Anodizing metals improves their surface hardness, and corrosion resistance while providing an appealing look that every manufacturer desires.
So what exactly is anodizing and how do you perform it?
Anodizing is a surface treatment process in which an oxide layer is deposited over the metal workpiece to enhance its corrosion resistance, durability, and aesthetic appearance. It is an electrochemical process that also allows for the addition of colors and further enhances the appearance of the workpiece.
This article discusses the anodizing process by going through its types, advantages, and limitation.
In the end, the article also explains the difference between anodization and electroplating.
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Anodizing is an electrolytic operation that is generally performed on Aluminum and its alloys.
Though challenging, anodizing of steel and stainless steel can also be achieved by following proper procedures to attain a protective coating over steel workpieces.
Apart from steel and aluminum, anodizing can also be performed on non-magnetic metals like gold and bronze.
It deposits an oxide layer over the base metal thereby preventing it from corrosion and scratches even under abrasive conditions.
The thick oxide layer that forms at the end of the procedure acts as a resistive layer increasing the hardness of the final product.
Apart from offering better resistance to wear and corrosion, the process is known to be used for enhancing the aesthetics of the product, making it more appealing.
Generally, the anodized layer has a thickness of around 5µm to 100µm.
However, an anodized layer of over 40µm thickness is preferred for applications where wear and corrosion resistance are the primary objectives.
On the other hand, an anodized layer of under 40µm thickness is used for applications that are not subjected to extreme conditions, and enhancing the look is the only purpose of anodizing.
The thickness of the anodizing coating plays an important role in determining the cost of anodizing process.
For workpieces made from metals that don't rust easily, a thin coating is preferable as it provides the required aesthetics at lower costs.
Types of Anodizing
|Type of Anodizing||Characteristics||Applications|
|Type 1||A thin layer with good corrosion resistance||Mask for type 3 anodized parts|
|Type 2||Comparatively thicker layer with the ability to add colors||Military, aerospace, and decorative purposes|
|Type 3||Thickest layer with high durability but no option to add colors||Military, aerospace, and utensils|
The type 1 anodizing process involves the use of sulfuric acid anodizing solution with a concentration in the range of 10-15% precisely.
This type of anodizing produces a thin coating (around 0.0001") with very high corrosion resistance.
As a result, it is also used as a masking layer over type 3 anodized parts.
Type 2 Anodizing
The Type 2 process utilizes a mixture of sulfuric acid and oxalic acid as the anodizing solution.
It produces a porous anodized layer which is suitable for adding colors to the anodizing.
Generally, type 2 anodizing is performed for aerospace and military equipment where specific colors are to be added to the anodized layer.
It develops good corrosion resistance- and heat resistance properties to the workpiece.
Generally, irrespective of the type of anodizing, the color options of anodized aluminum are comparatively more than other metals.
Type 3 Anodizing
The third process is better known as the hard coating as it produces an anodized layer that is thick and provides excellent resistance to wear.
This process is similar to type 1 anodizing but varies in the thickness of the coating by varying the voltage, temperature, and cooling rates of the workpiece.
As a result, it is the slowest anodizing process and does not provide the ability to add color to the layer.
However, you can achieve different shades of black and grey by varying the process parameters.
How to Anodize a Material?
The most crucial steps to the anodizing process can be summarized as pre-process treatment, electrolytic operation, oxidation to form the coating, and finally post-processing techniques.
|Preparing the workpiece||Remove unwanted material and achieve a smooth surface|
|Etching the workpiece||Remove impurities and projections to get a uniform coating|
|Desmutting the workpiece||Remove any residual alloying material|
|Anodizing the workpiece||Get the desired oxide layer|
|Inspection and Cleaning||Check for flaws and clean the workpiece|
|Adding Color||Select a suitable process to add color to the workpiece|
|Sealing||Seal the pores to prevent leaking of color and protect anodized layer|
Step 1: Prepare the Workpiece for Anodization
It is a vital process that has significant after-effects on the physical characteristics of the final product and most importantly its appearance.
The properties of the coating are also affected by preprocessing. This step is the most specific in terms of the method to be followed and takes an ample amount of time.
Surface treatment using mechanical means is vital to the procedure. The most prominent processes amongst others include abrasive polishing.
Bead blasting is another process used in surface treatment to achieve a high surface finish.
