Anodized Aluminum, how to anodize aluminum?

Mosi Li
Mosi Li

Director Project Engineer at 3Q Machining - China top CNC Machining Custom Part Manufacturer

What is anodizing aluminum?

Generally speaking, aluminum or aluminum alloy is used as the anode for the anode, and the lead plate is selected for the cathode. The aluminum and lead plate are placed together in the aqueous solution. There are sulfuric acid, oxalic acid, chromic acid, etc., for electrolysis. An oxide film is formed on the surface. If anodizing is not specified, it usually refers to sulfuric acid anodizing.

Anodizing Aluminum is accomplished by immersing the aluminum into an acid electrolyte bath and passing an electric current through the medium. A cathode is mounted to the inside of the anodizing tank; the aluminum acts as an anode, so that oxygen ions are released from the electrolyte to combine with the aluminum atoms at the surface of the part being anodized. Anodizing is, therefore, a matter of highly controlled oxidation the enhancement of a naturally occurring phenomenon.

Sources: https://www.anodizing.org/page/what-is-anodizing 

Why do aluminum alloys need to be anodized?

As one of the most popular and widely used metal material, aluminum is good for CNC Machining and it is always the top choice for engineering designer, but most of aluminum machined parts require surface treatment and the best recommendation is anodizing, but why aluminum parts needs to be anodized?

The purpose of aluminum anodization is to overcome the defects of aluminum surface properties and expand the application range and service life of aluminum. Fundamentally speaking, it is to improve the protective, decorative and functional properties of aluminum alloy. Protection mainly refers to preventing the corrosion of aluminum and protecting the appearance of aluminum. The decoration mainly starts from the aesthetics, and improves the appearance quality, such as improving the metallic luster and various colors. Functionality refers to increasing surface hardness, abrasion resistance, electrical insulation, etc. The porous type of anodized film can also be used to impart electromagnetic and photoelectric functions to the surface.

What is the benefit of anodizing aluminum

We all know that aluminum is widely used in industry, and there are many kinds of surface treatment methods for aluminum. There are usually anodizing treatment, chemical conversion treatment, micro-arc oxidation treatment, electroplating treatment, plating treatment or surface organic coating (spraying). Powder or spray paint) and so on. And the most widely used process is anodizing, so why is anodizing is most popular for aluminum?  

The anodizing process of aluminum and aluminum alloys has a wide range of applications in industry. It can be used to prevent corrosion of products or achieve the dual purpose of protection and decoration, such as wear-resistant layer, electrical insulation layer, spray paint bottom layer and electroplating bottom layer.

  • Improve hardness and wear risistance: The hardness of aluminum is relatively low, and at the same time wear resistance is relatively poor. Anodizing can improve the hardness and wearing resistance for Aluminum .
  • Prevent corrosion of aluminum products: Aluminium is poor of corrosion resistance especially when it is pure aluminum, Anodizing can improve corrosion resistance of aluminum. ” The aluminum alloy anodic oxide film has passed the copper salt accelerated acid salt spray test (CASS) for 16 hours, and the protection level of the film layer is not less than 9. Corresponding to the neutral salt spray test (NSS), it is almost the standard of 480h.”
  • As an electrical insulating layer: The oxide film obtained after anodization of aluminum and aluminum alloy products has a large resistance, so it has a certain effect on improving the electrical insulation of some products. Anodization can be used to prepare the dielectric layer of the capacitor, or it can be used Aluminum oxide prepares an insulating layer for its surface.
  • As the bottom layer of spray paint or electroplating:

    Due to the porosity and good adsorption capacity of the anodic oxide film, it can be used as the bottom layer of spray paint and other organic films, so that the paint film and organic film can be firmly combined with the product, thereby increasing its corrosion resistance.

    Before aluminum and aluminum alloy products are electroplated, a bottom layer must be applied to them before electroplating. There are many methods for applying the bottom layer on the surface of the substrate. In addition to electro-galvanizing, dipping zinc, and electroless nickel plating, anodizing is also one of the important methods.

  • Protection-decorative effect: For most aluminum and its alloy products that require surface finishing, after chemical or electrochemical polishing, anodized oxidation film with sulfuric acid solution can be used to obtain an oxide film with higher transparency. This kind of oxide film can absorb many kinds of organic dyes and inorganic dyes, so it has a variety of bright colors. This color film is both an anti-corrosion layer and a decorative layer. Under some special process conditions, a protective and decorative oxide film similar in appearance to porcelain can also be obtained.

