Have you ever wondered why the aluminum-made casing of the smartphone in your hand presents such a matte and delicate texture with good scratch resistance? The answer lies in one of the most used surface finishing methods -anodizing. This article will discuss what it is, its types and processes.
What is Anodizing？
Anodizing is a universal surface finish method for aluminum parts. This process involves placing an aluminum or aluminum alloy product as the anode in an electrolyte solution, where an electrochemical reaction forms a layer of aluminum oxide on its surface. The resulting anodic film can be anywhere from tens to hundreds of micrometers thick and boasts mechanical properties, corrosion resistance, wear resistance, and weatherability. Additionally, the film has a strong adsorption capacity, which allows for further processes like electrolytic coloring. By adjusting the coloring time, you can produce a wide range of shades from light to dark, resulting in a beautiful and decorative finish.
Characteristics of Anodizing
Porosity: The anodic film has a porous, honeycomb-like structure. The porosity is determined by the type of electrolyte and the specific anodizing conditions. This porous structure gives the film an excellent ability to absorb various organic materials, resins, waxes, inorganic substances, dyes, and paints. It can serve as a strong base layer for coatings or be dyed in a variety of colors to enhance the metal’s decorative appeal.
Wear Resistance: The aluminum oxide film is hard, which improves the metal’s surface wear resistance. When the film absorbs lubricants, its wear resistance is further enhanced.
Corrosion Resistance: The aluminum oxide film is very stable in the atmosphere, providing excellent corrosion resistance. The level of resistance depends on the film’s thickness, composition, porosity, the base material’s composition, and the integrity of its structure. To further improve its corrosion resistance, the anodic film is typically sealed or painted after treatment.
Electrical Insulation: The anodic film has high insulation resistance and breakdown voltage, making it suitable for use as a dielectric layer in electrolytic capacitors or an insulating layer in electrical products.
Thermal Insulation: The aluminum oxide film is an effective thermal insulator with stability up to 1500°C. This allows it to prevent aluminum from melting in parts that operate under momentary high temperatures.
Adhesion: The bond between the anodic film and the base metal is strong. It’s very difficult to separate them by mechanical means. Even if the film cracks as the base metal bends, it will maintain its excellent bond.
Mechanism of Anodizing
Aluminum anodizing uses an acidic solution with a moderate dissolving capacity. Lead serves as the cathode and acts only as a conductor. During the anodizing process, the following reactions occur:
At the Anode:
At the Cathode:
Simultaneously, the acid chemically dissolves the aluminum and the newly formed oxide film. The reaction is as follows:
The formation and dissolution of the oxide film happen at the same time. In the initial stage, the film’s formation rate is greater than its dissolution rate, causing the film thickness to increase continuously. As the thickness increases, so does its electrical resistance, which slows down the film’s growth. Eventually, the growth rate equals the dissolution rate, and the film’s thickness stabilizes.
The formation process can be further understood by examining the voltage-time curve of the anodizing process.
The entire voltage-time curve for anodizing can be divided into three main stages:
Stage A: Formation of a Non-Porous Layer. This is the ab segment of the curve. In the first few seconds or tens of seconds after the current is applied, the voltage rises sharply from zero to a maximum value, known as the critical voltage. This indicates that a continuous, non-porous thin film has formed on the anode’s surface. This film’s presence prevents further thickening of the layer. The thickness of this non-porous layer is directly proportional to the formation voltage and inversely proportional to the film’s dissolution rate in the electrolyte.
Stage B: Formation of the Porous Layer. This is the bc segment. After reaching its maximum, the voltage begins to drop by about 10% to 15% of its peak value. This signifies that the non-porous film has started to dissolve, and a porous layer is forming.
Stage C: Thickening of the Porous Layer. This is the cd segment. After about 20 seconds of anodizing, the voltage enters a stable, slow-rising phase. This indicates that as the non-porous layer is continuously dissolved to form the porous layer, a new non-porous layer is growing simultaneously. In other words, the porous layer is continuously thickening. The process of film formation and dissolution occurs at the bottom of each pore cell. When the film formation and dissolution rates reach a dynamic equilibrium, the thickness of the anodic film will no longer increase, even if the anodizing time is extended. At this point, the anodizing process should be stopped.
Types of Anodizing and Processes
There are many methods for anodizing aluminum and its alloys. Here, we’ll focus on the most common ones: sulfuric acid, chromic acid, and oxalic acid anodizing. Other methods include hard anodizing and porcelain anodizing.
