For previous article, please refer to Surface treatments 1: electroplating.

There are two main types of oxidation treatment methods for aluminum and aluminum alloys:

Oxide film is thin, with a thickness of about 0.5 to 4 microns, is porous, soft, has good adsorption properties. It can be used as bottom layer of organic coatings, but its wear resistance and corrosion resistance are not as good as anodized films;

 

Thickness of oxide film is about 5 to 20 microns (thickness of hard anodized film can reach 60 to 200 microns). It has high hardness, good heat resistance and insulation, and its corrosion resistance is higher than that of chemical oxide film. It is porous and has good adsorption capacity.

Chemical oxidation treatment equipment for aluminum and aluminum alloys is simple, easy to operate, has high production efficiency, does not consume electricity, has a wide range of applications, is not limited by size and shape of parts.
Chemical oxidation process of aluminum and aluminum alloys can be divided into two categories: alkaline oxidation method and acidic oxidation method according to properties of solution.
According to properties of film layer, it can be divided into: oxide film, phosphate film, chromate film, and chromic acid-phosphate film.

Note: ① Formulas 1 and 2 are suitable for chemical oxidation of pure aluminum, aluminum-magnesium alloy, aluminum-manganese alloy and aluminum-silicon alloy. Color of film is golden yellow, but color of oxide film obtained on the latter two alloys is darker. Film layer obtained in alkaline oxidizing solution is soft, has poor corrosion resistance, high porosity and good adsorption, and is suitable as a coating base layer.
② Add sodium silicate to formula 3, and oxide film obtained is colorless, with slightly higher hardness and corrosion resistance, slightly lower porosity and adsorption. It can be sealed in a solution with a mass fraction of 2% sodium silicate. Used alone as a protective layer, suitable for oxidation of aluminum alloys containing heavy metals.
③ In order to improve corrosion resistance of workpiece after oxidation treatment, it can be passivated in 20g/L CrO3 solution at room temperature for 5 to 15 seconds, and then dried at a temperature below 50℃.

Note: ① Oxide film obtained by formula 1 is thin, has good toughness and good corrosion resistance. It is suitable for aluminum and aluminum alloys that need to be deformed after oxidation. It can also be used for surface protection of aluminum castings. It does not require passivation or filling treatment after oxidation.
②pH value of solution in Formula 2 is 1.5 to 2.2. Resulting oxide film is thicker, about 1 to 3 microns, with good density and corrosion resistance. There is no change in size of parts after oxidation, and color of oxide film is colorless to light blue. It is suitable for oxidation treatment of various aluminum and aluminum alloys. After oxidation treatment in formula 2 solution, parts should be cleaned immediately with cold water, then filled with potassium dichromate 40~50g/L solution (when pH=4.5~6.5, use sodium carbonate to adjust), temperature is 90~95℃, time 5 to 10 minutes, wash and dry at 70℃.
③Oxide film obtained in solution of Formula 3 is colorless and transparent, with a thickness of about 0.3 to 0.5 microns. Film layer has good conductivity and is mainly used for deformed aluminum electrical parts.
④Formula 4 is suitable for pure aluminum, rust-proof aluminum, cast aluminum and other alloys. Oxide film is very thin, has good conductivity and corrosion resistance, low hardness, and is not wear-resistant. It can be spot welded or argon arc welded, but cannot be soldered. It is mainly used for aluminum alloy parts that require certain conductive properties.
⑤Oxide film obtained by Formula 5 is thin, about 0.5 microns, has good conductivity and corrosion resistance, has few pores, and can be used as a separate protective layer.

 

Aluminum is a relatively active metal with a standard potential of -1.66v. It can naturally form an oxide film with a thickness of about 0.01 to 0.1 microns in air. This oxide film is amorphous, thin and porous, and has poor corrosion resistance. However, if aluminum and its alloys are placed in an appropriate electrolyte, aluminum product is used as anode, and an oxide film is formed on the surface under action of an external current. This method is called anodizing.
By selecting different types and concentrations of electrolytes, and controlling process conditions during oxidation, anodized films with different properties and thicknesses of about tens to hundreds of microns can be obtained, their corrosion resistance, wear resistance and decorative properties have been significantly improved.

