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Aluminum anodizing process

Aluminum anodizing process

Most metals are in need of rust and corrosion prevention. Aluminum, used in household, is no exception. A dense film of oxide on its surface suffices to inhibit further corrosion. This is achieved in the result of the eloxal process.  The most mechanically endurable and uninfluenced coatings are formed at the low-temperature thin-layer aluminum anodizing, what we are going to tell you about.

 

SAFETY MATTERS

It is not too complicated to conduct high-quality anodizing at home. But it’s safer and more convenient to do it outdoors – in the yard or on the balcony. But you must keep in mind that you need to strictly observe safety measures in the process as you will work with acid.

Acid is a very caustic thing though you will work with a diluted solution, which causes only mild itching when in contact with the skin. But it can cause serious injuries if it gets into your eyes! Therefore, it is advisable to wear safety goggles when anodizing and always have a bucket of water or a weak soda solution on hand.

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During the anodizing process, oxygen is emitted on the anode, and hydrogen on the cathode. Mixing of these two gases might result in the detonating gas formation. The detonating gas is highly explosive. That is why DON’T do anodize indoors! It’s potentially dangerous. It might explode just on its own accord.

 

PRE-WORK

Remember: after anodizing the size of nuts and bolts slightly increase. The thickness of the protective anode coating is usually 0,05 mm.

It is worthwhile high-polishing items before anodizing to make them look perfect. The anode coating does not conceal surface defects.

Before electroplating, aluminum must be well degreased. You should not keep the metal in hot caustic sodium or potassium, as recommended in factory technologies, because it noticeably spoils the surface cleanliness. It is better to use a piece of laundry soap and a toothbrush because you have to work with small workpieces. Rinse the product in warm water first, then in cold.

Washing powder is very effective: dissolve it into some hot water in a plastic tank. Put into it the workpieces to be anodized and shake the tank well. After washing dry the items with hot air. Don’t fear to touch the items after degreasing. Oxygen oxidizes finger fat stains instantly.

ELECTROLYTE PREPARATION

The electrolyte for anodizing at home is a solution of sulfuric acid in distilled water. You can also use tap water, but distilled water is better since tap water might spoil the uniformity of the process – the distribution of the current density on the surface of the workpieces being anodized.

You can prepare distilled water by yourself. It’s easy! You can use rainwater,  melt snow or the ice from your fridge. Anyway, you can buy sulfuric acid and distilled water in any auto parts shop, as these ingredients are needed for car batteries.

However, they sell diluted acid with a density of 1.27 grams per cubic centimeter under the name “Electrolyte for a Lead Battery”. You need to mix this electrolyte with distilled water in a ratio of 1: 1.

If you take a regular 5-liter tank with the electrolyte and the same quantity of water you will have 10 liters of the anodizing solution.  This will be sufficient to process small parts, for large ones you will need twice as much.

Remember, that mixing an acid with water produces much heat. DON’T pour water into acid! It will boil up instantly, splashing it into your face. Trickle the electrolyte into the container with water stirring it constantly with a stirring rod. Don’t forget to wear safety goggles! If the case of acid contact with clothing or skin, wash it off immediately with water and rinse with a solution of soda.

 

PROCESSING MODES

The temperature range of the metal anodizing process is -10 – +10ºC.  The growing layer below -10ºC is also very good, but the power adapter voltage will not be sufficient to support the necessary amperage. At the temperature above    +10ºC, the protective coating will be also forming, but it will be soft and colorless.

It is recommended to stop the anodizing process at 5ºC. The point is that the temperature is not the same in the corners of the tank and on the surface of the workpieces being anodized. Much heat is produced during the anodizing process.

But don’t believe the thermometer data if there is no forced mixing of the electrolyte! One needs to mix the electrolyte all the time with a spoon, stick, airflow, pump, or whatever you have at hand. This is important for temperature equalizing at the surface of the processed aluminum workpiece. Otherwise, the workpiece will have locally overheated spots, followed by electrolytic breakdowns and local etching of the workpiece surface.