Sulfuric Acid Anodizing
This process uses a direct or alternating current in a dilute sulfuric acid electrolyte. It produces a colorless, transparent oxide film between 5 and 20 μm thick with good adsorption properties. The process is simple, the solution is stable, and it’s easy to operate.
| Solution Composition | Direct Current Method (Formula 1) | Direct Current Method (Formula 2) | Alternating Current Method |
| Sulfuric Acid（g/L） | 150-200 | 160～170 | 100～150 |
| Aluminum Ions | < 20 | < 15 | < 25 |
| Temperature (°C) | 15～25 | 0-3 | 15～25 |
| Anode Current Density (A/) | 0.8～1.5 | 0.4～0.6 | 2～4 |
| Voltage (V) | 18～25 | 16～20 | 18～30 |
| Anodizing Time (min) | 20～40 | 60 | 20～40 |
Sulfuric Acid Concentration: A high concentration of sulfuric acid increases the film’s chemical dissolution rate, resulting in a thin, soft, and porous film with strong adsorption and good dyeing properties. Lowering the concentration leads to a faster film growth rate, lower porosity, higher hardness, and better wear resistance and reflectivity.
Temperature: The electrolyte’s temperature impacts the quality of the oxide film. Between 10°C and 20°C, the film is porous, highly adsorbent, and elastic, making it suitable for dyeing, but its hardness and wear resistance are lower. If the temperature exceeds 26°C, the film becomes loose and soft. Below 10°C, the film is thicker and harder with better wear resistance, but its porosity is lower. Therefore, it’s crucial to control the electrolyte temperature during production.
Current Density: Increasing the current density accelerates film growth, shortens anodizing time, and reduces chemical dissolution, resulting in a harder and more wear-resistant film. However, if the current density is too high, the Joule effect can increase the dissolution of the film, which can slow down its growth. If the current density is too low, the anodizing time will be very long, and the resulting film will be loose and have lower hardness.
Time: The anodizing time is determined by the electrolyte concentration, temperature, current density, and the desired film thickness. Under the same conditions, as the time increases, the oxide film thickens, and its porosity increases. However, after reaching a certain thickness, the growth rate slows down and eventually stops.
Agitation: Agitation promotes solution convection, ensuring a uniform temperature and preventing localized overheating that could degrade the oxide film’s quality. Agitation is done with air compressors or water pumps.
Alloy Composition: The composition of the aluminum alloy affects the film’s quality, thickness, and color. Generally, other elements in the aluminum alloy can lower the film’s quality. For Al-Mg alloys with more than 5% magnesium and a non-uniform structure, proper heat treatment must be used to homogenize the alloy; otherwise, the film’s transparency will be affected. In Al-Mg-Si alloys, as the silicon content increases, the film’s color changes from colorless and transparent to gray, purple, and eventually black, making it difficult to get a uniform color. In Al-Cu-Mg-Mn alloys, copper decreases the film’s hardness, increases porosity, and results in a looser, lower-quality film. Under the same anodizing conditions, pure aluminum produces the thickest, hardest, and most corrosion-resistant film.
Chromic Acid Anodizing
The oxide film produced by chromic acid anodizing is 2 to 5 μm thick. It has low porosity and is soft, with poor wear resistance. Since there’s very little aluminum dissolution, the parts maintain their original precision and surface roughness after the film is formed. This process is therefore suitable for precision parts.
| Solution Composition | Formula 1 | Formula 2 | Formula 3 |
| Chromic Anhydride(g/L) | 50～60 | 30～40 | 95～100 |
| Temperature (°C) | 33～37 | 38～42 | 35～39 |
| Anode Current Density (A/) | 1.5～2.5 | 0.2～0.6 | 0.3～2.5 |
| Voltage (V) | 0～40 | 0～40 | 0～40 |
| Anodizing Time (min) | 60 | 60 | 35 |
| Anode Material | Lead Plate or Graphite | Lead Plate or Graphite | Lead Plate or Graphite |
Chromic Anhydride Concentration: Both high or low chromic anhydride content will reduce the anodizing capacity, though a high concentration is acceptable. Electrolytes with too little chromic anhydride are unstable, which can lead to a drop in film quality.
Impurities: Chloride ions, sulfate ions, and trivalent chromium ions are all harmful impurities in the chromic acid anodizing electrolyte. Chloride ions can cause part etching; an increase in sulfate ions will make the oxide film go from transparent to opaque and shorten the bath’s lifespan; and too many trivalent chromium ions will result in a dark, dull film.
Voltage: For the first 15 minutes of the anodizing process, the voltage should be gradually increased from 0V to 40V, in increments of no more than 5V, to keep the current within the specified range. Once the bath voltage reaches 40V, it should be maintained for the rest of the anodizing process.