3Q MACHINING offers one stop solution for your custom machined aluminum parts from Raw Material to finished parts with premium surface treatment finishes which including of anodizing, you can utilize anodizing to enhance the functionality and look of your aluminum parts and prototypes. To understand more information what is anodizing aluminum, keep reading this article here:

Aluminum Alloy Anodized - Aluminum Surface Treatment

Aluminum Anodizing Process Steps

The pre-treatment of machined aluminum parts is must in order to anodizing aluminum parts can achieve high quality surface, we usually remove the flash of the aluminum then Ultrasonic-cleaning and drying the aluminum parts before anodizing or other surface treatments, this can prevent poor quality surface problem happens.

Anodizing is a relatively simple electrochemical process, which has a history of nearly a century, and is used to increase the thickness of the natural oxide layer on the aluminum surface.

There are many types of anodizing aluminum, which can be divided into sulfuric acid (hard) anodization, chromic acid anodization, phosphoric acid anodization, and environmental protection anodization according to the composition of the bath.

  1. Anodizing Type I: Sulfuric acid anodization is the most commonly used. The thickness of the film is generally (5-14) microns. It is also relatively simple and has good corrosion resistance. Because of its porous structure, it can usually be filled with chromate, boiling water or dyeing. Can be used as the bottom layer for spraying.  1.2: Hard anodization can be said to be a special sulfuric acid anodization. Its thickness can reach 40 microns or more. It has good corrosion resistance and abrasion resistance. However, in the production process, the current is large, and the requirements for tooling and clamping are higher. Good conduction, otherwise it is easy to damage the parts or burn the parts. Sulfuric acid anodization will reduce the fatigue performance of parts.
  2. Anodizing Type II: Chromic acid anodization, the bath composition is mainly high-priced chromium, and the thickness of the film is generally (3~5) microns, which is suitable for the production of precision-sized products, and has little effect on the fatigue of parts.
  3. Anodizing Tyep III: Phosphoric acid anodization, basically no independently produced products, most of them are used as bonding bottom layer, and its bonding performance is good.
  4. Anodizing Type IIII: As environmental protection requirements become more and more stringent, environmental anodization research is currently being carried out on the market, and Airbus has already applied it to aircraft manufacturing. At this stage, environmental protection anodization is mainly based on the sulfuric acid-organic acid system. Organic acids include oxalic acid, adipic acid, etc., which can replace chromic acid anodization and produce precision-sized products.

Sources: https://www.youtube.com/channel/UCS5cWyINMDe4ZyaZcJki76Q

The most and widely use aluminum anodizing is Sulfuric acid anodization, especailly the hard anodization, process steps of aluminum anodizing as follow:

How to anodize aluminum?

1. Pretreatment:

Aluminum alloy parts are stained with more grease, a small amount of abrasives, dust, and defective oxide films during multiple machining processes. These materials have poor electrical conductivity and cannot be anodized, so they need to be pre-treated.

The method is to use carbon tetrachloride, trichloroethylene, gasoline or toluene as a cleaning agent, immerse the aluminum alloy die castings, scrub with a brush, then air dry, and then immerse in water for multiple cleanings. After the oil is gone, rinse immediately with hot water. If a black film is formed on the surface, soak it in a 32% nitric acid solution for 20 seconds to remove the black film, and finally rinse it off with cold water. Immerse in distilled water to prepare oxide film.

2. Anodizing:

⑴Sulfuric acid electrolyte preparation: It is made by mixing sulfuric acid and deionized water. At this time, the specific gravity of the solution is about 1.125-1.140. Sometimes, in order to obtain a good protective oxygen oxide film, a small amount of oxalic acid is usually added to the sulfuric acid electrolyte.

⑵Oxidation process: Install the line and instrument, the aluminum alloy parts to be processed are used as anodes and all are immersed in the electrolyte, then the power is turned on, and the following process conditions are controlled.

The temperature of the electrolyte is controlled at 12-25℃, the anode current density is 1-2A/dm2, and the voltage in the tank is between 13-23V. It takes about 30-40 minutes.

After the above-mentioned process is completed, remove the aluminum alloy pressed part from the electrolyte at any time, and rinse the acid with clean water. Pay more attention to the concave part, otherwise white spots will appear.

1. Polishing–2. degreasing–3. washing–4. anodic oxidation–5. washing–6. sealing–7. mechanical bright

Anodized aluminum parts

When should you anodize aluminum?

Now you know what is aluminum anodizing, why we need anodizing for our aluminum parts, and the process steps of anodizing, but when should your aluminum parts need to be anodized?

Here you can reach 3Q MACHINING team to discuss the end use of your aluminum machined parts, that we can find out what surfacture treatment is need to determine if your aluminum parts should be anodized or not.

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