Home Version of Aluminum Alloy Anodizing Coloring

The aluminum alloy anodic oxidation process generates a dense layer of oxide film on the surface of aluminum through electrochemical means, resulting in corrosion resistance, wear resistance, insulation, and decorative coloring.

In the daily DIY non-standard structural parts sampling, it is inevitable to meet the demand for anodic oxidation surface treatment of aluminum alloy workpiece, the amount of small outsourcing processing is not cost-effective, and the cycle is long.

After reviewing the process literature and watching related videos, I found that the raw materials and equipment required for aluminum alloy anodic oxidation are relatively simple and easy to obtain.

Under the premise of not considering batch cost, pollution, and process consistency, many process links can be omitted, combined, or simplified, with the feasibility of practical operation in the family (workshop).

Now I practice the process and related notes are recorded as follows. One is to give themselves a memo, the second is for interested friends reference.

Reagent raw materials: sodium hydroxide, sodium bicarbonate, dilute sulfuric acid (available lead-acid battery electrolyte dilution to get), dilute nitric acid (can not find available dilute sulfuric acid instead), reverse osmosis purified water (home reverse osmosis water production mechanism is prepared, the conductivity needs to be less than 50 μS / cm), water-soluble dyes, nickel-based sealer. The concentration of each solution temperature, etc. in the latter text in detail.

Instruments and auxiliary materials: adjustable constant current DC power supply, temperature furnace, beaker (or other acid and alkali containers) a number of thermometers, ice cubes, water bath, ultrasonic cleaner (non-essential), lead skin, aluminum wire, wire and alligator clips, bamboo skewer, 316 stainless steel tweezers, chemical-resistant gloves, goggles and so on.

Process Flow: Degreasing→Washing→Alkaline Etching→Acid Washing & Degreasing→Washing→Oxidation→Neutralization→Washing→Dyeing→Washing→Closure→Washing & Drying

Operation steps: (Note: the concentrations mentioned below are all mass concentrations unless otherwise specified)

Thanks for Chenyang Hardware, providing metal materials and CNC machining services. Capablemaching provide technical support.

In the process of workpiece processing, it is inevitable to be contaminated with cutting fluid, machine oil, tapping oil, etc., this kind of oil pollution is not removed, it will hinder the workpiece surface electrochemical reaction occurs.

Therefore, aluminum parts need to be immersed in hot alkaline degreasing solution (1L of pure water dissolved in 60g of sodium hydroxide), immersed for 10-15 minutes, in the saponification reaction to remove the workpiece surface oil, so that the workpiece is well exposed to the degreasing process, the temperature control of the process at 60-70 ℃, such as the workpiece is not easy to clean the region of the deep holes, narrow crevices, etc., need to be equipped with ultrasonic vibration device in order to remove deep holes in the oil and grease.

Wear chemical-proof gloves and goggles during operation to avoid lye burns.

The workpiece at the end of degreasing should be rinsed immediately with room temperature pure water for 1 minute to avoid corrosion bloom caused by lye retention, holes and crevices should be rinsed carefully.

Aluminum parts will be immersed in hot alkaline etching solution (120g of sodium hydroxide dissolved in 1L of pure water, in order to save raw materials, you can continue to add a quantitative amount of sodium hydroxide solids in the above degreasing step) at a temperature of 50-60 ℃, immersed for 3-8 minutes, 6061, 6063 aluminum alloy is recommended to deal with the time of no more than 5 minutes.

This process further removes the oxidized layer and slight scratches, extrusion lines, knife lines and flying edges on the surface of the workpiece through the corrosive polishing effect of concentrated alkali on aluminum.

If the immersion time is too long, it may lead to excessive corrosion of the aluminum. Wear chemical resistant gloves and goggles to avoid burns from the lye.

Alkaline etching end of the workpiece should be immediately immersed in room temperature dilute nitric acid solution (concentration of 30%) to neutralize the residual lye, immersion time of 30-60 seconds, to prevent the residual lye corrosion of the substrate, resulting in subsequent oxidation film bloom.

