The anodizing process is perfect for several materials, however, the most important and commonly used one is aluminum. If you are new to machining, you might not be familiar with anodizing. The anodizing process can make a layer of oxide on metal parts and prove helpful for increasing the visual qualities of metal parts.
This guide will take you through everything you need to know about anodizing aluminum. We will discuss what anodizing aluminum is, how to anodize aluminum and the benefits of aluminum anodization.
What is Anodizing aluminum?
Anodizing Aluminum is the electrochemical process of creating an oxide film on the surface of aluminum or aluminum alloy products when used as the anode in an electrolytic solution. This process utilizes electrolysis to form a thin layer of aluminum oxide on the surface, which is commonly known as anodizing for aluminum and its alloys. After undergoing anodization, the aluminum surface can develop an oxide film ranging from a few micrometers to several hundred micrometers in thickness. Compared to the natural oxide film found on aluminum alloys, anodized surfaces exhibit significantly enhanced corrosion resistance, wear resistance, and decorative properties.
The Principle of Aluminum Anodization
Among all the surface treatment methods for aluminum and aluminum alloys, anodization is the most widely applied. Aluminum anodization involves placing aluminum or its alloys in a suitable electrolyte solution (such as sulfuric acid, chromic acid, oxalic acid, etc.) as the anode. Under specific conditions and the influence of an external electric current, the aluminum or its alloy undergoes electrolysis. During this process, the aluminum or its alloy anodizes, forming a thin layer of aluminum oxide on the surface, with a thickness ranging from 5 to 20μm. In the case of hard anodization, the oxide layer can reach 60 to 200μm in thickness.
The reaction principles of anodization are as follows:
H2O – 2e- → [O] + 2H+ 2Al + 3[O] → Al2O3
The growth of the oxide film occurs simultaneously with its dissolution. However, the rates of growth and dissolution differ during different stages of oxidation. The film’s thickness only reaches a stable value when the growth rate and dissolution rate are equal.
Classification of Anodized Oxide Film
Oxide films are divided into two main categories: barrier-type oxide films and porous-type oxide films. Barrier-type oxide films are dense, poreless thin oxide films that adhere closely to the metal surface. Their thickness, generally not exceeding 0.1μm, depends on the applied voltage. Porous-type oxide films consist of two layers: a barrier layer and a porous layer. The thickness of the barrier layer is related to the applied voltage, while the thickness of the porous layer depends on the quantity of electricity passed through. The porous-type oxide film is the one most commonly used.
Structure of Oxide Films
The diagram below illustrates the porous honeycomb structure of oxide films on aluminum and aluminum alloys. These regular micro-pores are perpendicular to the surface, and parameters such as structural unit size, pore diameter, wall thickness, and barrier layer thickness can all be controlled by the composition of the electrolyte and process parameters.
In general, the length of the pores (film thickness) is more than 1000 times the pore diameter. The porosity is usually around 10%, while the porosity of hard films can be reduced to 2% to 4%. The porosity of oxide films used in construction is approximately 11%.
Porosity is related to the composition of the electrolyte. In commonly used electrolytes such as sulfuric acid, chromic acid, and oxalic acid, sulfuric acid has the greatest dissolution effect on the oxide film, while oxalic acid has the least.
Therefore, the anodized oxide film obtained in sulfuric acid electrolyte has the highest porosity, which can reach 20% to 30%. As a result, this type of film is relatively soft. However, it is highly elastic and has the strongest adsorption capacity.
Characteristics of Anodized Oxide Films
- Special Structure: Anodized oxide films exhibit a porous honeycomb-like structure, providing excellent adsorption capacity. They can serve as the base for coatings and can be dyed to enhance the decorative effect of metals.
- High Hardness: Anodized oxide films are characterized by high hardness, typically ranging from 196 to 490 HV. This high hardness translates to excellent wear resistance.
- High Corrosion Resistance: Aluminum oxide films are stable in both air and soil. They exhibit strong adhesion to the substrate. Typically, after anodization, they undergo sealing or painting treatments to further enhance their corrosion resistance.
- Strong Adhesion: Anodized oxide films adhere strongly to the base metal and are difficult to separate using mechanical means. Even when subjected to bending, the film maintains a strong bond with the base metal. However, these films have low plasticity and high brittleness. When subjected to significant impact loads and bending deformation, they may develop cracks. Therefore, such oxide films are not suitable for applications involving mechanical stresses and are often used as a base layer for paint coatings.
- High Insulation: Anodized oxide films on aluminum have high impedance and low thermal conductivity. They exhibit high thermal stability, with a temperature resistance of up to 1500 degrees Celsius. Their thermal conductivity ranges from 0.419 W/(m•K) to 1.26 W/(m•K). These properties make them suitable for use as the dielectric layer in electrolytic capacitors or as insulation layers in electrical products.
Four Types of Anodizing Processes
Sulfuric Acid Anodizing
Anodizing in a sulfuric acid electrolyte result in oxide films with thicknesses ranging from 5 to 20μm. These films possess strong adsorption capability, higher hardness, good wear resistance, and corrosion resistance. The films are colorless and transparent, allowing them to be easily dyed in various colors.