The process is very common and is used to remove any machining marks which are generally present after milling, turning or other subtractive machining operations.
Step 2: Etching the Workpiece
Chemical means on the other hand include etching and Desmutting. Without the use of chemicals in surface treatment certain desirable characteristics are impossible to achieve.
Etching uses acid etchants to remove a fine layer of material making the surface smooth and uniform.
The Chemical etching process provides clearance to the workpiece while removing some material so that an oxide layer can cover the part without significantly affecting the thickness parameter.
A special etching procedure can also be adopted to achieve a bright surface finish, known as the bright dip anodizing technique.
Step 3: Desmutting the Workpiece
Desmutting is the follow-up process after etching to remove the excessive alloyed metal and achieve a precise thickness according to the application.
The process can be achieved by rinsing the workpiece in a tank consisting of any inorganic acid such as sulfuric or nitric acid.
A pivotal instruction to note before performing the surface treatment processes is that the part dimensions must be accurate and the geometry complete.
Any subtractive machining should be performed before the start of any surface treatment.
Step 4: Anodize the Part
Anodizing the workpiece involves submerging it into the anodizing tank containing the electrolyte.
The workpiece is connected to the anode to facilitate the deposition of oxide ions and achieve the oxidation process.
Sodium phosphate is used as the electrolyte solution to introduce ions essential for the passage of current.
After the part has been submerged and ready to be anodized the cathode is applied with a negative potential.
The ions from the workpiece (anode) flow toward the cathode.
As the ions/charged molecules leave the workpiece metal, the part becomes porous.
The anode has an increased porosity and forms an oxide layer that is very adhesive and coherent.
Step 5: Inspect the Quality and Clean the Workpiece
As the process proceeds, the part is frequently removed from the electrolyte solution or from the surface treatment bath to check the physical parameters such as the surface finish and thickness.
There are various quality checks to ensure that a sufficient amount of anodization has been achieved.
The most common cheque is electrical testing. Using an ohm meter if the metal workpiece shows a very high resistance (approaching infinity), then it is conducive that the part has been anodized.
This is because the anodized surface is nonconductive in nature and is often used for making electrical enclosures.
On the other hand, the ohm meter will display a small amount of resistance if the part has not been properly anodized.
A simple scratch test is another fast and easy-to-use method. Where an operator can try and scratch the surface using a sharp edge. A completely anodized surface would not be marked as it provides high resistance to scratches.
Apart from that, to ensure the quality of the final part, a simple color dispersal test can be performed. A properly anodized surface would have an even dispersion of color.
Anodizing burns can be visually observed to register any faults. Which are due to an uneven current density application.
Step 6: Add Color to the Finished Product
Before the part can be colored and the pores sealed, rinsing is essential for preventing the formation of any ‘crud’. Which might leave the final surface distorted and nonuniform.
Rinsing is done alternatively in cold and hot water to open the pores and remove any residues from the anodizing solution.
After rinsing the part is submerged in a dye at a specific temperature. The anodizing dye seeps into the pores imparting a desired color on the surface.
Generally, color anodizing is performed for aluminum workpieces, and depending on your requirement, there are various techniques to achieve different anodized aluminum colors.
Step 7: Seal the Pores
Sealing the pores is usually done using either hot water sealing or cold water sealing combined with a nickel acid solution.
The color gets sealed in the pores preventing any leaching of color and ensuring that the workpiece has achieved its required chemical resistance.
If any bleeding or marks are not present during the final inspection of the aluminum surface, the part is ready to be used for your application.
Generally, when comparing anodizing with powder coating, anodizing is comparatively more durable but requires a tedious and costly procedure.
Factors Affecting the Anodization Process
The anodization process can reach its full potential if all of its parameters are monitored and controlled.
Selection of Workpiece Material
Base metal, which is the anode on which the oxide layer is deposited, can affect the quality of the anodized layer.
Therefore, it is important to select the right metal because different metals and their alloys produce different results under the same anodizing parameters.
Generally, 6061 Aluminum is a popular choice amongst manufacturers given its affinity for forming an oxide layer very easily, accompanied by high strength and corrosion resistance.
The surface of the part needs to be prepared for the precise application of chemical and mechanical surface treatments.
Anodization demands proper handling and care as the whole substrate surface is exposed throughout the procedure.
