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Anodizing is an electrolytic passivation process used to increase the thickness of the natural oxide layer on the surface of metal parts.

The process is called anodizing because the part to be treated forms the anode electrode of an electrolytic cell. Anodizing increases resistance to corrosion and wear, and provides better adhesion for paint primers and glues than bare metal does. Anodic films can also be used for several cosmetic effects, either with thick porous coatings that can absorb dyes or with thin transparent coatings that add reflected light wave interference effects.

Anodizing is also used to prevent galling of threaded components and to make dielectric films for electrolytic capacitors. Anodic films are most commonly applied to protect aluminium alloys, although processes also exist for titanium, zinc, magnesium, niobium, zirconium, hafnium, and tantalum. Iron or carbon steel metal exfoliates when oxidized under neutral or alkaline micro-electrolytic conditions; i.e., the iron oxide (actually ferric hydroxide or hydrated iron oxide, also known as rust) forms by anoxic anodic pits and large cathodic surface, these pits concentrate anions such as sulfate and chloride accelerating the underlying metal to corrosion. Carbon flakes or nodules in iron or steel with high carbon content (high-carbon steel, cast iron) may cause an electrolytic potential and interfere with coating or plating. Ferrous metals are commonly anodized electrolytically in nitric acid or by treatment with red fuming nitric acid to form hard black Iron(II,III) oxide. This oxide remains conformal even when plated on wiring and the wiring is bent.

Anodizing changes the microscopic texture of the surface and the crystal structure of the metal near the surface. Thick coatings are normally porous, so a sealing process is often needed to achieve corrosion resistance. Anodized aluminium surfaces, for example, are harder than aluminium but have low to moderate wear resistance that can be improved with increasing thickness or by applying suitable sealing substances. Anodic films are generally much stronger and more adherent than most types of paint and metal plating, but also more brittle. This makes them less likely to crack and peel from ageing and wear, but more susceptible to cracking from thermal stress.

Before choosing the oxidation process, you should understand the aluminum or aluminum alloy material, because the quality of the material and the difference in the ingredients will directly affect the quality of the aluminum product after anodization. For example, if there are defects such as bubbles, scratches, peeling, and roughness on the surface of aluminum, all defects will still be revealed after anodizing. The alloy composition also has a direct impact on the surface appearance after anodization. For example, the aluminum alloy containing 1-2% manganese is brown-blue after oxidation. With the increase of manganese content in the aluminum material, the surface color after oxidation changes from brown-blue to dark brown. Aluminum alloys containing 0.6 to 1.5% silicon are gray after oxidation, and white gray when they contain 3 to 6% silicon. The zinc-containing ones are opalescent, the chromium-containing ones are golden to gray in uneven shades, and the nickel-containing ones are pale yellow. Generally speaking, only aluminum containing magnesium and titanium containing more than 5% gold can obtain a colorless, transparent, bright and clean appearance after oxidation.

After selecting aluminum and aluminum alloy materials, it is natural to consider the selection of a suitable anodizing process. At present, the sulfuric acid oxidation method, oxalic acid oxidation method and chromic acid oxidation method widely used in our country have all been introduced in detail in manuals and books, so it is unnecessary to repeat them. This article would like to give a brief introduction to some new technologies currently under development in China, as well as some methods in foreign countries.

1. The new technology of anodizing has been developed in China

(1) Rapid oxidation of oxalic acid-formic acid mixed solution

The use of oxalic acid-formic acid mixture is because formic acid is a strong oxidant, in such a bath, formic acid accelerates the dissolution of the inner layer (barrier layer and barrier layer) of the oxide film, thereby making it a porous layer (ie. the outer layer of the oxide film). The conductivity of the bath can be improved (ie, the current density can be increased), so that the oxide film can be rapidly formed. Compared with the pure oxalic acid oxidation method, this solution can increase the productivity by 37.5%, reduce the power consumption (the power consumption of the oxalic acid oxidation method is 3.32 kWh/m2, this method is 2 kWh/m2), and save electricity by 40%.