The anode current density should fall within 1,6 – 4 А/dm². In this case, a beautifully colored, thick anode layer will grow. The best result is obtained at   2 –  2,2 А/dm². If the amperage is less, the protective layer will grow slowly and remain thin. The amperage over 4 А/dm² is fraught with breakdowns and etching of the workpiece.

The cathode amperage should below. The less is this parameter, the better, because this provides an even and soft mode of the current density distribution over the workpiece surface, especially if it is large. Just remember, the size of the lead cathode must be twice as big as the workpiece (anode). Anyway, if your chain does not have a zero resistance you need a high voltage power adapter with a few output voltage parameters, at that.  Because:

The growing protective layer on the workpiece is a dielectric. As it continues to grow its electric resistance is increasing.  To maintain the necessary current density during the whole process it is needed to regulate the amperage several times using a variable resistor.

However, the voltage may not be sufficient when the anode layer becomes thick enough. In this case, you need to add voltage. Therefore, the power supply must provide at least two voltages at the output.

 

A TANK FOR ANODIZING

Before you start it is necessary to prepare all the equipment for anodizing. Usually, several tanks are needed: for small items, for long and short workpieces.  It’s preferable to use aluminum tanks. Polyethylene is also good. For small workpieces, you can use a food container or any other plastic container.

It is advisable to cover the bottom and walls of the tank with aluminum sheets or cut a corresponding pattern from an aluminum sheet and make a fake “tank”.  It is needed to ensure a stable (similar) current density on all sides of the workpiece.

The tank should have good warmth keeping properties, otherwise, the electrolyte in it will get hot soon; and you’ll have to change it more often. The simplest way to give it thermal insulation properties is to line it with foam plastic 2-3 cm thick. Another option is to put the tank inside a box and fill the emptiness between the walls with foam insulation.

After that, you have to make a lead cathode. It is possible to make it out of a lead plate. Just remember that the cathode size should be twice as much as the workpiece size. Mind, that the side of the cathode surface leaned against the wall is not counted. The cathode plate should have holes for gas emission.

It is possible to assemble a cathode using several lead pieces if you don’t have a suitable one. Solder the pieces with a powerful soldering iron, making a thick seam along the joints. Try to form the cathode similar to the form of the processed workpiece surface facing it.  Use a strip of the same material for a tank contact lead.  Although, it is customary to use thick isolated copper wire.  Seal the soldering point with silicone sealant.

 

AN ANODIZING PROCESS

So, you have a tank with an electrolyte and a power supply unit at the output. To regulate the amperage when anodizing titan or aluminum you need to connect a variable wired resistor. There are two items in your tank: a plate-shaped lead cathode and an anode – a workpiece to be treated. At current supply, oxygen is released and the anode protective layer begins to grow.

At a good electrolytic contact between the lead plate and the workpiece, you will observe oxygen microbubbles going slowly up from the workpiece surface. They are so minuscule that they resemble wisps of smoke. You should check visually the duration of the process by the color of the workpiece.

It takes 20-30 minutes for smaller items, and 1-1,5 hours for big ones. When the workpiece is completely covered with a blue-grey coating take it out of the tank, rinse with cold tap water and wipe with a cloth sprinkled with a strong manganese solution to remove the reaction by-products. The surface should be sheen, smooth, light-grey.

After the process of home anodizing, some workpieces acquire dark dim hue, depending on the anodizing mode. To color, the anodized parts dip them into a filtered aniline dye solution warmed up to 50-60 degrees Celsius. Filtration is needed to get rid of undissolved dye particles, which can stain the metal surface.   Dyeing will take 15-20 minutes. After the passivation and tinting of the workpiece, you need to solidify the protective layer.  At a micro level, the protective layer has an air- and water-permeable porous structure. Such a layer of metal protects well from mechanical damage but is weak against a chemical one.

There are several methods to close the micropores. The simplest one is to boil the workpieces in distilled water for 30 minutes after anodizing. Or you can keep your workpieces in a steam bath for half an hour.

As you know there is a number of technologies of aluminum anodizing. They differ in the conditions of the process, in the electrolyte temperature, to be more precise. The temperature is the main factor having an effect on the quality of the protective anode layer. When anodizing at home, it is preferable to choose cold anodizing to get a thicker coating with beautiful color and sheen.

 

 

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