And through the corrosive effect of dilute nitric acid on a variety of metals to remove the alkaline etching link in the workpiece surface left behind by the black ash (workpiece surface residual magnesium, iron, zinc, copper and other components), the operation of the need to wear chemical-resistant gloves, goggles, etc., and pay attention to the ventilation to avoid the gathering of acid mist.

Neutralization and ash removal end of the workpiece should be immediately rinsed with room temperature water for 1 minute, to avoid acid retention caused by corrosion bloom, holes and crevices should be carefully rinsed.

Electrolyte: Prepare sulfuric acid solution (concentration 20%) with pure water, temperature 18-22℃. Too high temperature will lead to loose oxide film, too low temperature will prolong the reaction time.

Use an aluminum workpiece as the anode. Use fine aluminum or titanium wire, bamboo sticks, silicone plugs, and other wedges to assist in clamping.

Suspend the workpiece in the electrolytic bath through hidden holes or other non-visible locations. For example, insert the aluminum wire into small holes on the workpiece.

Then, use a toothpick to forcefully tighten it. The area where the aluminum wire contacts the workpiece will not be oxidized.

Therefore, the fewer the contact points, the better—under the condition that carrying capacity and stability are ensured. Avoid placing contact points near important visible surfaces.

The cathode is made of pure lead sheet. Current density 1.2-1.8 A/dm² (to calculate the entire surface area of the workpiece), voltage 12-20 V, treatment time 30-60 minutes. The thickness of the oxide film is positively correlated with the time, and each micron film thickness requires about 3 minutes of treatment time.

Conditions can be driven by anticorrosive pump tank liquid filtration cycle, to avoid the accumulation of local aluminum ions affect the overall effect of homework intermittent mixing can be.

Operation need to wear chemical resistant gloves, goggles, etc., to avoid acid corrosion.

Special attention is paid to the oxidation process system will have a large number of Joule heat generation, the need for real-time monitoring of the electrolyte temperature does not exceed 22 ℃, if necessary, in the water bath into the ice cooling.

The surface area of the workpiece is about 2dm², set the voltage limit to 18V and the current limit to 3A.

With the oxidation time, the thickness of the oxide film on the surface of the workpiece gradually increases, hindering the electrolysis channel, the current will gradually decline, the cut-off current is only tens of milliamps.

he workpiece after oxidation should be immediately immersed in the neutralization solution (10% sodium bicarbonate solution at room temperature), and the workpiece should be continuously pulled and stirred until no bubbles are generated on the surface of the workpiece.

The residual acid on the surface is removed by neutralization reaction to avoid corrosion spots caused by acid retention. Holes and gaps should be carefully washed.

Oxidation of the end of the workpiece should be immediately rinsed carefully with room temperature water for more than 1 minute, to avoid acid retention caused by corrosion bloom, holes and crevices should be carefully rinsed.

Dyeing solution: Use pure water to prepare water-soluble organic dye solution (light color dye dosage 2-6g/L, black or dark color 10-12g/L), firstly warm up the pure water to 80℃, slowly put in quantitative dye powder under vigorous stirring, keep warm at 80℃ for spare.

Dyeing: Depending on the required coloring depth, the aluminum workpiece will be immersed in the dyeing solution for 1-10 minutes, and the black color usually needs to be immersed for 5-7 minutes, during which the workpiece should be fully shaken or the dyeing solution should be stirred, such as the consistency of the appearance of the workpiece in multiple batches of dyeing has strict requirements, and it is necessary to strictly control the dye composition (batch), the conductivity of the water, the concentration of the dyeing solution, the temperature, the pH value, the dyeing time and the intensity of stirring and other factors.

Workpieces finished with dyeing should be immediately rinsed carefully with pure water at room temperature for more than 1 minute, and holes and crevices should be rinsed carefully.

