Your Ultimate Guide on How to Anodizing Aluminum

Anodizing Aluminum-feature image

Posted on August, 27th, 2020 | By WayKen Rapid Manufacturing

Once you’ve finished with the functionality of your product, you will inevitably get to the point where finishing your parts is concerned. Painting seems to be the most obvious option but if your part is made of aluminum you have a different option: aluminum anodizing. Also, aluminum anodizing is a very durable post-processing technique.

What is Anodized?

Anodizing is one of the most reliable and versatile parts finishing processes applied by different manufacturers to non-ferrous metals like aluminum, titanium, and magnesium as it adds a new level of durability, corrosion resistance, and surface finish.

This process adds a layer of anodic oxide structure onto the metal surface by submerging it into an acid electrolyte bath while an electric current passes through it. To induce oxidation, the metal part will act as an anode while a cathode is placed inside the anodizing tank. As the part is being processed, its surface texture gradually changes and forms an oxidized surface.

What is Aluminum Anodizing?

Aluminum is the most suitable metal for anodizing because of its mechanical properties and composition. The aluminum oxide finish produced adds a protective factor that enhances the part’s surface hardness, corrosion resistance, scratch resistance, and aesthetic appearance. Unlike other coated or paint finishes, anodized aluminum doesn’t chip or peel off because the anodized layer is a part of the aluminum itself.

How does aluminum anodizing work?

Naturally, the surface of an aluminum metal forms a protective layer when it is oxidized, and yes, aluminum anodizing is a highly controlled application of this natural occurrence. During the aluminum anodizing process, a layer of anodic oxide structure is formed through the controlled build up of aluminum oxide on the part’s surface. The layer formed is porous which can be subjected to other processes like dyeing and sealing.

principle of anodized aluminum

The working principle behind aluminum anodizing involves inducing the oxidation of aluminum by submerging it into an electrically conductive acid electrolyte solution where oxygen ions are released.

How to anodize aluminum: Step by step

To further understand the process of anodizing aluminum parts, here are 4 major steps involved:

Step 1: Pre-treating

The pretreatment process for aluminum involves achieving a visible finish and a clean workpart.

Cleaning. Pretreatment cleaning is important to remove residual grease, oils, and other impurities from a previous fabrication process like extrusion or CNC aluminum machining to avoid impurities and inconsistencies to the finished part.

Etching. The surface finish of a part prior to anodizing is relevant because it will dictate the quality of your final result. Tweaking your desired surface finish can be done through a process called etching, where minor imperfections on the surface are corrected while being prepared for anodizing.

Step 2: Anodic Oxide

After the surface is prepared, the parts will now go through the anodizing step where they will be submerged into a bath of sulfuric acid electrolyte (may vary depending on the type of anodizing process chosen). The aluminum parts will serve as an anode while a cathode is installed within the tank to allow an active passing of electrical current through the system and induce the release of oxygen ions from the electrolyte solution. This process will cause an anodic coating to form into the substrate and create a fully integrated layer. The anodic layer thickness is being measured in this process

Step 3: Coloring

The anodic coating formed on the surface is porous by the structure which allows the addition of color into the part. Various ways of adding colors to an anodized part involve submerging it into dyes or dissolved metal salts. It is through this process that finishes like black anodized aluminum, gold, black, nickel, and stainless are achieved.

Step 4: Sealing

To close the porous surface created in anodizing and to provide a uniform surface, the parts are subjected to a final step that will submerge them into a solution of nickel acetate. Sealing ensures a long-lasting color and prevents further corrosion for the anodized part.

