A Comprehensive Guide to Aluminum Anodizing

Aluminum is a go-to material in many part manufacturing industries because of its excellent mechanical properties, myriads of alloys, low cost, and availability. Yet still, it undergoes surface treatment via anodizing for increased hardness, corrosion resistance, and sometimes aesthetics.

aluminum anodized parts

The aluminum anodizing process is a reputable industrial surface treatment because it produces a durable and beautiful product. However, you might struggle with the process without a fair chemistry background. As a result, this article will guide you through aluminum anodization, how it works, its benefits, and subsequent applications.

What is Aluminum Anodizing?

aluminum anodizing process

Aluminum anodizing is an anodizing process tailored towards aluminum alloys. It is an electrochemical process that involves coating an aluminum part with a layer of hard, corrosion-resistant, and wear-resistant aluminum oxide. As a result, anodized aluminum parts have excellent durability, corrosion resistance, wear resistance, a shiny finish, and, if applicable, a variety of colors that improve the part’s aesthetics.

Anodizing aluminum parts is a unique technique because, unlike the other surface treatment techniques, the addition of protective covers to the material occurs during the process.

Importance of Anodizing Aluminum in Various Industries

industrial aluminum anodized parts

Aluminum anodizing is used by part manufacturers that rely on non-ferrous materials such as aluminum. In this section, we discuss the importance of the process to different industries.

Aerospace

The aerospace industry uses aluminum parts because of their strength and lightweight. However, the anodized aluminum finish improves corrosion protection and hardness without compromising its strength-to-weight ratio. Consequently, the aluminum anodized finish is a popular surface treatment option for aircraft components.

Automotive

The automotive industry uses aluminum materials to make parts like the wheel and engine components. Since the parts become exposed to the external environment frequently, it can lead to rusting or corroding. An aluminum anodized finish provides an extra layer of protection against corrosion. Therefore, the industry relies more on it due to its durability and longevity. Additionally, it allows various color options, which can improve aesthetics.

Architecture

Because aluminum anodized parts are corrosion-resistant, will not rust or corrode, and can have different color options, they are attractive for beautiful construction exteriors or interiors.

The architecture and construction industry relies on the process’s ability to deliver different color options and textures and its excellent mechanical properties, such as a high strength-to-weight ratio.

Consumer Goods

Consumer goods are often a mix of quality, aesthetics, and functionality, all of which aluminum anodizing can deliver. Anodization is an eco-friendly and versatile process, evident in the popularity of anodized aluminum products in making microwaves, coffee makers, and other consumer goods. Manufacturers can use the anodizing process to ensure unparalleled beauty, longevity, and durability.

Homeware Sectors

Homeware products also use anodized parts due to their durability and aesthetic appeal. Consequently, products like aluminum lampshades and decorative exude luxury without compromising their quality, durability, and lightweight.

How Does Aluminum Anodizing Work?

aluminum anodizing working process

Anodizing an aluminum part is a multiple-step process aimed at improving the part’s properties, such as hardness, corrosion resistance, and aesthetics. Below is a stepwise procedure on how it works:

Cleaning

Clean the untreated aluminum surface using a mild detergent and pad to remove contaminants like dirt, grease, or oils. Proper cleaning will improve the adhesion of the anodic coating to the aluminum and ensure a quality coating.

Pre-treatment

Pre-treatment will allow you to remove surface impurities not removed by the cleaning step, surface irregularities, and excess alloying elements. Etching can clean and remove surface irregularities while de-smutting can remove smuts (thin layers of alloying elements) on the aluminum components.

Anodizing

Immerse the aluminum component in an electrolytic system with a bath containing an acid (chromic or sulfuric acid, depending on the type of anodizing), the aluminum part as the anode (aluminum anode), and an inert material like stainless steel as the cathode.

Passing a direct current (DC) through the electrolyte solution will cause it to release oxygen ions. At the same time, the aluminum anode goes into the solution and becomes aluminum ions. The oxygen ion and the aluminum cations react to form the layer of aluminum oxide (Al2O3) coating deposited on the part surface.

The summary of the electrochemical process at each electrode is summarized below:

Electrolyte

H2SO4 → H

At the Aluminum anode

Oxidation of the aluminum atoms occurs at the anode

2Al(s)+3H2​O(aq)→Al2​O3​(s) + 6H+(aq) + 6e−

At the cathode

Reduction occurs at the cathode

6H+(aq) + 6e− → 3H2(g)

Complete equation

The complete electrochemical reaction is:

2Al(s)+3H2​O(aq)→Al2​O3​(s) + 6H2(aq)

When aluminum goes into solution, it forms a microscopic groove or pore with size and shape depending on the metal alloy and the depth depending on the temperature, solution concentration, and voltage control.

