Anodic oxidation 101: Every thing you need to know about Anodizing


Anodic oxidation 101: Every thing you need to know about Anodizing

Anodizing can deposit a chemically stable oxide layer on a metal surface. The resulting oxide film is thicker and stronger than the natural oxide coating of the metal. It is hard, porous, transparent and is an integral part of the metal surface, so it does not peel or flake. Once deposited, the anodic coating can be colored in a number of ways prior to sealing.

In this article, we will explore what anodizing is and how it works. We will also look at the different types of anodizing and their respective colors. Finally, we will discuss the advantages of anodizing over other finishing processes.

Also Read More:  Surface Finishes

What is Anodizing?

Anodizing Aluminum Parts

Anodizing is a process that is used to increase the thickness of the natural oxide layer on the surface of metal parts. It can be used on aluminum, magnesium, titanium, and other metals. The anodic layer can be dyed any color, and can also be textured or mirrored.

How Does Anodizing Work?

Anodizing is an electrochemical process that creates a protective oxide coating on metal. The metal is placed in an acidic solution and connected to a positive electrode (anode). A negative electrode (cathode) is also placed in the solution. When an electric current is passed through the solution, the metal ions are attracted to the cathode and the oxide coating forms on the anode.

The steps of Anodizing:

Step 1: The metal is cleaned and placed in an acidic solution. (The metal is connected to a positive electrode (anode). A negative electrode (cathode) is also placed in the solution. )

Step 2: When an electric current is passed through the solution, the metal ions are attracted to the cathode and the oxide coating forms on the anode.

The Types of Anodizing Process

There are 3 types of anodizing processes: decorative and industrial.

anodising aluminium process
  • Type I – Chromic Acid Anodize: 

Chromic acid anodizes (CAA) is a process that is used to improve the surface finish and corrosion resistance of alloys. 

This anodizing creates a thin, dense oxide coating on the surface of the metal. The process can be used on both bare and painted aluminium surfaces.

Chromic acid anodize produces a deep, satin black finish that is very resistant to corrosion. And it is often used for architectural applications or products that will be exposed to harsh environments.

  • Type II – Sulfuric Acid Anodize: 

This anodizing process involves immersing the aluminium part in a sulfuric acid bath and then applying an electrical current. This causes a thinner coatings of oxide to form on the surface of the metal.

This anodized layer can be anywhere from 0.0002 inches to 0.0050 inches, and it helps protect the aluminium from corrosion and wear. The sulfuric acid anodizing process can also improve aluminium parts’ appearance and mechanical properties.

This treatment is often used in the aerospace and automotive industries, as well as in medical devices and other high-tech equipment manufacturing.

  • Type III – hardcoat Anodize: 

The hard oxidation process refers to hard oxidation, the full name of hard anodic oxidation treatment. The hard anodized film is generally required to be 25-150um thick.

This process creates a thick, wear-resistant oxide layer on the surface of aluminum parts. This type of anodizing is often used on products that will be exposed to high levels of wear and tear, such as automotive engine blocks and piston rings.

The hardcoat anodize process can also be used to improve the appearance of aluminum parts. This treatment can produce a variety of colors, including black, bronze, and gold.

Hard anodizing is often used in the aerospace industry, as well as in the manufacturing of military equipment and other high-tech products.

Type I  Vs Type II Vs Type III, What’s the difference?

Chromic acid anodizing (CAA), sulfuric acid anodize (SAA), and hardcoat anodize are three common types of anodizing. Chromic acid anodizing is a process that uses chromic acid to create a thin, protective oxide layer on the surface of aluminum.

Type II is also a thin film coating, but it uses sulfuric acid instead of chromic acid. Hardcoat anodizing is a thicker film than the other two processes, and it offers more protection against wear and tear. So, which type of anodizing should you choose for your project?

Causes of defects in anodic oxidation coloring:

Anodizing process

In addition to natural oxidation, aluminium anodization can also be coloured, which is to adsorb non-ferrous metal salts in the pores of the oxide film before oxidation sealing. And that gives colour to the oxide film. However, improper treatment of oxidation colouring will cause uneven colouring, colouring difficulties, colour difference and other problems. Let’s analyze the reasons for these problems.


If the oil removal process of pretreatment is not complete, it will cause obvious white spots in the film layer, which will bring difficulties to coloring.

Electrolyte Concentration

The concentration increases, the dissolution rate of the film increases, the growth rate of the film decreases, the porosity of the film is high, the adsorption force is strong, elastic, good staining (easy to dye dark), but the hardness, wear resistance is slightly poor; When the concentration of sulfuric acid is reduced, the growth rate of the oxide film is accelerated, the pores of the film are less, the hardness is high, and the wear resistance is good.


The length of the colouring time will also affect the quality of coloring and color resistance, such as coloring time is short, color is easy to fade, time is long, color is too deep, and the surface is easy to flower.


The temperature has a great influence on the coloring. When the temperature is below 15℃, the coloring speed is slow, and when the temperature is too high, the coloring film will fog, and the Sn salt is easy to hydrolyze and reverse, resulting in the turbidity of the tank.


When the voltage is low, the coloring speed is slow, the color change is slow, and it is easy to produce color imbalance. When the voltage is high, the coloring speed is fast, and the coloring film is easy to peel off.


Whether in anodic oxidation film formation or electrolytic coloring, surfactant-based additives and stabilizing agents should be added to stabilize the film formation speed and film thickness, inhibit the dissolution of the oxide film and improve the uniformity of coloring.

The surface finishes of Anodizing

Anodizing Colors

Anodizing is a finishing process that can be applied to aluminum in order to give it a coloured finish. Many different colours can be achieved through anodizing, and each one has its own unique look and feel. 

