A Comprehensive Guide on How to Anodize Aluminum

Expand your metal working knowledge by learning how to anodize aluminum! Read this comprehensive guide and find out what you need to know about the process.

Anodizing aluminum is a process used to create a hard and durable finish on the surface of aluminum parts. It is a complex procedure that requires special chemicals, safety gear, and equipment, but once you learn the basics you’ll find it’s not difficult to do. This guide will provide an overview of the steps involved in properly anodizing aluminum.

What is Anodizing?

Anodizing is an electrolytic process that creates a protective oxide layer on the surface of a metal, typically aluminum. During anodizing, the aluminum part is submerged in an electrolyte solution and connected to a positive electrical charge while a negative charge is applied to a cathode in the solution. This causes an oxidation reaction to occur on the surface of the aluminum, creating a hard, durable oxide layer that is integral to the metal.

Anodizing can be used to improve the corrosion resistance, wear resistance, and appearance of aluminum parts. The thickness and color of the oxide layer can be controlled by adjusting factors such as electrolyte composition, temperature, and voltage during the anodizing process.

After anodizing, the aluminum part may undergo post-treatment processes such as dyeing or sealing to further enhance its properties or aesthetic appeal. Dyeing involves applying colored dyes to the porous oxide layer for decorative purposes while sealing involves closing off any remaining pores in the oxide layer to improve corrosion resistance.

Overall, anodizing is a widely used surface finishing technique for aluminum parts in industries ranging from aerospace and automotive to consumer goods due to its ability to enhance performance and appearance.

Necessary Materials and Preparation

Before beginning the anodizing process, you’ll need to make sure you have all the tools and materials necessary. Make sure to have the aluminum parts that will be anodized, a plastic or glass container (such as a bucket or storage bin), rubber gloves, protective eyewear, two charging posts for each part, as well as more posts for other shapes and sizes of metals to be anodized at the same time. You’ll also need battery charger/rectifier, distilled water – or rainwater that has been boiled and cooled – baking soda or acid bath solution depending on your desired finish look, measuring cups, spoon stirrers and thermometer.

The Anodization Process

The anodizing process is a multi-step procedure that involves cleaning, etching, and finally the anodizing in which the parts are placed into an electrolytic tank filled with a sulfuric acid solution, along with charging posts. An electric current is then passed through the tank and aluminum parts to create a strong oxide layer that provides corrosion and weather resistance. The voltage level and time will be determined by the desired look although it is usually around 4 volts. Keeping your solution at between 12°C–35°C (53°F–95°E) temperature range during the process helps ensure consistent results.

Aluminum is a relatively active metal with a standard potential of -1.66v, which can naturally form a layer of oxide film with a thickness of about 0.01~0.1 micron in air. This oxide film is poorly resistant to corrosion because it is thin, porous and amorphous. However, if aluminum and its alloys are placed in a suitable electrolyte, with aluminum products as the anode, under the action of the applied current, the surface of which generates an oxide film, this method is called anodic oxidation.

By choosing different types and concentrations of electrolyte and controlling the process conditions during oxidation, anodic oxide films with different properties and thicknesses of tens to hundreds of microns can be obtained, and their corrosion resistance, wear resistance and decorative properties can be significantly improved and enhanced.

The electrolyte used for the anodic oxidation of Al and aluminum alloys is generally an acidic solution of medium solubility, with lead as the cathode, which only acts as a conductor. When aluminum and its alloys are anodized, the following reactions occur at the anode:

2Al —> 6e + 2Al

At the cathode, the following reactions occur:

6H2O +6e —> 3H2 + 6OH

At the same time the acid chemically dissolves the aluminum and the resulting oxide film, the reaction is:

2Al + 6H —> 2Al3+ +3H2

Al2O3 + 6H —> 2Al + 3H2O

The growth process of oxide film is the process of continuous generation and dissolution of oxide film.