Oxalic Acid Anodizing
Oxalic acid is a weak acid with minimal corrosive effects on aluminum and its alloys. Therefore, oxalic acid anodizing produces a harder, thicker (up to 60 μm) film with good corrosion resistance and excellent electrical insulation. Depending on the alloy elements and their content, the film can take on a variety of vibrant colors. The oxalic acid electrolyte is very sensitive to chloride ions. If their concentration exceeds 0.04 g/L, corrosion spots can appear on the film. The concentration of trivalent aluminum ions also shouldn’t exceed 3 g/L.
| Solution Composition (g/L) | Formula 1 | Formula 2 | Formula 3 |
| Oxalic Acid | 27～33 | 50-100 | 50 |
| Temperature (°C) | 15～21 | 35 | 35 |
| Anode Current Density (A/) | 1～2 | 2-3 | 1-2 |
| Voltage (V) | 110～120 | 40～60 | 30～35 |
| Anodizing Time (min) | 120 | 30～60 | 30～60 |
| Power Source | DC | AC | DC |
Coloring and Sealing
After anodizing, aluminum and its alloys have a porous oxide film on their surface. By applying coloring and sealing treatments, the parts can take on a variety of colors, while also improving the film’s corrosion resistance and durability.
Inorganic Pigment Coloring
The mechanism of inorganic pigment coloring is physical adsorption, where inorganic pigment molecules are adsorbed and fill the pores of the film. This method produces less vibrant colors and has a weaker bond with the substrate, but it offers better lightfastness. The dyes used in this method are a two-step process. The anodized metal is immersed in two different solutions alternately until the reaction between the two salts within the oxide film produces enough pigment to achieve the desired shade.
| Color | Composition | Mass Concentration (g/L) | Temperature (°C) | Time (min) | Formed Colored Salt |
| Red | Cobalt Acetate | 50~100 | Room Temp. | 5~10 | Potassium Ferricyanide |
| Potassium Ferricyanide | 10~50 | Room Temp. | 5~10 | Potassium Ferricyanide | |
| Blue | Potassium Ferrocyanide | 10~50 | Room Temp. | 5~10 | Prussian Blue |
| Ferric Chloride | 10~100 | Room Temp. | 5~10 | Prussian Blue | |
| Yellow | Potassium Chromate | 50~100 | Room Temp. | 5~10 | Lead Chromate |
| Potassium Chromate Lead Acetate | 100~200 | Room Temp. | 5~10 | Lead Chromate | |
| Black | Cobalt Acetate | 50~100 | 5~10 | Cobalt Oxide | |
| Potassium Permanganate | 12~25 | Room Temp. | 5~10 | Cobalt Oxide |
Organic Dye Coloring
The mechanism for organic dye coloring is more complex and is considered to involve both physical adsorption and chemical reactions. The chemical bonding between the aluminum oxide and the dye molecules can occur in several ways: covalent bonds between the oxide and the dye’s sulfonic groups, hydrogen bonds between the oxide and the dye’s phenolic groups, or the formation of a complex between the oxide and the dye molecules. Organic dye coloring produces vibrant colors and offers a wide range of hues, but it has poor lightfastness. Distilled or deionized water is recommended for preparing the dye solution, as tap water contains calcium and magnesium ions that can form complexes with the dye molecules, rendering the solution useless.
| Color | Dye Name | Mass Concentration (g/L) | Temperature (°C) | Time (min) | pH Value |
| Red | Rhodamine B (R) | 5～10 | 60～70 | 10～20 | / |
| Acid Magenta (GR) | 6～8 | Room Temp. | 2～15 | 4.5～5.5 | |
| Reactive Red | 2～5 | 70～80 | / | / | |
| Alizarin Red S (GLW) | 3～5 | Room Temp. | 5～10 | 5～6 | |
| Blue | Direct Fast Blue | 3～5 | 15～30 | 15～20 | 4.5～5.5 |
| Reactive Blue | 5 | Room Temp. | 1～5 | 4.5～5.5 | |
| Acid Blue | 2～5 | 60～70 | 2～15 | 4.5～5.5 | |
| Golden Yellow | Rhodamine S (S) | 0.3 | 70～80 | 1～3 | 5～6 |
| Rhodamine B (R) | 0.5 | / | / | / | |
| Reactive Yellow | 0.5 | 70～80 | 5～15 | / | |
| Alizarin Yellow (GLW) | 2.5 | Room Temp. | 2～5 | 5～5.5 | |
| Black | Acid Black (ATT) | 10 | Room Temp. | 3～10 | 4.5～5.5 |
| Acid Element | 10～12 | 60～70 | 10～15 | ||
| Naphthol Green Black | 5～10 | 60～70 | 15～30 | 5～5.5 |
Electrolytic Coloring
In this process, the anodized aluminum or alloy is placed in an electrolyte solution containing metal salts and is subjected to electrolysis. An electrochemical reaction reduces the heavy metal ions that have entered the pores of the oxide film into metal atoms, which are deposited at the bottom of the pores on the non-porous layer, giving the film its color. The colored oxide film produced by this method has good wear resistance, lightfastness, heat resistance, corrosion resistance, and long-lasting color stability. It is used for architectural aluminum profiles. The higher the voltage and the longer the duration of electrolytic coloring, the deeper the color.