The oxide coating depends on the acid anodizing type, voltage, and duration. Furthermore, thicker coatings ensure better corrosion resistance and higher surface hardness.

Coloring (if applicable)

Customizing the anodized aluminum components by introducing colors during or after the anodizing process is possible. This involves introducing dyes or pigments into its microscopic grooves created by the electrochemical process.

Coloring anodized aluminum is achieved by:

  • Electrolytic coloring
  • Dip coloring
  • Integral coloring
  • Interference coloring

Sealing

Seal the part using hot water (200ºF or 93ºC), which leads to the formation of hydrated aluminum oxide crystal responsible for sealing the pores. Other methods include steam and chemical sealing processes. Sealing allows you to lock in the colorants/dyes and improve the finished parts’ durability and corrosion resistance.

Types of Aluminum Anodizing

how aluminum anodizing works

Aluminum anodizing has three types with different specifications due to different materials, electrodes, and voltage. Making a quality anodized part requires knowledge of the three types of anodization, which are highlighted below:

Type I (Chromic Acid Anodizing)

Type I anodizing, also known as light or chromic acid anodizing uses chromic acid as an electrolyte with other setups previously explained constant.

On passing a current through the electrolyte, aluminum atoms leave the part’s surface, causing microscopic grooves, reacting with oxygen, and forming a protective oxide layer of about 0.08 to 0.25 um.

Type I anodizing offers more decorative applications. Nevertheless, Type I anodized parts have better heat and corrosion resistance compared with non-anodized aluminum parts.

Type II Sulfuric Acid Anodizing

Type II or sulfuric acid anodizing is similar to Type I anodization. The only difference is the use of sulfuric acid instead of chromic acid.

Sulfuric acid’s higher potency causes the formation of deeper grooves leading to a thicker oxide layer (2.54-25 um) than Type I parts. Consequently, Type 2 anodized parts have better mechanical properties than non-anodized and Type 1 anodized parts. Moreover, forming a deeper groove means the parts have better paint retention.

Type III Hardcoat Anodizing

Type III or hard coat anodizing is an advanced technique using higher voltage and sulfuric acid. It produces an oxide layer of 25 to 150 microns thicker than other anodizing types. Additionally, the oxide layer is extremely hard with a surface hardness of 70 Rockwell C or higher than base aluminum’s with 60 Rockwell C hardness.

Type III anodized aluminum components are used in heavy part manufacturing due to their high abrasion resistance and excellent corrosion resistance.

Differences Between Type I, Type II, and Type III Aluminum Anodizing Process

The major difference between the two types of aluminum anodizing processes stems from the use of different materials, electrodes, and energy. The table below shows a summary of their differences:

PropertiesType-I (ChromicAcid)Type-ll (SulphuricAcid)Type-Ill (HardAnodize)ApplicationsDecorativeDecorativeCorrosion ResistanceCorrosionResistanceGoodGoodExcellentCoating Thickness0.08-0.25 um2.54-25 um12.7-80 umWear ResistancePoorModerateExcellentPorosityLowModerate to HighHighEnvironment FriendlyNoYesYesAppearancePrimary CoatClear or dyedHard Clear or HardBlack

How to Identify If the Anodizing Process Is Successful

A successfully anodized part will pass the four tests.

Check For Matte Finish

Hold the anodized part in sunlight and check for a matte finish formed by the microscopic grooves. An absence of a matte finish can indicate an incomplete or unsuccessful anodizing process. Singly, the matte finish is not a concluding test.

Scratch Resistance Test

Scratch the parts with a sharp object, such as a hard nail. A completely anodized component will be scratch-resistant than uncoated aluminum. The degree of hardness, however, depends on the aluminum alloy and the anodizing type. For example, Type 3 anodized parts have the highest scratch resistance.

Eddy Current Test

Using the eddy current thickness tester will show the completion of the anodizing process and the coating thickness. Choose a tester, clean the anodized part, adjust the probe settings to suit the aluminum alloy for accuracy and sensitivity, and test the part. An unsuccessfully anodized component will have an unstable electromagnetic field.

Conductivity Test

Using a basic voltmeter, you can check the success of the anodizing process. Clean the anodized part and test the part. A significant deviation in the conductivity of the part shows a problem with the process. You can further use visual inspection to crosscheck the part.

Advantages of Anodized Aluminum vs Non-Anodized Aluminum

Anodizing aluminum has benefits like enhanced corrosion resistance and better aesthetics, which makes it better than other techniques. The benefits include:

Enhanced Corrosion Resistance

Anodizing an aluminum part increases corrosion resistance due to the formation of an unreactive oxide coating. Therefore, they are more resistant to chemical reactions even at high temperatures than the non-anodized aluminum part.