The first section a (curve ab): non-porous layer formation. Within a few seconds to a few tens of seconds at the beginning of energization, the aluminum surface immediately generates a dense, highly insulating oxide film with a thickness of about 0.01~0.1 microns, which is a continuous, non-porous film layer called the non-porous layer or blocking layer, and the appearance of this film prevents the passage of current and the continued thickening of the film. The thickness of the non-porous layer is proportional to the formation voltage and inversely proportional to the dissolution rate of the oxide film in the electrolyte. Therefore, the voltage of the curve section ab then shows a sharp increase from zero to a maximum.

The second section b (section bc of the curve):Porous layer formation. As the oxide film is generated, the dissolution of the film by the electrolyte begins. Since the generated oxide film is not uniform, cavities will be dissolved first in the thinnest part of the film, and the electrolyte can reach the fresh surface of aluminum through these cavities, and the electrochemical reaction can continue, the resistance decreases, the voltage drops (by 10~15% of the highest value), and a porous layer appears on the film.

The third section c (curve cd section):The porous layer thickens. After about 20s of anodic oxidation, the voltage enters a relatively smooth and slow rising stage. It shows that the non-porous layer is continuously dissolved to form a porous layer at the same time, the new non-porous layer is growing, which means that the generation rate of the non-porous layer in the oxide film has basically reached a balance with the dissolution rate, so the thickness of the non-porous layer no longer increases, and the voltage change is also very small. However, at this time, the generation and dissolution of the oxide film at the bottom of the hole does not stop, they are still going on, and as a result, the bottom of the hole gradually moves to the inside of the metal matrix. As the oxidation time continues, the pores deepen to form pores, and the film layer with pores gradually thickens. When the film generation rate and dissolution rate reach a dynamic equilibrium, even if the oxidation time is extended, the thickness of the oxide film will not increase again, and the anodic oxidation process should be stopped. The anodic oxidation characteristic curve and oxide film growth process are shown in the figure below.

Color Dyeing and Sealant Application

After the anodizing process is complete, you’ll usually want to further enhance your aluminum parts with coloring and sealants. Color dyeing or painting gives the aluminum pieces a vibrant hue that can’t be achieved by anodizing alone, while sealant application creates a protective coating that helps to keep the color looking fresh for longer periods of time. There are many options when it comes to color dyeing and sealant application, so do some research to find out which ones will best suit your needs.

Anodizing color dyeing and sealant application are post-treatment processes that can be applied after the anodizing process to further enhance the appearance and properties of an anodized aluminum part.

Color dyeing involves applying a colored dye to the porous oxide layer of an anodized aluminum part. The dye is absorbed into the pores of the oxide layer, creating a permanent, fade-resistant color. A wide range of colors can be achieved through dyeing, including metallic finishes, bright colors, and pastels.

Sealant application involves closing off any remaining pores in the oxide layer to improve corrosion resistance. This is typically done by immersing the anodized part in a hot water or chemical bath that causes the remaining pores to close up. Sealant application can also improve abrasion resistance and make cleaning easier.

The order in which these processes are performed may vary depending on the specific requirements of the part being produced. For example, if a decorative finish is desired, color dyeing may be performed before sealant application. Alternatively, if maximum corrosion resistance is required, sealant application may be performed before or without color dyeing.

Overall, anodizing color dyeing and sealant application are important steps in achieving high-quality finished aluminum parts that meet specific performance and aesthetic requirements.

Processes

There are many methods of anodizing aluminum and its aluminum alloys, commonly used are sulfuric acid anodizing, chromic acid anodizing, oxalic acid anodizing, hard anodizing and porcelain anodizing.

Sulfuric acid

Anodizing aluminum and its alloys by passing DC and AC electricity in dilute sulfuric acid electrolyte, a colorless and transparent oxide film of 5~20 microns thickness with good adsorption can be obtained.

