Aluminum alloy materials are widely used in various fields due to their low relative density, weldability, easy forming and high mechanical strength, especially in aviation industry. Aluminum has a relatively strong affinity with oxygen in the air. Even in dry air, it is very easy to react with oxygen, and a thin layer of non-porous amorphous Al2O3 is formed on the surface. However, due to thinner oxide film naturally produced , poor wear resistance and corrosion resistance, far from meeting its application requirements, and cannot be used as a reliable protective layer. Therefore, in order to improve corrosion resistance of aluminum alloys, it is necessary to carry out surface treatment on aluminum alloys, and anodic oxidation is the most commonly used surface treatment method for aluminum and aluminum alloys. Anodized aluminum films have good mechanical properties, corrosion resistance and friction resistance. At the same time, surface of membrane has strong adsorption. Contact us at Sales02@szxinraowujin.com.cn.

1. Development of Aluminum Alloy Anodizing Technology
Aluminum alloy materials are widely used in various fields due to their low relative density,
weldability, easy forming and high mechanical strength, especially in aviation industry. Aluminum
has a relatively strong affinity with oxygen in the air. Even in dry air, it is very easy to react with
oxygen, and a thin layer of non-porous amorphous Al2O3 is formed on the surface. However, due
to thinner oxide film naturally produced , poor wear resistance and corrosion resistance, far from
meeting its application requirements, and cannot be used as a reliable protective layer. Therefore,
in order to improve corrosion resistance of aluminum alloys, it is necessary to carry out surface
treatment on aluminum alloys, and anodic oxidation is the most commonly used surface
treatment method for aluminum and aluminum alloys. Anodized aluminum films have good
mechanical properties, corrosion resistance and friction resistance. At the same time, surface of
membrane has strong adsorption.
In aviation, aluminum alloy is main material of aircraft body structure. Selection of structural
materials for aircraft design should have high specific strength and specific stiffness to reduce
structural weight of aircraft or increase economic benefits, and it should also have good
machinability. Skins, beams, ribs, stringers, bulkheads and landing gear on aircraft can all be
made of aluminum alloy. Aluminum alloys are divided into 9 types of structural materials
according to different alloying elements, as shown in Table 1.
Representation method of aluminum and aluminum alloy grades
Group Grade
Pure aluminum (aluminum content not less than 99.00%) 1XXX
Aluminum alloy with copper as main alloying element 2XXX
Aluminum alloys with manganese as main alloying element 3XXX
Aluminum alloys with silicon as main alloying element 4XXX
Aluminum alloys with magnesium as main alloying element 5XXX
Aluminum alloys with magnesium and silicon as main alloying elements 6XXX
Aluminum alloys with zinc as main alloying element 7XXX
Aluminum alloys with other alloying elements as main alloying elements 8XXX
Alternate Alloy Group 9XXX
In Table 1, 7XXX and 2XXX aluminum alloys are the most widely used. 7075 aluminum alloy has
advantages of high strength, good mechanical properties, and good corrosion resistance. 2024
aluminum alloy is a heat-treatable Al-Cu-Mg alloy with high strength and good machinability. It is
mainly used in important structures such as aircraft skins, frames, ribs and beams. Domestically
produced 2024-T351 large-scale thick plate will be selected for the first time in the development
of a certain type of aircraft. Whether this material can be installed and used on aircraft plays a
decisive role in the level of domestic production of aircraft materials.
In order to overcome shortcomings of aluminum alloy surface properties, expand scope of
application, and prolong service life, surface treatment technology is an indispensable part of use
of aluminum alloys. Aluminum alloy anodized film has good corrosion resistance, good organic
coating adhesion, high hardness and wear resistance, and is also a high-resistance insulating film.
Aluminum alloy anodizing technology can meet a variety of needs, so that aluminum alloy
surface can obtain many excellent qualities such as good corrosion resistance, good wear
resistance, good decoration, good adhesion and good functionality. It is a relatively in-depth and
comprehensive surface treatment technology researched and developed at present.

2.  1 Aluminum alloy anodizing technology
Anodizing of aluminum alloy is a dynamic equilibrium process of growth and dissolution of oxide
film. During anodizing, it is necessary to promote growth rate of oxide film to be greater than
dissolution rate of surface film layer. Aluminum alloy anodizing technology can be divided into
direct current anodizing, alternating current anodizing and pulse current anodizing according to
current form. According to properties of film layer, it can be divided into ordinary film, hard film,
porcelain film, bright modification layer and semiconductor barrier layer. According to electrolyte,
it can be divided into natural colored anodizing with sulfuric acid, chromic acid, oxalic acid and
mixed acid as main component of electrolyte.