The anodic oxide film has a porous structure, and these pores will reduce the corrosion resistance, wear resistance and other properties of the oxide film.

The nickel salt in the sealant will have chemical reaction in the pores of the oxidized film under certain conditions to generate insoluble compounds.

These compounds will fill in the pores, like small plugs to block the pores, thus reducing the contact channel between external substances and the inside of the oxide film, improving the closure of the oxide film.

In the case of fluoride-containing sealers, fluoride ions will react with the alumina in the oxide film to form new compounds such as fluoroaluminate, which covers the surface of the oxide film and enhances its protective properties.

(1) Medium temperature sealing

Sealing solution: Use pure water and medium temperature sealant YM-300 (containing nickel acetate and fluoride) to formulate the sealant (sealant concentration of 6g/L), firstly, warm up the pure water to 65 ℃, and then put in the quantitative sealant slowly under vigorous stirring, detect the pH value of the solution with the test paper, and then use glacial acetic acid or ammonia to adjust the pH value to 5.5-6.5 if necessary, and then keep warm at 65 ℃ for spare time.

Soak the aluminum workpiece in the sealing solution for 10-15 minutes, the temperature of the sealing treatment is preferably above 55℃, but do not exceed 70℃, and keep clean during the process to avoid contamination by impurities, especially chlorine, iron and other ions.

During the sealing process, a small amount of dye will diffuse from the surface of the workpiece and pollute the sealing liquid.

(2) Boiling water closure

Boil in pH5.5-6.5 pure water for 30-40 minutes to promote hydration and expansion of the oxide film, pore closure. Boiling water closure method requires deionized water conductivity <5μS/cm, temperature 95-100 ℃, immersed for 25-35 minutes, this method is suitable for weathering requirements of the workpiece treatment is not high.

The workpiece at the end of the closure is rinsed with room temperature tap water for 1 minute, and the holes and crevices should be rinsed carefully. Subsequently dry naturally or in a 60℃ blast drying oven.

Finally the finished product:

It can be seen that even if it is a simple process, as long as you control the temperature, concentration, current density and so on, and don't skip the cumbersome water washing process, you can also get a good surface treatment effect.

In addition, except for the sodium hydroxide solution in the oil removal and alkali etching process, which needs to be prepared on the spot (a lot of flocculent precipitate "sodium aluminate" will be produced after each process. Fortunately, caustic soda is quite cheap and easy to obtain, so there is no need to be stingy about recycling), the reagent solutions in other links consume very little effective ingredients in the entire process, and can be recycled after use. It is no problem to seal and store it away from light for a year or so.

In addition, before the workpiece is anodized, add a sandblasting process, the surface has a frosted texture, does not get fingerprints, and the effect is better. Take a few more photos when you have time.

De-filming: If you are not satisfied with the color of oxidation, or there are defects on the surface after anodization and need to be re-oxidized, you can perform de-filming. Put the workpiece directly into a 10% concentration sodium hydroxide solution at 60-70℃, shake and stir continuously, and the de-filming can be completed within 1 minute. After washing and neutralization, it can be oxidized again.

After the shielding cavity is processed, it will be oxidized again. This time, it is sandblasted with 180 mesh silicon carbide to remove the knife marks.

This is the detail of the aluminum wire inserting the toothpick.

I still repeat the process on the first floor, but I add a cooling circulating water to the electrolytic tank, and I am too lazy to keep adding ice cubes.

The effect after oxidation dyeing.

The finished product is still very beautiful. The matte texture after sandblasting feels very comfortable. After sealing, there will be some black and gray on the surface, which looks a bit mottled. You have to brush it hard with a brush under running water. The surface in contact with the PCB is milled off 3-5 wires with a flying knife to ensure good electrical contact

Family portrait, and it can be assembled after the PCB proofing comes back.

The base surface of the heat sink is also milled off 3-5 wires with a flying knife to ensure good electrical contact and heat conduction.