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Some problems in aluminum anodizing process

Aluminum has the term"" metals of the 20th century"" , and its greatest advantage is its smaller specific gravity, which is only 2.702, which is about 1/3 of that of copper (specific gravity, 8.9), iron (specific gravity, 7.9), and aluminum products, therefore, have the advantages of being light weight.Aluminum and aluminum alloys also have a series of advantages, such as higher mechanical strength, ease of processing, excellent thermal and electrical conductivity, and no magnetic properties, so they are widely used in aviationIndustries such as daily hardware, instrument and instrument and machine manufacturing.Aluminum and its alloy surface is easy to generate an extremely thin oxide film, in the atmosphere has some corrosion resistance, but this layer of oxide film is amorphous, it loses the original gloss on the surface of aluminum pieces, and the film layer is loose and porous, corrosion resistance is not strong, but also extremely easy to stain.In order to improve the anti-corrosion ability of aluminum parts and increase the wear resistance of parts, aluminum and aluminum alloy products usually need to be treated with sulfuric acid anodization or hard anodization (for ease of narrative convenience, hereafter all are referred to as anodization treatment).In this paper, the author combined the five aspects of equipment and installation, solution formulation and maintenance, material selection, processing margin, quality failure prevention of the anodization processing process of aluminum pieces in machine manufacturing industry, to talk about several opinions and opinions.

1 Aluminum anodization equipment and installation

Compared with other metal plating processes, the equipment of aluminum anodization is more complex, and requires a freezer, air press, an air stirring tube and a cryovial to be placed inside the tank, in addition to an oxidation tank, rectifier, heater.Generally, the oxidation bath is a titanium trough, plastic trough or lined with lead iron trough, the cooling tube is a lead or titanium tube, and the heating tube is a titanium heating tube or an electric heater.The air stirring tube uses more plastic tubes, and the diameter is generally 20-25 mm, and the stirring tube is about 25 mm from the bottom of the tank, the diameter of small pores on the tube is 1-2 mm, and the pore distance is 50-100 mm.The total area of small pores should be within 80% of the cross-sectional area of the stirring tube.The consumption of compressed air is calculated as 0.8 m3 per minute per square meter of liquid surface, and the pressure of compressed air can be estimated as 1.47-1.96 kPa per meter of deep solution.For the selection of bar materials in a production line, one view is that no copper or copper alloy is permitted, whereas bars made of aluminum or aluminum alloy should be used to prevent contamination of the solution by copper ions caused by drops of acid liquid held at the bars where parts enter and exit the troughs.The author argues that even when aluminum alloy poles are employed, they will cause accelerated accumulation of aluminum ions in solution.Effective ways are to reduce drops of acid onto the pole bar or to protect the pole bar.That is, we could properly extend the length of stay above the slots after the parts had fallen out of the slots so that the acid solution would drop as much back into the slots as possible, while also cutting the jacket at the poles with a plastic tube comparable to the pole thickness to protect the poles from acid corrosion.It is worth mentioning that titanium troughs, titanium heating tubes, and cooling tubes, which are currently adopted by many manufacturers, have a rough sand surface on the surface due to the poor quality of titanium, at which point, they are corroded by the dissolution of solution.So it turned out to be more reliable to use a lead iron slot and lead cooling tube.However, the lead cooling tube is adapted to be thick walled in case it is touched by the cathode plate.Nevertheless, due to the low melting point of lead, which is highly differentiated from the melting points of other metals, the welding technique and welding methods are highly demanding.

2 Aolution formulation and maintenance

Distilled and deionized water should be used for the preparation of solutions.Considering that distilled water preparation is slow and has little quantity, deionized water production is cumbersome, and the water output is not limited, the author applies the steam cooling method to obtain distilled water:put titanium vapor heating tube in cold water to steam and turn on tap water, the steam cooling inside the titanium tube into distilled water outflow, the method is simple and the water output is large.

Two aspects should be noted when formulating a solution:① sulfuric acid should be added slowly to the water with constant stirring, and it is cut that it is impossible to add water to concentrated sulfuric acid in order to prevent acid splashing;② The solution cannot be dispensed at once, preferably in several portions, and the freezer and air press are turned on for stirring when formulated.Corrode in case of exothermic induced increase in solution temperature.Titanium heat tubes, cooling tubes and slots, in severe cases, also cause leakage.Even with plastic slots, warming will deform slots and age by heat.Therefore, the right way to formulate the solution is to calculate the good tank capacity and the amount of sulfuric acid, add about 3/4 volume of water into the tank, open such equipment as a pump, freezer and air press, pour the sulfuric acid slowly into the tank, and when the solution temperature reaches 40 to 50 ℃, the sulfuric acid should be stopped, and the sulfuric acid should be added again after the solution has cooled, so repeatedly until the sulfuric acid is added to completion, then add water to the prescribed volume.

The maintenance of the solution is mainly to analytically adjust the sulfuric acid content.Due to the dissolution of aluminum during anodization, there will always be aluminum ions in the solution, so the sulfuric acid content cannot be controlled by the method of measuring density.In actual production, what we call sulfuric acid content refers to the content of free sulfuric acid.The method of determination of free sulfuric acid can be referred to the book analysis of common plating solutions, which was edited by Wuhan materia medica press, Wuhan, China.