 1.1 Anodic oxidation in sulfuric acid solution
Ordinary sulfuric acid anodizing can obtain 0.5 ~ 20.0μm film layer with good adsorption, which
is suitable for general protection or as bonding bottom layer of paint coating (such as aircraft
outer skin, etc.); sulfuric acid anodic oxidation film is porous, and porosity is about 35%; strong
adsorption capacity, easy to dye, widely used for decorative purposes. Sulfuric acid anodized film
has high corrosion resistance but has a great influence on fatigue performance of base material,
is not suitable for spot welds, riveted assemblies and parts that are prone to retaining electrolyte.
 1.2 Anodizing in chromic acid solution
Aluminum alloy chromic acid oxide film is much thinner than sulfuric acid oxide film, usually only
2 ~ 5μm, color is from off-white to dark gray, generally cannot be dyed, can maintain precision
and surface roughness of original parts. Film layer is soft and highly elastic, and will not
significantly reduce fatigue strength of substrate, but its wear resistance is not as good as that of
sulfuric acid anodized film. Chromic acid oxide film is dense and has a tree-like branch structure.
After oxidation, it can be used without sealing treatment, and has good adhesion to coating.
Under same thickness, its corrosion resistance is higher than that of unsealed sulfuric acid oxide
film. Because of high solubility of chromic acid to copper, aluminum alloys with a copper mass
fraction greater than 4% are generally not suitable for chromic acid anodizing. Chromic acid
anodization is more expensive than sulfuric acid anodization in terms of solution cost and power
consumption, and it will cause environmental pollution. Even if environmental protection
measures are taken, cost of process will be increased. Therefore, use is subject to certain
restrictions.
 1.3 Boric acid-sulfuric acid solution anodic oxidation
In addition to advantages of chromic acid anodized film layer, boric acid-sulfuric acid anodized
film layer also has good covering ability, low concentration of bath components, no Cr( Ⅵ ),
convenient bath treatment, less environmental pollution and energy saving, and is called an
"environmentally friendly" surface treatment method. Boric acid-sulfuric acid anodic oxidation
film is thinner, and stress of film layer is smaller. It is not prone to cracks like sulfuric acid anodic
oxidation, and like chromic acid anodic oxidation, oxide film has high elasticity and dense
structure.
 1.4 Oxalic acid solution anodizing
Aluminum alloy oxalic acid anodic oxidation can obtain an oxide film of 8-20 μm. Oxalic acid
hard anodic oxidation was widely used in Japan and Germany in the early days. Because oxalic
acid has a weak dissolving ability for aluminum alloy and its oxide film, obtained film layer is
relatively thick, high hardness, good wear resistance and corrosion resistance, has good electrical

3. insulation and protection properties. Conventional oxalic acid anodic oxidation process is very
easy to electric breakdown and ablation phenomenon occurs, qualified rate is low, and solution is
sensitive to chloride ions, required external voltage is higher, and energy consumption is higher.
Therefore, production cost is 3 to 5 times higher than that of sulfuric acid anodic oxidation. in
addition, oxalic acid is easily reduced to glycolic acid on cathode, and oxidized to carbon dioxide
on anode, stability of electrolyte is also poor, and color of oxalic acid oxide film is also easy to
change with process conditions, resulting in color difference in product, which is limited in
application.
 1.5 Phosphoric acid solution anodizing
Phosphoric acid anodization method of aluminum alloy was first studied and adopted by Boeing
Company of the United States. Phosphoric acid anodization treatment process is a weak acid
anodization treatment method. Compared with chromic acid or sulfuric acid anodization method,
it has advantages of environmental friendliness, low toxicity, low cost and easy control of process
parameters. Pore size of film layer formed by phosphoric acid anodic oxidation is relatively large,
which is convenient for filling functional materials such as lubricating substances. However,
compared with sulfuric acid film and oxalic acid film, phosphoric acid oxide film has a smaller
thickness, generally only a few microns, which is limited in application.
 1.6 Mixed acid anodizing
Mixed acid anodization is based on sulfuric acid, oxalic acid, etc., various organic acids and
inorganic salts are added. Mixed acid anodization can improve temperature range and oxidation
efficiency of anodic oxidation, improve hardness and wear resistance of film layer. However,
composition of electrolyte is complex and cost is usually higher than that of sulfuric acid
oxidation. Therefore, application of mixed acid anodic oxidation is also limited to a certain extent.