Tips for Surface preparation prior to anodizing

Aluminum Anodizing Finishing Examples

Here are some useful tips and best practices applied when anodizing aluminum:

  1. Make use of mechanical finishing techniques when necessary. Etching alone isn’t enough to correct pre-existing surface defects like scratches and dents. If these are evident in a part’s surface, might as well use mechanical polishing techniques like grinding, buffing, and sandblasting.
  • Be mindful of your packaging. Before sending out your parts for anodizing, make sure that you have packed them in such a way that they are dent-proof and well cushioned. Additionally, avoid sticking a tape directly to the surface to be anodized.
  • Ensure that your parts are properly cleaned and degreased. Look out for uncleaned swarf seated on blind holes, fingerprints on the surface (avoid handling parts with bare hands!), and residual oils from the fabrication processes.
  • Remove steel inserts like rivets and pins on parts to be anodized as these can corrode differently throughout the different treatment processes.

What Are the Materials You Need to Anodize Aluminum?

For typical aluminum anodizing process, here are the major materials you’ll need:

  • Treatment tanks for degreasing, cleaning, etching, anodizing, coloring, and sealing
  • Power source
  • Distilled water
  • Sulfuric acid as your electrolyte solution (may vary depending on the type of process chosen)
  • Cathode
  • Aluminum wire (Titanium wire may also be used if available)
  • Lye
  • Degreasing detergent
  • Acid neutralizer
  • Coloring Agents
  • Parts mounting hangers

Other materials

Other materials for easy handling and processing may also include:

  • Abrasive pads for cleaning
  • Small ball floaters to minimize acid fumes
  • Heating instrument for dyes
  • Agitating instrument for the acid bath
  • Suspension fixtures for the aluminum parts
  • Thermometer for regular temperature monitoring
  • Sets of personal protective equipment

What materials can also be anodized other than Aluminum?

Anodized finish is not fully limited to aluminum only, it can also be done on other metals like magnesium, titanium, niobium, tantalum, and zinc. These can be seen on various architectural fittings, interior design fixtures, and other decorative structures because of its elegant surface finish and corrosion resistance.

How Different Types of Anodizing Aluminum Process Work

So, what’s so different about anodizing aluminum compared to other finishes is its process. The majority of protective covers are added to the material while in anodization, the cover is formed by removing positive ions from the surface of aluminum parts. There are three major types of anodizing processes done on aluminum parts which result in the different finishes and appearances we see on several products.

Type I – Chromic Acid Anodize

This type uses a chromic acid solution to create a thin coating (from 0.5 up to 2.5 microns) on anodized parts. Chromic acid anodizing produces the thinnest coating and least color absorption among the three major types. Although the coating is relatively thin, it still provides viable protection for the aluminum part against corrosion and an effective first coat for powder-coated or painted finishes.

Type II Sulfuric Acid Anodizing

Sulfuric Acid Anodizing is the most commonly used method for anodizing aluminum parts, where sulfuric acid is used as the anodizing solution. It produces an anodized layer of 2.5 up to 25 microns. The porous nature of this process is perfect for absorbing dyes well.

Type III Hardcoat Anodizing

This anodizing process is also done on a sulfuric acid solution, however, the layer produced is much thicker and denser than that of the normal sulfuric acid anodization. This is used for tough applications where superior abrasion and corrosion resistance are needed.

Which type of Anodizing is the best for you?

Each anodizing type has its own advantages depending on what application it will be used for.

  • Chromic anodizing is favored for its thin layer coating while providing high corrosion resistance to coated parts. Choose chromic anodizing:
  • For parts that have tight dimensional tolerances
  • For parts that involve welding and assemblies
  • If used as a coating or paint primer
  • Sulfuric anodizing produces an adequately porous structure that is good for dyeing and providing matte finish. This is harder than chromic anodizing. Choose sulfuric acid anodizing:
  • If you are looking for  lesser expensive way of anodizing
  • If your application requires components to be hard and abrasion resistant.
  • For optical components, military weapons, hydraulic components, electronic enclosures, and other mechanical hardwares.
  • Hard anodized parts have a harder and thicker coating. Choose hard anodizing:
  • If your components need improved wear resistance
  • For components like large valves, pistons, cams, gears, and other hinging mechanisms.