The technological formula is: oxalic acid 4-5%, formic acid 0.55%, three-phase AC 44±2 volts, current density 2-2.5A/d㎡, temperature 30±2℃.

(2) Mixed acid oxidation

This method was officially incorporated into the Japanese national standard in 1976, and was adopted by Japan North Star Nikkei Household Products Co., Ltd. Its characteristics are that the film is formed quickly, the hardness, wear resistance and corrosion resistance of the film are higher than those of the ordinary sulfuric acid oxidation method, and the film layer is silver-white, which is suitable for printing and coloring products. After my country’s aluminum product industry visited Japan, it was recommended for use in 1979. The recommended process formula is: H2SO4 10～20%, COOHCOOH·2H2O 1～2%, voltage 10～20V, current density 1～3A/d㎡, temperature 15～30℃, time 30 minutes.

(3) Porcelain oxidation

Porcelain oxidation mainly uses chromic acid, boric acid and potassium titanium oxalate as electrolytes, and uses high voltage and high temperature for electrolytic treatment. The appearance of the film layer is like the glaze on the porcelain, which has high corrosion resistance and good wear resistance. The film layer can be dyed with organic or inorganic dyes, so that the appearance has a special luster and color. At present, it is mostly used in aluminum cookware, lighters, gold pens and other products, and is very popular among the masses.

(4) National Defense Color Oxidation

National defense color oxidation is mainly used in the decoration of military aluminum products, so it requires special protection. The oxide film is military green, matt, wear-resistant and durable, and has good protective performance. The process is as follows: firstly, oxalic acid is oxidized to form a golden yellow film layer, and then anodized with a solution of potassium permanganate 20g/l and H2SO41g/l. Shenyang Aluminum Products Factory used this process to produce military kettles and cooking utensils.

(5) Multi-color oxidation

Wet the dyed but unsealed anodic oxide layer with chromic acid or oxalic acid to spread CrO3. Part of the surface of the dyed product will fade after being wetted by CrO3. Add oxalic acid or chromium to any part of the product as needed. Acid wash away can generally stop the reaction with the image. Then dye the second color or repeat the procedures of CrO3 wiping, flushing, dyeing, etc., and patterns such as flowers and clouds can appear as needed. At present, it is mostly used in gold cups, water cups, tea boxes, lighters and other products.

(6) Marble pattern dyeing process

The oxidized product is first dyed with the first base color, dried, and then immersed in water with oil floating on the surface. When it is lifted or immersed, the oil and water will sag naturally, making the film layer irregularly striped. Contaminated by grease. When re-dyeing the second color, the parts of the oxide film that are stained with grease will not be dyed, and the parts without grease will be dyed with the second color, forming an irregular pattern like a marble pattern. This method can be seen in the articles of Comrade Zhou Shouyu of the Guangdong State-owned Yangjiang Knife Factory (“Electroplating and Finishing”, No. 2, 1982).

(7) Chemical etching and oxidation

After mechanical polishing and degreasing, the aluminum products are coated with masking agent or photosensitive, and then chemically etched (fluoride or iron salt etchant) after drying to form a concave-convex pattern. After electrochemical polishing and anodizing, it presents a surface pattern with a strong sense of body, which is comparable to the surface appearance of stainless steel. Now it is mostly used in gold pens, tea boxes and screens.

(8) Rapid anodic oxidation at room temperature

Usually, H2SO4 oxidation requires cooling equipment, which consumes a lot of electricity. After adding α-hydroxypropionic acid and glycerol, the dissolution of the oxide film can be inhibited, so that the oxidation can be carried out at normal temperature. Compared with the common sulfuric acid oxidation method, the film thickness can be increased by 2 times. The recommended process formula is:

H2SO4                        150～160g/l

CH3CH(OH)COOH              18ml/l

CH2OHCHOHCH2OH            12ml/l

Current density                  0.8～12A/d㎡

Voltage                        12-18 volts

Temperature                    18～22℃

(9) Chemical oxidation method (also known as conductive oxide film)

The corrosion resistance of the film is close to that of the sulfuric acid anodized film. The conductive oxide film has a small contact resistance and can conduct electricity, while the H2SO4 anodic oxide film cannot conduct electricity due to its large contact resistance. The corrosion resistance of the conductive oxide film is much stronger than that of copper, silver or tin plating on aluminum. The disadvantage is that the film layer cannot be soldered, only spot welding can be used. The recommended process formula is:

CrO3 4g/l, K4Fe(CN)6·3H2O 0.5g/l, NaF 1g/l, temperature 20～40℃, time 20～60 seconds.