The sulfuric acid anodizing process is simple, the solution is stable, the operation is convenient, the allowable impurity content range is wide, the power consumption is low, the cost is low, and it can be applied to the processing of aluminum and various aluminum alloys almost, so it has been widely used in China.

The following table shows several typical anodic oxidation processes:

Oxalic Acid

Oxalic acid anodizing is an oxidation process using 2% to 10% oxalic acid electrolyte by direct or alternating current.

When using direct current for anodizing, the hardness and corrosion resistance of the resulting film is as good as that of H2SO4 anodized film, and because the oxalic acid solution is less soluble in aluminum and oxide film, a thicker oxide film can be obtained than that in sulfuric acid solution; if the oxidation is carried out with AC current, a softer, more elastic film can be obtained. Oxalic acid anodizing film is generally 8~20 microns, up to 60 microns thick.

The oxidation process can be done by changing the process conditions (such as oxalic acid concentration, temperature, current density, waveform, etc.) to obtain a decorative film such as silvery white, golden yellow to brown, etc. No further dyeing is required.

Oxalic acid anodizing electrolyte is very sensitive to chloride ions, and corrosion spots will appear on the film when its mass concentration exceeds 0.04g/L. The mass concentration of trivalent aluminum ions is also not allowed to exceed 3g/L.

However, oxalic acid anodic oxidation is more expensive and energy-consuming (because the resistance of oxalic acid electrolyte is larger than that of sulfuric acid and chromic acid), the solution is toxic, and the stability of the electrolyte is poor. Several oxalic acid anodizing processes are shown in the table below.

Porcelain

By adding certain substances to the electrolyte, the oxide film is formed while being adsorbed in the film, resulting in a smooth, lustrous, uniformly opaque oxide film similar to porcelain enamel and enamel color, called “porcelain anodic oxide film” or “porcelain oxide film”. This oxide film has good elasticity, good corrosion resistance, and can be dyed to give a plastic appearance. The resulting film thickness is about 6~25 microns.

The following are two methods of porcelain oxidation:

① Adding salts of certain rare metal elements (such as titanium, thorium, etc.) to sulfuric acid or oxalic acid solutions: During oxidation, the hydrolysis of these salts produces color-emitting substances deposited in the pores of the oxide film, forming a film layer similar to porcelain enamel with high hardness, which can maintain the high precision and high finish of the parts, but the cost is expensive, the cycle time of the solution is short, and the process conditions are strict.

②Anodizing solution with a mixture of chromic anhydride and boric acid: simple composition, low cost, good elasticity of the oxide film, but lower hardness than the previous one, can be used for general decorative porcelain oxide surface treatment. Porcelain anodizing solution and process conditions are shown in the table below.

Boost time <5 Hold:35~55 Total time:40~60[/av_cell][av_cell col_style='' av_uid='av-1r2l2d']1. Good solution stability, easy to operate, 2. Gray film layer 3. 10~15μm thickness 4. Suitable for general decorative parts, low cost[/av_cell][/av_row] [av_row row_style='' av_uid='av-fs03hx'][av_cell col_style='' av_uid='av-566hh']3[/av_cell][av_cell col_style='' av_uid='av-caaujp']Potassium titanium oxalate Boric acid Oxalic acid Citric acid[/av_cell][av_cell col_style='' av_uid='av-b1omh1']35~45 8~10 2~5 1~1.5[/av_cell][av_cell col_style='' av_uid='av-9coz9h']24~28[/av_cell][av_cell col_style='' av_uid='av-72o391']Start:2~3 Termination:0.6~1.2[/av_cell][av_cell col_style='' av_uid='av-nhorp']Gradual increase 90~110 Maintain 90~110[/av_cell][av_cell col_style='' av_uid='av-3tzrtx']Boost time 5~10 Hold:25~30 Total time:40~60[/av_cell][av_cell col_style='' av_uid='av-ax86t']1.Film layer is off-white, high hardness 2.Film thickness 8~16μm 3.Suitable for wear-resistant high-precision parts decoration 4.High cost and short solution life[/av_cell][/av_row] [/av_table] [av_textblock textblock_styling_align='' textblock_styling='' textblock_styling_gap='' textblock_styling_mobile='' size='' av-medium-font-size='' av-small-font-size='' av-mini-font-size='' font_color='' color='' id='' custom_class='' template_class='' av_uid='av-lefp1mo4' sc_version='1.0' admin_preview_bg=''] Note: The cathode material can be pure aluminum, lead plate or stainless steel plate. In the oxidation solution, the change of various components will play a decisive role in the color of the oxide film: for example, with the rise of chromic anhydride, the color of the film transforms to opaque gray; with the rise of boric acid, the color of the film transforms to milky white; and with the rise of oxalic acid, the color of the film transforms to yellow. [/av_textblock] [av_textblock textblock_styling_align='' textblock_styling='' textblock_styling_gap='' textblock_styling_mobile='' size='' av-medium-font-size='' av-small-font-size='' av-mini-font-size='' font_color='' color='' id='' custom_class='' template_class='' av_uid='av-lefp5pzm' sc_version='1.0' admin_preview_bg='']

