Application of Surface Pretreatment Technology for Aluminum Etching

[China Aluminum Industry Network] 1. Process Overview In recent years, the production and sales volume of aluminum alloy matt profiles has been increasing, with a market share of over 80%. At present, the production of matt profiles generally uses alkaline etching methods, and the aluminum dissolved losses range from 3% to 5%, which results in huge waste of resources, greatly increases the difficulty of environmental protection management, and aggravates the “three wastes” processing burden of the company. Acid etching technology is developed for the lack of alkali etching method, in principle, completely different from the alkaline pitting matt surface treatment. The process mainly adopts a unique method to generate a highly uniform and high-density molecular-point corrosion on the surface of the aluminum profile. By improving the material structure of the profile surface of the profile, the effect of smoothing the surface and eliminating the surface defects caused by the metallurgical process can be achieved (for example, the extrusion can be eliminated. Indentation, masking of the weld line, and mitigation of defects such as coarse grains, make the aluminum surface smooth and delicate, soft metallic luster, greatly improving the surface quality of aluminum profiles. With the acid-etched matt process, the aluminum melt loss is between 0.5% and 0.8%, which reduces the production cost to less than half of the traditional matt process, and the production efficiency has a substantial increase, which is of great practicality. Economic Value.

2. Acid etching mechanism Acid etching is chemical etching under certain conditions. The aluminum profile contacts the etching solution and reacts rapidly to form the complex in a very short time. The reaction equation is as follows:




The resulting complex has a certain viscosity and easily adheres to the surface of the aluminum substrate to form a protective film. Extruded profiles generally have many slight marks. They are peaks observed under high power microscope. The protective film adhering to the aluminum matrix is ​​thicker in the recesses, thinner at the bulge, and has a greater chance of contact with the etching solution at the thin parts. , can continue to dissolve faster; thick areas are shielded by adherents to slow down the dope reaction rate, due to the difference in the rate of dissolution of the concave and convex parts, so as to achieve a smooth surface, eliminate extrusion marks (sand surface) Asia Light effect. Relieving coarse grain and weld line is directly related to the specific properties of acid etching. In the production of aluminum profiles, coarse grain and weld line are closely related to the die, extrusion process and the homogenization of cast rods. The deeper the alkali etching, the more severe the coarse crystal and the weld line. Due to the fast acid etching time and short time, the reaction has ended without severe coarse crystal and weld line, so the acid etching process can effectively relieve the coarse crystal and the weld line.

Acid etching additives in the acid etching process has the following main aspects: (1) Corrosion Inhibition: Since F- reacts with natural oxide film and aluminum matrix very quickly, and the reaction is severe, coarse grains and weld lines are prone to appear. Surface quality defects such as. In order to not only maintain a certain reaction speed but also not cause quality defects, organic compounds such as sodium citrate, sodium citrate, and glycerin are added to suppress the speed of acid etching, thereby achieving the corrosion inhibition effect. (2) Leveling effect: Extrusion profiles have a lot of slight marks, which are observed at high magnification under the peaks. Adding trisodium phosphate, diatomaceous earth, polyhydric alcohol and other compounds can protect the bottom of the profile and the peaks. Flatten to achieve a leveling effect. (3) Promoting effect: The addition of accelerating agent can fully spread the acid etching additive into the bath, and effectively enter the inner layer of the surface of the aluminum substrate covered by the complex, so that the reaction can be continued so as not to cause uneven corrosion of the profile. (4) Surfactant action; The addition of surfactants such as sodium dodecyl benzene sulphonate, sodium lauryl sulphate and the like can further enhance the deoiling ability, as well as generate a layer of foam on the surface of the bath, inhibit heat loss and troughs Liquid volatilization, more important role is to increase the stability of the bath, because the negatively charged surfactant can be adsorbed around the particles, so that the particles produce electrostatic reaction, so that the particles can be well suspended in the bath and not easy to precipitate . (5) Metal ions: The addition of Cu2+, Ag+, and other metal ions can prevent over-corrosion, making the sand surface more delicate, uniform, and clear sand surface.

3. Test 3.1 Test purposes Through the acid etching and alkaline etching process tests on aluminum profiles, the surface quality of profiles obtained in the two processes and the amount of aluminum dissolved in the production process were compared.

3.2 Test material Take the same batch, the same type of curtain wall profile as the test material. Curtain wall profiles, especially those with large geometric dimensions and thicker wall thicknesses, are prone to defects such as coarse grains and weld lines after the conventional alkali etching. They are representative of acid and alkali etching process tests.