This process offers stability in the solution, allows for a wide range of impurity content, consumes less energy compared to chromic acid anodizing, is operationally convenient, and cost-effective. Factors affecting the quality of the oxide film include material factors, sulfuric acid concentration, impurities, current density, temperature, and processing time.
Chromic Acid Anodizing
Chromic acid anodizing results in thinner oxide films, typically measuring only 2 to 5μm. Consequently, products can maintain their original precision and surface roughness, making this process suitable for precision components. The films have lower porosity and softer quality, with less wear resistance. Compared to sulfuric acid anodizing, chromic acid anodizing incurs higher solution costs and greater energy consumption, limiting its use.
Oxalic Acid Anodizing
Oxalic acid anodizing produces thicker oxide films, typically around 8 to 20μm. These films are elastic, exhibit good corrosion resistance, and possess excellent electrical insulation properties. However, this method is costly, approximately three to five times more expensive than sulfuric acid anodizing. The solution is toxic, and its stability is comparatively poor.
Phosphoric Acid Anodizing
Oxide films formed in a phosphoric acid electrolyte dissolve more readily than in sulfuric acid, resulting in thinner films (approximately 3μm) with larger pores. Due to the strong water-resistant nature of phosphoric acid films, they prevent adhesive aging through hydration, leading to better adhesive bonding. Therefore, this method is mainly used for surface treatment of printed metal sheets and as a pretreatment for adhesive bonding of aluminum components.
Coloring and Sealing of Aluminum Anodized Films
The unique honeycomb-like structure of anodized aluminum alloy films can be utilized to achieve a variety of vibrant colors and enhance the corrosion resistance and wear resistance of the film through coloring and sealing treatments.
Coloring must be carried out immediately after anodization, and the oxide film should be thoroughly cleaned with cold water before coloring. In industrial production, both colored and uncolored aluminum and its alloy products are subjected to sealing treatments to prevent contamination of the oxide film and improve its corrosion resistance and insulating properties.
The most suitable oxide films for coloring are those obtained from sulfuric acid electrolytes. They can form colorless and transparent film layers on most aluminum and aluminum alloys, with strong pore adsorption capacity.
Common methods for coloring oxide films include adsorption coloring, which uses inorganic pigments or organic dyes, and electrochemical methods, such as the anodic coloring method. Anodic coloring involves placing aluminum and its alloys that have undergone anodization into an electrolyte solution containing metal salts and conducting electrolysis.
Through electrochemical reactions, heavy metal ions entering the micro-pores of the oxide film are reduced to metal atoms and deposited on the pore bottom, resulting in coloration. Oxide films obtained through this process exhibit excellent wear resistance, weather resistance, heat resistance, corrosion resistance, and color stability.
Oxide films obtained from electrolytes with moderate dissolving capabilities typically require further sealing treatments. The purpose of sealing is to close the pores in the film, thereby enhancing the film’s protective properties and preserving the coloring effect over an extended period.
Benefits Of Anodizing Aluminum Parts
There are numerous advantages to anodizing your parts, and here are some compelling reasons to consider anodization:
Anodized parts exude elegance and significantly enhance the overall surface finish of a component.
The anodizing process greatly improves a part’s corrosion and abrasion resistance, substantially extending its lifespan.
Ease of Maintenance
Anodized aluminum is highly resistant to corrosion and abrasion, making it resistant to dents and wear, which simplifies maintenance.
Unlike some other plating methods, anodized parts maintain their color integrity and do not chip or peel due to their unique structure.
Anodizing aluminum is a cost-efficient way to achieve a desirable finish with impressive surface properties, making it an economical choice.
The outer anodized layer of anodized aluminum parts possesses insulating properties and exhibits low electrical conductivity, making aluminum an ideal choice for anodized components.
Application Of Anodized Aluminum parts
Anodized aluminum parts find a wide range of applications across various industries due to their enhanced properties and durability. Some common applications of anodized aluminum parts include:
- Aerospace: Lightweight and corrosion-resistant components for aircraft and spacecraft.
- Automotive: Corrosion-resistant trim and engine parts.
- Construction: Durable building facades, window frames, and handrails.
- Electronics: Heat sinks and casings with insulating properties.
- Medical: Biocompatible equipment components.
- Consumer Goods: Scratch-resistant items like cookware and smartphones.
- Sporting Goods: Lightweight and durable equipment.
- Military: Corrosion-resistant parts for weaponry.
- Marine: Saltwater-resistant boat components.
- Decorative Items: Vibrant and long-lasting trophies and art.
- Electrical Industry: Insulating enclosures and connectors.
- Solar Panels: Weather-resistant frames for solar installations.
“Anodizing is a crucial manufacturing process involving the immersion of aluminum components in electrolytic solutions and chemical baths. Its applications span various industries, contributing to a safer way of living. Anodizing can vary in cost and complexity but guarantees superior, high-quality end products.
At KUSLA, we provide professional aluminum anodizing services, ensuring top-notch quality, fast turnaround times, and competitive prices. If you have projects or quotes to discuss, please reach out. We’re here to offer expert advice on different parts finishing services that can elevate your prototypes and components.”