Adding the Right Color Options to Anodized Aluminum and other Metals

Aluminum

different color in aluminum anodizing

Color anodizing options are affected by the alloying element used on the aluminum, the effects are as follows:

  • Copper- This element tends to increase the occurrence of galvanic corrosion pitting during the coloring process which may sometimes result in blotchy grey and brownish appearance.
  • Manganese- This element causes a yellow hint to the surface. The more manganese and chromium content, the darker the colors will appear.
  • Magnesium- This element causes the color to appear dull
  • Silicon- The higher its content on the alloy, the more it approaches the grey tint color.
  • Zinc- This element doesn’t cause much dulling into the anodized surface.

Titanium

Coloring anodized titanium doesn’t involve using dyes, but it involves differing the voltage applied on the system. Because of this process, the colors on the anodized part do not fade like the dyed ones.

Magnesium

The coloring of this metal is harder to achieve as the oxide layer isn’t thick enough for the dye to settle.

The Standard Specification of Anodized Aluminum

One of the most common standards used in anodizing aluminum is the US Military Specification MIL-A-8625.  This standard has become a basis for many applications involving components with highly critical applications like that in the aerospace and medical industry. This specification covers the standard requirements needed for the six types of anodic coatings typically used on highly precise applications.

Aside from the MIL-spec, there is also another standard used for architectural purposes, the AAMA 611-98. This specification is a basis for components used for decorative and commercial applications.

The Benefits of Anodizing Aluminum Parts and Other Metals

aluminum oxide

There are many advantages to anodizing your aluminum parts and here are some of the reasons why you may need to anodize:

Aesthetics- Anodized parts add a little hint of elegance and enhance a part’s overall surface finish.

Durability- The whole process of anodizing improves a part’s overall corrosion and abrasion resistance which heavily contributes to the extension of the lifespan of the workpart.

Safety- This coating process doesn’t result in significant destruction of environmental resources as it is chemically stable and non-toxic.

Ease of maintenance- The corrosion and abrasion resistance of anodized aluminum make it hard for the part to incur dents and wear that may cause additional fabrication and restoration processes.

Color Stability- Unlike other plating methodologies used in the industry, anodized parts do not chip off or peel because of their structure.

Economical- Anodizing aluminum cost is a cheap way of achieving a desirable finish with impressive surface properties.

Why should you consider anodizing aluminum rather than painting or another type of finish? There are actually a few points for that.

  1. The anodized layer is very hard. Oxides are usually harder than their base metal. The same thing can be said for aluminum dioxide. When we anodize a part, we increase the surface area of its oxide layer. So, The surface of the part becomes much harder. With Type 3 anodizing it can reach some metal cutting tool steels.
  2. The coating is very resistant to any kind of chemical reaction. The aluminum dioxide is extremely stable and doesn’t react with salts and the majority of other natural elements. Even at high temperatures, the oxide film perseveres.
  3. If you want to color your part and not be afraid that it peels off, you can anodize the part. The colorant stays deep inside the pores of the surface and it is impossible to chip or scratch unless you grind off the whole outer layer of the part.
  4. The process is extremely eco-friendly. The by-products are salts and the same remaining electrolyte. It makes anodizing stand out from toxic paints that are often used in the industry.
  5. Apart from paint, you can make adhesives fill the pores so anodized aluminum parts can be glued together.
  6. Anodization offers a beautiful unique finish that has a rich color and yet retains the original metallic shine.

Despite all the advantages, aluminum anodizing has some downsides as well. Firstly, anodized aluminum alloys are not suitable for kitchenware because they conduct heat poorly. In some extreme cases, it can result in temperature cracks when such parts encounter heat. Secondly, the fatigue resistance of anodized parts is lower because fatigue cracks are prone to concentrate in the pores.

