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The Anodizing Process Explained

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Anodizing is an electrochemical process that creates a protective oxide layer on aluminum. This oxide layer helps to prevent corrosion and wear and can also be used to give the aluminum a decorative finish. Anodizing is often used on products such as jewelry, kitchen utensils, and automotive parts.

 

How Does Anodizing Work?

The anodizing process can be divided into five basic parts: preparing the surface, anodizing itself, cleaning the parts, adding colour and sealing the pores.

 

Step 1: Preparing the Surface

Before you anodize a part, you have to prepare its surface through mechanical and chemical means. First, polish or bead blast the surface to ensure your part has the desired visual appearance. Bead blasting will give your part a natural matte finish, while brushing will give your part a brushed appearance.

 

Step 2: Anodizing Process

First, the metal part is connected to the positive terminal on an electric circuit. This positive terminal is called an anode (this is where the name of this process comes from).

 

the anodizing process explained. Submerging aluminium

 

With the part submerged and secured to ensure it doesn’t move around, the negative end of the electrical circuit (called a cathode) is connected to a metal electrode that is also submerged in the bath. A charge is sent through the circuit, and the cathode attracts positive ions from the metal part while the metal part attracts negative oxygen ions from the solution.

When the metal ions leave the metal part, the surface of the part becomes porous which allows the oxygen ions to bind to it. This forms the layer of oxide.

 

Step 3: Cleaning the Parts

After anodization, the parts have must be cleaned with deionized water and solvents and dried. This removes excess solution and readies the part for its chromatic finish.

 

Step 4: Adding Colour (Optional)

Anodized aluminum offers a wide range of colour and gloss options for aesthetic appeal, while still maintaining its metallic appearance. The colour of the metal can be changed in one of three ways: dip colouring, altering the thickness of the oxide layer, and electrolytic colouring.

 

the anodizing process explained: color anodizing blue

 

Dip colouring

The name “dip colouring” is pretty self-explanatory. The anodized part is dipped in a dye solution and the colour fills the pores. This method of adding colour to anodized parts is considered the least durable, since the dye degrades over time as it is exposed to UV rays.

 

Altering The Thickness Of The Oxide Layer

Light reflects differently on a layer of oxide depending on its thickness. By altering the thickness of the oxide layer, you can change the colour slightly.

 

Electrolytic colouring

If you want your metal part to have a black or bronze finish, you can accomplish this visual aesthetic by submerging your anodized part in a bath of metallic salts. The salts react with the part’s surface and fill the pores with a chemical compound that is black or brown in colour.

 

Part 5: Sealing Pores

Sealing is an important step of the anodization process used to preserve the aesthetics and improve the corrosion resistance of the anodized part by ensuring that the pores of the porous oxide layer are sealed. After the oxide layer forms, the part must be cleaned in a deionized solution before sealing the open pores. This step can be done using hot DI sealing, mid-temperature sealing, or cold or room temperature sealing.

 

the anodizing process explained: sealing pores

Hot Sealing

Hot DI sealing involves submerging the anodized metal part in deionized water that has been heated up almost to its boiling point. A reaction between the water and the part forms a mineral called boehmite that fills up the open pores.

The downside of hot DI sealing is that it can cause dyed parts to bleed their colour. It yields better results for clear anodized parts and parts that have been electrolytically colored.

 

Mid-Temperature Sealing

This method of sealing requires the part to be submerged in a solution containing metal salts, which fill the pores. It doesn’t cause as much bleeding of dyes as hot DI sealing. The only problem is it’s hard to control and to repeat with the same level of accuracy every time.

 

Cold Or Room Temperature Sealing

When an anodized part is cold or room-temperature sealed, it is set in a bath of a solution containing fluoride. The fluoride reacts with the surface layer of the part and then deposits a layer of fluoroaluminate, which seals off the pores.

Cold sealing can achieve the highest quality seal, which results in a more durable finish, but it’s a much slower process and is more challenging to control than hot water seals.

 

Materials That Can Be Anodized

Not all metals can be anodized. This is because of their molecular composition. The materials that can undergo this electrochemical process include aluminium, titanium, zinc, tantalum, and niobium.

Anodizing And Aluminium – Anodizing is one of the most common surface finishes applied to aluminium parts. Anodized aluminum is more durable than uncoated aluminum and is resistant to corrosion and wear. It is also easy to repair if the oxide layer is damaged.

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