Mismanagement of the material surface can deem the surface unable to be treated without any defects or need for suitable rectification usually by the use of chemicals.
Chemistry of the Baths Used
The material being anodized is submerged for each process including etching, anodizing, coloring, sealing, and rinsing.
Some of the fundamental controls are PH, temperature, concentration, and time duration of the material submersion within the solution.
The specific chemistry of each bath needs to be controlled for the chemical reactions to be performed adequately.
There are various software available for monitoring the chemical composition and even managing the chemical composition as per your requirement.
Timely and regular calibration of the electrical supply is an imminent control task.
The anodizing time, bath temperature, agitation, and cathode quality are some other factors that need to be considered for proper anodization of the workpiece.
Advantages of Anodization
Anodized aluminum alloys or other metals have a long life span due to their toughness and surface hardness properties.
The oxide layer adds to the wear resistance and prevents the metal from corrosion.
As the color is permanently sealed off within the pores, the color does not leach away or peel off even when subjected to chemical or abrasive conditions.
Apart from that, it provides a long life and is resistant to damage under UV radiations of the sun.
Ease of Maintenance
The scars and marks on the surface are practically nonexistent. The dirt usually does not accumulate but can be cleaned off easily in any case.
Rinsing and washing using mild soap cleans the surface of the anodized workpiece and restores the original appearance of the material.
Mild abrasives may be required sometimes as well for scraping off some unusual persistent dirt.
The process allows the material to retain its metallic appearance providing a glossy look while enhancing its luster.
Apart from that, the ability to add uniform layers of different colors, makes the workpiece appealing.
Although anodizing the workpiece requires special baths to perform cleaning and etching operations, the cost compared to its value is extremely low due to the materials’ long-life span.
Limitations of Anodization
Anodization of aluminum requires very skillful workers with good knowledge of the procedure to avoid any possible fundamental mistakes.
Apart from that, color matching of the workpiece can be difficult when following a batch-type production system.
Furthermore, it is a time-consuming process and requires large tanks to anodize multiple workpieces simultaneously to save time.
Applications of Anodization
The anodized final product is durable, weather resistant, and with a pleasing appearance. This creates endless applications for the process.
One of the most common applications includes structural parts, utensils, furniture, architectural components, and machine components e.g., motors, jewelry, and artwork.
Apart from that, the ability to add color makes it suitable for making signboards for outdoor applications, providing excellent wear resistance.
The electrical resistance of the anodized layer also makes it suitable for making casings for electrical components.
Anodization vs Electroplating
|Purpose||Addition of an oxide layer||Coating a metal on another|
|Properties||Corrosion and electrical resistance||Corrosion and wear resistance|
|Material Flexibility||Only metals||Both metals and nonmetals|
It involves the addition of an oxide layer or increasing the thickness of the existing oxide layer on a metal, while electroplating is the process of coating a specific metal on another with no involvement of an oxide layer.
The method involves electrochemical treatment in which only the material to be used is the anode and is of importance.
Whereas in electroplating, the process is similar to electrolysis with both the anode and cathode determining the plating layer formed over the workpiece.
Only metals can be anodized, whereas electroplating can be used to add a metallic layer over metal and non-metal workpieces.
Both metals and nonmetals can be electroplated.
Anodizing process is widely used with aluminum and its alloys along with some other metals such as zinc, titanium, and niobium.
Different types of anodizing produce different results, therefore it is important to identify your requirements before executing the process.
As hangers are used to submerge the metal part in the electrolyte, the edges are often clamped and do not participate in the formation of the oxide layer.
Pre-processing which includes etching, bead blasting, and abrasive polishing is a crucial step in determining the quality of anodized surface, and therefore must be monitored carefully.
Frequently Asked Questions (FAQ)
What materials can be anodized?
Metals that can react with oxygen and form an oxide layer can be anodized. Aluminum is the most frequently used metal in anodization. Other metals such as titanium, tantalum, and zinc are often anodized as well.
Which type of anodizing is the cheapest?
Type 2 anodizing is the cheapest of all and provides the ability to add colors to the anodized surface. However, it provides low hardness compared to the type 3 process.
Can the anodized surface wear off?
yes, the anodized surface can wear off. Although the process provides a very hard surface that does not wear off easily, an acid reaction can wash the oxide layer. Apart from that, the oxide layer can also wear off under normal conditions, after prolonged use.
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