 2 Research status of aluminum alloy anodizing technology
In actual production of anodic oxidation of aluminum alloys, due to influence of factors such as
pretreatment process, anodic oxidation process and film sealing treatment, sometimes corrosion
resistance of anodized film cannot meet requirements specified by user, and failure of salt spray
test of test piece will directly affect normal production. Therefore, in order to overcome problems
and deficiencies of typical aluminum alloy anodic oxidation technology, improve performance of
anodized film, meet needs of high-efficiency and energy-saving production, researchers at home
and abroad have conducted extensive research mainly on pretreatment process, anodic oxidation
process and film sealing treatment.
 2.1 Pretreatment process
Purpose of pretreatment is to remove oxide film and oil stain on the surface of sample, and
prepare a clean, smooth and activated surface for anodic oxidation treatment. Pretreatment
process is basic guarantee for obtaining a good oxide film. Selection of pretreatment process
needs to consider processing conditions of parts, alloy composition, pollution degree and
product requirements, etc. In addition, attention should be paid to influence of operational
factors such as temperature, time and type of pretreatment materials. Usually, pre-oxidation
treatment of aluminum alloy mainly includes degreasing (removing oil), alkali etching, lightening
and water washing and other processes. Among them, the more critical and most prone to
problems is alkali etching step, which can remove oxides and other dirt on aluminum alloy
surface, not only has a cleaning effect on aluminum alloy surface, but also has an activation effect.

4. Temperature and time during alkali etching process should be strictly controlled. Low
temperature will cause uneven corrosion on aluminum surface, and high temperature will cause
excessive corrosion on aluminum surface; oil stain on the surface of bath liquid needs to be
cleaned up in time, water film should be confirmed to be continuous during inspection to ensure
that oil stain on the surface of parts is completely removed.
Yang Peixia and others found that effect of using acetone solution for degreasing is better than
that of detergent and alkaline degreasing. In order to achieve high-efficiency degreasing at room
temperature, ultrasonic cleaning technology is used in acetone solution. Ultrasonic waves can
strengthen degreasing process and shorten degreasing time. Zhao Yunqiang and others
conducted comparative analysis through experimental research and found that method of
removing film layer with acidic solution is better than that of alkaline solution. Xiao Zuodong et al.
changed traditional three processes of degreasing, alkali etching, and light emitting into one
process and used a bath solution to obtain a new process for removing surface oil, natural oxide
film and residual black hanging ash, exposing a smooth substrate. The process has less aluminum
loss and bath precipitation, does not need to wash bath frequently, reduces pollution, is
beneficial to environmental protection, saves resources and costs. Qiu Zuoqun uses a
three-in-one treatment agent (degreasing, alkali etching, one-time treatment in same tank for
light emission), which has a good light effect, eliminates toxic and harmful gases, simplifies
pre-treatment process, shortens process cycle, and reduces costs. Li Zhenfang carried out sulfuric
acid anodic oxidation treatment on ZL102 aluminum alloy. Through analysis, it was found that
pretreatment process had a great influence on quality of anodic oxide film. By improving
pretreatment process, thickness and corrosion resistance of oxide film were increased. Water
washing part is mainly to remove a large number of impurity ions brought in by the first three
steps, to ensure cleanliness of workpiece surface, to improve quality of oxide film layer, to avoid
contamination of bath by impurity ions, and to prolong use of bath.
 2.2 Anodizing process
Aluminum alloy anodizing mainly refers to process of forming an oxide film on aluminum
products under action of external current under corresponding electrolyte and specific process
conditions. It can be seen that factors affecting performance of anodized film in anodic oxidation
process mainly include electrolyte and additives, power supply type and optimal design, and
specific process conditions.
Sulfuric acid anodic oxidation process with sulfuric acid solution as main bath liquid has
characteristics of low cost, simple composition, stable process, convenient operation and wide
applicability. Problems of low temperature environment and high energy consumption are more
prominent. Anodic oxidation film prepared by chromic acid anodizing process with chromic acid
as main bath has characteristics of thin film layer, good corrosion resistance and excellent paint
adhesion, but chromic acid pollutes environment heavily and is not conducive to environmental
protection. In a patent, CM Wong et al. of United States proposed a room-temperature anodizing
method using H2SO4 and H3BO3 as electrolytes. Obtained film layer has excellent corrosion
resistance and bonding force with paint, and will not cause stress fatigue of substrate. It not only
retains advantages of sulfuric acid anodic oxidation and chromic acid anodic oxidation, but also
overcomes shortcomings of these two processes. Wang Guoyang et al. studied process of hard
anodic oxidation of LC9 aluminum alloy using a mixed acid solution based on sulfuric acid and
organic acids as additives, and obtained a uniform, dense, and flat oxide film layer. Zhang

5. Liaoyuan et al. added a certain amount of liquid additive WL-99 to conventional sulfuric acid
anodizing bath to conduct wide-temperature rapid anodizing experiments on 6063 aluminum
alloys. They found that adding additive could speed up film formation speed, increase upper limit
of operating temperature, and prolong service life of bath. Moutarlier et al. added molybdate or
permanganate to sulfuric acid anodic oxidation solution to increase thickness of barrier layer of
oxide film and improve corrosion resistance of film.
