Anodising increases the surface hardness of aluminium parts and protects them from corrosion, improving the usefulness of this light, convenient metal. It can also be used to create surface colour on aluminium objects. Cast aluminium is more difficult to anodise successfully at home. The process is the same as for extruded aluminium, but the results may be of lower quality. Expect less predictable colour results when using cast pieces due to their uneven surface texture.
- Skill level:
- Moderately Challenging
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Things you need
- Cast aluminium parts
- Lye or nitric acid
- Nonreactive container
- Battery acid
- Aluminium plate
- 6- to 12-volt car battery charger
- Fabric dye, such as Rit
- 2 nonreactive pots
- Heat source
Clean the cast aluminium parts. Mix 3 tablespoons of lye or 29.6 to 59.1ml of nitric acid in a gallon of water. Add lye to water, never water to lye, to prevent a dangerous chemical reaction. Submerge aluminium parts in the mixture to remove oils and other contaminants.
Fill a large, nonreactive container, such as a plastic tub, with three parts water and one part battery acid. Add the acid to the water, as with the lye.
Use wire to suspend the part and an aluminium plate in the battery acid solution. Attach the negative lead of the battery charger to the aluminium plate. Attach the positive lead to the part to be anodised.
Switch the battery charger on, at a rate of at least 2 volts. A higher voltage rate will cause the piece to anodise more quickly. You should see bubbles or a white fog come off the aluminium plate. This indicates that electricity is passing through the parts.
Allow the parts to rest in the solution for around 2 hours, or until the surface of the part to be anodised looks dull and slightly yellowed. Bubbles should decrease in frequency, and may move in a different direction.
Switch off the power. Remove the part from the acid bath and rinse it in distilled water. Low spots in a cast aluminium surface will retain acid unless cleaned thoroughly.
Combine fabric dye and distilled water in a nonreactive pot. Place the part into the dye solution, then heat to 100 degrees Fahrenheit(37.7 degrees Centigrade) for 15 minutes. Remove the part and rinse it in clean water.
Boil distilled water in a separate pot large enough to hold the part. Place the part in the boiling water for 30 minutes.
Remove the cast aluminium part from the boiling water. Allow it to dry fully and cool to room temperature. The part should now be fully anodised. Colours may appear lighter or less even on cast aluminium than on extruded parts.
Tips and warnings
- Professional anodising kits may be more effective than homemade methods on cast parts.
- Powder coating cast aluminium produces a more even finish.
- Always wear gloves, goggles, and protective clothing when working with caustic or acidic substances.
- Anodise in a well-ventilated place, or wear a chemical respirator.
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