Alodine Coating Process for Aerospace Aluminum Parts

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Alodine Coating Process for Aerospace Aluminum Parts

What is Alodine Coating

What is Alodine Coating

The “Alodine” surface treatment process means that aluminum machining parts and aluminum alloys are treated with chromium phosphating technology. The phosphating solution is mainly composed of phosphate radicals and hexavalent chromium, and fluoride ions are generally added. The phosphating film formed is emerald green, which has good matching and corrosion resistance with the coating.

Aluminum Alloy Chemical Oxidation Alodine chemical oxidation was first used in the United States in 1945. Subsequently, France, Japan, and the Soviet Union all adopted it successively. The formulas they use vary, but they are basically chrome, phosphoric acid, fluoride, and various additives. In such a solution, a chemical conversion coating is formed on the surface of the aluminum alloy. Although the wear resistance of this film is only 20% of that of the anodized film, the former is more resistant to bending and torsion, especially with excellent paint adhesion.

The material used for chemical oxidation implements the US military standard MIL-C-81706. Colorless conversion film, namely CC film (Col.:-lessCon, ersionCoating) according to MIL-C-5541B class requirements, using Alodine 1500 (liquid); colored conversion film, namely MC film (Mu[tico sub oredConver- SionCoating) according to MIL-C-55 IBI class requirements, using Alodine 1200 (solid). The standard also stipulates: 6061-T6 (equivalent to LD2) uncoated aluminum test piece for CC oxidation; 2024-T3 (equivalent to LY12) uncoated test piece for MC oxidation. It can be seen that the requirements are very strict, which also shows that the corrosion resistance of the MC film is better than that of the CC film.

Secondly, it is clearly stipulated in the standard that a salt spray test of the conversion coating should be done once a month, and it is only allowed to be put into production if it is qualified. If it is not qualified, the reason should be found out, and the process can be adjusted until it is qualified before continuing production. We do not have such clear regulations and regular inspections.

Finally, it is also stipulated that different processes of dipping, brushing and spraying can be used as repair measures for the oxide film of aluminum parts. Brushing and spraying require special personnel to operate, and can only be produced after passing the examination.

Alodine anodizing treatment is a kind of aluminum alloy surface treatment, which is mainly used to improve the corrosion resistance of aluminum alloy structures and the adhesion with primers. Alodine is a trade name for the surface treatment of aluminum alloys, which can form a very thin (0.5~4 micron) anodic oxide film through chemical reactions on the surface of aluminum alloys. The principle of Alodine anodizing is the chemical reaction of chromate passivation. The main components of Alodine are mixed with chromic anhydride, methyl tetrafluoroborate, sodium fluoride and other substances in a certain proportion.

Purpose of using Alodine:

1. Improve the corrosion resistance of the aluminum alloy surface.

2. Improve the adhesion to the primer.

3. Can be used as a coating to change conductivity.

Alodine series:

1. Alodine 600, powder, for use on fuel tank structural surfaces coated with fuel-resistant paint (BMS10-20), recommended for producing low resistivity oxide films. Color: Yellow.

2. Alodine 1000, concentrate, used for aluminum-clad structural parts that do not require painting. Color: Colorless.

3. Alodine 1200/1200S, powder, most commonly used for repair parts that have been cut or filed, has good corrosion resistance and can provide better bonding properties. Color: Pale golden to tan

4. Alodine 1500, powder, color: colorless

Alodin features

1. The resulting oxide film is 0.5~4 microns thin, soft, porous, and has good adsorption properties, does not change the mechanical properties of the material, does not affect the size of the workpiece, and is usually used as the bottom layer of paint or other coatings.

2. The operation time is short, from a few minutes to an hour. No power consumption, simple equipment and low cost.

3. Alodine oxidation still has certain conductivity.

4. The Alodine process is mainly carried out at room temperature, with low energy consumption and less pollution.

5. Alodine solution is stable, long-term use, easy to maintain, suitable for large-scale continuous production.

6. Alodine solution is poisonous and cannot be in contact with human body and cannot be disposed of at will. It must be fully diluted and discarded.

7. Alodine is flammable and should be kept away from fire sources. Cloths that have been used with alotin solution should also be isolated to avoid fire.

About the use and precautions of Alodine:


First, the precautions for coating Alodine are:

1) Usually Alodine is packaged in powder form when it is sold. Before use, it needs to be prepared into a certain concentration of acid water-soluble.

liquid (1 gallon of water plus 3 ounces of Alodine 600 powder) and let sit for at least an hour before applying.

2) Special care should be taken during use, it is a strong oxidant and has a risk of cancer, so it should not be exposed to work

the facial features and other exposed parts of the victim. During use and configuration, gas masks, rubber or polychloroprene gloves must be worn;

3) If the eye contact cannot be rubbed, immediately rinse with water for 15 minutes, and go to the hospital for treatment in time. Rinse other parts with plenty of water and go to the hospital for treatment in time;

4) Alodine should be kept away from fire, heat and sparks. And should be placed in isolation from other substances. Also be airtight and well ventilated. Dry cloths that have been used with Alodine solution are prone to fire. Therefore, it must be immediately rinsed with water and stored in a container filled with clean water. Alodine powder may spark if in contact with strong acids. Therefore, Alodine solution should be prepared in designated places.

5) Choose stainless steel or acid-resistant plastic or rubber (preferably polyhexene) container for alotin. Glass and lead products and paper cups are prohibited.

6) Alodine solution must be diluted with a large amount of water before being scrapped, otherwise it will cause pollution.

2. The general procedure for coating Alodine is as follows:

1) Using a cleaning brush or rag, cleanly remove the liquid grease solvent from the damaged surface and dry the surface with hot air or wipe it dry.

2) Remove debris from the surface with tools such as alumina sandpaper or emery cloth until a clean and smooth aluminum surface is obtained.

3) Wipe and clean the surface with a cotton cloth moistened with MEK (not soaked) until no remaining particles are found on the cotton cloth.

4) Let the surface dry for at least 15 minutes, and perform a water film test to ensure that the surface is clean enough.

5) While keeping the surface wet, apply a generous coat of Alodine by brush or spray and let the solution stay on the surface until a specific color is found (after about 30 seconds). Keep the area moist with a cotton cloth with a small amount of solution (has an easy to see color from pale yellow iridescent to tan, you can use color for identification, but apply Alodine 600 for little or no color) . Be careful not to let the alotin solution dry until the surface color changes.

6) Rinse the surface carefully with a clean cotton cloth and water. Be careful when you dry the surface, don’t remove the coating and don’t scratch the surface. Gently remove excess liquid with a clean, dry cotton cloth until completely free of liquid.

7) Air dry the surface. (Generally 1 to 3 hours). Note that drying at 120F is most suitable, and the maximum temperature should not exceed 130F. Wear clean gloves when touching a part or surface and keep the part or surface dry and clean to prevent surface contamination.

Author: Mose Li

Author: Mose Li

Director of Project Engineering at 3Q Machining

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