During formation of film layer, reaction heat and Joule heat will be generated. When heat reaches
a certain level, sparks will appear, resulting in "ablation" at the place where current concentration
of film layer is concentrated, affecting quality. Therefore, it is necessary to stir and increase heat
dissipation to improve anodic oxidation efficiency and improving film performance. At present,
there are many measures for stirring and heat dissipation at home and abroad, mainly including
high-speed pump to accelerate circulation of bath liquid, compressed air stirring or bath liquid
high-speed jet stirring and heat dissipation. Li Jie et al. used "vibration flow agitation" system
composed of vibration generation, vibration flow agitation and ceramic diffusion ventilator, which
can increase oxidation efficiency by 3 to 5 times. In addition, selection of hanger is also very
important. Material of hanging fixture must ensure good electrical conductivity. Generally, a hard
aluminum alloy rod is used. Plate must have a certain degree of elasticity and strength, hook
should be made of copper or copper alloy. If used special or general-purpose fixture is used again
during anodizing treatment, oxide film on the surface must be completely removed to ensure
good contact. Fixture must not only ensure sufficient conductive contact area, but also minimize
fixture marks. If contact surface is too small, it will cause burning and corrosion of anodized parts.
Zhou Chunhua and others used two kinds of hanging methods: hook type and clamp type.
Through research, they found that clamp type has characteristics of large contact area and good
electrical conductivity, but current calculation area is not accurate, and oxidation of clamp cannot
be carried out. In this way, hook must be treated before and after each oxidation to make it
conductive.
In terms of power supply type and optimal design, DC power supply only needs to be equipped
with a DC generator or a rectifier, and working current is set to a constant value, while oxidation
voltage changes with time, and its application is wider. Zhang Xing et al. used industrial pure
aluminum L2 as the experimental material, and prepared a black film layer on the surface of
aluminum alloy by sulfuric acid direct current anodic oxidation-electrolytic coloring process.
Wang Dizhen used AC anodizing technology to study aluminum alloys with high copper content
such as LY12, solved problems of low hardness of anodized film, breakdown, burning or edge
cracking and successfully applied it in production. Tang Shengguo et al. used AC and DC
alternating oxidation methods to change structure and composition of anodic oxidation film of
3005 aluminum alloy in sulfuric acid medium. K.Yokoyama et al. systematically discussed
advantages of using pulse power supply for anodization, pulse anodization can improve
performance of anodized film. Gu Lin et al. used self-made power supply equipment and
oxidation devices to study hard anodic oxidation of cast aluminum alloy ZL301 in 4 different
electrolytes, and concluded that the pulse power supply can improve the film formation speed,
hardness and wear resistance of hard oxide film. sexual advantage.
At present, typical anodizing technologies used for aluminum alloy protection in aviation industry
mainly include sulfuric acid anodizing, chromic acid anodizing and boric acid-sulfuric acid
anodizing, which have characteristics of simple electrolyte composition, easy analysis and

6. adjustment, easy control of oxide film thickness, simple and convenient process operation, etc.
Aiming at aviation aluminum alloy anodizing technology, AVIC Aircraft Xi’an Aircraft Branch has
also done a lot of work. Figure 1 shows surface morphology of oxide film of aircraft skin
aluminum alloy material after three different oxidation techniques. Figure 1(a) is Sulfuric acid
anodization, Figure 1(b) is chromic acid anodization, Figure 1(c) is boric acid-sulfuric acid
anodization. It can be seen from Figure 1 that pore diameter and density of the porous layer on
the surface of anodic oxide film are quite different, boric acid-sulfuric acid oxide film has fewer
pores and smaller pore size. Strip-shaped pores parallel to surface of sample can also be seen in
Figure 1, which is caused by unevenness of original surface and growth of pores along side.
Although sulfuric acid anodization will not cause pollution, impact on fatigue properties of
materials has always been a concern. Chromic acid anodizing has dense film layer and low
porosity. After anodizing, dimensional change of parts is small, which will not damage fatigue
strength of material. Electrical insulation of film layer is good, preventing galvanic coupling when
aluminum and other metals are in contact. It is widely used due to its advantages of corrosion.