| Color | Dye Name | Mass Concentration (g/L) | Temperature (°C) | AC Voltage (V) | Time (min) |
| Red | Silver Nitrate | 0.4-10 | Room Temp. | 8~20 | 0.5~1.5 |
| Sulfuric Acid | 5~30 | Room Temp. | 8~20 | 0.5~1.5 | |
| Bronze →Brown →Black | Nickel Sulfate | 25 | 20 | 7~15 | 2~15 |
| Boric Acid | 25 | 20 | 7~15 | 2~15 | |
| Ammonium Sulfate | 15 | 20 | 7~15 | 2~15 | |
| Magnesium Sulfate | 20 | 20 | 7~15 | 2~15 | |
| Bronze →Brown →Black | Tin(II) Sulfate | 20 | 15~25 | 13~20 | 5~20 |
| Sulfuric Acid | 10 | 15~25 | 13~20 | 5~20 | |
| Boric Acid | 10 | 15~25 | 13~20 | 5~20 | |
| Purple →Reddish Brown | Copper Sulfate | 35 | 20 | 10 | 5~20 |
| Magnesium Sulfate | 20 | 20 | 10 | 5~20 | |
| Sulfuric Acid | 5 | 20 | 10 | 5~20 | |
| Black | Copper Sulfate | 25 | 20 | 17 | 13 |
| Magnesium Sulfate | 15 | 20 | 17 | 13 | |
| Sulfuric Acid | 25 | 20 | 17 | 13 |
Sealing Treatment
After anodizing, whether it’s colored or not, the aluminum must be sealed promptly. The purpose of sealing is to fix the dye in the pores to prevent it from bleeding out, while also improving the film’s wear resistance, lightfastness, corrosion resistance, and insulation. Sealing methods include hot water sealing, steam sealing, dichromate sealing, hydrolysis sealing, and filling sealing.
Hot Water Sealing
The principle of hot water sealing is based on the hydration of amorphous:
Here, n can be 1 or 3. The hydration of forms boehmite () and, when forms hydrargillite (). The formation of hydrargillite increases its volume by nearly 100%. As a result of the hydration on the surface and pore walls of the oxide film, the pores are sealed due to the volume expansion. The process for hot water sealing involves a temperature of 90°C to 100°C, a pH of 6 to 7.5, and a duration of 15 to 30 minutes. The water used for sealing must be distilled or deionized, not tap water, as tap water can reduce the transparency and color of the oxide film. The principle of steam sealing is the same as hot water sealing, but the results are much better, although the cost is higher.
Dichromate Sealing
This method is performed in a hot potassium dichromate solution, which is a strong oxidizing agent. When the anodized aluminum part is submerged in the solution, the aluminum oxide on the film and pore walls reacts with the potassium dichromate, as follows:
The resulting aluminum chromate hydroxide and aluminum dichromate hydroxide precipitates, along with the boehmite and hydrargillite formed by the reaction of the hot water molecules with the aluminum oxide, seal the pores of the film. The sealed oxide film is yellow and has good corrosion resistance. This method is suitable for sealing anodized aluminum alloys intended for protection, but it’s not suitable for sealing colored decorative films.
| Potassium Dichromate | 50-70 g/L |
| Temperature | 90°C～95°C |
| Time | 15～25 min |
| pH Value | 6-7 |
Hydrolysis Sealing
When a very dilute solution of nickel or cobalt salt is adsorbed by the oxide film, a hydrolysis reaction occurs:
The resulting nickel or cobalt hydroxide precipitates into the pores of the oxide film, sealing them. Since the small amounts of nickel and cobalt hydroxide are almost colorless, this method is suitable for sealing colored oxide films.
| Solution Composition (g/L) | Formula 1 | Formula 2 | Formula 3 |
| Nickel Sulfate | 4～6 | 3-5 | / |
| Cobalt Sulfate | 0.5 ～0.8 | / | / |
| Cobalt Acetate | / | / | 1～2 |
| Sodium Acetate | 4～6 | 3～5 | 3～4 |
| Boric Acid | 4～5 | 3～4 | 5～6 |
| pH Value | 4～6 | 5～6 | 4.5～5.5 |
| Temperature (°C) | 80～85 | 70～80 | 80～90 |
| Sealing Time (min) | 10～20 | 10～15 | 10～25 |
Filling Sealing
In addition to the methods mentioned above, anodic oxide films can also be sealed using organic substances such as clear lacquer, molten paraffin wax, various resins, and drying oils.
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