Increased Surface Hardness and Durability

The oxide coating is harder than base aluminum. Hence, anodized parts have higher surface hardness and durability. The hardness, however, depends on the type of anodizing, with type 3 anodizing reaching some metal cutting tool steels.

Improved Aesthetic Appeal

Anodizing an aluminum part will improve its aesthetic appeal, offering a beautiful finish that complements its use compared to the base aluminum with its dull finish.

Custom Color Options

The principle of color also applies to the anodizing process, although with a slight addition. Anodizing colors are subtractive, i.e., after reflecting the light on the part, the anodized film transmits light to the underlying aluminum substrate, which also reflects it to the film and outside.

This means that the film is more of a filter and plays an important role in color matching.

To choose a color option, you need to look out for the aluminum grade, as every aluminum grade has its color and shades, type of finish, the type of dyes available, layering, and parts of crystalline nature, which affect the reflection of light.

Electrical Insulation Benefits

The oxide coating has insulative properties, making the parts suitable for parts that need dampening of electrical conductivity, for example, in electronic enclosures.

Easier to Clean

Anodized parts are easier to clean because the coating seals the materials, preventing them from reacting with chemical cleaning agents. However, the use of strong chemicals can slowly compromise the parts.

Disadvantages of Anodizing Aluminum

Even though an anodized part’s excellent properties make it a recommended surface treatment process. A few disadvantages can limit its use. They include:

Limited to Aluminum and its Alloys

Anodizing is only suitable for non-ferrous materials such as aluminum, magnesium, and titanium. These materials can form oxides, which are hard and offer corrosion resistance.

Thickness Limitations

Anodized parts have a thin layer. Anodizing creates a thin layer of oxide ranging from 0.08um to 80 um, depending on the type of anodizing, for a thicker anodized coating improves corrosion resistance and hardness. However, it can incur a high cost, leading to relying on other processes. In contrast, the thinner anodized coating is also costly due to the extreme control needed to achieve that.

Wear Over Time

Anodized parts can wear over time, especially in high-wear operations that expose them to abrasion and harsh chemicals or environmental conditions.

Aluminum Alloys for Anodizing

Below are the common anodizing aluminum alloys, their constituents, properties, and applications.

Series 1000 (Pure Aluminum)

Alloys in the series contain 99% aluminum and impurities like iron and silicon. As a result, they have high electrical conductivity, excellent corrosion resistance, and flexibility. Anodizing a pure aluminum alloy will produce clear or translucent coatings and improve its hardness and corrosion resistance. The allows are applicable in making electrical components (e.g., connectors, wiring), nameplates, and signage.

Series 2000 (Aluminum-Copper Alloys)

Series 2000 alloys comprise aluminum and copper (2% to 7%). They can also contain small amounts of manganese and magnesium. The alloys are strong, machinable, and moderately corrosion-resistant.

Anodizing the alloy will improve corrosion resistance and wear resistance. Furthermore, it provides better decorative finishes, hence their architectural and automotive applications

Series 3000 (Aluminum-Manganese Alloys)

The series contains aluminum and manganese (1% to 1.5%). They have good formability, moderate strength, and excellent corrosion resistance. Anodizing the parts makes them better suited for making parts like heat exchangers, architectural panels, and facades.

Series 5000 (Aluminum-Magnesium Alloys)

The series alloys are made of aluminum and magnesium (2% to 6%) and have a high strength-to-weight ratio and excellent corrosion resistance. Anodizing the part will improve its corrosion resistance and durability but its anodized form is applicable in making architectural window frames and marine parts such as the boat hull.

Series 6000 (Aluminum-Magnesium-Silicon Alloys)

These are alloys of aluminum, magnesium (0.6% to 1.2%), and silicon (0.4% to 1.2%). They have good extrudability, a high strength-to-weight ratio, and excellent corrosion resistance. Common applications of the series 6000 alloys are in architectural extrusions such as window frames, door frames, and automotive parts such as body panels and wheels.

Series 7000 (Aluminum-Zinc Alloys)

The 7000 alloys mix aluminum and zinc (5% to 8%). They have a high strength-to-weight ratio, fatigue, and corrosion resistance. Anodizing improves corrosion resistance and enhances its surface finish for decorative and aerospace applications such as aircraft skins and structural components.

Considerations If Choosing Anodizing Aluminum

anodizing aluminum products with polishing finish

The anodizing aluminum process is now a go-to method for many parts manufacturing industries. However, choosing the process should come after considering the following factors.

Thickness of Anodized Layer

The anodized part thickness depends on the aluminum alloy and the type of anodizing used. Basic thickness is cheaper than thicker coating because the thinner the layer, the higher the anodizing cost.

This relationship is due to the control, experience, professionalism, and other factors that affect the cost. Part manufacturers often go for thicker coatings and employ lapping or honing to achieve a final thickness.