Sealing Treatment

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As the surface oxide film has high porosity and adsorption properties, it is easily contaminated, so after anodizing, the film should be closed to improve the corrosion resistance, wear resistance and insulation of the film. Commonly used sealing methods are:

boiling water and steam sealing method

The principle is: the hydration of anhydrous alumina at higher temperatures:

Al2O3 + nH2O —> Al2O3-nH2O

When γ-AL2O3 with a density of 3.42 is hydrated into a monohydrate compound, the volume of oxide increases by 33%, and when it is hydrated into a trihydrate compound, the volume of oxide increases by 310%, therefore, when the oxidized good parts are placed in hot water, the barrier layer and the oxide film layer on the inner wall of the porous layer are first hydrated, and after a period of time, the bottom of the pore is gradually closed by the hydrated film, and when the whole pore is completely closed, the water of the pore will stop circulating, the surface layer of the film layer continues to be hydrated until the entire mouth of the pore is blocked by the hydration film. Such as the use of steam, can be more effective in closing all the pores, but the steam method used equipment and costs are higher than the boiling water method, unless special requirements, should be used as far as possible boiling water method.

Dichromate sealing method

This method is suitable for sealing the film layer of anodic oxidation in sulfuric acid solution and the film layer of chemical oxidation. The oxide film treated with this method shows yellow color, it has high corrosion resistance, but it is not suitable for decorative use.

The principle is: at a higher temperature, the oxide film and dichromate produce a chemical reaction, the reaction products alkali aluminum chromate and aluminum dichromate precipitate in the film pores, while the hot solution makes the oxide film layer surface hydration, strengthening the sealing effect. Therefore, it can be considered as the double sealing effect of filling and hydration.

The usually used sealing solution is 5~10% aqueous dichromate solution, the operating temperature is 90~95℃, the sealing time is 30 minutes, and there should be no chloride or sulfate in the solution.

Hydrolysis Salt Closure Method

Principle:After the very dilute solution of zirconium salt and nickel salt is adsorbed by the oxide film, the following hydrolysis reaction occurs:

Ni+2H2O —> Ni(OH)2 +2H

Co+2H2O —> Co(OH)2 +2H

The generated nickel hydroxide or cobalt hydroxide is deposited in the micro-pores of the oxide film and the pores are closed. Because a small amount of nickel hydroxide or cobalt hydroxide is almost colorless, it is especially suitable for the sealing of dyed oxide films, which will not affect the color of the products, and will also form complexes with organic dyes, thus increasing the sunlight resistance of the color.

Folded Filled Closure Method

In addition to the sealing methods described above, anodic oxide films can also be sealed with organic substances such as clear varnish, molten paraffin, various resins and dry oils.