Design Tips and Considerations for Anodizing Aluminum

Anodizing color for your aluminum parts

       Dimensional Considerations

Do bear in mind that you need to compensate for the anodizing thickness in arriving at your final part dimension and designation of feature tolerances.

       Hardness Requirements

Be mindful of your applications and their tensile strength requirements so that you’ll know what hardness you need to call out on your parts design.

       Color Matching

Manage your expectations when setting up your desired color as different metals and alloys react to various dyeing compounds and coloring parameters.

       Combined Coatings

Anodized parts may also be subjected to secondary coating processes like painting and Teflon impregnation to further enhance its corrosion resistance and structural integrity.

Difference Between Simple Aluminum And Anodized Aluminum

metal finishing of anodizing

The visual difference between a machined and an anodized surface is obvious. Anodized aluminum is matted compared to the shiny finish of the machined aluminum. It is rougher and looks more porous, which it is, basically. However, the part still has its metallic look even after painting, which makes anodizing such a lucrative option. Worn aluminum parts may have discrepancies in their surface where the oxide layer was damaged and restored itself later having contacted with oxygen, this is not seen in anodized parts since the protective layer is much thicker and can’t be damaged easily.

Application of Anodized Aluminum

Anodizing has, in such a way, revolutionized the aluminum fabrication industry due to its excellent and impressive enhancement of mechanical and aesthetic properties. This can be seen on almost any walk of life:

  • Aerospace and defense Industry
  • Medical Industry
  • Automotive Industry
  • Consumer electronics
  • Custom parts
  • Architecture and construction industries
  • Prototyping and product development
  • Housewares
  • Hydraulic fittings

Examples of end products using anodized aluminum:

Basically, when you need to harden aluminum alloys and make them more durable, You will look toward aluminum anodizing.

Window panes. Modern windows are constructed from lightweight and corrosion-resistant aluminum alloys. If you anodize the panes, it will be much harder to bend them, making the window more airproof.

Handheld devices. A lot of handheld devices (such as phones, MP3 players, speakers, etc.)  must be strong because they are often dropped or we accidentally strike something with them, we scratch them on something ( like keys or furniture), and not only should they resist these damages, they must not be deformed too much. By constructing appliance enclosures from aluminum with an anodized finish not only do they become durable but light as well.

Is Anodizing Aluminum Expensive?

anodizing process

Anodizing aluminum is not a complex process. You can even do it at home and sometimes you maybe can anodizing aluminum cost. You’ll need a tank that can withstand acids, a constant source of electrical current with high voltage, and the cathodes usually brass or steel. However, you won’t get a surface quality of industrial grade. The layer will have a different thickness in different areas and so the finish will look different in certain areas.

For the best surface quality it is best you consider using anodizing aluminum services. They will have the best equipment and the quality will be much higher. In addition, if you have a batch of parts to be anodized, you can do it much faster by outsourcing rather than purchasing your own equipment. Professional anodizing tanks can process tens of parts simultaneously in the same tank. They are just connected to the current as one large anode.

For the best surface quality it is best you consider using anodizing aluminum services. They will have the best equipment and the quality will be much higher. In addition, if you have a batch of parts to be anodized, you can do it much faster by outsourcing rather than purchasing your own equipment. Professional anodizing tanks can process tens of parts simultaneously in the same tank. They are just connected to the current as one large anode.

To Anodize, Or Not to Be Anodized?

Anodizing aluminum is one of the most popular and highly adapted methods of finishing aluminum parts due to its many advantages including enhanced surface finish, wear resistance, hardness, and corrosion resistance.

At Wayken Rapid Manufacturing, we offer the best aluminum anodizing service that will give you the best value with our high-quality processes, quick turn-around time, and competitive pricing. We provide practical and innovative solutions for achieving each of our client’s most demanding requirements.

If you have any quotations or projects to review, please don’t hesitate to contact us. Here, we can offer more cost-effective and practical advice on different surface finishing services that can help enhance your prototypes and parts.

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