Anodized film colour from brown to dark brown, transparent to black, generally according to the different aluminium alloy materials, processing process differences and different. The thicker the oxide film, the lower the temperature of the electrolyte, and the darker the colour of the oxide film.

The most common colors are black, bronze, and gold. However, other colors, such as blue and green, can also be achieved.

Surface Smoothness

After anodic oxidation, the surface of parts is changed. For a rough surface, after this treatment can appear than the original level off some, but for the original finish higher parts, often after this treatment, according to the smooth surface brightness decreases instead, lower amplitude in 1 ~ 2 magnitude.

The film thickness of Anodize oxide

The thickness of the anodized oxide film is graded, and the grade specification varies from 5 to 25 microns, depending on the application. Exterior finishes require a film thickness of 25 microns. Anodizing is suitable for extrusion, casting, rolling, drawing and forging aluminium products.

  • Anodization of sulfuric acid

Generally, the film thickness is 10 ~ 35μm, with high hardness, wear resistance, and good protective performance after sealing. This method should be the most widely used, and the cost is low. In general, the surface roughness after sulfuric acid anodization is 0.4 ~ 0.8, and additional processing is needed to meet the surface roughness requirement of 0.1…

  • Chromic acid anodizing

The general film thickness of 2 ~ 10μm, good corrosion resistance, and adequate electrical insulation can prevent contact with other metal galvanic corrosion. Under normal circumstances, the surface roughness after chromic acid anodization is 0.4 ~ 0.8, which needs to be supplemented to reach the surface roughness of 0.1, but because of the film…

  • Hardcoat anodization

Generally, the coating thickness is 10 ~ 60μm, which has good corrosion resistance, wear resistance and insulation. Different film layers with higher hardness or better toughness can be obtained by adjusting the process parameters.

Benefits of anodizing metals

anodizing coating

Anodizing process creates a protective oxide layer on the surface of metal parts. This layer helps to improve the durability and corrosion resistance of the metal, and it can also be used to give aluminum a colored finish.

There are many advantages to using anodized aluminum, including:


Anodize aluminum parts is more resistant to wear and tear than non-anodized aluminum.

The hardness of the anodic oxide film is much higher than that of the aluminum oxide:

  • The hardness of the matrix is about HV100.

  • The hardness of the ordinary anodic oxide film is about HV300.

  • The hard anodic oxide film can reach about HV500.  

The anodic oxide film not has higher hardness and better wear resistance, especially the porous oxide film on the surface layer, which can adsorb lubricant. It can further improve the wear resistance of the surface.

Corrosion resistant

The anodized coating surface has strong corrosion resistance because its anodic oxidation after the formation of the film has a relatively stable chemical reaction. Not easily disturbed by some elements of the outside world. If the surface is treated in other ways, it is easy to be corroded under some process conditions.

Easy to maintain

Anodized aluminum is easier to clean and maintain than non-anodized aluminum. Anodic oxide film has good stability, especially in the atmosphere, its corrosion resistance is much better than other methods of surface treatment. The anodic oxide film can be used as a decorative surface layer and does not require regular maintenance. 


Anodized aluminum can be given a variety of different finishes, including a matt, brushed or polished look. This surface finish can enhance the appearance of the aluminum and make it more visually appealing.


Anodizing is a relatively inexpensive process, especially compared to other surface treatment methods such as bead blasting, powder coating, painting,or metal plating.

Improved adhesion

The aluminum anodizing can improve the bonding strength of coatings, inks and adhesives on the surface of the aluminum.

Electrical insulation

The anodic films acts as an insulator and can help improve aluminium parts’ electrical insulation properties.

Thermal resistance

This is because the thermal conductivity of the anodic oxide film is much lower than that of pure aluminium: the anodic oxide film can withstand a temperature of about 1500℃. In contrast, pure aluminum can only withstand 660℃.

These are just a few of the advantages that anodizing aluminum can offer. As you can see, there are many reasons why you might want to consider anodizing your next aluminum project. Anodizing can improve the durability, appearance and performance of your aluminum parts, and it is a relatively inexpensive process. 

Electroplating Vs Anodizing, What is the difference?

Electroplating and anodizing are both electrochemical processes that can be used to change the surface of a metal. They both have their own unique benefits, and it can sometimes be difficult to decide which process is the best option for a particular application. We will compare and contrast electroplating and anodizing, and help you decide which process is right for your needs.

  1. Electroplating is depositing a thin layer of metal onto the surface of another metal. On the other hand, anodising is an electrochemical process that changes the surface of a metal

2. Electroplating is typically used to improve the appearance of metal, or to protect it from corrosion. Anodizing is often used for the same purposes, but can also be used to increase the hardness of a metal.

3. Electroplating is a relatively simple process that can be done at home with some basic equipment. Anodizing is a more complex process that usually requires specialized equipment.

4. Electroplating can be used on a wide variety of metals, including steel, aluminium, and brass. Anodizing can only be used on metals that are good conductors of electricity, such as aluminum and titanium.

So, which process is right for you? If you need to electroplate a metal for appearance or corrosion protection, electroplating is the way. Anodizing is the better option if you need to anodize a metal for increased hardness or corrosion resistance. Still not sure which process to use? Contact us today and we’ll help you decide.


Anodizing is an electrochemical process that changes the surface of a metal in a way that makes it more resistant to corrosion. Electroplating is typically used to improve a metal’s appearance or protect it from corrosion. Anodizing can only be used on metals that are good conductors of electricity, such as aluminum and titanium.

It offers many advantages over other surface treatments and is relatively inexpensive. If you are unsure about which process to use, contact us, and we will be happy to help you decide. Thanks for reading!

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