Solution maintenance also includes control of impurity ion content.During the anodization of parts, various heavy metals in aluminum alloy will also enter the solution, and a part of them will precipitate on the cathode plate, so the cathode plate should be brushed frequently to remove the heavy metals.Practice has proven that as long as we can effectively avoid copper ion contamination from dissolution of the solution by copper bars and copper ticks on the oxidation tank and frequently brush the cathode plate, the effect of heavy metal impurities on the lifetime of the solution is generally modest.In this way, the main threat to the solution lifetime would come from the aluminum ions generated from the dissolution of aluminum pieces.The aluminum materials used for parts of machine manufacturing industry are more complex and there are both pure aluminum, antirust aluminum, hard aluminum, wrought aluminum and other deformed aluminum alloys, and cast aluminum alloys.Tank treatments (i.e., compounding several oxidation tanks with different concentrations) for parts of different materials would obviously be unethical and unrealistic.That is, generally only one oxidation slot can be used to treat parts of different materials separately.The concentration of the solution in this case should be controlled around 220 g/L, which takes care of hard aluminum positive polarization and in turn minimizes the dissolution of aluminum as much as possible.To avoid the accumulation of aluminum impurities to the concentration limit, the anodization solution should be regularly filtered once a year for clarification and the bottom precipitate discarded, so that both the solution was partially replaced and the accumulation of aluminum can be retarded.

One easily overlooked issue in solution maintenance is liquid level control.Generally, the spring summer season is characterized by high air humidity and temperature, with the water in the air becoming cold and condensed on the liquid surface, which will gradually increase and the concentration will gradually decrease;The climate is dry in autumn and winter, the air temperature is not very different from the liquid temperature, the solution volatilization is large, the liquid level will gradually become lower, and the concentration will gradually increase, and the skilled person should pay attention to make a good adjustment of the solution concentration according to this change.

3 Material selection

The composition and heat treatment state of aluminum alloys have a great influence on the anodic oxide film appearance and properties produced on its surface.So, if there are special requirements for the appearance and performance of the part, the material should be selected specifically.The effect of aluminum alloy composition is mainly the effect of other elements in the alloy on the appearance of the membrane layer, such as high-purity aluminum produced membrane is colorless and transparent, copper in the alloy will make the membrane layer dark, zinc will make the membrane layer opaque opalescent, and high silicon content will make the oxide film dark gray.In addition, the hard anodization of aluminum copper alloy, aluminum silicon alloy, and aluminum manganese alloy is more difficult and easily ablated.When the copper content in the alloy is more than 5% or the silicon content is more than 7%, direct current oxidation is not suitable to be adopted, but instead, alternating current superimposition or pulse current should be employed for the oxidation.The thicker hard aluminum plate has one part of cladding and bare aluminum, the cladding is only a thin layer of purer aluminum at the surface, the inner layer has many impurities, so the surface is not suitable for mechanical treatment, even the alkali treatment and chemical polishing treatment are careful, not excessive, because once the bottom layer is exposed a color oxide film is generated to cause end-of-life.The heat treatment affects the structure and properties of the oxide film by changing the internal microstructure, such as lattice composition, arrangement sequence, and grain size of aluminum alloys.For example, aluminum materials tempered or qualitatively treated by heat treatment eliminate the internal stress inside the part, so that the lattice composition is more stable and more orderly arranged, and the part has darker color after hard oxidation, while the part that is not tempered or qualitatively treated, has lighter color of hard oxide film.Aluminum alloy parts can be refined by the method of heating above the critical temperature and then cooling in still air, so that the grains, the fine grains react uniformly during the anodization process and have consistent color and appearance, in contrast, the coarse grains react unevenly during the anodization process and easily show orange peel appearance.It is worth noting that the oxidation film color can also be inconsistent with the same material grade but different status.Table 1 lists several common aluminum alloys state symbols and state meanings.Table 2 lists the temperatures for solution treatment and artificial and natural aging for several common materials.The surface of aluminum materials that have been heat-treated (including wrought) has a thick layer of oxide crust that is often overlooked due to insignificant color and should be treated clean before anodization by mechanical or chemical methods.

  Table 1 common aluminum alloy status symbols and cast aluminum status codes

MaterialStatus Symbol,Code StatusMeaning
Aluminum AlloyRHot Processing
SArtificial Aging
YCold Hardening
Y2Semi-cold Hardening
ZNatural Aging
MOAnnealed high quality surface (plate)
JMThickened Aluminum-Coated Annealing (Plate)
JCZThickened Aluminum Coated Solution Treatment plus Natural Aging (Plate)
CZSolution Treatment with Natural Aging
CZYSolution Treatment with Natural Aging and Cold Hardening
CSSolution Treatment with Natural Aging
CSYSolution Treatment with Artificial Aging and Cold Work Hardening
CSolid Solution Treatment
BMAluminum-Free Annealing (Plate)
BCZAluminum-free Solution Treatment plus Natural Aging (Plate)
BCZOAluminum-Free Solution Treatment with Manual Aging High Quality Surface (Plate)
BCZYAluminum-Free Solution Treatment with Natural Aging and Cold Hardening
Cast aluminumT1Artificial aging of T1 without solution treatment
T4Solution treatment
T5Solution treatment followed by incomplete artificial aging
T6Solution treatment followed by artificial aging
T7Solution treatment followed by stabilization treatment
T8Solution treatment followed by softening treatment

