Electrolysis is used to form a thin oxide film on the surface of metal or alloy parts by using the parts as anodes.
The metal oxide film changes the surface state and properties, such as surface coloring, improved corrosion resistance, increased wear resistance and hardness, and protection of the metal surface.
For example, in aluminum anodizing, aluminum and its alloys are placed in corresponding electrolytes (such as sulfuric acid, chromic acid, oxalic acid, etc.) as anodes, and electrolysis is performed under specific conditions and with external current applied.
The aluminum or its alloy at the anode oxidizes, forming a thin layer of aluminum oxide on the surface, with a thickness of 5-20 microns. Hard anodizing films can reach 60-200 microns in thickness.
After anodizing, the hardness and wear resistance of the aluminum or its alloy are improved, reaching 250-500 kg/square millimeter. The hard anodizing film also has good heat resistance, with a melting point as high as 2
In practical applications, anodizing of aluminum alloys is quite common and can be used in daily life, as this process creates a hard protective layer on the surface of aluminum parts, making it suitable for producing kitchen utensils and other household items.
However, anodizing of cast aluminum has poor results, with uneven surfaces and only black coloration. Anodizing of aluminum alloy profiles is relatively better.
In recent years, China’s aluminum oxidation coloring technology has developed rapidly, and many factories have adopted new process technologies and accumulated rich experience in actual production.
There are many mature and developing methods for anodizing of aluminum and its alloys, from which suitable processes can be selected based on production needs.
Before selecting an oxidation process, it is important to understand the aluminum or aluminum alloy material, as the quality of the material and its composition directly affects the quality of the anodized aluminum product.
For example, if there are defects such as bubbles, scratches, peeling, roughness, etc. on the surface of the aluminum, they will still be visible after anodizing. The alloy composition also directly affects the appearance of the anodized surface.
For instance, aluminum alloys containing 1-2% manganese turn brownish-blue after oxidation, and increased manganese content leads to a transformation from brownish-blue to dark brown color.
Aluminum alloys containing 0.6-1.5% silicon turn gray after oxidation, while those containing 3-6% silicon become white-gray. Zinc-containing alloys appear milky while chromium-containing alloys exhibit uneven colors ranging from golden-yellow to gray, and nickel-containing alloys appear light yellow.
Generally speaking, only aluminum alloys containing more than 5% magnesium and titanium can achieve a transparent and bright appearance after oxidation.
After selecting suitable aluminum and aluminum alloy materials, it is necessary to consider choosing the appropriate anodizing process.
Currently, sulfuric acid oxidation, oxalic acid oxidation, and chromic acid oxidation are widely used in China and have been thoroughly documented in manuals and books. This article briefly introduces some new processes currently under development in China and foreign methods.
1. New processes developed in China:
(1) Rapid AC oxidation in oxalic-methanoic acid mixture.
Using an oxalic-methanoic acid mixture is based on the idea that methanoic acid is a strong oxidizing agent and can accelerate the dissolution of the inner layer (barrier layer and blocking layer) of the oxide film, resulting in the formation of a porous outer layer.
This type of solution can increase conductivity (i.e., increase current density), allowing the oxide film to form quickly. Compared with pure oxalic acid oxidation, this solution can increase productivity by 37.5% and reduce power consumption (3.32 kWh per square meter for oxalic acid oxidation compared to 2 kWh per square meter for this process), saving 40% of electricity.
The process formulation is as follows: oxalic acid 4-5%, methanoic acid 0.55%, three-phase AC 44 V, current density 2-2.5 A/d㎡, temperature 30±2℃.
(2) Mixed acid oxidation.
This method was officially included in the Japanese national standard in 1976 and adopted by Kita-sei Nissho Co., Ltd. Its characteristics are fast film formation, higher hardness, wear resistance, and corrosion resistance of the film compared to conventional sulfuric acid oxidation.
The film is silver-white and suitable for printing and coloring products. After China’s aluminum products industry visited Japan, this method was recommended for use in 1979.
The recommended process formulation is: H2SO4 10-20%, COOHCOOH·2H2O 1-2%, voltage 10-20 V, current density 1-3 A/d㎡, temperature 15-30℃, time 30 minutes.
(3) Ceramic oxidation.
Ceramic oxidation mainly uses chromic acid, boric acid, and potassium titanium oxalate as electrolytes and undergoes electrolytic treatment at high voltage and temperature.
The film is like glaze on ceramics, with high corrosion resistance, good wear resistance, and can be colored with organic or inorganic dyes, giving it a special luster and color. It is mainly used in aluminum cookware, lighters, and gold pens, and is very popular among consumers.
(4) Military color oxidation.
Military color oxidation is mainly used for decoration on military aluminum products, so it requires special protective effects. The oxide film is army green, non-glossy, wear-resistant, durable, and has good protective properties.