However, chromic acid anodization will cause environmental pollution, and timely adoption of
environmental protection measures will also increase cost of process. For this reason, boric
acid-sulfuric acid anodizing process of Boeing standard BAC5632, which can replace chromic acid
anodizing, is used, and the anodized parts are shown in Figure 2. Practice has proved that in
addition to advantages of chromic acid anodized film layer, sulfuric acid-boric acid anodized film
layer also has good adsorption capacity, easy to dye various colors, and good covering ability,
which can maintain high precision and low properties such as surface roughness.
 2.3 Membrane sealing treatment

7. At present, common film sealing methods include hot water, steam, dichromate, inorganic salt
and organic matter, but there are common problems such as high toxicity, high energy
consumption, low efficiency or poor film quality, so development is useless. Green closed process
with pollution, stable process and low energy consumption has great application value. Green
sealing technology mainly includes microwave sealing, nickel-free medium temperature sealing,
rare earth salt sealing, applied voltage sealing and bidirectional pulse sealing, etc. Wang Zhutang
studied a new type of microwave hydration sealing method, which has advantages of
conventional sealing methods and overcomes their shortcomings, proves its effectiveness in
aluminum alloy anodic oxidation sealing treatment from both theoretical and experimental
aspects. Zhang Yongguang has developed a new nickel-free room temperature sealing agent with
potassium fluozirconate as main salt. This sealing process can not only achieve effective sealing
under condition of low fluoride ion concentration, but also eliminate pink frost and green film
surface that appear in Ni-F sealing. Zhang Jintao et al. performed anodic oxidation treatment on
LY12 aluminum alloy to obtain an oxide film, and added rare earth salt cerium nitrate in process
of sealing oxide film by using rare earth salt sealing method.
Results show that addition of Ce ions enhances corrosion resistance of aluminum alloy matrix. Li
Yang et al. carried out preparation and corrosion resistance of aluminum-based
superhydrophobic surface. After aluminum-based superhydrophobic surface was anodized, a
micro-nano structure was constructed on the surface, and hydrophobicity of aluminum-based
surface was enhanced after silanization treatment, and corrosion resistance of substrate sexual
enhancement. Zhao Jingmao and others studied sealing effect of cerium salt solution on LY12
aluminum alloy anodized film under action of pulsed electric field. Aluminum alloy anodized film
obtained by this sealing process has a smooth and even surface, is superior to traditional sealing
method in terms of corrosion resistance, reduces cost and energy consumption, and has no
pollution to environment.
Bautista et al. studied sealing effect of triethanolamine solution on anodic oxidation film of
aluminum alloy, and believed that triethanolamine can accelerate sealing of anodic oxidation film.
Yang Weichun et al. used acid dipping mass loss method to study influence of magnetic field on
formation of anodic oxide film of aluminum and its sealing treatment. Magnetic field can increase
membrane cells, reduce porosity of oxide film, improve normal temperature sealing and
hydrolysis salt sealing of oxide film. Sealing degree improves sealing quality of oxide film. Li
Yongxing et al. studied sealing of ammonium adipate on 2024-T3 aluminum alloy anodic
oxidation film under condition of DC voltage. Results show that ammonium adipate sealing can
effectively close micropores of porous layer under condition of DC voltage, fill defect holes in
anodized film, repair defects in barrier layer, and increase thickness of barrier layer. Sealing effect
of potassium chromate is equivalent, and it is better than that of boiling water.
 3 Conclusion
Anodizing technology is an important surface treatment process in production of aluminum alloy
parts on aircraft. In recent years, with development of modern industry, improvement of people's
living standards and enhancement of environmental protection awareness, aluminum and its
alloys will play an increasingly important role in people's daily production and life. Therefore,
requirements for anodizing process of aluminum alloys are getting higher and higher, functional
requirements are becoming more and more diverse. In production practice, it is necessary to

8. conduct long-term observation, analysis and summary of many factors that affect performance of
anodized films, to master methods to improve performance of anodized films, and to provide
guarantee for obtaining satisfactory product quality.
Domestic aluminum alloy anodizing focuses on oxide film thickness, hardness and corrosion
resistance. In addition to focusing on oxide film thickness, hardness and corrosion resistance,
foreign countries are also actively developing process closed-cycle recovery systems, moving
towards direction of zero-emission clean technology. Therefore, we need to actively carry out
new process research on the basis of existing aluminum and aluminum alloy anodic oxidation
research, improve film quality, reduce energy consumption, reduce production costs, and strive
to develop high-efficiency, energy-saving, simple process and environmentally friendly
treatments. process to meet needs of my country's aviation industry for anodized films with
excellent performance and diversified functions.
For more details, contact me at thomascasting001@gmail.com