What Is an Alodine Treatment?
Alodine treatment is a chromate treatment used for the surface preparation of aluminum materials.
Alodine is a trademark registered by Nippon Parkerizing, and the alodine process was initially developed by Amchem Products in the United States. Alodine treatment, also known as al-surf treatment, is a conversion treatment applied to aluminum materials.
This treatment forms an excellent corrosion-resistant coating on aluminum and its alloys. One of the key features of Alodine is its electrical conductivity, which sets it apart from general anodized aluminum.
However, due to the presence of hexavalent chromium, known as an environmental pollutant, alternative processes like trivalent chromium conversion coatings, non-chromium alodine, and palcoat treatments have been developed in recent years.
Uses of Alodine Treatments
Alodine treatment is utilized for various applications, including:
- Heat sinks (electronic devices)
- Equipment parts
- Automotive parts (for weight reduction)
- Aluminum alloys in general
- Aircraft parts
It can be applied to complex-shaped products, forming a thin film on the surface through a chemical reaction. The color of the coating can be adjusted depending on the chemicals used, with colorless-type (#1000) and yellow-type (#1200) being common choices. Alodine treatment is highly effective for aluminum materials intended to conduct electricity and serves as a valuable primer for paints, enhancing adhesion between aluminum and paint.
Principle of Alodine Treatments
Alodine treatment is a chromate treatment that involves applying a treatment solution containing primarily phosphoric acid, chromic acid, or dichromic acid to the surface of aluminum. While it is an effective surface treatment, it contains hexavalent chromium, prompting the development of more environmentally friendly alternatives.
Two common methods are:
- Phosphate Chromate (using phosphoric acid): Forms a relatively thick green film.
- Chromic Acid Chromate (using chromic acid): Produces a thin, golden-colored film.
Alodine treatment is known for its ability to produce highly corrosion-resistant coatings under relatively simple conditions. The process is typically conducted at room temperature and does not require electricity. The treatment is completed within minutes and includes steps such as degreasing, rust removal, cleaning, surface conditioning, film formation, hot water rinsing, and drying.
Keeping the surface clean before coating is crucial for achieving optimal performance. Additionally, the contact electrical resistance of Alodine (#1200) can be adjusted by varying the treatment time.
Other Information on Alodine Treatment
1. Difference Between Alodine Treatment and Anodizing
Aside from alodine treatment, which involves chromating to create a chemical conversion coat containing hexavalent chromium, there is another process called anodizing, which generates an anodic oxide film on the aluminum surface. The oxide film formed by anodizing is highly corrosion-resistant.
Aluminum is prone to corrosion and discoloration due to its high ionization tendency, making it susceptible to chemical reactions with water, oxygen, and various other substances. Anodizing effectively addresses this issue.
Furthermore, anodizing allows for a wide range of color options by introducing dyes into the microscopic pores within the anodized film. In contrast, the oxide film produced by alodine treatment is very thin, typically ranging from 0.1 to 0.3 microns, and offers lower corrosion and wear resistance compared to the anodic oxide film created through anodizing.
Alodine treatment, however, does not involve electrolytic processes, and the treatment time is relatively short. Therefore, it is widely employed to protect items that do not require high levels of corrosion or wear resistance.
2. Chemical and Electropolishing Before Aluminum Surface Treatment
Before applying surface treatments to aluminum, chemical, and electropolishing processes are often used as pretreatment steps to achieve a smooth surface by melting protrusions on uneven metal surfaces.
Chemical polishing involves immersing aluminum in a polishing solution to chemically polish the metal surface. Electropolishing, on the other hand, uses an electrolytic process in a polishing solution to achieve metal surface polishing. Before chemical or electropolishing, the aluminum surface can be intentionally roughened through methods like sandblasting to attain a shiny and smooth surface.