Table 2 solution treatment, artificial aging and natural aging temperature of several common aluminum materials

MaterialSolid solution
treatment temperature
Artificial agingNatural aging
LY11495~505℃ water-cooling——96 hours at room temperature
LY1290~503℃ water-cooling185~195℃  12hour96 hours at room temperature
LY10515~520℃ water-cooling75±5℃    24hour4 days and nights at room temperature
LY2495~505℃ water-cooling165~175℃  16hour——
LY16530~540℃ water-cooling160~170℃  16hour——
LD2510~530℃ water-cooling150~165℃  15hour96 hours at room temperature
LD5505~525℃ water-cooling150~160℃  15hour96 hours at room temperature
LD7525~535℃ water-cooling185~195℃  12hour——
ZL104535±5℃  water-cooling175±5℃   10hour——
ZL105525±5℃  water-cooling180±5℃   10hour——
ZL201540±5℃  water-cooling175±5℃    5hour——
LC4465~475℃ water-cooling115~125℃  24hour——
LC9460~475℃ water-cooling135±5℃   16hour——

4 Processing margin  

Most parts in the machine manufacturing industry have high accuracy requirements, and the size of parts will increase after hard anodization.Therefore, in mechanical processing, the size before anodization should be determined according to the film thickness and dimension tolerance, so that the parts can meet the prescribed tolerance range after treatment.In practice, in order to achieve standardization and standardization, the method of ensuring the quality of parts by process is often used, that is, when operating strictly according to the technical requirements of the process, the oxidation time is set to 50-60 minutes, the thickness of the film is no longer specified, and the hardness of the film is checked regularly.In this way, the uniform volume can be achieved in determining the dimension tolerance of the machine.Considering the operating habits and psychology of the operators in the process of machining," upper tolerance of shaft type, lower tolerance of hole type" , the upper tolerance diameter of the dimension can be reduced by 60 after the axle parts are oxidized to survive according to the design.μm, lower limit minus 50μm;Reduce the upper diameter of porous parts by 50 after they have been oxidized to survive as designedμm, lower limit minus 60μm, as the margin of hard oxidation process.

5 Quality failure prevention

Generally speaking, as long as the concentration of solution, temperature and impurity concentration are well controlled, as well as the current, voltage and time of operation, the quality of aluminum anodization is stable.However, due to the obvious quality failure of aluminum anode oxidation, once quality problems occur, they often need to be re-treated or scrapped.Therefore, the most important thing is to focus on prevention, to do a good job in daily maintenance, and to do a good job in various pretreatment work.Common tasks are:

(1) The parts should be anodized as soon as possible after pre-treatment, preferably no more than 4 hours;

(2) Workers who attach parts should wear clean gloves to avoid contaminating the aluminium parts;

(3) Brush" V" type conductive base, conductive pole and fixture hook once a week to ensure good electrical contact;

(4) Remove the cathode plate and scrub it carefully once a week;

(5) Remove foam oil and other floating objects floating on the liquid surface regularly every month;

_To ensure that the parts and hangers are clamped firmly and the parts that fall into the bottom of the slot should be retrieved in time, the fixture for each mounting part should be retracted from the old oxide film before it can be used;

The electrolyte is clarified and filtered at least once a year, and the bottom sediment is removed to keep the solution clear and transparent;

_Apart from the requirement of conduction between the parts and the contacts, other parts of the fixture should be protected by insulation and checked regularly, and broken fixtures should be discarded and not used resolutely.

The ablation of hard anodized parts usually occurs at about 28V, and is not easy to ablate beyond 30V.Therefore, the rate of current rise can be slowed down when the current is about 25V, and then increased when the current is stable to avoid ablation.

Many quality failures are caused by mixtures, which can be found and avoided in the pre-treatment process if attention is paid to:materials with low content of copper and silicon, such as pure aluminum, rust-proof aluminium, etc., show white, Brown floating ash on the surface after alkali etching, while materials with high content of copper and silicon, such as forged aluminium, hard aluminium and cast aluminium, have brown floating ash on the surface after alkali etching.

6 Concluding remarks

The oxide film formed on the surface of aluminium and aluminium alloy after anodization has good protective decoration and wear resistance, and has been widely used in aviation, machinery, electronics and light industry.In recent years, our country's machine manufacturing industry has absorbed the experience of foreign counterparts and used the advantages of light weight and easy processing of aluminium and aluminium alloy to replace traditional steel parts and steel parts in machine parts.The proportion of aluminium parts in the whole machine parts has increased rapidly, greatly reducing the weight of the product, improving the processing efficiency and shortening the product manufacturing cycle.This paper talks about some problems in aluminum anodization from several aspects for your reference only.

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