What Is Metal Anodizing
Metal anodizing is an electrochemical process that creates an oxide layer on the surface of a metal. This process enhances both the mechanical and visual properties of a metal.
During the process, the metal part forms an anode electrode which reacts with the electrolytic solution when direct current is applied. Metal anodizing improves metal properties like; corrosion and scratch resistance, metal durability as well as aesthetic appearance.
Type I-Chromic Acid Metal Anodizing
This is because it exhibits good anti-corrosive properties and is decorative. It is less preferred because it is not environment friendly, has poor wear resistance, and produces very thin layer (0.08-0.25 micrometer).
Type II-Sulfuric Acid Metal Anodizing
It produces a slightly thicker film (2.54-25 micrometer) on the metal surface. This type of anodizing uses sulfuric acid in place of electrolyte solution. This type of metal anodizing is mostly applied where hardness and abrasion resistant properties are desirable.
You can use it in kitchenware, computer and electronic covering, military weapons, etc. Sulfuric acid anodizing enables colored surface finishing due to the porous nature of the unsealed film. The porous oxide film absorbs dye well creating a decorative surface finish on the metal part. It is affordable, fast and allows for color dyeing with waste management.
Type III-Hard Metal Anodizing
Type III anodizing is similar to type II anodizing but produces a thicker coating of up to 150 micrometers. This anodizing type is used to produce metals with excellent corrosion resistance, wear resistance and is environment friendly.
Sealing or dyeing is not recommended with type III anodizing because it reduces metal’s wear resistance. As a result, most hard anodized metal surfaces have a hard clear or black appearance. This type of anodizing is applied to metals that are exposed to extreme temperatures and chemicals such as gears and valves.
Metal Anodizing Process
Metal anodizing process involve the below steps.
Step 1: Pre-treatment
This is the first step before a metal can be anodized. The purpose of pre-treatment is to eliminate any impurities like oil, dust, etc. that may prevent the process. For instance, aluminum metal is cleaned using an alkaline detergent heated at approximately 63 degrees Celsius.
Step 2: Rinsing
Next, the metal is rinsed severally in a de-ionized water.
Step 3: Etching
Etching is a chemical process that removes a thin layer or film from a metal surface by eating it up. Etching also known as chemical milling uses strong chemicals such as sodium hydroxide for aluminum metals at this stage.
Step 4: Desmutting
This process removes excessive alloy metals from aluminum surface using inorganic acids such as hydrochloric acid or sulfuric acid. This is the final stage before anodizing process occurs.
Step 5: Anodizing
An electrolyte bath containing is prepared in a tank, the metal is then immersed and a direct current applied.
In this case, the tank forms the cathode while the metal forms an anode electrode. Once voltage is applied, negatively charged ions from the electrolyte solution bind with the anode (metal surface) forming an oxide layer. This results into an anodized metal.
Anodizing process results into pores on the metal surface to enable ions to bond and form a protective film. Therefore, sealing process is necessary to ensure maximum anti-corrosive properties.
Step 6: Sealing
Sealing can be done through hydrothermal treatment in chemical baths or precipitating metal salts in the pore openings.
Additionally, color pigments can be used as sealants leaving behind a permanent color pigment that cannot be scratched.
Benefits Of Anodizing Metals
Anodized metals have a prolonged lifespan due to the improved abrasion and corrosion resistance from the protective layer formed. They can be used regularly and in harsh environments without wearing out compared to the untreated metals.
Anodizing process is a safe process because it releases minimal harmful emissions to the environment. The chemically formed layer is stable and can be recycled compared to naturally formed oxide layers that wear out with time.
Anodizing process is a very economical process despite having complex steps. It results in a durable metal for example aluminum which requires minimal maintenance compared to un-anodized aluminum metal.
Types Of Metals Suitable For Anodizing
Metal anodizing is a durable surface finishing process that can be applied to a range of metals and substrates. Some of the metal types that are commonly used for metal anodizing include;
Aluminum anodizing is the most popular metal anodizing due to the resulting desirable mechanical properties and aesthetic appearance. Aluminum naturally forms an oxide layer with atmospheric oxygen which prevents further aluminum oxidation and does not rust. This layer however wears out easily because it does not adhere to the surface of aluminum hence the need to anodize.
How Aluminum Anodizing Works
Aluminum metal undergoes pre-treatment before anodizing process occurs. This include; cleaning, rinsing and etching. Just like other metals, aluminum metal is immersed in a chemical solution either chromic acid or sulfuric acid. It then forms an anode (positive) which then attracts the negative ions to bond when electric current is passed.
This electrochemical process results into an aluminum oxide layer. The type of electrolyte used (chromic or sulfuric acid) influences the thickness and properties of anodized aluminum.
Applications Of Anodized Aluminum
Anodized aluminum is used across various industries due to its beneficial properties such as durability and aesthetic appearance.
Some of the areas of use include:
- Aerospace; aircraft parts.
- Consumer goods: photo and radio equipment, light fixtures, etc.
- Architecture: duct covers
- Kitchenware: food preparation products, kitchen equipment, etc.