Electrolyte used in anodizing of Al and aluminum alloys is generally an acidic solution with medium solubility, lead serves as cathode and only plays a conductive role. When aluminum and its alloys are anodized, following reactions occur at anode:
2Al ---> 6e-+ 2Al3+
Following reactions occur at cathode:
6H2O +6e----> 3H2 + 6OH-
At the same time, acid chemically dissolves aluminum and resulting oxide film, and reaction is:
2Al + 6H+---> 2Al3+ +3H2
Al2O3 + 6H+---> 2Al3+ + 3H2O
Growth process of oxide film is process of continuous generation and dissolution of oxide film.
The first section a (curve section ab): non-porous layer is formed. Within a few seconds to tens of seconds after power is turned on, a dense, highly insulating oxide film immediately forms on aluminum surface, with a thickness of about 0.01 to 0.1 microns. It is a continuous, non-porous film layer, called a non-porous layer or barrier layer. Appearance of this film hinders passage of current and continued thickening of film layer. Thickness of nonporous layer is directly proportional to formation voltage and inversely proportional to dissolution rate of oxide film in electrolyte. Therefore, voltage in section ab of curve shows a sharp increase from zero to maximum value.
The second section b (curve bc section): porous layer is formed. With formation of oxide film, dissolution of film by electrolyte begins. Since generated oxide film is not uniform, holes will be dissolved first in the thinnest part of film. Electrolyte can reach fresh surface of aluminum through these holes, electrochemical reaction can continue, resistance decreases, voltage decreases (decrease is 10 to 15% of maximum value), and a porous layer appears on membrane.
The third section c (curve section cd): porous layer thickens. After anodizing for about 20 seconds, voltage enters a relatively stable and slow rising stage. It shows that while non-porous layer is continuously being dissolved to form a porous layer, a new non-porous layer is growing. That is to say, formation speed and dissolution speed of non-porous layer in oxide film have basically reached a balance, so thickness of non-porous layer no longer increases and voltage change is also very small. However, formation and dissolution of oxide film at the bottom of hole did not stop at this time. They were still continuing, causing bottom of hole to gradually move toward inside of metal matrix. As oxidation time continues, holes deepen to form pores, and film layer with pores gradually thickens. When film formation rate and dissolution rate reach a dynamic balance, thickness of oxide film will not increase even if oxidation time is extended, and anodizing process should be stopped at this time. Anodizing characteristic curve and oxide film growth process are shown in figure below.

There are many methods for anodizing aluminum and its aluminum alloys. Commonly used ones include sulfuric acid anodizing, chromic acid anodizing, oxalic acid anodizing, hard anodizing and porcelain anodizing.

Aluminum and its alloys are anodized by direct current and alternating current in dilute sulfuric acid electrolyte, a colorless and transparent oxide film with a thickness of 5 to 20 microns and good adsorption can be obtained.
Sulfuric acid anodizing process is simple, solution is stable, easy to operate, allows a wide range of impurity content, consumes less electricity, has low cost, can be applied to processing of almost all aluminum and various aluminum alloys, so it has been widely used in China.
Following table shows several typical anodizing processes:

Main factors affecting quality of oxide film are:
① Sulfuric acid concentration: usually 15% to 20%. As concentration increases, dissolution rate of membrane increases, growth rate of membrane decreases, membrane has high porosity, strong adsorption, elasticity, good dyeability (easy to dye dark colors), but slightly poor hardness and wear resistance; when concentration of sulfuric acid is reduced, growth rate of oxide film is accelerated, film has fewer pores, high hardness, and good wear resistance.
Therefore, when used for protection, decoration and pure decorative processing, upper limit of allowable concentration, that is, 20% concentration of sulfuric acid is often used as electrolyte.
② Electrolyte temperature: Electrolyte temperature has a great influence on quality of oxide film. As temperature increases, dissolution rate of film increases and film thickness decreases. When temperature is 22 to 30℃, resulting membrane is soft and has good adsorption capacity, but poor wear resistance; when temperature is greater than 30℃, film becomes loose and uneven, sometimes even discontinuous, and has low hardness, thus losing its use value; when temperature is between 10 and 20℃, oxide film generated is porous, has strong adsorption capacity, is elastic, suitable for dyeing, but hardness of film is low and wear resistance is poor; when temperature is lower than 10℃, thickness of oxide film increases, hardness is high, and wear resistance is good, but porosity is low. Therefore, temperature of electrolyte must be strictly controlled during production. To produce a thick and hard oxide film, operating temperature must be lowered. Compressed air stirring and relatively low temperature are used during oxidation process, usually around zero for hard oxidation.

 

