contributes to high-quality anodized aluminum products. So why is anodized aluminum so important?
Anodizing aluminum improves corrosion resistance, adds long-lasting color, and improves hardness, durability, and wear resistance.
The detailed anodizing process is covered below:
Anodized aluminum is often produced in large batch productions, as the process is both time-consuming and expensive.
The main principle of anodizing is the electrolytic process. The mill finish aluminum extrusions or aluminum trims are submerged into a temperature-regulated sulphuric acid bath. Here a current passes through the solution releasing oxygen atoms from the acid that combine with the aluminum to grow an oxide film on the mill finish.
Anodized coatings can be colored because they are porous and can entrap the dyes within their structures which are sealed by a final coating of Nickel acetate.
What Is The Difference Between Mill Finish Aluminum & Anodized Aluminum?
Mill finish aluminum is the natural, untreated, unchanged aluminum surface from the rolling mill. The aluminum finish resulting from the extrusion process at aluminum mills characterizes extruded aluminum.
Anodized aluminum has a chemically altered surface. The anodizing process is coating a micron-level thick protective layer that provides hardness, and abrasion resistance and prevents corrosion.
- Mill finish is less durable than anodized aluminum finish
- Mill finish has die lines and imperfections while anodizing makes the surface uniform
- Mill finish aluminum can be easily scratched while anodized protective coating is not easily peeled
- Oxidized film on mill finish has barely any thickness. Anodic surface treatment develops a 25um thick protective layer of oxide film.
- Mill finish comes in a dull color and anodized aluminum can have multiple glossy colors.
Pros and Cons of Various Aluminum Finishes
Mill Finish Pros
- Mill-finish aluminum is cheap because it can be sourced from recycled aluminum
- It is ductile
- It can be bent and fabricated easily
- Mill finish preserves the metallic look of aluminum
- It can be alloyed
- It can undergo various surface treatment process
- Economical choice
Mill Finish Cons
- Mill finish is extremely thin and can be easily removed
- Mill finish aluminum can be oxidized under salt and moisture to give a rustic look
- Surface has imperfections and die lines
- It comes in a dull silvery color
Anodized Aluminum Pros
- Anodizing aluminum products has good wear and abrasion resistance
- Anodizing aluminum increases the hardness of aluminum
- Colored anodizing can give aluminum a deep black, bronze, or champagne shade which is etched into the surface
- Anodized aluminum surface has greater adhesion properties
- It has a glossy mirror-like finish
- Ideal for marine and salty conditions
- Corrosion resistant, suitable for external use in UV light.
Anodized Aluminum Cons
- Anodizing aluminum adds to the production cost
- For multiple batches, color variation can occur
- Aluminum oxide layer is grown from the base metal instead of other electroplating methods
- It makes aluminum extrusion products harder to bend and fabricate
- Limited dyes are available. If you’re looking for a wider color range with a protective coating, take a look at this PVDF guide for aluminum.
What Other Aluminum Finishes Are Available?
There are a few other aluminum finishes that are popular in the industry.
PVDF is a layering of a fluoropolymer-based plastic on top of aluminum to give it a protective finish that is both colorful and glossy.
PVDF coatings are widely used in construction and you can spot them from their colorful range. PVDF doesn’t protect against wear and abrasion and is mostly done for aesthetics.
Powder Coating Vs Brush Finish Vs Polished Finish
Powder coating is another popular protective coating with many colors. An electrostatically charged powder is sprayed onto aluminum surface that sticks to the freshly chromatid surface.
This enhances rust protection and prevents abrasive damage. Powder coatings can be chipped out revealing aluminum’s natural color.
Brush finish is the linear or diagonal-grained finish visible on aluminum railings and surfaces. It is purely done for aesthetics to get an even surface finish and remove marks and spots.
Polished finish aluminum can be achieved using a polishing liquid and buffing aluminum. Then aluminum makes a mirror-like shiny surface. Polished finish is used on decorations and accessories.
Conclusion & Helpful Tip
Mill finish aluminum has a rough texture with visible die lines and marks. This isn’t a lack of quality but how a factory floor product looks. Mill finish is mostly further processed into extrusions, trims, and components. It is also used to make products like aluminum foils and soda cans where surface finish is not an issue.
To enhance the properties of aluminum like corrosion resistance, abrasion, and weather resistance you can have your aluminum products anodized. If you’re thinking anodizing aluminum will give it a particular color or yellowish hue, you can choose clear anodizing.