Anodizing is an electrochemical process by which a metal surface is given an anodic oxide finish made of aluminum oxide. Unlike other forms of protective coating, the process of anodizing means the protective layer is fully integrated with the aluminum substrate underneath so that it does not chip or peel. Anodizing is used across a wide variety of industries for things such as: implants in the medical industry, microwaves and refrigerators in the electronics industry, and helicopter rotor blades in the aerospace industry to name a few. Materials suitable for anodizing include: aluminum, magnesium, zinc, and titanium.
This article will discuss anodizing, how it works, its uses, material compatibility, and its advantages and disadvantages.
What Is Anodizing?
Anodizing is an electrochemical process used to create a protective oxide layer on a metal surface. This metal oxide creates a protective coating which is part of the metal rather than a separate coating. This creates a protective barrier that is durable and chip-resistant. Metals that can be oxidized include: magnesium, aluminum, zinc, titanium, niobium, and tantalum alloys.
How Does Anodizing Work?
Anodizing works by using an electrochemical process to treat a bare metal surface and create a corrosion-resistant and durable finish. In this process, the metal is bathed in an acid (electrolyte solution), and an electrical current is passed through a cathode rod in the bath to the anode (the metal being treated). In doing so, the oxygen atoms can be released from the electrolyte and attached to the surface of the metal, which forms a metal oxide.
What Is the Use of Anodizing?
One of the main reasons for anodization is to increase a metal’s corrosion resistance. By creating an oxide layer on the surface of the metal, the oxygen in the environment cannot get to the metal, which in turn prevents corrosion. The process of anodization is used for corrosion resistance because of its durability. The oxide layer that forms due to anodization is a part of the metal, rather than a coating that is applied to the metal. This makes the oxide layer much stronger and more durable than coatings. Finally, anodization can be used to protect a metal while keeping the metal dimensionally stable. This is because anodization does not significantly increase the thickness of a part when compared to coatings used to protect the metal. Such a thin proactive layer can be achieved with anodization because the oxide layer penetrates the metal as the treatment is part of the metal substrate and not a coating that is applied to the substrate.
What Is the Advantage of Anodizing?
There are several advantages to anodizing metal which can include heightened corrosion resistance. Anodizing the metal will create an oxide layer to increase the corrosion resistance of the metal. Since anodization uses an oxide layer, it is UV resistant which can result in a more durable finish. Additionally, anodization requires no maintenance which is ideal for hard-to-reach parts. Finally, anodization is chemically stable and non-toxic, which is great for the environment.
What Is the Process of Anodizing?
There are five mandatory steps and one optional step to anodizing metal parts. These steps are discussed in more detail below:
1. Cleaning
The first step in anodization is to clean the metal to be treated. The first stage of cleaning is to remove any dirt or debris from the metal. The second is to degrease the surface. This is important to ensure an oxide layer can form evenly across the surface. Additionally, the metal can be chemically etched to remove a very thin layer of material to give the oxygen a fresh surface to bond to. Etching will create a smooth and consistent surface finish, and remove any small damage to the surface. If this step is missed, then the oxygen from the electrolyte will not be able to form an oxide layer and will leave an area of metal exposed or poorly anodized.
2. Preparation
After cleaning, the equipment needs to be prepared to anodize the metal. An anodizing tank large enough to fit the part to be anodized needs to be ready with the chosen acid solution. The temperature and concentration of the acid solution is a key parameter that needs to be decided before starting the process, as these parameters will affect the final thickness and properties of the protective layer. If these conditions are not properly prepared and the process is delayed then the newly etched metal surface will start to corrode due to the oxygen in the atmosphere. Additionally, the final thickness of the anodized layer will be incorrect if the parameters are not controlled.
3. Anodizing
Anodizing is the main step of the process. The part is bathed in an acid electrolyte solution. The cathode in the acid bath is commonly sheet aluminum, and the part to be anodized becomes an anode. The length of time the part is anodized, the intensity of the current used, the acid temperature, and the acid concentration all affect the thickness of the oxide layer. By monitoring the parameters, the part can be removed at the right time ensuring the appropriate oxide layer is formed.
4. Optional Coloring
There will be pores left from the creation of the oxide layer, these pores can be filled with a dye to color the part. To do this, the part needs to be dipped in the dye for a period to allow the dye to seep into the pores. This step is optional and only adds an aesthetic to the part as the pores are sealed in the next step.
5. Sealing
The pores created during the anodization of the parts must be closed to prevent corrosion. To do this, the parts can be dipped in either boiling water or a cold nickel acid solution. This will close the pores, and if the part has been dyed it will prevent the dye from leaking out. This step is essential to prevent the oxide layer from being penetrated and to stop the dye from leaking.