In recent years, the surface treatment of aluminum materials has developed rapidly internationally. Some old processes that cost manpower, electricity and resources have been reformed, and some new processes and technologies have been widely used in industrial production.Here are typical rizing methods:

(1) High-speed anodizing method

The high-speed anodizing process mainly reduces the impedance of the electrolyte by changing the composition of the electrolytic solution, thereby enabling high-speed anodizing with a higher current density. The solution of the old process used a current density of 1A/d㎡ to form a film at a rate of 0.2 to 0.25μ/min. After using this new process solution, even if the current density of 1A/d㎡ was still used, the film forming speed could be improved. Increase to 0.4 ~ 0.5μ/min, greatly shorten the processing time and improve the production efficiency.

(2) Tomita type (high-speed oxidation) method

The Tomita method is much shorter than the old process, and the production efficiency can be increased by more than 33%. This method is not only suitable for ordinary anodic oxide film, but also for hard film oxidation.

If a hard film is to be produced, it is achieved by reducing the temperature of the solution, and the film forming speed is roughly the same as that listed in the table above. The relationship between film hardness and solution temperature is as follows:

10℃——Hardness 500H, 20℃——400H, 30℃——30H

(3) Ruby film

The process of forming a ruby film on the surface of aluminum is a novel process. The color of the film can be comparable to that of artificial ruby, so the decorative effect is excellent, and the corrosion resistance and wear resistance are also good. The appearance of different colors can also be prepared by different types of metal oxides contained in the solution. The process method is as follows: First, use 15% sulfuric acid for anodic oxidation, the current density used is 1A/d㎡, and the time is 80 minutes. After taking it out, the workpiece can be immersed in (NH4)2CrO4 solutions of different concentrations according to the requirements of the color depth, the temperature is 40 ℃, and the time is 30 minutes, mainly to let the metal ions enter the porous anodic oxide film hole source. Then add sodium hydrogen sulfate (1 g molecular weight), ammonium hydrogen sulfate (1.5 g molecular weight), the temperature is 170 ℃, the current density is 1A/d㎡, after the above treatment, a purple-red and flashing fluorescent ruby film can be obtained . If the immersion is Fe2(CrO4)3, Na2CrO4, the resulting film is blue with deep purple fluorescence.

(4) Asada method electrolytic coloring

Asada method electrolytic coloring is to make metal cations (nickel salts, copper salts, cobalt salts, etc.) penetrate into the bottom of the pinholes of the oxide film after anodizing, through electric current electrolysis, thereby coloring. This process has developed rapidly in recent years, mainly because it can obtain bronze tones and blacks, which are welcomed by the construction industry. The color has very stable light fastness and can also withstand harsh weather conditions. Compared with the natural coloring method, this process can save electric energy. Almost all aluminum profiles for construction in Japan have been colored by this method. my country’s Tianjin, Yingkou, Guangdong and other places have also introduced such technology and a complete set of equipment. Some units in Guangdong have also successfully tested and applied to production.

(5) Natural coloring method

The natural coloring method is completed by one electrolysis. There are also several types of solutions, including sulfosalicylic acid and sulfuric acid, sulfotitanic acid and sulfuric acid, and sulfosalicylic acid and maleic acid. Since most of the natural coloring methods use organic acids, the oxide film is relatively dense, and the film layer has excellent light resistance, wear resistance and corrosion resistance. But the disadvantage of this method is: to get good color, the composition of the aluminum alloy material must be strictly controlled.

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