Anodize aluminum metal has the following properties:
- Improved insulation properties
- Better visual appearance because of the possibility of adding colors during anodizing process
- Improved corrosion, abrasion and wear resistances. They also do not scratch easily because the oxide layer is permanent unlike coatings.
- Does not rust.
Titanium is anodized using electric current to modify the oxide film on the titanium surface. Titanium anodizing can be done using type 2 or type 3 anodizing.
Titanium anodizing is done mainly for decorative and functional purposes, and its majorly used in aerospace and medical industries. Aerospace uses anodized titanium to prevent aircrafts from wear and corrosion, while medical devices use it due to its nontoxic nature.
Type 2 Titanium Anodizing
This type of titanium anodizing protects metals against wear and corrosion. Untreated titanium metals produce titanium dust when rubbed against each other. Therefore, medical devices especially orthopedic implants use type 2 anodized titanium for patient mobility.
Aerospace uses type 2 anodized titanium because they can withstand the extreme temperatures between (-70 to 270 degrees Celsius). Additionally, they are not corroded with salt water and humidity present in the atmosphere.
Type 2 anodized titanium have a distinctive grey color that makes them easily identifiable from stainless steel or untreated titanium.
Type 3 Titanium Anodizing
This type of titanium anodizing is also known as ‘color titanium’ because it results into colored titanium. The resulting anodized titanium is mainly used in medical field for orthopedic implants due to easy visual identification.
For example, a surgeon can easily identify anatomical difference between a right and left bone plate by their colors. Another area of application is in jewelry manufacturing.
How Titanium Anodizing Works
Titanium anodizing is an electrochemical process that uses both chemicals and electricity to manipulate the surface oxide layer.
Titanium metal is immersed in a chemical solution mostly Trisodium phosphate (TSP) or any other relevant salt solution. Electric current is then passed and the water undergo hydrolysis (splitting) producing oxygen ions that bond with the anode (Titanium).
An oxide layer is then formed on the titanium surface with the color depending on the layer thickness. The layer thickness can be manipulated to produce various colors with the thinnest layer being bronze and thickest green color respectively. Voltage of the electricity current dictates the color of the resulting layer and it ranges from 15 to 110 volts.
During this process, color interference occurs within certain wavelengths that either cancel each other or combine to form a color. As a result, titanium anodizing does not require dyeing hence it is considered bio medically safe for use in medical industry.
Anodized Stainless Steel
Stainless steel is rarely anodized like other metals despite being a conductive metal. This is because this process creates an iron oxide (rust) on its surface. Rust eats away the metal surface instead of protecting it.
Anodizing magnesium follows similar principles of the anodized metals and forms a thick oxide layer on its surface. This oxide layer aids in successful metal dyeing or paint adhesion. Anodized magnesium is therefore used as a paint primer due to this desirable quality.
Anodizing zinc is possible though rare and is done using lower voltages (20-30 volts) in silicate baths. The resulting oxide layer are hard and exhibit good corrosion resistance.
Challenges In Metal Anodizing
There are few challenges associated with metal anodizing process
The surface of the metal being anodized adds a significant thickness to the metal which can impact on allowed thickness tolerance.
Electrochemical process impacts on the hardness of the metal changing some metal hardness scale.
Color fading is common during metal anodizing process which eventually impacts on the overall color match. Strict adherence to process dynamics like batch anodizing is important when color anodizing.
Some metals have their conductivity is impacted for example, anodized aluminum surface becomes less conductive compared to bare aluminum surface. A mask can be used to ensure the metal surface does not lose its conductivity quality.
Applications Of Metal Anodizing
Metal anodizing improves certain qualities of a metal thus making its applications diverse.
- Aerospace industry for making air craft parts.
- Automotive; gears, valves, etc.
- Medical industry for orthopedic implants
- Consumer goods like jewelry, photo equipment, radio equipment.
- Architect; duct coating, light fixtures,
- Kitchenware; food preparation products.
Metal Anodizing Vs Metal Electroplating
Metal anodizing is the formation of an oxide layer on the metal surface to achieve functional and aesthetic properties. Metal electroplating is the process of adding a metal coating with desirable properties from anode to cathode. Both processes are electrochemically achieved using electric current and chemical baths.
Metal Anodizing Color Options
There are various metal anodizing color options that one can apply during the process. The colors can be checked from the Pantone color scheme and the pigment added to the oxide layer during anodizing and sealed.
Porous oxide layers are dyed by adding color pigments to the open pores and sealed to prevent bleed out. This method uses organ dyes which are intense and offer excellent weather and light-fastness. The colors include; yellow, green, black, blue, orange, pink, etc. and it is relatively cheap.
Metal dye colors like bronze and tin are electrically deposited to the anode pores during anodizing process. Titanium anodizing does not need dyeing as the color options can be achieved by varying the voltage applied. This is known as interference coloring.
Factors Determining Cost Of Metal Anodizing
Metal anodizing is a cost effective metal surface finishing process. The overall cost is however affected by factors like, type anodizing, part dimension, shape and quantity and coloring.
Adding color to anodized metal surface is the most complex step in anodizing process. Therefore, it costs much higher to produce a colored anodized metal surface than a non-color anodized metal.