Dimensions

Anodizing an aluminum part will lead to surface growth – outer diameters get bigger and the hole smaller – due to the aluminum oxide layer. The extent of deposition of the coating depends on the type of anodization. However, it is estimated to be approximately 50% of the anodic layer’s total thickness.

Additionally, the process is unsuitable for sharp corners, especially Type III anodizing; hence, part manufacturers should consider the radii of outside edges and inside corners.

Conductivity

Anodized parts are good insulators due to their coating insulative properties. You can, however, mask the area that needs to be electrically conductive using a clear chemical conversion coating.

Combined Coatings

Anodizing is a prerequisite for several surface treatment processes because it improves adhesion and bonding. Common post-finishing processes after anodizing a part include:

  • Painting to increase the part’s corrosion resistance and to get colors not achievable using dyes. The microscopic grooves ensure the paint sticks compared to painting on a non-anodized metal through dyeing.
  • Teflon impregnation on Type III hard coats for reduced friction coefficient.
  • Electroplating nickel and chromium on an anodized aluminum part to get an aesthetically pleasing shiny appearance and improved corrosion resistance.

Color and Aesthetics

You can color anodized parts for improved aesthetics, light reduction, and part identification. However, before dyeing the part, you must discuss the following with the anodizing service provider.

Coloring methods

There are four common coloring methods for aluminum anodized parts. They include:

  • Electrolytic coloring involves immersive the part in a bath containing metallic salts, passing a current through the bath to allow the metal deposits to fill the anodic pores. This can enable you to get different UV-resistant colors, although it depends on the alloy and the duration of exposure.
  • Dip coloring involves immersing the part in a heated dye bath, allowing the die to fill the pores, and using de-ionized water to end the process. You can get various non-UV-resistant colors with this process.
  • Integral coloring involves integrating the dyeing process into the anodizing process. This provides shades of bronze and black and more abrasion resistance.
  • Interference coloring involves using sulfuric acid to increase the anodized part’s pore size and depositing metals of different colors on the pores. The result is due to optical interference effects rather than light scattering.

Color matching

Color matching can be challenging, especially if an assembly has multiple anodized pieces with different colors. You should dye them together to get the desired consistency.

Fading

The coatings can fade due to UV exposure and heat, especially when using organic dyes. Since many colors require organic dyes, you must navigate operational use and choose your hues accordingly.

Dye responsiveness

Dye responsiveness depends on the types of anodizing and the subsequent thickness. Type 1 anodized parts have a thin coating, challenging to reach true black. Consequently, the part will appear gray even when using black due. The same problem is seen in Type III thick coating. The best coating is achieved using Type II anodizing, allowing a wide range of color options.

Cost Considerations

Factors that affect the cost of anodizing an aluminum part include the part’s complexity, size, color option, and desired thickness. These factors further influence the choice of materials and type of anodizing. Therefore, you should consider the factors and weigh them against the desired outcome and budget.

Conclusion

Aluminum anodizing is a reputable surface treatment process because it guarantees the durability and aesthetic value of the finished product. You can anodize your aluminum part with the right materials and tools, such as tanks, chemicals, and stable electricity. However, for quality anodizing, consider using an anodizing aluminum service provider.

With us at RapidDirect, you get access to a quality-focused anodization process, short lead time, and easy quotation, all at a competitive price. Are you thinking of a smooth operation and high-quality aluminum anodizing at a low price? Contact us!

Try RapidDirect Now!

All information and uploads are secure and confidential.

FAQs

Does anodized aluminum rust?

No, anodized aluminum parts do not rust due to the presence of an aluminum oxide coating that is impervious to oxidation and corroding agents.

Does anodized aluminum wear off?

No, it does not wear off under normal use because the coating is a part of the base aluminum material, and it occurs at a microscopic level. However, abrasion can cause it to wear off, and exposure to sunlight can lead to color fading. Wearing off depends on the type of anodizing. For example, Type III anodized parts retain their color and wear resistance longer than Type II and Type I.

What happens if aluminum is not anodized?

Base or non-anodized aluminum is susceptible to corrosion, wear, and abrasion. As a result, some products, especially those exposed to seawater and other extreme conditions, need to be anodized.

Is anodized aluminum better?

Yes, anodizing aluminum has better mechanical properties than base aluminum. It is more durable, harder, corrosion resistant, and offers more color options, which leads to better aesthetic appeal.

Will anodized aluminum fade?

Yes, constant abrasion, exposure to sunlight (especially for organic dyes), and harsh environmental conditions can lead to an anodized part fading.

What can damage anodized aluminum?

You can damage an anodized part by using highly abrasive materials (materials with higher hardness) or corrosive chemicals.



Source link