The process involves first performing oxalic acid oxidation to generate a golden-yellow film layer, then subjecting it to anodic oxidation treatment using a solution of 20g/l of potassium permanganate and 1g/l of H2SO4. Shenyang Aluminum Products Factory has used this process to produce military water bottles and cookware.
(5) Multicolor oxidation.
The already dyed but unclosed anodic oxide layer is wetted with chromic acid or oxalic acid so that CrO3 spreads out.
The surface of the dyed product fades when it is wetted by CrO3, and the oxalic acid or chromic acid is washed away with water in any part as needed, generally stopping the reaction with the image.
Then, the second dye is applied, or the CrO3 wiping, rinsing, dyeing process is repeated to produce patterns such as flowers and clouds as needed.
Currently, this method is widely used in products such as gold cups, water cups, tea boxes, and lighters.
(6) Marble pattern dyeing process.
After the product is oxidized and dyed with the first color, it is dried and then immersed in water with grease on the surface.
When lifted or immersed, the grease and water flow down naturally, causing irregular stripe-shaped stains on the film. When the second dye is applied, the oxidized film cannot be stained where it is stained with grease, while the part without grease is dyed with the second tone, forming a marble-like irregular pattern.
This method can be found in the article by Comrade Zhou Shouyu from Guangdong State-owned Yangjiang Knife Factory (Electroplating and Coating, 1982, Issue 2).
(7) Chemical etching oxidation.
After mechanical polishing and degreasing, aluminum products are coated with masking agents or photosensitive materials and dried, then subjected to chemical etching (fluoride or iron salt etchants) to form concave-convex patterns.
After electrochemical polishing and anodic oxidation, the surface pattern with a strong sense of the main body is presented, which can be comparable to the appearance of stainless steel. It is currently used in products such as gold pens, tea boxes, and screens.
(8) Room temperature rapid anodic oxidation.
Typically, H2SO4 oxidation requires a cooling device, resulting in high power consumption. Adding alpha-hydroxypropionic acid and glycerol can suppress the dissolution of the oxide film, making it possible to carry out oxidation at room temperature.
Compared with ordinary sulfuric acid oxidation, the film thickness can be increased by twice. The recommended process formulation is:
|Current density||0.8-12 A/d㎡|
(9) Chemical oxidation method (also known as conductive oxide film).
The corrosion resistance of the film layer is similar to that of sulfuric acid anodic oxide film. The conductive oxide film has a lower contact resistance and can conduct electricity, while the H2SO4 anodic oxide film cannot conduct electricity due to its high contact resistance.
The corrosion resistance of the conductive oxide film is much stronger than that of copper-plated, silver-plated or tin-plated aluminum.
The disadvantage is that tin soldering cannot be performed on the film layer, only spot welding can be used. The recommended process formulation is: CrO3 4g/l, K4Fe(CN)6·3H2O 0.5g/l, NaF 1g/l, temperature 20-40℃, time 20-60 seconds.
When selecting aluminum for anodic oxidation, the following should also be noted:
(1) The surface of the selected aluminum should not have severe scratches, structural defects, or inclusions. They will affect the appearance and corrosion resistance of the oxide film layer.
(2) Some aluminum alloys should be heat-treated according to reasonable specifications. The size of the grains has a certain impact on the structure and properties of the oxide film. Coarse grains react unevenly during oxidation, often resulting in an orange peel-like appearance. Therefore, it is generally desired that the aluminum has a fine grain structure.
2. Introduction to New Surface Treatment Processes Abroad
In recent years, foreign countries have developed rapidly in aluminum surface treatment. Old processes that were once labor-intensive, power-intensive and resource-intensive have been reformed, and new processes and technologies have been widely applied in industrial production.
(1) High-speed anodic oxidation method.
The high-speed anodic oxidation process mainly changes the composition of the electrolyte solution and reduces the impedance of the electrolyte solution, thereby allowing higher current densities for high-speed anodic oxidation.
The film formation speed of the old process using a 1A/d㎡ current density was 0.2~0.25μ/min, while the film formation speed of this new process using the modified solution can be increased to 0.4~0.5μ/min even with a 1A/dm2 current density, greatly reducing processing time and improving production efficiency.
(2) Tomita-style (high-speed oxidation) method.
The Tomita-style method has a much shorter processing time than the old process, and its production efficiency can be increased by more than 33%. This method is suitable not only for ordinary anodic oxide films but also for hard oxide films.
If a hard film is to be produced, a method of reducing the solution temperature is used. The film formation speed is generally the same as that listed in the table above. The relationship between film hardness and solution temperature is as follows:
- 10℃ – hardness 500H
- 20℃ – hardness 400H
- 30℃ – hardness 300H
(3) Ruby film.
The process of producing a ruby film on the surface of aluminum is a novel process. The color of the film can be comparable to that of artificial rubies, making it ideal for decorative purposes. It also has good corrosion resistance and wear resistance.