3.3 Test Contents 3.3.1 Basic Alkaline Erosion Process Test Two samples were taken for alkali etching treatment in alkaline solution, and other processes were operated by conventional oxidation process.

3.3.2 acid etching process test Take two samples in the etching solution for acid etching. After the profile has been etched, washed, it is alkali-washed in caustic solution for 1 min, and other processes are operated by conventional oxidation processes.

3.4 Test results and analysis Observe the surface conditions of the 1# and 2# samples prepared by the alkaline etching process. The surface of 1# sample is finely surfaced with slight squeezing marks, and the coarse crystal and weld line are not obvious. The surface of 2# sample is clear on the surface of the sand, and no extrusion marks can be seen, but there is slight coarse crystal and the weld line is exposed. . It shows that with the increase of alkali etching time, the extrusion marks of the profile surface are flattened and the sand surface is more detailed and uniform, but defects such as coarse grain and weld line are easily exposed.

Observe the surface state of the 3# and 4# samples prepared by the acid etching process. The surface quality of the two sample parts is almost the same, and all of them show that the sand surface is delicate and soft, and the existence of extrusion marks, coarse crystals, and weld lines can hardly be seen. It shows that the acid etching process can make the profile get a good quality of the sand surface in a few minutes, the extrusion marks are eliminated, the welding line can be effectively covered, the coarse grain phenomenon is relieved, and the profile surface is not easy with the increase of etching time. Has caused corrosion.

The wall thickness of the 1# and 2# specimens before oxidation was measured to be 3.0mm, that of the 1# specimen after oxidation was 2.96mm, and that of the wall thickness was 0.04mm; the wall thickness of the 2# specimen was 2.92mm, and the wall thickness was measured. The loss of 0.08mm indicates that the alkali etching time will increase, the amount of corrosion of the profile will increase, and the production of sand matt profiles will be more than 3%.

The wall thickness of the 3# and 4# specimens before oxidation was 3.0mm, that of the 3# specimens after oxidation was not less than 2.99mm, and the wall thickness loss was not more than 0.01mm; the wall thickness measured on the 4# specimen was 2.99 Mm, the loss of wall thickness is not more than 0.01mm. It shows that the time of acid etching increases, the amount of molten aluminum in the profile increases little, and the production of sand-faced matt profiles is more than 0.5%.

3.5 Test conclusions Based on the above test results and analysis, it can be concluded that it is feasible to use acid etching technology for aluminum oxide surface treatment. Compared with alkaline etching, the loss of aluminum material after matt surface treatment of sandstone profiles is greatly reduced, and the surface quality of the profiles is improved, the pretreatment time is shortened, the production efficiency is improved, and the overall cost is reduced.

4. Production Applications 4.1 Process Degreasing - Washing - Etching - Washing - Alkali Washing - Washing - Washing - Neutralization - Washing - Conventional Anodizing (Circulation Filtration for Etching)

After the aluminum section is degreased and rinsed, it enters the etching process. Etching and washing with water followed by caustic washing for 30 s to 1 min to remove surface-bound complexes. Anodic oxidation (coloring) and sealing treatment are performed in the normal oxidation step after washing, neutralization, and the like.

The filter device can effectively ensure the slag removal effect of the bath. Some manufacturers circulating filtration device is only a plate and frame filter press. With the increase of filter residue, the circulation flow rate of the bath liquid is insufficient, resulting in bad slag removal and affecting the use of bath liquid.

4.3 Common Faults and Countermeasures (1) The surface of the spotted spot has local spotting on the surface of the spot profile, which indicates that the slag in the bath is bad, and the etched slot has aluminum slag. It is required to improve the liquid discharging method of the bath and increase the amount of aluminum slag recovered.

(2) The matte surface produced by the matte surface is rough and matt, indicating that the content of the accelerator in the acid etching additive is insufficient, and the accelerator content needs to be added.

(3) There are mechanical lines on the surface of the profile produced by mechanical patterns on the surface, indicating that the etching time is not enough, or the content of acid etching additive in the bath is low, and the content of ammonium hydrogen fluoride or additives must be added.

(4) Excessive surface corrosion on the surface of the profile indicates that the etching time is too long, or the amount of corrosion inhibitors and metal ions in the acid etching additive is insufficient. The etching time needs to be adjusted and the content of the acid etching additive in the bath is replenished.

5. Conclusion The application of acid etching surface pretreatment technology has received good surface quality effects and considerable economic benefits. In order to improve the surface quality of products, reduce production costs and open up new ways for intensive and intensive production.

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