③Current density: Within a certain limit, when current density increases, film growth rate increases, oxidation time shortens, resulting film has many pores, is easy to color, hardness and wear resistance increase; if current density is too high, surface of part will be overheated and local solution temperature will increase due to influence of Joule heat, dissolution rate of film will increase, and there is a possibility of burning part; If current density is too low, film growth rate will be slow, but resulting film will be denser, hardness and wear resistance will be reduced.
④ Oxidation time: Selection of oxidation time depends on electrolyte concentration, temperature, anode current density and required film thickness. Under same conditions, when current density is constant, growth rate of film is proportional to oxidation time; but when film grows to a certain thickness, film resistance increases, which affects conductivity, film's dissolution rate increases due to temperature rise, so film growth rate will gradually decrease and will no longer increase in the end.
⑤ Stirring and moving: It can promote convection of electrolyte, strengthen cooling effect, ensure uniformity of solution temperature, and will not cause quality of oxide film to decrease due to local heating of metal.
⑥Impurities in electrolyte: Impurities that may exist in electrolyte used for aluminum anodization include Clˉ, Fˉ, NO3ˉ, Cu2+, Al3+, Fe2+, etc. Among them, Clˉ, Fˉ, NO3ˉ increase porosity of membrane, make surface rough and loose. If its content exceeds limit value, it may even cause corrosion and perforation of workpiece (Clˉ should be less than 0.05g/L, Fˉ should be less than 0.01g/L); when Al3+ content in electrolyte exceeds a certain value, white spots or patchy white patches often appear on the surface of workpiece, which reduces adsorption performance of film and makes dyeing difficult (Al3+ should be less than 20g/L);  when Cu2+ content reaches 0.02g/L, dark stripes or black spots will appear on oxide film; Si2+ often exists in a suspended state in electrolyte, making electrolyte slightly turbid and adsorbed on film as brown powder.
⑦ Aluminum alloy composition: Generally speaking, other elements in aluminum metal reduce quality of film, obtained oxide film is not as thick as that obtained on pure aluminum, and hardness is also low. When anodizing aluminum alloys with different compositions, care should be taken not to perform them in same tank.

Chromic acid anodizing refers to technology of anodizing aluminum and its alloys using 5 to 10% chromic acid electrolyte. Oxide film obtained by this method has following characteristics: ① Thinner (compared to sulfuric acid and oxalic acid oxide films), about 2 to 5 microns, which can maintain original accuracy and roughness of workpiece; ② Soft and highly elastic, with almost no pores, and stronger corrosion resistance than sulfuric acid anodized films; ③ Opaque, color ranges from gray to dark gray, or even rainbow, so it is not easy to dye; ④ Due to small number of pores, film layer can be used without sealing treatment; ⑤ It has good binding force with organic matter, so it is often used as base layer of paint; ⑥ Compared with sulfuric acid anodization, cost is higher, and its use is subject to certain restrictions.
Following table shows several chromic acid anodizing processes:

Oxalic acid anodizing is an oxidation process that uses 2% to 10% oxalic acid electrolyte and passes through direct current or alternating current.
When using direct current for anodizing, hardness and corrosion resistance of resulting film are no less than those of H2SO4 anodized films. Moreover, since solubility of oxalic acid solution to aluminum and oxide films is small, a thicker oxide film layer can be obtained than in sulfuric acid solution; If AC current is used for oxidation, a softer and more elastic film layer can be obtained. Film layer of oxalic acid anodization is generally 8 to 20 microns, and maximum thickness can be 60 microns.

 

During oxidation process, as long as process conditions (such as oxalic acid concentration, temperature, current density, waveform, etc.) are changed, decorative film layers such as silvery white, golden yellow to brown, etc. can be obtained without need for dyeing.
Oxalic acid anodizing electrolyte is very sensitive to chloride ions, and if its mass concentration exceeds 0.04g/L, corrosion spots will appear on film layer. Mass concentration of trivalent aluminum ions is also not allowed to exceed 3g/L.
However, oxalic acid anodizing costs more and consumes more energy (because resistance of oxalic acid electrolyte is greater than that of sulfuric acid and chromic acid), solution is toxic, and stability of electrolyte is poor. Several oxalic acid anodizing processes are shown in table below.

Certain substances are added to electrolyte so that they are adsorbed in film layer while forming an oxide film, thereby obtaining a smooth, shiny, uniform and opaque oxide film similar to porcelain glaze and enamel color, which is called "porcelain anodic oxidation film" or “porcelain oxide film”. This kind of oxide film has good elasticity and good corrosion resistance, and can have a plastic appearance after dyeing. Resulting film thickness is approximately 6 to 25 microns.
Following are two methods of porcelain oxidation:
① Add salts of certain rare metal elements (such as titanium, thorium, etc.) to sulfuric acid or oxalic acid solutions: During oxidation process, due to hydrolysis of these salts, chromogenic substances are deposited in pores of oxide film, forming a glaze-like glaze. Film layer has high hardness, can maintain high precision and smoothness of parts, but it is expensive, solution has a short service life, and process conditions are strict.
② Use a mixture of chromic anhydride and boric acid as anodizing liquid: simple ingredients, low cost, good elasticity of the oxide film, but lower hardness than previous one, and can be used for general decorative porcelain oxidation surface treatment. Porcelain anodizing solution and process conditions are shown in table below.

Note: Cathode material can be pure aluminum, lead plate or stainless steel plate.
In oxidation solution, changes in various components will determine color of oxide film: for example, as amount of chromic anhydride increases, color of film changes to opaque gray; as amount of boric acid increases, color of film changes to milky white; With increase of oxalic acid, color of film layer changes to yellow.
For rear more, please refer to Surface treatment 3: hard oxidation.