6. Quality Control
The parts must be inspected to ensure the process has worked. In the quality control, factors such as the oxide-layer thickness and surface finish are checked. The part can also be visually inspected for any defects. Adhesion tests can be used to confirm the anodized layer is fully adhering to the substrate, and corrosion tests can be performed to ensure the part performs as expected. Without quality control, any breakdown in the process will not be spotted and the parts will not perform as expected.
What Organization Sets the Standards for Anodizing?
The Aluminium Anodizers Council (AAC) provides a variety of process controls that can monitor and control the correct anodized coat for the specific application. The AAC has quality criteria that include: anodized-film uniformity and density; abrasion, corrosion, and fade resistance; reflectivity and image clarity; adhesion and sealing. Applicable testing includes: weight tests, microscopic tests, and dielectric tests. In addition, anodizing companies can also obtain ISO accreditations (e.g., ISO 9001) to certify that their processes meet the required standard.
How Much Does Anodizing Cost?
The cost of anodization will vary depending on the specific job. The cost of anodization is likely to increase with the size of the part and the thickness of the anodized layer required. The price may also vary with the type of metal being used, the number of parts being anodized, and with the use of optional color dyes. The price of anodization could be in the range of $2–120 per part depending on these factors.
What Are the Different Materials Suitable for Anodizing?
Several different materials can be anodized. Anodization can be used to create corrosion resistance or to improve aesthetic . Below are six metals that are commonly anodized:
1. Magnesium
Magnesium is a soft silver/white earth metal which is the eighth most abundant metal on earth. Magnesium is one of the lightest structural metals at a density of 1.737 g/cm³, with a strength of up to 280 MPa and a modulus of 45 GPa. Pure magnesium has a melting point of 650 °C and magnesium alloys will have varying melting points based on the composition. The corrosion resistance of magnesium is poor and can be made worse by the inclusion of alloying metals which cause intermetallic corrosion. Magnesium’s biggest advantage is its light weight which gives it a high strength-to-weight ratio, however, it is let down by its poor resistance to degradation. Due to this reason magnesium and all of its alloys can be and are anodized to prevent corrosion. The cost of magnesium before anodizing is $25 per kg.
magnesium
Magnesium. Image Credit: Shutterstock.com/RHJPhtotos
2. Aluminum
Aluminum is one of the most abundant metals on earth. It has a light and silver appearance and is known for its low cost and light weight. Aluminum weighs 2.71 g/cm³, with a strength of up to 570 MPa, and a modulus of 70 GPa. The melting point of aluminum is 660 °C. The corrosion resistance of aluminum is moderate but is much improved with anodization. Aluminum and all of its alloys can be anodized, and commonly are, to protect from corrosion. The advantages of aluminum are its high strength-to-weight ratio, low cost ($4 per kg), and recyclability. The disadvantages are aluminum’s high thermal conductivity which makes it hard to weld and its low strength compared to other metals such as steel or titanium.
aluminio
Aluminum. Image Credit: Shutterstock.com/BigTunaOnline
3. Zinc
cURL Too many subrequests.
cURL Too many subrequests.
cURL Too many subrequests.
cURL Too many subrequests.
cURL Too many subrequests.
cURL Too many subrequests.
cURL Too many subrequests.
cURL Too many subrequests.
cURL Too many subrequests.
cURL Too many subrequests.
cURL Too many subrequests.
cURL Too many subrequests.
cURL Too many subrequests.
cURL Too many subrequests.
cURL Too many subrequests.
cURL Too many subrequests.
cURL Too many subrequests.
cURL Too many subrequests.
cURL Too many subrequests.
cURL Too many subrequests.
cURL Too many subrequests.
cURL Too many subrequests.
cURL Too many subrequests.
cURL Too many subrequests.
cURL Too many subrequests.
cURL Too many subrequests.
cURL Too many subrequests.
cURL Too many subrequests.
cURL Too many subrequests.
cURL Too many subrequests.
cURL Too many subrequests.
cURL Too many subrequests.
cURL Too many subrequests.
cURL Too many subrequests.
cURL Too many subrequests.
cURL Too many subrequests.
cURL Too many subrequests.
XTJ es un fabricante OEM líder dedicado a ofrecer soluciones integrales de fabricación de mecanizado de aluminio 6061, desde prototipos hasta producción. Nos enorgullece ser una empresa certificada con sistema de gestión de calidad ISO 9001 y estamos decididos a crear valor en cada relación con el cliente. Lo logramos a través de la colaboración, innovación, mejoras en los procesos y una mano de obra excepcional.