Different types of metal oxides in the solution can be used to produce a variety of appearances. The process involves first anodizing with 15% sulfuric acid using a current density of 1A/dm2 for 80 minutes.
Then, the workpiece is immersed in a (NH4)2CrO4 solution at different concentrations for 30 minutes at 40℃, depending on the desired color intensity, to allow the metal ions to enter the pores of the anodic oxide film.
After that, the workpiece is immersed in a sodium bisulfate (1 gram molecular weight) and ammonium bisulfate (1.5 gram molecular weight) solution at 170℃ with a current density of 1A/dm2. The resulting film is a purple-red color with a fluorescent shimmer, while Fe2(CrO4)3 or Na2CrO4 solutions will produce blue films with deep purple fluorescence.
(4) Asada electrocoloring.
Asada electrocoloring is a process where, after anodizing, metal cations (nickel salts, copper salts, cobalt salts, etc.) are electrolyzed into the bottom of the pinholes of the oxide film to produce color. This process has developed rapidly in recent years, mainly because it can obtain bronze and black colors, which are popular in the construction industry.
The colors produced are stable and resistant to harsh weather conditions. This process can save energy compared to natural coloring methods.
Almost all of Japan’s architectural aluminum profiles are colored using this method.
(5) Natural coloring method.
The natural coloring method completes the coloring in one electrolysis.
There are several types of solutions used, including salicylic acid and sulfuric acid, sulfonic acid and titanium acid, and sulfonic acid and maleic acid.
Since organic acids are mostly used in the natural coloring method, the oxide film is relatively dense and has excellent light resistance, wear resistance, and corrosion resistance.
However, the disadvantage of this method is that to obtain excellent colors, the composition of the aluminum alloy material must be strictly controlled.
1. Sulfuric acid anodizing.
Sulfuric acid anodizing has the following characteristics:
(1) Low cost of the solution, simple composition, easy operation and maintenance.
Generally, only diluting sulfuric acid to a certain concentration is required, without adding other chemical agents. Chemically pure sulfuric acid or industrial sulfuric acid with fewer impurities is recommended for use, so the cost is particularly low.
(2) High transparency of the oxide film.
The sulfuric acid anodized film of pure aluminum is colorless and transparent. For aluminum alloys, as the alloying elements Si, Fe, Cu, and Mn increase, the transparency will decrease. Compared with other electrolytes, the color of the sulfuric acid anodized film is the lightest.
(3) High coloring performance.
The sulfuric acid oxide film is transparent, and the porous layer has strong adsorption and is easy to dye and color. The color is bright and not easy to fade, with a strong decorative effect.
(4) Operating conditions for sulfuric acid anodizing are:
2. Oxalic acid and chromic acid anodizing.
Oxalic acid anodization is widely used in Japan, and the characteristics of the oxide film are similar to sulfuric acid anodization, with a lower porosity than sulfuric acid anodization, high corrosion resistance, and hardness. The cost of the oxalic acid solution and the operating voltage are higher than that of sulfuric acid, and the color of the oxide film of some alloys may be darker. Both oxalic acid and sulfuric acid anodization require a good cooling system.
The operating conditions for oxalic acid anodizing are:
|Oxalic acid (volume fraction)||2%~10%|
|Temperature / ℃||15~35|
|Current density / A.dm-2||0.5~3|
Chromic acid anodized films are particularly resistant to corrosion and are mainly used in the aerospace industry. The adhesion of chromic acid oxide films and paint is strong, making it suitable as a base for paint. The gray opaque chromic acid anodized film is generally not used for decorative purposes.
The operating conditions for chromic acid anodizing are:
3. Hard anodizing.
In the late World War II, in order to increase the hardness and thickness of the anodized film, the temperature of the sulfuric acid anodizing tank was lowered to 0℃, and the current density was increased to 2.7~4.0A/dm2, obtaining a “hard oxide film” of 25~50μm. A hard anodized film can be obtained at 5~15℃ using oxalic acid with a small amount of sulfuric acid. Some patents use optimized sulfuric acid concentration, organic acids, or other additives such as benzene hexacarboxylic acid for hard anodizing.
In Scotland, Campbell invented the use of AC-DC superimposed power supply, high-speed flow of electrolyte, 0℃, and current density of 25~35A/dm2 to obtain a hard anodized film of 100μm.
Nowadays, pulse current is used for hard anodizing, especially for high-copper aluminum alloys, which are generally difficult to hard anodize. The use of pulse current can prevent “burning”. There are also many power supplies used for hard anodizing, such as AC-DC, various frequencies of single-phase or three-phase pulse currents, reverse currents, etc.
In traditional DC hard anodizing, the current density generally cannot exceed 4.0A/dm2. For single-phase rectifier pulse power supply, the peak value of the current pulse can be very large, but maintaining the uniformity of the oxide film thickness is an important issue.